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Neuroimaging 1 > MR Physics > Flashcards

MR Physics Flashcards

1
Q

When was MRI scanning developed?

A

Between 1950 and 1973
First MR images of human tissue was published by Peter Mansfield in 1977

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2
Q

What is a magnetic moment?

A

The magnetic moment (µ) is a vector quantity used to measure the tendency of an object to interact with an external magnetic field - in MRI the object is the hydrogen nucleus

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3
Q

What is Larmor frequency?

A

Refers to the rate of precession of the magnetic moment of the proton around the external magnetic field

The equation states that the frequency of precession of the nuclear magnetic moment is directly proportional to the product of the magnetic field strength (B0) and the gyromagnetic ratio (g)

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4
Q

What is the contrast in an MRI scan generated by?

A

Different body tissues exhibiting different magnetic properties
Scans visualise this difference in magnetic behaviour between brain matter, fat etc
The contrast is the ratio between high magnetic signal and low magnetic signal

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5
Q

What type of signal do bone and fluid have?

A

Bone = high signal (bright)
Fluid = low signal (dark)

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6
Q

What contrast is used in MRI and why are they used?

A

Gadolinium
Can be used to highlight certain tissues or pathologies

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7
Q

What are the two types of contrast?

A

Exogenous (gadolinium injection)
Endogenous (natural properties of tissue, i.e., without injections)

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8
Q

How does the value of the pixel reflect the MR signal?

A

Higher the pixel value, the brighter the signal

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9
Q

When we do image processing, we are manipulating sets of numbers, what is the arrangement of the numbers called?

A

Matrix or sometimes called a two dimensional array

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10
Q

How many axes do MRI scans have?

A

3 = x, y, z
This means we are dealing with stacks of matrices

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11
Q

What is the spatial resolution of an MR scan?

A

Each pixel quantifies an amount of physical space - in structural brain MRI scan this is typically 1mmx1mm
The size of the pixels is referred to as the spatial resolution

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12
Q

What does higher resolution mean in terms of pixels?

A

Higher resolution = smaller pixels = clearer image

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13
Q

What is a voxel?

A

3D volume of anatomy rather than a flat square (pixel)

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14
Q

What is the raw data format of MRI?

A

DICOM file

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15
Q

To perform analysis on the raw data what needs to happen?

A

Need to convert raw scans into other types which are more standardised and compatible with image processing tools
The single most common of these is the nifti format (.nii) which can be further compressed down into nifti-gz (.nii.gz) to save storage space

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16
Q

Describe the basic MRI hardware

A

Made up of 4 components
1. The magnet
2. The gradient coils
3. Radiofrequency transmitter and receiver
4. Computer

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17
Q

What is the function of the gradient coils?

A

The main function is to spatially modulate the main magnetic field in a predictable way, thereby causing the Larmor frequency of spins to vary as a function of position

The use of stronger gradients permits smaller anatomical features to be detected and enables faster scanning

Gradient coils are used to produce deliberate variations in the main magnetic field (B0). There are three sets of gradient coils, one for each direction. The variation in the magnetic field permits localisation of image slices as well as phase encoding and frequency encoding

18
Q

What is the RF transmitter and receiver?

A

The radiofrequency (RF) transmitter is the generator of the radiofrequency current which is delivered to the transmitting coil

This creates a signal which is used to excite protons in the imaging field

Radiofrequency coils can be both transmitters and receivers of the radiofrequency signal or receivers alone

A radiofrequency coil that performs both of these actions is called a transmit receiver coil or a transceiver coil

19
Q

How are the gradients generated?

A

Three sets of gradient coils one for each direction/axes, through which large electrical currents are applied repeatedly in a carefully controlled pulse sequence

20
Q

What is the basis of MRI scanning?

A

Placed in a magnetic field which affects the water atoms in the brain
Excites hydrogen nuclei in water atoms with radio wave
Receive radio wave responses from atoms and dump data into a matrix (K-space)
Perform mathematical operation to convert the raw data into an image

21
Q

What is nuclear spin?

A

Nuclei with unpaired neutrons and/or protons have nuclear spin

The nuclear spin state is the orientation of the spin-generated magnetic field relative to an external magnetic field

Spin combines with nuclear charge to produce a magnetic moment

22
Q

Describe the basic principle of MR imaging

A

For imaging purposes the hydrogen nucleus (a single proton) is used because of its abundance in water and fat

The hydrogen proton can be likened to the planet earth, spinning on its axis, with a north-south pole. In this respect it behaves like a small bar magnet. Under normal circumstances, these hydrogen proton “bar magnets” spin in the body with their axes randomly aligned

When the body is placed in a strong magnetic field, such as an MRI scanner, the protons’ axes all line up. This uniform alignment creates a magnetic vector oriented along the axis of the MRI scanner

When additional energy (in the form of a radio wave) is added to the magnetic field, the magnetic vector is deflected. The radio wave frequency (RF) that causes the hydrogen nuclei to resonate is dependent on the element sought (hydrogen in this case) and the strength of the magnetic field

The strength of the magnetic field can be altered electronically from head to toe using a series of gradient electric coils, and, by altering the local magnetic field by these small increments, different slices of the body will resonate as different frequencies are applied

When the radiofrequency source is switched off the magnetic vector returns to its resting state, and this causes a signal (also a radio wave) to be emitted. It is this signal which is used to create the MR images.

Receiver coils are used around the body part in question to act as aerials to improve the detection of the emitted signal. The intensity of the received signal is then plotted on a grey scale and cross sectional images are built up

23
Q

What is a magnetic moment?

A

The measure of the object’s tendency to align with a magnetic field

24
Q

What are spin up and spin down states?

A

Protons align in two orientations separated in energy by an amount proportional to the magnetic field and in relative numbers determined by Boltzmann statistics

In the spin-up state, the detected component of nuclear angular momentum is in the same direction as the external magnetic field. In the spin-down state, it is in the opposite direction

25
Q

What is the preferred energy state?

A

It is preferred to be in a lower energy state (aligned with the magnetic field) which means there are an excess of protons in the spin up state which means we can manipulate the signal

26
Q

What is the magnetic field strength measured in?

A

Tesla

27
Q

What is the advantage of higher field strengths?

A

Better signal-to-noise ratio (increased spatial resolution and reduced imaging time) and increased chemical shift effects, improving spectral fat suppression and spectroscopy

28
Q

What does increasing the field strength achieve?

A

As you increase the field strength, the amount of spins in the low energy state increases which increases the excess of spins in this state - this means the signal increases

29
Q

What is a magnetisation vector?

A

The summation of all the magnetic moments of the individual hydrogen nuclei

If hydrogen nuclei are placed within a strong external magnetic field, they become aligned within the field in one of two directions parallel to the direction of the field

In MRI, the main magnetic field is termed B0.

  • aligned in the direction of B0 (parallel)
  • aligned in the opposite direction of B0 (antiparallel)

A parallel and antiparallel hydrogen nuclei have equal but opposite magnetic moments and cancel each other out. However, there are always slightly more hydrogen nuclei parallel to B0 and this slight difference is termed the NMV (net magnetisation vector) and given the symbol M.

30
Q

What happens to the spins when placed in an external magnetic field?

A

The spins precess (rotate) about the external field
The precession frequency obeys the Larmor equation
fo = γ B0

31
Q

What is chemical shift?

A

An artefact - spatial displacement of water and fat due to their slightly different resonant frequencies - seen as slightly dark bands flanking an object along the frequency encoding direction

Chemical shift refers to small changes in resonant frequency due to different molecular environments of nuclei
The ¹H protons of fat, for example, are nestled within long-chain triglycerides and covered by electron clouds. These clouds partially shield the fat protons from the full effects of an externally applied magnetic field. The ¹H protons of water, however, are less shielded because their electron clouds are pulled away from them by the highly electronegative oxygen atom

32
Q

How can we detect the MRI signal?

A

Protons can be excited from the lower to higher energy state by applying radiofrequency waves that match the energy gap
Need to supply energy from a transmitter
Energy supplied by a pulse of radio waves at a frequency, f = g.B0

Energy couples to nucleus at resonance condition (frequency of transmitter matches Larmor frequency)

33
Q

How do we measure induction?

A

Nuclear spins precess like a spinning magnet
Changing magnetisation induces voltage in detector coil
Measure rate of oscillation as a voltage change

34
Q

What is free induction decay?

A

Free induction decay (FID) refers to a short-lived NMR signal which appears immediately following the 90° pulse

It is induced in the receiver coil by the rotating component of the magnetization vector in the x-y plane which crosses the coil loops perpendicularly

In pulsed methods the main magnetic field is held constant while an RF-field at the Larmor frequency is pulsed on and off - FID is observed following the pulse being turned off

35
Q

How do we measure free induction decay?

A

Changing magnetisation induces voltage in detector coil
Detect analogue oscillating voltage
Analogue signal digitised at discreet points - analogue to digital converter (ADC)

36
Q

What changes the image contrast?

A

When the pulse is switched off, the spins are not at equilibrium - more spins in the upper energy level
Once the radiofrequency field is switched off the nuclei undergo two processes due to microscopic motion of water and are collectively known as relaxation
- T2 (spin-spin or transverse relaxation)
- T1 (spin-lattice or longitudinal relaxation)

Different tissues have different T1 and T2’s and influence the appearance of the image

37
Q

Explain T2 relaxation

A

Free induction decay
Spin experience tiny difference in magnetic field strength
Each spin has a slightly different frequency from each other
After the RF pulse, bulk vector spreads out, dephases, in the XY plane losing signal
Decay rate exponential with a time constant T2 (ms)
Different tissues have different T2
CSF decays more slowly than white matter - it has a longer T2 relaxation time

38
Q

Explain T2* relaxation

A

T2 effects transverse magnetisation in the XY plane
Signal decay rate exponential with a time constant T2
T2* = T2 loss + extra loss due to variation in applied magnetic field (e.g., imperfect magnet)
T2* always shorter than T2 - signal disappears faster
Influences the appearance of the image

39
Q

Explain T1 relaxation

A

Back to equilibrium
Spins give up energy to their local environment to return to a low energy state
Requires the correct energy (frequency) for exchange to occur
Over a longer period the magnetic moments realign with the magnetic field with an exponential recovery time constant T1
Bulk magnetisation vector returns to the z-axis
Tissues have different T1 - another source of contrast
CSF recovers alignment with B0 more slowly than white matter as it has a longer T1 relaxation time

40
Q

What do inhomogeneities in B0 and B1 cause?

A

Large inhomogeneities in the B0 field can cause loss of signal

Inhomogeneities in either the transmitted or received RF fields (B1) result in changes in the image intensity and manifest as brighter or darker areas in the image

41
Q

How are inhomogeneities reduced?

A

Specific coils are built into the scanner called shim coils which are there to create fields that can cancel the bulk of the inhomogeneities

Neuroimaging 1 (16 decks)

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