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Neuroimaging 1 > MR Physics 3 > Flashcards

MR Physics 3 Flashcards

1
Q

What natural properties of hydrogen does MRI utilise to create images?

A

Proton density - related to the number of hydrogen atoms in a particular volume
2 characteristic times - T1 and T2 - how long the tissue takes to return to equilibrium after an RF pulse

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2
Q

What are the different proton densities of fluids and bone?

A

Fluids such as blood and CSF have higher PD compared to bones and tendons

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3
Q

What T1 times do fluids and bone have?

A

Fluids have a long T1 (1500-2000ms)

Water-based tissues are usually mid-range (400-1200ms)

Fat-based tissues have short T1 (100-150ms)

Water resonant a faster than fat and therefore takes longer to return to resting state

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4
Q

What T2 times do fluids and bone have?

A

T2 is always shorter than T1 for a given tissue

Fluids have the longest T2 (700-1200ms)

Water-based tissues have longer T2 than fat-based tissues (40-200ms and 10-100ms)

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5
Q

Why is MRI contrast developed?

A

Due to the differences in relaxation times of T1, T2, T2* and proton density (amount of water at a particular location) between different tissue types

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6
Q

What are the two principle types of pulse sequence?

A

Spin echo (SE)
Gradient echo (GE)

SE sequences use two RF pulses to create the echo which measures the signal intensity, SE can produce T1, T2 or proton density weighted images, generally produce the best quality images
GE sequences use a single RF pulse followed by a gradient pulse to create the echo which also measures signal intensity
GE sequences can produce T1, T2 or proton density weighted images - generally have much shorter SE and TR, however, they are affected by inhomogeneities of the magnetic field creating T2*

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7
Q

What does the recovery time of T1 and T2 depend on?

A

The molecular motion present at the Larmor frequency

The relaxation times are related to the molecular motions

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8
Q

How can we change MRI contrast?

A

By altering the time at which the signal is measured after excitation and the time allowed to recover between pulses (changing TE and TR)

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9
Q

What are TE and TR?

A

TR = time to repetition - how quickly the entire pulse sequence is repeated

TE = time to echo - the time between the signal excitation and maximum signal

Both TE and TR are under the operator’s control

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10
Q

What provides image contrast?

A

Differences in proton density and T1 between tissues in each image voxel provide image contrast

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11
Q

What is the difference between a long TR and a short TR?

A

Long TR = proton density weighted

Short TR = T1 weighted

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12
Q

What is a pulse sequence?

A

Pulse sequence is used to create all MR images

It consists of a radiofrequency pulse (RF pulses) and gradient pulses which have carefully controlled duration and timings (TE and TR) - these can be modified to change the image contrast

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13
Q

What types of relaxation are T1 and T2?

A

T1 = spin-lattice relaxation
T2 = spin-spin relaxation

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14
Q

What is the difference between T2 and T2*?

A

T2 = spin echo sequence
T2* = gradient echo

In a gradient echo sequence, we see the combined effect of T2 and magnetic field inhomogeneities - we call this relaxation the apparent relaxation time. This is T2*

T2 can be considered the true T2 of the tissue being imaged as it has been corrected for magnetic inhomogeneities

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15
Q

What is the reason why we have T2* images?

A

Due to the effects of an imperfect magnetic field
Even if we had a perfect magnetic field, the patient would make it imperfect due to susceptibility effects (air pockets like sinuses, dense bone and iron-rich blood break-down products all change the main magnetic field
These inhomogeneities affect the relaxation of tissues after an RF pulse, speeding up the apparent spin-spin relaxation
A spin echo sequence can correct for this effect but gradient echo sequences can’t and so gradient echo images depend on the apparent spin-spin relaxation time T2*

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16
Q

What weighted image takes longest to scan?

A

T2 weighted images because they require long TR and long TE

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17
Q

How does the length of T2 affect the image?

A

Long T2s are brighter (high signal) at long echo times than short T2s (low signal)

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18
Q

What are IR sequences?

A

A variation of spin echo sequences called inversion recovery (IR sequence) which has an extra 180 degree RF pulse separated by a new timing parameter (the inversion time T1) before the 90 degree pulse

19
Q

What are IR sequences used to obtain?

A

IR sequences can produce excellent T1 weighted images however they are most often used for tissue suppression images, since by selecting the T1 carefully, the signal from any particular issue can be nulled

20
Q

What are FLAIR images?

A

Fluid Attenuated Inversion Recovery

An IR sequence with a different T1 to produce T2 weighted images with a suppressed CSF signal

21
Q

What is FLAIR imaging used for?

A

Commonly used for neurological imaging where lesions may be close to the ventricles

It is more sensitive than simple T2 weighted images for MS lesions

22
Q

What is SNR and CNR and what is the difference?

A

SNR = signal to noise ratio - measure of the image signal in an area of tissue with respect to the background tissue

CNR = contrast to noise ratio - is the contrast between the average image values in a tissue of interest relative to the background

23
Q

How does the TE and TR times affect SNR?

A

Increasing the TE will reduce the SNR - long TE causes transverse magnetisation to decay to very low values which result in signal loss

Increasing TR increases SNR as a high TR allows the longitudinal magnetisation to approach its maximum and produces high signal intensities

24
Q

What is isointensity?

A

Having the same intensity/brightness as the thing we are comparing it to

25
Q

What type of scan has a short TR and short TE?

A

T1 weighted

26
Q

What type of scan has a short TR but a long TE?

A

Noisy scan

TE is always before TR as you wouldn’t repeat the RF pulse before you’ve even measured it

27
Q

What type of scan has a long TR and short TE?

A

Proton density

28
Q

What type of scan has a long TR and TE

A

T2 weighted

29
Q

What substances have a long T1 and T2?

A

Water
CSF
Certain pathologies
Edema

30
Q

What substances have intermediate T1 and T2?

A

Muscle
Grey matter
White matter

31
Q

What substances have a short T2 and long T1?

A

Air
Cortical bone
Heavy Ca+
Deoxyhaemoglobin
Tendons

32
Q

What substances have short T2 and short T1?

A

Fat
Proteinaceous solution
Paramagnetic agents

33
Q

What are the pros and cons of GE and SE imaging?

A

Gradient echo
PROS
- Faster imaging
- Shorter TE

CONS
- Poor magnetic field e.g., air pockets homogeneity distorts the image

Spin echo
PROS
- Less susceptible to magnetic field homogeneity

CONS
- Usually longer TE
- Slower imaging than GE

34
Q

Define diamagnetic, paramagnetic and ferromagnetic

A

Diamagnetic = very weak susceptibility which actually produces an internal field in the opposite direction to the applied field
(most body tissues are diamagnetic)

Paramagnetic = have stronger susceptibility and produce a field in the same direction as the main field (gadolinium contrast, deoxyhaemoglobin)

Ferromagnetic = become strongly magnetised and experience a large force when placed in an external field (metal alloys containing iron, nickel and cobalt)

35
Q

What are 3D FatSat FLASH images?

A

Acquiring 2, 3 or more echoes at different TE’s, “water only” and “fat only” images can be extracted through sophisticated mathematical processing

Exploit chemical shift difference between water and fat for image editing
A chemical shift selective pulse before the acquisition saturates the fat signal

36
Q

What is the Dixon method?

A

Allows 4 types of images to be produced
1. Fat only
2. Water only
3. In-phase
4. Out-phase

Modern Dixon methods combine 3 echoes acquired at different TEs to create water only and fat only images

37
Q

How does the Dixon method create in phase and out phase images?

A

Relies on the chemical shift frequency difference between fat and water to create two types of images
For a particular TE, the fat and water signal can be in or out of phase
Two images are acquired with in-phase fat water and out of phase combined with GE and SE sequences

Water only =
in-phase(water + fat) + out of phase(water-fat)

Fat only =
in-phase(water + fat) - out of phase(water - fat)

38
Q

What is another way of changing the image contrast?

A

Inject substances that change the T1/T2 of tissue

== Gadolinium-chelate

39
Q

What are the properties of Gadolinium which make it a good contrast agent?

A

Paramagnetic substance (weakly magnetic)
Shortens T1
Increases signal on T1 weighted image
Useful in detecting lesions and tumours
Dynamic contrast enhanced (DCE) MRI

40
Q

What is the reason for injecting chelate with gadolinium?

A

Gadolinium is toxic so needs to be chelated to make it safe for injection - other chemical ions mixed with gadolinium surrounding the toxic metal, protecting the body

41
Q

What is magnetisation transfer?

A

An MRI technique that can be used to exploit the contrast between tissues where 1H protons are present in three states:

  • Bound to macromolecules
  • In free water
  • As water in the hydration layer between the macromolecules and the free water

Irradiating tissue with a long RF pulse saturates the signal macromolecule proteins
Protein protons exchange with mobile protons - reducing the water signal
Change the image contrast dependent on protein concentration

42
Q

What is CEST?

A

Chemical exchange saturation transfer

  • measures the transfer of magnetisation from molecular protons to the solvent water protons, an effect that becomes apparent as an MRI signal loss
43
Q

How can hyperpolarisation be utilised as a contrast agent?

A

MR image mostly concerned with the hydrogen signal
MRI lung imaging is confounded by low tissue density, short T2* and magnetic field inhomogeneity
Can use gases such as xenon or helium which are hyperpolarised to increase image contrast by filling the lungs with the gas

44
Q

What is pyruvate metabolism imaging?

A

Can track the evolution of the pyruvate signal as it metabolises to other molecules such as lactate

Lactate is a hallmark indicator of cancer so can measure lactate signal to detect cancer

Neuroimaging 1 (16 decks)

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