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Year 1: Medical Imaging > Overall > Flashcards

Overall Flashcards

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USMLE® Step 1 flashcards
1
Q

What is an analogue image type?

A

Photograph film

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2
Q

What are the two types of medical imaging?

A

Anatomical and functional/physiological

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3
Q

X-ray modalities use photons to measure differences in what?

A

Attenuation coefficient

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4
Q

When can x-ray radiographs or CT be considered functional imaging?

A

If the attenuation coefficients can change eg multiple phases

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5
Q

Images can be formed via emission or transmission, what are these and which modalities are they related to?

A

Emission of information from object to detector eg nuclear medicine
Transmission of information from source through object to detector eg CT

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6
Q

What is the point spread function in an imaging system?

A

The finite distribution of signal in the image domain after a point source has been imaged

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7
Q

Does an imaging system have a 1:1 ratio of each point in an object to its image or does all parts of an image get contribution from all parts of an object?

A

The second type, with the amount of contribution varying across the image so most of the contribution goes to the right place

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8
Q

What is it called when the point spread function depends only on the relative displacement of the points in the image and the object?

A

Shift invariant

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9
Q

What does it mean for a point spread function to be a linear system?

A

The contribution to the image from any point is proportional to the strength of signal at that point

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10
Q

For linear, shift invariant imaging systems, the image is a convolution of the object with what?

A

The point spread function (PSF)

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11
Q

What are different measures of spatial resolution (or sharpness)?

A

Number of details per unit distance (lp/mm), width of the PSF (FWHM), modulation transfer function

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12
Q

What is spatial resolution?

A

Ability to separate object details within an image

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13
Q

Is the line pair spatial resolution test subjective or objective?

A

Subjective

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14
Q

Why is the full width half max (FWHM) of the PSF a useful measure of spatial resolution?

A

Two points need to be at least FWHM distance apart to be seen as two peaks on an image and be resolved

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15
Q

Is spatial frequency the frequency of change per unit distance across an image?

A

Yes, it is a measure of how often sinusoidal components of the structure repeat per unit of distance.

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16
Q

If the line pairs are closer together on a test object, is this considered low or high spatial frequency and does this have high or low standard deviation?

A

High and low (eg less variation in signal as harder to resolve)

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17
Q

What is the square wave response as a measure of spatial resolution and what would you expect to see?

A

It plots a measure of amplitude of variation (eg standard deviation) against spatial frequency. Low frequency = high standard deviation (or modulation) and high frequency = low standard deviation (or modulation) (eg less difference between
objects - harder to resolve)

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18
Q

What is the modulation transfer function?

A

The ability to transfer contrast at a particular resolution from the object to the image and it is a curve as a function of spatial frequency. Fourier transform of the line spread function.

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19
Q

What modulation transfer points do we usually refer to?

A

50 or 10

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20
Q

What is aliasing?

A

The overlapping of frequency components resulting from a sample rate below the Nyquist rate, which creates artefacts and/or distortion

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21
Q

What is the Nyquist frequency, as referred to with aliasing?

A

The highest frequency that can be coded at a given sampling rate in order to fully reconstruct the signal, and it is half the sampling rate, eg sampling rate needs to be twice the frequency

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22
Q

What is the definition of contrast?

A

Difference in image signal between a feature in the image and its neighbourhood

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23
Q

What is the minimum detectable contrast?

A

The contrast level which can be distinguished from noise

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24
Q

What are the two definitions of contrast and their equations?

A

Michelson (absolute difference between max intensity and min intensity divided by sum of them)
Weber (absolute difference between max intensity and min intensity divided by minimum intensity). Sometimes defined where max intensity = feature and min intensity = background

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25
Q

When is it most useful to use Michelson and Weber contrast?

A

Michelson = periodic contrast
Weber = localised contrast against uniform background (more clinically relevant)

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26
Q

Does adding background information (eg scatter) increase or decrease contrast?

A

Decrease

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27
Q

What is the definition of noise?

A

Variation is measured signal for a constant input signal

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28
Q

What are the two types of noise and what is examples of each?

A

Systemic (eg structural interference) and random (eg quantum, poisson)

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29
Q

What is the equation for Poisson noise?

A

The square root of the signal, so an increase in signal increases the noise

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30
Q

Does the signal to noise ratio increase or decrease with increased signal?

A

Increase

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31
Q

How are contrast, resolution and noise all interrelated?

A

Resolution defined in terms of relative contrast. Poor contrast = hard to resolve. Contrast for small details degraded by poor resolution. Contrast detectability is noise limited.

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32
Q

What is geometric linearity in relation to image quality and another name for it?

A

Ability to image without spatial distortion so all point in the object are relatively in the same place in the image. Spatial accuracy

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33
Q

What are the two main methods of image reconstruction?

A

Filtered back-projection and iterative reconstruction

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34
Q

What is a sinogram in the image reconstruction process?

A

The 2D array of data of the projections around a cross section of the object in terms of projection angle and position

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35
Q

Simple back projection suffers from what image quality issue and what does this mean?

A

1/r blurring, where the image cross section is the convolution of the original cross section and the 2D 1/r function, which means the intensity of an imaged point source decreases with increasing distance from its centre

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36
Q

Where is the ‘filter’ applied in filtered back-projection?

A

Filter before the sinogram is back projected

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37
Q

How is the sinogram used in simple back projection?

A

Each projection slice on the sinogram is back projected by splitting the intensity of the sinogram slice at each point along the pixels it is aligned with, and this makes a set of lines of equal shade at each point, which is repeated for each projection angle

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38
Q

How does filtered back projection work?

A

Each projection angle profile is Fourier transformed and an appropriate filter is applied to each k-space profile. The inverse Fourier transform is then performed on each filtered profile and this is then back projected

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39
Q

What are the most common filter types in filtered back projection?

A

Ramp (Ram-Lak), Shepp-Logan, Cosine, Hamming and Hann

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40
Q

What does the ramp filter look like in k-space and what is a advantage and disadvantage of this?

A

Linear ‘ramp’ and it reduces the 1/r blurring effect but it also increases high-frequency noise

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41
Q

What solution is there in filtered back projection to reduce the high-frequency noise amplification from the ramp filter?

A

A filter that decreases the amplification at very high frequencies

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42
Q

Why are filters needed in image reconstruction?

A

Unfiltered images have high amplitude, low frequency data and little high frequency data

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43
Q

Is there more or less noise when there are more iterations in the iteration reconstruction technique?

A

More

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44
Q

When does the iterative reconstruction loop end?

A

When the simulated projections match measured projections

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45
Q

What is Ordered Subset Expectation Maximum (OSEM) in iterative reconstruction?

A

A subset of projections is used rather than all projections. Compare one subset, adjust estimate, compare next subset etc

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46
Q

Does using OSEM improve or reduce computation time in iterative reconstruction compared to other IR techniques?

A

Improve

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47
Q

What are the advantages of filtered back projection (FBP) and OSEM IR?

A

FBP: fast and uses real projection data
OSEM: better image quality

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48
Q

What are the disadvantages of filtered back projection (FBP) and OSEM IR?

A

FBP: artefacts likely and difficult to model physics
OSEM: computationally expensive and estimates projections not object

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49
Q

Is noise typically high or low spatial frequencies?

A

High

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50
Q

What do smoothing filters do for images?

A

Average nearby pixel values and reduce deviation

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51
Q

What is image segmentation?

A

It is a method of dividing a digital image into segments

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52
Q

What are parametric images?

A

A pixel in an image contains a calculated parameter (other than the customary parameter), an example of this is time to max value for functional imaging

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53
Q

What are CT numbers and what is there unit?

A

It is related to the linear attenuation coefficient of a type of tissue or material and Hounsfield units (HU)

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54
Q

Most medical images are digital with how many bits?

A

16 (2 bytes)

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55
Q

How is the greyscale formed from digital images?

A

Pixel values are assigned a grey scale

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56
Q

What is image windowing?

A

The CT image greyscale component of an image is manipulated via the CT numbers and is done via window width and window level

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57
Q

Why is image windowing necessary?

A

Monitors can display a lot more grey shades and images can contain way more unique values than humans can distinguish, so need to compress information for human ability

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58
Q

What is the window width?

A

The range of CT numbers that an image contains. How many pixels either side of window level to spread greyscale over and outside these values the pixels are black or white

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59
Q

What is the window level?

A

Midpoint of the range of the CT numbers displayed

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60
Q

In image windowing, there is a trade-off between visible contrast and what?

A

Dynamic range

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61
Q

What image features can be affected by viewing conditions and why?

A

Light and glare can reduce perception of low contrast. Small details affected by viewing distance

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62
Q

What is signal detection theory?

A

There is a threshold above which an image is classified as positive, above which there may be false positive and below which there may be false negatives. This threshold can be moved to change the amounts of each and is dependent on the individual

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63
Q

What is the Receiver Operator Characteristic (ROC) curve and what would be ideal?

A

A graph of true positive against false positive, where ideally the area would be 1 (ie only true positives)

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64
Q

What is the difference between x-rays and gamma rays?

A

X-rays are emitted from processes outside the nucleus (electron shells) and gamma rays originate from inside the nucleus

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65
Q

What are characteristic x-rays?

A

Electrons moving into different energy states (usually K-shell after its electron has been ejected) and emit photons as they do so with discrete energies

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66
Q

What are alpha and beta characteristic x-rays?

A

Alpha x-rays occur when an electron from the shell above fills the gap, whilst beta x-rays are from two shells above. There are typically K-alpha, K-beta, L-alpha and L-beta x-rays.

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67
Q

What are bremsstahlung x-rays?

A

They occur in a spectrum when the electrons decelerate and a photon is emitted to conserve momentum

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68
Q

Where is the primary and secondary beam in the imaging process?

A

Primary beam is before it interacts with the imaged object and secondary beam is what has come out after the imaged object, which will then hit the detector

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69
Q

What is the basic premise of an x-ray tube?

A

A cathode and anode sealed within a vacuum

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70
Q

Why does an x-ray tube contain a vacuum?

A

The electrons are not stopped along the way from the cathode to anode

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71
Q

Why is there a copper stem attached to the target in an x-ray tube?

A

To get rid of the heat as it has high conduction

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72
Q

Where is the x-ray window in an x-ray tube and why?

A

X-rays are produced predominantly 90 degrees from the anode

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73
Q

What contains the vacuum in the x-ray tube?

A

The glass envelope

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74
Q

What is the source of the electrons in an x-ray tube and how?

A

The cathode, which is a heated filament, and this produced electrons via thermionic emission

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75
Q

What is the cathode typically made of in an x-ray tube and what is its melting point?

A

Tungsten and 3400 degrees

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76
Q

What is a focusing cup in an x-ray tube and what does it do?

A

It is a negatively charged, shallow depression on the surface of the cathode to concentrate the electron beam towards the focal spot on the anode

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77
Q

Is the anode positively or negatively charged?

A

Positive

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78
Q

What is the efficiency of the anode to create x-rays?

A

Less than 1%

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79
Q

How do we prevent heating of the x-ray tube?

A

High melting point of target, high thermal conduction (moves heat away from focal spot), larger focal spot and larger anode to conduct heat to

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80
Q

What is the anode typically made of?

A

Tungsten (set in copper), can also be copper or molybdenum depending on what energies are required

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81
Q

Why is the anode angled in an x-ray tube?

A

Smaller effective focal spot (out of window) with a larger actual focal spot and this equals higher heat dissipation

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82
Q

Why do we spin the anode in an x-ray tube?

A

It spreads out the electron impact to reduce the heat to an individual area and increase heat dissipation

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83
Q

Why is tungsten used in the anode of an x-ray tube?

A

It is high Z

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84
Q

What characteristic must an anode have in an x-ray tube?

A

High Z, high heat dissipation and ability to take a lot of mechanical strain (spinning at high rpm)

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85
Q

How do you change the size of the effective focal spot?

A

Size of filament (usually have 2 that can change between), angle of anode and construction of focussing cup

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86
Q

What are the differences in resolution and exposure time for small and large focal spot?

A

Small focal spot has a higher resolution than larger focal spot. Small focal spot has a slower exposure time than large focal spot to allow the heat to dissipate

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87
Q
A
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88
Q

What is the tube window made of in an x-ray tube and what does it do?

A

Typically beryllium and it allows the x-rays to exit whilst maintaining the vacuum

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88
Q

Why is it an issue to use mains power for an x-ray tube potential difference?

A

Alternating current is not good as the charge needs to move in one direction

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89
Q

What do we do to the mains power AC to be able to use it for x-ray tube potential difference?

A

Rectifier to flip over the negative side (charge moves in one direction) and layer waveforms to keep a constant waveform

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90
Q

Is the mAs of an x-ray tube referring to the current of the electrons traversing the tube or the applied current to the filament?

A

The current of electrons traversing the tube (tube current)

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91
Q

The potential difference of the x-ray tube is what?

A

The maximum kV of the electrons

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92
Q

What is the inherent filtration in an x-ray tube?

A

Beryllium window (minimal absorption) and self-absorption in the target

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93
Q

Why do we deliberately add filtration after an x-ray tube?

A

Reduces low energy soft x-rays that would be absorbed by the patients skin so it reduces the dose and measured tube output

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94
Q

How much added filtration is there after an x-ray tube?

A

At least 2.5 mm of aluminium equivalent

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95
Q

What is the heel effect?

A

Variation in x-ray intensity across the field in the cathode-anode direction (low at anode end)

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96
Q

What creates the heel effect?

A

X-rays generate in the target get re-absorbed in the target itself

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97
Q

When is the heel effect used to reduce patient dose?

A

Mammography by putting soft tissue at the anode side (decreased dose towards nipple)

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98
Q

Which characteristic governs the overall intensity of the whole x-ray spectrum?

A

Tube current measured in mA

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99
Q

How is the total number of electrons calculated in the x-ray tube?

A

Current times time

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100
Q

What is the difference between kV and kVp in an x-ray tube?

A

kV: mean voltage across the tube
kVp: peak voltage across the tube
(can be approx the same for a low voltage ripple)

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101
Q

What does increasing the kVp of the x-ray tube do?

A

Maximum and average energies are higher and more photons (higher intensity)

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102
Q

What type of x-ray interactions are desirable and undesirable in medical imaging?

A

Desirable: no interaction and complete absorption
Undesirable: scatter/deflection (inelastic is the main type as elastic scatter is minimal at these energies)

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103
Q

What effect is the absorption of x-rays in medical imaging?

A

Photoelectric effect

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104
Q

Photoelectric absorption probability is proportional to what?

A

Cube of atomic number divided by energy cubed

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105
Q

What is the k-absorption edge?

A

The abrupt increase in the photoelectric absorption of x-ray photons at energies just beyond the binding energy of k-shell electrons

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106
Q

Why is it a high z material (like lead) used for shielding?

A

Photoelectric absorption occurs more for higher Z (higher probability)

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107
Q

What is the mass attenuation coefficient of a material and its unit? (can also be mass absorption coefficient)

A

The linear attenuation coefficient normalised by the density of the material. Attenuation per unit mass (rather than per unit distance) (cm^2/g)

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108
Q

Compton (inelastic) scatter does what to the incident photon?

A

Changes the direction and energy (change in energy depends on scattering angle)

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109
Q

Is compton scatter directly or inversely proportional to the number of electrons/protons?

A

Directly

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110
Q

What happens to the contrast if we increase the kV and why?

A

It reduces because the chance of interactions (mostly photoelectric absorption) decreases so less attenuation

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111
Q

What ratio of the secondary beam (emerging from patient) is scattered radiation?

A

2:1 to 4:1

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112
Q

Is the secondary beam typically harder or softer?

A

Harder

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113
Q

Decreasing the kV can decrease the scatter (and increases contrast), but what other issues does this increase?

A

Scatter in patient is increased (interaction probability increased) so we need more mAs to get enough x-rays to the detector. The mean energy of the scatter is less so less leaves the patient

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114
Q

Why do we not want scattered radiation in the secondary beam?

A

It reduces contrast in the image and is a radiation risk

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115
Q

How can we reduce scatter in the patient?

A

Collimate the beam, reduce patient thickness, air gap between patient and detector (scatter doesn’t reach it), anti-scatter grid (physical collimation)

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116
Q

Why do some anti-scatter grids move?

A

It blurs the grid pattern on the image

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117
Q

What is the disadvantage of anti-scatter grids?

A

Increases patient dose as the collimator removes some useful radiation

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118
Q

Radiographic film is construction of what type of ions within an emulsion?

A

Silver Bromine

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119
Q

What is film screen?

A

Film is placed near a scintillator which converts x-rays to visible light, which the film is more sensitive to (so can use less dose) but decreased resolution

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120
Q

What is the equation for optical density?

A

The log of base 10 over the initial density divided by the transmitted intensity

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121
Q

What are the two types of digital x-ray imaging?

A

Computed radiography (CR) and digital radiography (DR)

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122
Q

Since digital imaging is pixel based, do we lose or gain information?

A

Lose as the signal has to fit into the pixel spaces so not continuous

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123
Q

What is the active material in computed radiography (CR) detectors?

A

Photostimulable phosphor (PSP)

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124
Q

How do CR detectors work?

A

X-ray excites electron in the PSP to the conduction band. Some drop to valence band and some drop to trapping sites (impurities), where they will stay until they are ‘read’ (processed)

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125
Q

What are the layers of a CR detector?

A

Front protective layer
Phosphor (PSP)
Electroconductive layer
Support (structural)
Light shielding layer
Back protective layer

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126
Q

Electrons are trapped in TLDs and CR detectors until they are processed, what is the difference in the process between CR and TLD?

A

TLDs use thermoluminescence so h eat, whilst CR uses photoluminescence so light

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127
Q

How is CR processing completed?

A

Fine focussed red light in raster fashion and emission of blue light which is collected via a PMT

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128
Q

What are the advantages and disadvantages to CR?

A

Advantages: similar workstyle to film and has the benefits of digital imaging (improved contrast)
Disadvantages: poorer resolution than film and takes time to process

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129
Q

What type of arrays are DR detectors built on as the readout layer?

A

Thin Film Transistor (TFT) arrays

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130
Q

What is the difference between direct and indirect DR detectors? (both are flat panel detectors commonly)

A

Direct: electron-hole pair generated in amorphous selenium (photoconductor)
Indirect: Phosphor based scintillator used to convert to visible photons and then electron-hole pair generated in amorphous silicon (photodiode)

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131
Q

What does the transistor do in DR detectors? (TFT is in readout layer of direct and indirect DR)

A

It amplifies the electric signal

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132
Q

How are DR detectors read?

A

On the matrix array, each pixel row is switched on in turn and the signal from the columns is recorded

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133
Q

Is it easier to determine over/under-exposure with film or digital? Why could this be an issue and how is it solved?

A

Film. This is an issue for digital imaging as under exposure reduces SNR and overexposure is unnecessary dose. Detector dose indicator (DDI)

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134
Q

What is the Detector Dose Indicator (DDI)?

A

It is a manufacturer specific measure of dose to the detector (not patient) and it is a retrospective number based on average useful signal

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135
Q

What device has historically been used in fluoroscopy to get real-time viewing of the x-ray image?

A

Image intensifiers (flat panel detectors have been used as an alterantive)

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136
Q

How do image intensifiers work?

A

X-rays to input phosphor (scintillator), which goes to photocathode which generate electrons that get accelerated to the anode and focussed to the output phosphor (scintillator) for light photons to video or CCD

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137
Q

Why are image intensifiers named the way they are?

A

The output phosphor is much smaller than input phosphor (2cm vs 40cm)

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138
Q

A primary image is a measure of what?

A

The mass attenuation coefficient and thickness of tissue traversed

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139
Q

Is the mass attenuation coefficient dependent on photon energy, Z of material or number of photons (mAs)?

A

Photon energy and Z of material = dependent
Number of photons = not dependent

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140
Q

What is the equation for the intensity of the signal after travelling through a homogeneous medium of thickness t and mass attenuation coefficient mu?

A

The initial intensity times by e to the power of minus mu times t (integrate power if not homogeneous like patients)

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141
Q

Is the detector more efficient for primary or scatter?

A

Scatter

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142
Q

Contrast shows physical and chemical differences, what are examples of each type?

A

Physical - eg density
Chemical - eg atomic number

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143
Q

What are the two types of contrast that we need to focus on? (not the equations)

A

Subject contrast (different amounts of radiation exiting the patient) and detector contrast (detector properties and image processing)

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144
Q

What is the threshold contrast test?

A

The limit at which cannot distinguish detail from background. Minimum perceptible contrast

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145
Q

What properties do we need to consider for artificial contrast?

A

High Z, choose appropriate kV for K-edges, viscosity and toxicity

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146
Q

Why does having a focal spot with finite size create image blur/geometric blur (limits spatial resolution)?

A

It creates a penumbra at the edge of objects because x-ray arrive from slightly different locations

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147
Q

How can motion blur be minimised?

A

Reduce the exposure time (increase mA) or use immobilisation or breath hold techniques

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148
Q

When is the difference between kerma and absorbed dose? (both have the same units)

A

Kerma measures the energy transferred from photons to electrons per unit mass at certain position, whereas absorbed dose measures the energy deposited in a unit mass at a certain position. Difference is transfer or deposition of energy (electron could move away and deposit energy elsewhere)

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149
Q

How do we reduce magnification in xray? (want to do this as magnification is a type of distortion)

A

Increase the focus to detector distance and reduce the object to detector distance

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150
Q

What is the typical amount of magnification in x-ray?

A

1.1

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151
Q

When would we want to deliberately increase the magnification in x-ray? Even though this would increase distortion and unsharpness

A

Mammography

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152
Q

How is pin cushion distortion related to magnification?

A

The focus to detector distance will be larger for parts of the image that are further away from the focal spot so these parts will look bigger on the image (more magnification at the periphery)

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153
Q

The detective quantum efficiency (DQE) is a measure of the combined effects of the modulation and noise, and is expressed as a function of what quantity?

A

Spatial frequency

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154
Q

What is the equation for the detective quantum efficiency (DQE)?

A

Output SNR divided by input SNR all squared

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155
Q

What is the ideal detective quantum efficiency (DQE) of a detector?

A

1 for all frequencies

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156
Q

What requirements are there for mammography?

A

Good spatial resolution (small objects), good contrast (different soft tissues), good dynamic range (variation in thickness across image), minimal patient dose (radiosensitive tissue)

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157
Q

Does mammography use DR detectors and AEC?

A

Yes to both

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158
Q

What is automatic exposure control (AEC)?

A

It terminates the exposure when the correct detector dose is reached to create consistent SNR across images

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159
Q

What is the approximate kV for mammography and why?

A

Low (20-30kV) to accentuate photoelectric effect

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160
Q

What is the typical target for mammography?

A

Molybdenum

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161
Q

What modalities do they use the half field imaging technique?

A

Mammography

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162
Q

Why is compression used in mammography?

A

Reduce dose, spread tissue for visualisation (reduces superimposed anatomy), more uniform thickness, reduce motion, reduce scatter and beam hardening

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163
Q

Why is there the potential for high skin doses in fluoroscopy?

A

Long screening times (even though low dose rate)

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164
Q

Why is pulse fluoroscopy used?

A

Reduces patient dose (and doe to staff) as we can’t see really fast pulse rate anyway

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165
Q

What is the normal pulse rate range in fluoroscopy?

A

7.5 - 15 pulses per second

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166
Q

What is virtual collimation? (used in fluoroscopy)

A

Collimator position can be inferred from previously screened image so don’t need to screen to move collimators (dose reduction technique). Provides graphical display of position of collimator blades on image without irradiation

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167
Q

What is last image hold and why does it reduce the dose? (used in fluoroscopy)

A

Displays most recently acquired image after radiation termination, so don’t have to continue screening

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168
Q

What is automatic brightness control (ABC)? (used in fluoroscopy)

A

The system will modulate the parameters to maintain the brightness (to accommodate large range of patent sizes and body parts)

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169
Q

In fluoroscopy, using ABC, what type of curve characterises how the system reacts?

A

kV/mA control curves

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170
Q

In fluoroscopy, why can there be an issue with automatic brightness control (ABC) when changing the field size?

A

There will be a reduced output so the system will increase the factors to maintain the output

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171
Q

The most common CT scanner nowadays is the third generation, what is this design?

A

Fan beam (x-axis only) with multiple rotating detectors (500-1000) and much faster than previous generations

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172
Q

What is the advantages and disadvantages of cone beam CT?

A

Advantages: large volume imaging so faster scanning
Disadvantages: loss of scatter rejection and tricky reconstruction

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173
Q

What are the two methods of CT scanning?

A

Axial scanning (stop and shoot, move patient one slice thickness, rotate and repeat)
Helical scanning - most common (move patient as CT assembly rotates, each rotation incomplete so need to interpolate. Faster imaging)

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174
Q

Modern CT scanners have several rings of detectors positioned in the z-direction, why is this useful?

A

Image a large volume in fewer rotations as image multiple slices simultaneously

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175
Q

What are adaptive arrays and why are they used in CT scanners?

A

The detector elements do not need to be the same size, so we can bin them for the required slice thickness

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176
Q

What is the equation for Hounsfield units?

A

Linear attenuation coefficient = mu.
mu of tissue minus mu of water divided by mu of water all times by 1000

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177
Q

What is the partial volume effect? (CT artefact)

A

Can’t contain information smaller than a voxel so average value of voxel is provided (worse artefact with increase in pixel size or slice thickness)

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178
Q

What causes streak artefacts in CT images?

A

Motion, edges of high attenuation medium and components of image outside fov. Type of beam hardening artefact

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179
Q

Why does beam hardening create artefacts?

A

High attenuation starves the tissues behind it

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180
Q

How are ring artefacts caused in CT images?

A

There are non-uniformities amongst detectors and each projection has a change in measured attenuation coefficient at one point

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181
Q

How is dose modulation achieved in CT scans?

A

An attenuation map of the patients density/thickness is created from the pre-scan radiograph, which adjusts (modulates) the output for each slice

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182
Q

What is the order of magnitude of the activity administered in nuclear medicine in diagnosis and therapy?

A

Diagnosis = kBq- MBq
Therapy = MBq-GBq

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183
Q

What types of half-life need to considered in the choice of radiopharmaceutical?

A

Radioactive half life of radionuclide and biological half life of pharmaceutical

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184
Q

What are the types of nuclear medicine?

A

Planar/static, dynamic/gated, wholebody, SPECT, PET, non-imaging, therapy

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185
Q

What is the most common type of PET scan?

A

Radiopharmaceutical FDG is used (which uses F-18), which simulates glucose and gets metabolised. Glucose uptake is shown, which is used for cancer detection

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186
Q

How do the ideal requirements change for nuclear medicine therapy and diagnosis?

A

Therapy: targeted dose to a pathway and minimise radiation protection issues for others
Diagnosis: good image quality in timely manner and minimal radiation dose

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187
Q

What considerations are required for choice of radionuclide?

A

Half-life (long enough for transport and biological pathway but not too long as excess dose after use), emissions (suitable for detection and dose considerations), cost, ease of manufacture, toxicity, chemistry (binding to pharmaceutical and stability of bond)

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188
Q

Why is technetium-99m used in 95% of nuclear medicine studies?

A

Metastable, mostly (98.6%) 140.5 keV gamma emission, produced onsite from molybdenum-99 generator (elution process), labels wide range of pharmaceuticals, 6 hour half-life (mostly decayed after a day, good for biological uptake, can be transported)

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189
Q

What are the typical radionuclide used in therapy and their type of emission?

A

I-131 (beta and some gamma), Y-90 (beta), Lu-177 (beta), Ra-223 (alpha)

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190
Q

Why are particle emissions mostly used in therapeutic nuclear medicine?

A

Short range in tissue so localised energy deposition, better for radiation protection (gamma emissions can be used for imaging, which could be directly like I-131 or via bremststrahlung)

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191
Q

What are gamma cameras known as?

A

Scintillation or anger camera

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192
Q

Can gamma cameras only process one event at a time or multiple and is this an issue?

A

Only one (single photon) but this is mostly okay as don’t want too high radioactivity

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193
Q

What is the typical field of view of gamma cameras?

A

40-50 cm

194
Q

What is the typical sensitivity of gamma cameras?

A

Hundreds of counts per second per MBq

195
Q

Do gamma cameras measure location or energy of incoming gamma ray?

A

Both

196
Q

What is the high Z scintillation crystal detector type used in gamma cameras?

A

Sodium iodide doped with thallium

197
Q

How does the detector work in gamma cameras?

A

Gamma photons deposit energy via photoelectric effect or Compton scatter and causes scintillation to create visible light which is then detected (1 photon per 30 eV of gamma energy)

198
Q

How thick are most crystals in gamma cameras and what is the benefits of thicker crystals?

A

3/8 inch or 9.5mm to capture most 140 keV gamma rays. Thicker crystals are more sensitive but have worse resolution

199
Q

What are the drawbacks of using a crystal in a gamma camera?

A

Fragile, hygroscopic (discolours and blocks photons when exposed to water) and sensitive to temperature changes

200
Q

What does the light guide do in a gamma camera?

A

Allows good optical contact between the crystal and the PMTs and minimises light loss

201
Q

How do photomultiplier tubes (PMT) work in gamma cameras?

A

They use electron cascades to convert light detector into a measurable signal and creates a gain of up to 10^6

202
Q

How many PMT are arranged in an array behind a crystal in a gamma camera?

A

Around 50 per head typically

203
Q

Why is anger logic required to determine positional information from PMTs?

A

Scintillation light spreads in all directions and multiple PMTs see each event but by different amounts depending on its distance from the event so this can be used to localise the event

204
Q

What does the collimator do in gamma cameras and why is it needed?

A

Only accepts gamma rays perpendicular to the crystal as it non-parallel photons will hit the septum (usually lead) because we get no direction information in the crystal detection (it does not reject scatter)

205
Q

What is an advantage and disadvantage of using collimators in gamma cameras?

A

It improves imaging performance by improving spatial mapping but reduces sensitivity

206
Q

What types of collimators can be used in gamma cameras and what is the most common type?

A

Parallel hole (most common), diverging hole, converging hole and pinhole

207
Q

Is more or less thick septae in collimators needed in gamma cameras for higher energy gamma rays and why?

A

More thick to ensure maximum absorption of angled photons

208
Q

Does increasing the distance from the patient to the collimator in nuclear medicine imaging increase or decrease the resolution?

A

Decrease

209
Q

The hole diameter and hole length of the collimators in nuclear medicine change the resolution of the image, how?

A

Increase diameter = worse resolution
Increase length = better resolution

210
Q

In nuclear medicine imaging, intrinsic and extrinsic resolution are related to what features?

A

Intrinsic = light spread and anger logic
Extrinsic = collimator design etc

211
Q

What is sensitivity dependent and independent from in gamma cameras?

A

Dependent on collimator geometry and independent with distance

212
Q

What is the dynamic range of an imaging system?

A

It describes the range of x-ray intensities a detector can differentiate

213
Q

What is different for solid state gamma cameras?

A

No PMTs and it uses a pixelated detector

214
Q

What are the two types of solid state gamma camera?

A

Direct conversion with CZT and indirect conversion with CsI crystal with a photodiode

215
Q

What are the advantages of solid state gamma cameras?

A

Better energy resolution, higher count rates, more compact and intrinsic resolution only limited by element size

216
Q

What are the disadvantages of solid state gamma cameras?

A

Expensive and fixed collimation (can’t manually change as need <mm accuracy)

217
Q

Do typical gamma cameras have a continuous crystal or discrete detector elements?

A

Continuous crystal

218
Q

How are pixel signals created in normal gamma cameras?

A

Binning signal into variable matrix sizes

219
Q

What is the typical matrix sizes for nuclear medicine imaging?

A

64 x 64 - 256 x 256

220
Q

Nuclear medicine scan lengths can be dependent on what quantities?

A

Time, counts or heartbeats

221
Q

What is the point of an energy windows in nuclear medicine and why does it work?

A

It reduces scatter in the image because gamma emissions are roughly monoenergetic when produced but scatter has lower energy. Energy of gamma is proportional to light photons

222
Q

How does an energy window work in nuclear medicine and why is it bigger than one energy?

A

Only photons of a certain range of energies are accepted, which is a broad photopeak around the main gamma energy because of poisson effects from scintillation

223
Q

How big are energy windows in nuclear medicine typically and why?

A

+/-10% of nominal photopeak because if its too narrow, the sensitivity is reduced but if it is too wide more scattered photons included

224
Q

Why might you need multiple energy windows in nuclear medicine?

A

Two photopeaks from one radionuclide or multiple radionuclide used simultaneously

225
Q

How do we measure the amount of radioactive material before nuclear medicine administration?

A

Radionuclide calibrators

226
Q

How do radionuclide calibrators work?

A

Source emits radiation and causes ionisation in chamber, which creates a current as there is a potential difference applied to the chamber.

227
Q

How is the current measured in a radionuclide calibrator used?

A

The current is corrected for the radionuclide and source geometry (calibration factors) to give a display in units of radioactivity

228
Q

How are the calibration factors checked for a radionuclide calibrator?

A

Known radioactive source used and compared against primary/secondary standard. This is done yearly

229
Q

Why could there be non-uniformity in nuclear medicine imaging?

A

Crystal construction, PMT gain, collimator design and non-linearity of anger logic

230
Q

How do gamma cameras correct for uniformity, linearity and energy?

A

Correction maps

231
Q

Why is an energy correction required for gamma cameras to create uniform images?

A

Events happening between PMTs have a lower output than those occurring directly over PMTs so there is a variation in energy across the detector

232
Q

How are spatial non-linearities from the PMT array and anger logic shown on an image?

A

Curvature added where straight lines are pulled towards the centre of the PMT

233
Q

Other than the reasons for the energy and linearity corrections, why else can gamma cameras be non-uniform?

A

Crystal non-uniformities and collimator design

234
Q

How is the uniformity correction performed on gamma cameras?

A

Obtain a large count uniform image (60-200 million counts) via either a flat flood source or point source with no collimator

235
Q

What type of source is used for intrinsic and extrinsic uniformity testing of gamma cameras?

A

Intrinsic = point source
Extrinsic = flat flood source

236
Q

What calculation is used for nuclear medicine uniformity testing?

A

Difference between highest and lowest pixel values divided by the sum of them and then all multiplied by 100%

237
Q

In nuclear medicine uniformity testing, what are the two ways it can be calculated?

A

Integral uniformity = highest and lowest values across the whole region
Differential uniformity = highest and lowest values in pixels of close proximity

238
Q

In a nuclear medicine image for uniformity testing, what are the two regions called?

A

CFOV- central field of view, which is central portion with good quality
UFOV - useful fov is the entire image including edges

239
Q

What is the tolerance of the uniformity testing in nuclear medicine imaging?

A

Few % (3-6%)

240
Q

Why are centre of rotation QC tests needed in nuclear medicine?

A

Gamma camera heads are heavy and come under mechanical strain when rotating but image reconstruction assume all views are projected equally back to the isocentre

241
Q

How is the centre of rotation test carried out with gamma cameras?

A

Image a point source and measure deviation across all views (should be equal)

242
Q

What other QA tests may be needed in nuclear medicine imaging systems?

A

Spatial resolution, count rate response, sensitivity, energy resolution, multiple window registration, SPECT-CT registration, CT QA

243
Q

What are the advantages of SPECT imaging?

A

3D localisation of source, improved quantification and background removal (from attenuation correction)

244
Q

What is required for SPECT to create good images?

A

Gamma cameras need to be able to rotate with good centre of rotation calibration and good uniformity (low ring artefacts) and stable pharmaceutical distribution over scan time

245
Q

For SPECT imaging, is rotation that is circular (fixed diameter) or contoured (moves close to patient) better for resolution?

A

Contoured

246
Q

How does contoured rotation in SPECT imaging work?

A

Beam of light across the detector and move until the outer light is broken and there are touch sensitive pads to prevent collision

247
Q

In SPECT acquisitions, the detectors can be in H or L mode, what do these mean?

A

H mode = cameras opposite one another
L mode = cameras perpendicular to one another

248
Q

Is iterative reconstruction or filtered back projection typically used in nuclear medicine imaging?

A

Iterative reconstruction

249
Q

When might the L-configuration of gamma cameras be preferred?

A

Heart scanning, which is more anterior so only 180 degrees might be used

250
Q

What is attenuation correction in nuclear medicine imaging?

A

A second form of imaging (eg CT or less typically using a transmission source) is used to develop a density map of each patient and corrects the SPECT image

251
Q

When is planar nuclear medicine imaging used?

A

For specific anatomy (not whole body) and there is not rapidly changing radiopharmaceutical distributions

252
Q

When is whole body nuclear medicine imaging used? (uses planar imaging with couch moving)

A

Looking at distribution through entirety of patient and may not know what you are looking for

253
Q

When is dynamic nuclear medicine imaging used?

A

Rapidly changing biodistribution is of interest and normal planar takes too long to acquire

254
Q

How is dynamic nuclear medicine images acquired?

A

Series of short planars in quick succession with acquisition dependent on biological pathway

255
Q

What image quality issues are seen in dynamic nuclear medicine scans?

A

Noisy due to lack of signal in each frame but bad resolution if take too long on each frame as radiopharmaceutical moves

256
Q

When is SPECT imaging used?

A

Distribution of radiopharmaceutical may be complex within an organ or may want to determine volumetric distribution

257
Q

What is a type of non-imaging nuclear medicine? (not therapy)

A

Inject radiation then collect patient samples to get information about biological pathways (eg rates of clearance over time)

258
Q

What is the main difference between PET and typical nuclear medicine?

A

In PET, radionuclide emit positrons which annihilate with electrons (within a few mm) to give back to back 511 keV gamma rays so different scanner design for this and generally shorter half life

259
Q

In PET scanning, how does the system know whether to accept or reject a pair of photons?

A

Energy window to check both gamma rays are 511 keV and detection must happen within a small time window of one another for them to be assumed the same interaction

260
Q

What is the typical PET scanner detector design?

A

Ring of crystal blocks that are etched to provide light paths towards the PMT and no need for a collimator

261
Q

How does time of flight PET work?

A

The difference in arrival time of corresponding photons is used to determine where the decay event happened

262
Q

What is standardised uptake value (SUV) in PET?

A

It is a ratio of tissue radioactivity at a specific ROI and the injected dose of radioactivity per kilogram of patients body weight to get voxel values. It is a standardised quantitative representation of radiotracer concentration

263
Q

What is the equation for the standardised uptake value (SUV) in PET?

A

Radioactive concentration divided by (injected activity divided by patients weight)

264
Q

What is the point of calculating the standardises uptake values (SUV) in PET?

A

Allows comparability across patients as it accounts for factors like injected dose and patients body weight

265
Q

What are the different types of standardised uptake values (SUV) in PET?

A

Mean (average voxel value within ROI), max (largest single voxel value in ROI) and peak (average of N voxel values surrounding highest value)

266
Q

What factors make dosimetry more difficult for internal radiation rather than external radiation?

A

Varied uptake and clearance rates, different organ doses, impossible to measure directly so need to calculate, radioactive decay reduces activity

267
Q

What is the S-factor that is used in nuclear medicine dosimetry?

A

Dose from one organ (source) to another (target) for a specific radionuclide and it is normalised to activity. It is calculated using monte-carlo techniques and phantoms

268
Q

What is the specific absorbed fraction (SAF) that is used in nuclear medicine dosimetry?

A

Fraction of emitted energy which is deposited in target organ

269
Q

For nuclear medicine dosimetry, how can cumulated activity be calculated, which is the total number of emission from an organ during the lifetime of radioactive material?

A

Area under time-activity curve

270
Q

In nuclear medicine dosimetry, the dose to a target organ from a source organ is calculated by multiplying the accumulated dose with what?

A

The S-factor (effective dose would includes weighting factors and summing over all target organs)

271
Q

In nuclear medicine dosimetry, cumulated activity can be as simple as instant uptake and exponential clearance. What is the equation for this?

A

The administered activity multiplied by the fraction of activity within the source organ and divide by effective decay constant for the organ

272
Q

What is the frequency range of ultrasound waves?

A

20 kHz - 18 MHz

273
Q

How do sound waves travel through air?

A

Compression and rarefaction (longitudinal wave). Propagates through air or other materials by vibration of matter

274
Q

What properties of sound waves stay constant as waves pass from one medium to another and what changes?

A

Frequency stays the same but speed and wavelength can change

275
Q

What determines the speed of sound in tissue?

A

Density and compressibility

276
Q

What is the speed of sound in soft tissue?

A

1540 ms^-1

277
Q

What is acoustic impedance?

A

How much resistance an ultrasound beam encounters as it passes through tissue and it is a physical property of tissue

278
Q

What does acoustic impedance depend on?

A

Density of the tissue and speed of sound wave

279
Q

The ability of an ultrasound wave to transfer from one tissue type to another depends on the difference in impedance, what if this difference is large?

A

Sound is reflected (only a fraction of energy is reflected)

280
Q

What is the units for characteristic acoustic impedance? (can use this to work out equations)

A

kg m^-2s^-1

281
Q

What is the typical value of characteristic acoustic impedance for water and soft tissue?

A

1.5 kg m^-2s^-1

282
Q

The reflection fraction of the ultrasound waves is the difference of acoustic impedances divided by what all squared?

A

The sum of the impedances (entire fraction squared)

283
Q

Refraction of ultrasound waves is described by what law?

A

Snells law n1sin(theta1)

284
Q

Why does refraction in ultrasound cause artefacts?

A

US machines assume waves and echoes travel along a direct path

285
Q

When does scattering of ultrasound waves occur?

A

Wave strikes a structure with a different acoustic impedance to surround tissue and a wavelength less than that of the incident wave

286
Q

What is the name of structures that scatter ultrasound waves?

A

Diffuse reflectors (like red blood cells)

287
Q

What creates speckle on ultrasound images?

A

Interference effects between overlapping echoes from scattering that occurs in all directions

288
Q

What is attenuation in ultrasound?

A

Reduction in amplitude of the ultrasound beam (loss of energy), mainly from scattering and tissue absorption

289
Q

What is the attenuation coefficient in ultrasound?

A

Intensity loss per unit distance (dBcm^-1)

290
Q

Is attenuation for ultrasound dependent on the medium and frequency?

A

Yes to both (directly proportional to frequency)

291
Q

Why is ultrasound gel used?

A

It overcomes the very high reflection coefficient of air

292
Q

How many piezoelectric crystals are in a transducer typically?

A

300

293
Q

How do ultrasound transducer produce waves?

A

Voltage is applied across the piezoelectric crystals, which makes it change shape rapidly and produce sound waves.

294
Q

How do ultrasound transducer receive waves?

A

Reflected sound waves hit piezoelectric crystals and produce an electric signal (voltage generated) that is formed into an image

295
Q

What is the purpose of the absorbing substance in ultrasound probes?

A

Eliminates back reflections from the probe itself

296
Q

What is the purpose of the acoustic lens in ultrasound probes?

A

Focuses emitted waves

297
Q

What do ultrasound scanners determine to create an image?

A

How long it took the echo to be received and how strong the echo was

298
Q

Are lower or higher ultrasound frequencies used for deeper penetration?

A

Lower

299
Q

Does diagnostic imaging ultrasound use high or low power levels and high or low frequencies?

A

Low power levels and high frequencies

300
Q

Does physiotherapy therapeutic ultrasound use high or low power levels and high or low frequencies?

A

High power levels and low frequencies

301
Q

What is lithotripsy?

A

Focused ultrasound waves to break large stones in the kidney and liver to pass through the urinary system

302
Q

How is ultrasound used for cleaning?

A

It agitates the cleaning fluid to produce cavitation bubbles

303
Q

What are the 3 imaging modes in ultrasound?

A

A (amplitude), B (brightness) and M (motion) modes

304
Q

What is A mode ultrasound?

A

Single transducer scans a line through the body (most simple version)

305
Q

What is B mode ultrasound?

A

Linear array of transducers simultaneously scans a plane through the body that can be viewed as a 2D image

306
Q

What is M mode ultrasound?

A

rapid sequence of B-mode scans whose image follows each other in sequence to image range of motion

307
Q

Why are high density materials not good for ultrasound imaging?

A

They reflect 100% of the incidnet sound wave

308
Q

Why are gas filled areas not good for ultrasound imaging?

A

They are opaque to sound waves

309
Q

What structures are good for ultrasound imaging?

A

Soft tissue and fluid filled spaces

310
Q

What is the doppler effect for ultrasound imaging?

A

If a tissue is moving towards the source of the wave, the reflected wave has a higher frequency than the incident wave and vice versa when it is moving away

311
Q

What is the doppler shift?

A

Difference in frequency between transmitted and received waves

312
Q

What are the advantages of ultrasound imaging?

A

Non-invasive, portable, inexpensive, non-ionising, no known damage to tissue, fast images generated in real time

313
Q

What are the disadvantages of ultrasound imaging?

A

Limited field of view, requires trained operator, difficulty imaging bone and gas-filled structures, increased depth = lower frequency = lower resolution

314
Q

Magnetic resonance imaging is derived from what concept?

A

Nuclear magnetic resonance

315
Q

What are the advantages of MRI?

A

No ionising radiation, excellent soft tissue contrast, obtain dynamic images in any plane

316
Q

What is the basic concept of MRI?

A
  1. Position body in a strong magnetic field
  2. Transmit radio waves from scanner
  3. Receive radio waves from hydrogen nuclei in the body
  4. Transform radio waves into image
317
Q

What is the range of field strengths for MRI scanners for humans?

A

1-7 T

318
Q

What are the magnet types for MRI scanners?

A

Superconducting, permanent and resistive (subcategories: human, small bore, open, portable, PET/MR, MR linac)

319
Q

What are the 6 parts of an MRI system?

A

Magnet, gradients, RF transmitter, RF receiver, image processor, host computer

320
Q

What is the purpose of gradients in MRI?

A

Spatial localisation by manipulating the static field

321
Q

What is the magnet in an MRI scanner that makes the static field?

A

Superconducting wire in helium at 2.4 Kelvin wrapped around a hollow bore

322
Q

What is the purpose of the RF coils in MRI?

A

They get energy into tissues by inducing transitions and detect resultant signals

323
Q

How do RF coils create a signal?

A

Flux linkage changes in coils

324
Q

What are the 3 types of RF coils in MRI?

A

Volume coils (includes in-built bod coil), surface coils and array coils

325
Q

What is the purpose of volume RF coils in MRI?

A

They provide a homogeneous RF excitation across a large volume and have deep imaging

326
Q

What is an advantage and disadvantage of using surface RF coils?

A

Advantage: higher SNR
Disadvantage: only images proximal tissue (can be a benefit in some scans)

327
Q

What is the typical resolution in MRI scans and what is the limit of on the resolution?

A

1 mm and time in scanner

328
Q

What are the disadvantages of MRI?

A

Many contraindicated patient groups, can be claustrophobic, possibility of projectiles, long scan times (discomfort and motion), waiting lists and expensive

329
Q

What is the nuclei in the body that MRI is mostly concerned with?

A

Hydrogen

330
Q

What is the Earths magnetic field and then the static field of MR magnets?

A

Earth: 0.06mT
MR magnets: 0.2-7 T (200-7000 mT)

331
Q

What is the MRI fringe field?

A

The stray magnetic field outside the bore of the magnet

332
Q

What are the temporary bioeffect hazards of static magnetic fields between 3-8 T that are associated with head movement?

A

Nausea, headaches, metallic taste, vertigo

333
Q

What are the hazards of the static magnetic field from the force fields? (separate from bioeffects)

A

Translational force and torque

334
Q

The translational force from the MRI static field is proportional to what?

A

The magnetic susceptibility multiplied by the volume and field strength and the fringe field gradient (dB/dr)

335
Q

The torque from the MRI static field is proportional to what?

A

Square of the magnetic susceptibility multiplied by the volume multiplied by the field strength squared

336
Q

Whilst all metals can cause image artefacts in MRI, what type is worse in an MRI scanner?

A

Ferromagnetic metals

337
Q

What are examples of ferromagnetic metals?

A

Iron, certain types of stainless steel, nickel, cobalt

338
Q

What is the approximate frequency of the time varying gradients in MRI?

A

1 kHz

339
Q

What is the slew rate in MRI?

A

The speed at which a gradient can be turned on and off. The speed at which the gradient reaches its maximum amplitude.

340
Q

What is the units for slew rate for MRI scanners?

A

Millitesla per meter per microsecond (mT/m/ms).

341
Q

What are the hazards of time varying gradients in MRI?

A

Faraday induction creates electrically conducting tissues, like peripheral nerve stimulation and acoustic noise

342
Q

What are the hazards of RF exposure in MRI scans?

A

Tissue heating and implant heating

343
Q

What is SAR in MRI scanning?

A

Specific Absorption Rate = Average power divided by weight (W/kg)

344
Q

The SAR in MRI increases with what factors?

A

Square of the Larmor frequency, square of the applied RF field, patient size and number of RF pulses in a given time

345
Q

What are normal SAR limits in clinical MRI scanning and this keeps heating below what limits?

A

2 W/kg, below 0.5 degrees

346
Q

What is a magnetic quench?

A

Rise in temp in magnetic coil, which introduces resistivity in wires and loses superconductivity , leading to a loss of field and boiling off liquid helium as a gas

347
Q

What do B_0, B_0xyz and B_1 stand for in MRI?

A

B_0 = static field
B_0xyz = gradient fields
B_1 = RF fields

348
Q

Why are we only concerned with hydrogen in MRI?

A

There’s lots of it (in water, fat etc) and it has a strong magnetic moment

349
Q

Why is there no net magnetic field in normal random alignment in the human body?

A

The individual magnetic moments cancel

350
Q

What do the magnetic moments of the protons do in an external magnetic field directed upwards?

A

They align with the field either parallel (spin up) or anti-parallel (spin down) with a few more spin up overall

351
Q

In an external magnetic field, is the spin up or spin down magnetic moments of proton in the stable energy state and which one is the lower energy state?

A

More stable and lower energy state = spin up (parallel with the external field) (spin down is quasi stable)

352
Q

In an external magnetic field, why is the alignment of the magnetic moments not exact and what do they do?

A

Uncertainty principle. The proton’s spin precesses around the external field with the tip of the magnetic vector tracing out a circle

353
Q

In MRI the z direction is defined by the direction of the external magnetic field (B_0) and is the longitudinal direction, what is the x-y plane called?

A

The transverse plane

354
Q

What does the transverse moment equal when the protons are in precession with a static magnetic field applied?

A

Zero because they cancel out

355
Q

What direction is the net magnetisation (M_0) after an external static magnetic field is applied?

A

Upwards as there are more spin up nuclei

356
Q

What is the phase of a magnetic moment vector?

A

The position of the vector tip on the precession circle at one particular time

357
Q

What is it called when the tips of the vectors of the magnetic moments are at the same position in the precession circle?

A

The are in phase

358
Q

Can antiparallel magnetic moments be in phase with parallel magnetic moments and why?

A

Yes because it is about the transverse component

359
Q

What is the net magnetisation (M_0) at equilibrium when an external magnetic field is applied?

A

It is in the z direction as there are more parallel than antiparallel spins and the transverse component is zero as there is no phase coherence

360
Q

Why can we not measure the net magnetisation (M_0) after a static external magnetic field has been applied?

A

It is a lot smaller than the main field in that direction so we can only measure magnetisation in the x-y plane

361
Q

What is the frequency of the precession of protons called?

A

The Larmor frequency

362
Q

What does the Larmor frequency depend on?

A

The external magnetic field strength and the nucleus

363
Q

For protons, what is the order of magnitude of the Larmor frequency for field strengths typical in MRI?

A

MHz

364
Q

What is the equation for the Larmor frequency (omega_0)? The equation is called the Larmor equation

A

The gyromagnetic ratio multiplied by B_0 (static magnetic field)

365
Q

What is the gyromagnetic ratio for protons?

A

42.57 MHz/T

366
Q

To cause resonance in MRI, what part of the electromagnetic spectrum has the same frequency as the Larmor frequency?

A

Radiofrequency range (MHz)

367
Q

What does the RF wave (B_1) do to the net magnetisation (M_0) vector?

A

It pushes the moment into the transverse plane, which rotates in the processional frequency in the x-y plane

368
Q

What is the flip angle in MRI?

A

The angle that the net magnetisation (M_0) moves through from the RF pulse

369
Q

A 90 degrees RF pulse (flip angle) puts the net magnetisation vector (M_0) in what direction?

A

In the transverse (x-y) plane

370
Q

What difference in the RF radiation turns a 90 degree pulse into an 180 degree pulse?

A

Double the strength or duration of the RF pulse

371
Q

After an RF pulse, what does the signal receiver coil detect and how?

A

The changing magnetic field as the magnetisation rotates and a current is generated in the coil, which produces the MR signal

372
Q

What is the free induction decay (FID) of the MR signal after a 90 degree pulse?

A

The signal oscillates at the Larmor frequency and decays exponentially to zero

373
Q

What is T1 relaxation (in brief)?

A

Recovery of longitudinal magnetisation (in Z direction)

374
Q

What is T2 relaxation (in brief)?

A

Decay of transverse magnetisation (in x-y plane). Rate at which M_xy decays to zero

375
Q

What will happen to protons precessing slower or faster than the Larmor frequency in the x-y plane after a 90 degree pulse?

A

They will dephase

376
Q

Is T1 relaxation independent or dependent of T2 relaxation?

A

Independent

377
Q

What is another name for T2 relaxation?

A

Spin-spin relaxation

378
Q

What does the spin-spin relaxation (T2) depend on?

A

Tissue composition and how easily protons can move within the lattice

379
Q

Do more solid tissues have a longer or shorter T2 and what does this mean?

A

Shorter so the protons lose their initial phase coherence quicker

380
Q

Why do different tissues have different T2 relaxation times?

A

Proton density in inhomogeneous, which causes small local variations in field strength

381
Q

The T2 relaxation time is the time when the transverse magnetisation (x-y plane) drops to what percentage of the net magnetisation (M_0)?

A

37% (1/e)

382
Q

What is additionally included T2* relaxation?

A

Inhomogeneities in the main field B_0

383
Q

What occurs in T2* relaxation?

A

Inhomogeneities affects the dephasing of protons by either increasing or decreasing the frequency of rotation

384
Q

Does T2* increase or decrease the rate at which the transverse magnetisation decays compared to T2 only? (T2* includes T2)

A

Increases

385
Q

Does spin-spin relaxation lose energy?

A

No

386
Q

In MRI, protons lose energy through interactions with the lattice, what does this do in terms of energy?

A

High to low energy transitions

387
Q

Is T2* relaxation reversible?

A

Yes

388
Q

What is T1 relaxation also called?

A

Spin-lattice relaxation

389
Q

What is the equation for transverse magnetisation which includes T2 time?

A

The net magnetisation (M_0) multiplied by e ^(-t/T2)

390
Q

What is the equation for longitudinal magnetisation (Z) which includes T1 time?

A

The net magnetisation multiplied by (1- e ^(-t/T1))

391
Q

The T1 relaxation time is the time when the longitudinal magnetisation (z) increases to what percentage of the net magnetisation (M_0)?

A

63% (1- 1/e)

392
Q

Is T1 or T2 times longer?

A

T1 (by 5 - 10 times)

393
Q

Does M_0 simply flip between the z axis and the xy plane, and what does this mean?

A

No so M_z and M_xy act independently

394
Q

What are M_z and M_xy controlled by?

A

M_z controlled by T1 and M_xy controlled by T2

395
Q

Does the resultant M (vector sum of M_z and M_xy) equal M_0 at all times?

A

No

396
Q

What do TR and TE stand for in MRI?

A

TR = Repetition time and TE = echo time

397
Q

What is a pulse sequence in MRI?

A

Set of RF and gradient pulses, including strength, duration and timing of all pulses

398
Q

What do all pulse sequences in MRI contain?

A

RF excitation pulse, slice selection gradients, phase encoding gradients, frequency encoding gradients and signal collection

399
Q

What is the echo time?

A

The time between the RF pulse and the peak of the signal induced in the coil

400
Q

What is the repetition time in MRI?

A

The time between corresponding consecutive points on a repeating series of pulses and echoes

401
Q

What are gradients in MRI?

A

Linear variations of magnetic field strength in one direction

402
Q

What is the units of gradients in MRI?

A

Millitesla per metre

403
Q

Each direction of gradient (G_x etc), can be represented by a differential of what wrt to that direction (x, y, z)?

A

The magnetic field in the Z direction B_z (eg dB_Z/dx)

404
Q

The total B field in a certain direction (eg x) is given by what equation?

A

The static field (B_0) + x G_x

405
Q

With a gradient applied, what is the equation for the resonant frequency?

A

The total B field in a certain direction B_z(x) multiplied by the gyromagnetic ratio

406
Q

What is a dephasing gradient?

A

It accelerates the dephasing and lose all our transverse signal (lose phase coherence)

407
Q

What are the two steps of a gradient echo (GRE)?

A

Step one is a dephasing gradient to destroy the signal, then a rephasing gradient is applied with the same strength but opposite polarity to the dephasing gradient, this brings the signal back

408
Q

Does a gradient echo (GRE) affect the T2 and T2* processes?

A

No

409
Q

What do Fourier Transforms do?

A

Convert waveforms from being time-based to their constituent fundamental frequencies, each with different amplitudes

410
Q

If time and frequency are fourier transform pairs, what is the pair for distance?

A

Spatial frequency (k = 1/x)

411
Q

What is spatial frequency?

A

A measure of how often sinusoidal components of the structure repeat per unit distance. Inverse of the periodicity with which the image intensity value changes

412
Q

Do image features that change in intensity over long image distances have high or low spatial frequencies?

A

Low

413
Q

Do images of high or low spatial frequencies have sharp edges and details?

A

High

414
Q

Why are gradients used in MRI?

A

It is impossible to differentiate positions of spins, so gradients are for slice selection and spatial encoding within a slice (frequency and phase)

415
Q

What directions do the spatial encoding gradients correspond with?

A

Frequency encoding (G_FE) = x
Phase encoding (G__PE) = y

416
Q

When is the only time slice selection gradients are switched on?

A

During the excitation RF pulse

417
Q

Does the RF pulse only have the Larmor frequency or a range of frequencies?

A

A finite frequency spread (BWrf = bandwidth rf?) to determine the size of the slice

418
Q

Which spins should be excited by the transmit RF pulse in MRI?

A

Those that precess with frequencies of the input RF

419
Q

A uniform excitation in frequencies (eg top hat function) will correspond to what function in the time domain after Fourier transform?

A

Sinc function

420
Q

What does the slice selection gradient do?

A

It varies the field in the z direction (parallel to patients height) with a linear gradient so that the speed of precession of the spins is faster at one end than the other

421
Q

How does the slice selection gradient mean a slice can be excited by itself?

A

The spins precess at different speeds depending on their z position and the RF pulse will only excite those of the same frequency, which will be in a slice

422
Q

What angle is the RF pulse sent in compared to the static field?

A

90 degrees so orthogonal to it

423
Q

In MRI, how do relaxation rates affect the resulting image?

A

They set the contrast

424
Q

What is the tumbling rate in MRI?

A

It is the energy communication frequency that affects T1 relaxation (eg tumbling rate close to Larmor frequency = fast T1)

425
Q

What is the slice width in MRI dependent on?

A

The bandwidth of the RF pulse and the slice selection gradient strength

426
Q

What is the range of slice widths in MRI?

A

3 - 5 mm

427
Q

What does the frequency encoding (usually x direction) gradient do?

A

The frequency of precession of the molecules will change depending on their x-position (one end will precess faster than the other)

428
Q

When is the frequency encoding gradient applied?

A

A negative gradient to dephase the signal, then a positive gradient during data acquisition

429
Q

How can you gain positional data from the affect of the frequency encoding gradient?

A

When you Fourier Transform the signal, the components of frequency will split up and the amplitude will depend on the amount of signal at each position

430
Q

Does a stronger frequency encoding gradient give a wider or smaller field of view?

A

Smaller

431
Q

Why can’t you frequency encode twice for the y direction?

A

It rotates the frequency encoding direction 45 degrees between the x and y direction, rather than resolving the position

432
Q

Is the phase encoding gradient one single value?

A

No, it is a range of values that needs to change every time

433
Q

What does the number of phase encoding gradient strengths correspond with on the image?

A

The number of pixels in that direction

434
Q

When is the phase encoding gradient turned on?

A

After the RF pulse but before data acquisition

435
Q

Whilst the phase encoding direction changes the speed of precession in the y direction, what are we more interested in?

A

The difference in phase that will occur after the rates of precessions have changed (controlled gradual dephase)

436
Q

After the phase encoding gradient is turned off, is the frequency and phase the same or different from before the gradient was applied?

A

Frequency is the same but the phase is the same as during the gradient

437
Q

During one data acquisition stage in MRI, do we fill up one frequency encoding direction or phase encoding direction and how we get the other?

A

Frequency encoding, so need to repeat to for each phase encoding strength to build up that data

438
Q

Is the phase encoding gradient increased continuously or discretely to get different spatial frequencies?

A

Discretely

439
Q

For a single slice in MRI, does the frequency or phase encoding gradient stay the same?

A

Frequency encoding

440
Q

What do the frequency and phase encoding gradients do in k space?

A

Moves you (phase encoding is y direction and frequency encoding is x direction)

441
Q

The field of view in the MRI image is inversely proportional to what?

A

The resolution of data in k space (opposite is true as well, resolution in final image is inversely proportional to the field of view in k space)

442
Q

Low spatial frequencies in k space corresponds to what on the final MRI image?

A

Contrast

443
Q

High spatial frequencies in k space corresponds to what on the final MRI image?

A

Image detail (edge detail)

444
Q

Can we collect an incomplete k-space data set (to save time) and still get a full image?

A

Only in some cases with special processing

445
Q

Is the acquired MRI data in k-space and real image data usually real or complex?

A

Complex but depends on the coil used

446
Q

The resolution in MRI is dependent on what?

A

Gradient strength and sampling time (inversely proportional so smaller resolution for larger gradient strength or sampling time)

447
Q

Can we split MRI images into imaginary and real components, and magnitude or phase components?

A

Yes, can split it in either of these ways

448
Q

Does a T1-weighted scan measure T1 and what does it do?

A

No but it is relate to it, it minimises the effects of all mechanisms except the T1 relaxation times

449
Q

On T1 weighted image, what parts of the image appear bright and dark?

A

Tissues with relatively short T1 times, like fat are bright. Long T1 values are darker

450
Q

What are T1-weighted images used for?

A

Looking at anatomical structures

451
Q

On T2 weighted image, what parts of the image appear bright and dark?

A

Tissues with relatively long T2 are bright (like fluids) and short T2 are darker

452
Q

What are T2-weighted images used for?

A

To look at pathological structures (eg tumours)

453
Q

For medical imaging, do we want high or low contrast in MRI?

A

High

454
Q

What influences the contrast and intensity of signal in MRI?

A

T1 and T2 relaxation times, proton density and additional mechanisms (diffusion, motion etc)

455
Q

Does a short echo time give more net transverse signal so what type of contrast does echo time control?

A

Yes and T2 contrast

456
Q

In an MRI image, will there be a short or long echo time (TE) for T2 contrast?

A

Long to maximise the difference in T2 recovery curves between tissue types

457
Q

In an MRI image, will there be a short or long repetition time (TR) for T1 contrast?

A

Short to have maximum difference in T1 recovery curves between tissue types

458
Q

Since T1 and T2 exist together, how do we just see one weighted?

A

Remove the effects of the other one

459
Q

What TE and TR do we need for T1-weighted images?

A

Short TE and short TR (short TE minimises T2 weighting and short TR maximises T1 weighting)

460
Q

What TE and TR do we need for T2-weighted images?

A

Long TE and long TR (long TR minimises T1 weighting and long TE maximises T2 weighting)

461
Q

What TE and TR do we need for proton density weighted images?

A

Short TE and long TR (minimises both T2 and T1 weighting)

462
Q

What does a gadolinium based contrast agent do in MRI?

A

Affects the T1 value of an image (T1 weighted image)

463
Q

What is the purpose of a spin echo sequence?

A

It recovers some lost magnetisation due to the inhomogeneities (to use T2 curve rather than T2* curve) and get more signal

464
Q

What is a spin echo (SE) sequence compared to a gradient echo sequence (GRE)?

A

It is basically a gradient echo sequence with a 180 degree pulse added

465
Q

What is a spin echo (SE) sequence?

A

It is a 90 degree pulse then a time delay of TE/2, then an additional 180 degree RF pulse, all repeated after TR

466
Q

What does the 180 degree pulse do in a spin echo sequence in general terms?

A

It generate a spin echo at time TE (echo time) to get more signal

467
Q

What else changes for a spin echo sequence?

A

The slice selection gradient is repeated for the 180 degree pulse with a negative gradient in between (negative gradient normal anyway after the first gradient)

468
Q

Why is a 180 degree RF pulse necessary in a spin echo sequence?

A

The protons dephase after a 90 degree due to T2* relaxation, so this helps them rephase

469
Q

What does the 180 degree pulse do in a spin echo sequence in specific terms?

A

It flips all the proton spin vectors across the transverse plane onto the other side, then the protons start dephasing in the same direction as before, which makes them rephase after TE/2

470
Q

Do spin echoes take more or less time than gradient echoes?

A

More

471
Q

What is included in an inversion recovery (IR) sequence?

A

A pre-pulse (also called inversion pulse) before a spin echo or gradient echo

472
Q

What are the main uses of inversion recovery (IR) sequences?

A

Heavily T1 weighted images, fat suppression or fluid suppression ( eg CSF = cerebrospinal fluid)

473
Q

What is the main drawback of inversion recovery (IR)?

A

Long scan times

474
Q

What is an inversion pulse in inversion recovery (IR) scans?

A

An extra 180 degree RF pulse

475
Q

What is the inversion time (TI) in inversion recovery MRI?

A

The time between the inversion pulse and the excitation (normal sequence)

476
Q

What does the inversion pulse do in inversion recovery MRI?

A

It flips the net magnetisation (M_0) by 180 degrees until it is aligned along the -z direction

477
Q

What happens to the net magnetisation after the inversion pulse do in inversion recovery MRI?

A

The magnetisation begins to recover along the z axis with T1 time (becomes more positive by passing through zero then upwards to +z)

478
Q

Between the inversion pulse and the excitation (90 degree RF pulse), is there any transverse component?

A

No

479
Q

With inversion recovery scans, if the 90 degree excitation pulse is applied when the magnetisation is passing through zero, what will happen?

A

No signal is produced so that tissue is suppressed (can be selective)

480
Q

How can we suppress a certain tissue type with an inversion recovery pulse?

A

Match the inversion time with the T1 time of a certain tissue (so that the magnetisation is zero when the excitation 90 degree pulse occurs)

481
Q

What are the two applications of inversion recovery MRI?

A

STIR = Short TI Inversion Recovery (fat suppression - fat is black)
FLAIR = FLuid Attenuated IR (fluid suppression)

482
Q

Does STIR or FLAIR have shorter inversion times and why?

A

STIR because fat has a short T1, whereas fluids (FLAIR) have a longer T1

Year 1: Medical Imaging (2 decks)

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