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PHYS4010: Functional Medical Imaging > MR 4 > Flashcards

MR 4 Flashcards

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USMLE® Step 1 flashcards
1
Q

Describe a magnetic field gradient varying along x

A

It varies from left to right

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2
Q

Give the equation for a magnetic field gradient in the x-direction

A
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3
Q

Describe a magnetic field gradient varying along y

A

It varies from back to nose

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4
Q

Give the equation for a magnetic field gradient in the y-direction

A
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5
Q

Describe a magnetic field gradient varying along z

A

It varies from toes to head

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6
Q

Give the equation for a magnetic field gradient in the z-direction

A
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7
Q

What is the impact of applying a magnetic field gradient?

A

Spins within the field will precess at slightly different frequencies.

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8
Q

Give the equation for the local magnetic field experienced by a spin in the presence of a magnetic field gradient

A

B(r) = local magnetic field
B₀ = applied static field
G = gradient
r = position

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9
Q

Give the equation for the Larmor frequency of a nucleus in the presence of a magnetic field gradient

A

ω = Larmor frequency
γ = Gyromagnetic ratio
B₀ = applied static field
G = gradient
r = position

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10
Q

What is the isocentre?

A

The position at which spins precess at the Larmor frequency.

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11
Q

_______ _____ are designed to produce linearly varying magnetic fields.

A

Gradient coils

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12
Q

Which gradient coil produces a gradient along z?

A

Maxwell pairs

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13
Q

Which gradient coil produces a gradient along x or y?

A

Straight wires

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14
Q

How can a magnetic gradient be produced in an arbitrary direction (i.e. at an angle)?

A

By using a linear combination of x and y gradients.

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15
Q

Describe an MRI scanner that contains gradient coils for gradients along x, y, and z

A
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16
Q

Why is MRI acoustically noisy?

A

Magnetic field gradients are created by currents that flow within the magnetic field and alternate rapidly, causing the gradient coils to vibrate.

17
Q

How does slice select work?

A

A magnetic field gradient, G, is applied in the direction of the chosen plane. An RF pulse is also applied over a narrow bandwidth of frequencies (∆ω) at the same time as the gradient to excite spins with that range of Larmor frequencies.

18
Q

Give the equation for the bandwidth of frequencies chosen in slice selection

A

∆ω = bandwidth of frequencies
γ = Gyromagnetic ratio
G = gradient
∆z = position range

19
Q

How are RF pulses created?

A

By modulating the carrier frequency at audio frequencies to generate the pulse shape (i.e. by multiplying the carrier frequency by an audio window).

20
Q

What is the frequency equivalent of multiplying two functions in time?

A

Convolution in the frequency space.

21
Q

How can a specific, narrow range of frequencies be excited?

A
  1. Choose a carrier frequency (ω₀) for the RF frequency.
  2. Use an RF pulse with a bandwidth, ∆ω.
  3. Fourier transform the signals to give a square wave centred at the Larmor frequency in frequency space that can be used to excite the frequencies.
22
Q

How can the thickness of an imaging slice be altered?

A

By varying the shape of the RF pulse. The longer the pulse (∆ω), the narrower the bandwidth.

23
Q

How can the slice position of an imaging slice be altered?

A

By varying the carrier frequency. The carrier frequency represents the centre of the slice.

24
Q

Give the equation that relates time to frequency

A

ω = frequency
t = time

25
Q

Give the equation that relates space to spatial frequency

A

k = spatial frequency
λ = space

26
Q

What is k-space?

A

An array of numbers representing spatial frequencies in an image.

27
Q

Why is k-space important in MRI imaging?

A

k-space must be filled to generate an MRI image.

28
Q

How does back-projection reconstruction (BPR) work for MRI imaging?

A
  1. A slice is selected by applying a gradient in the z-direction.
  2. Another gradient then is applied along in a different direction so that precessional frequency depends on this gradient.
  3. The signal depends on the density of spins, ρ(x), at a given position. This spin density is encoded using a third ‘readout’ gradient.
  4. This is repeated for many angles to map out lines of k-space.
  5. The projections are Fourier Transformed to process an image.
29
Q

What is the benefit of applying many gradients to generate an MRI image?

A

The signal can be localised

30
Q

What applications are best suited to back projection reconstruction?

A

Imaging things with a short T₂* like the lungs.

31
Q

What are the 3 main drawbacks of back projection reconstruction?

A
  • The low frequencies (central) are well sampled but the high frequencies are poorly sampled.
  • Images can be badly affected by inhomogeneities in the main magnetic field (B₀).
  • The total scan time is long.
32
Q

Define signal decay

A

The bright signal at the centre of k-space which drops away.

33
Q

What is spin-warp imaging?

A

A method of imaging that uniformly samples k-space. It uses an encode gradient initially (AB line) which causes a phase shift, then a frequency encode gradient (BC line) is applied.

34
Q

What are the advantages of spin-warp imaging?

A
  • Most widely used imaging sequence
  • Robust
  • Field errors cause distortion rather than blurring
35
Q

What are the disadvantages of spin-warp imaging?

A
  • Long imaging times
  • Motion causes artefacts (exemplified by long imaging times)
36
Q

What is echo-planar imaging?

A

An imaging technique that uniformly samples k-space similar to spin-warp imaging, however, it samples all of k-space in one FID by taking a spin echo and then changing the phase encoding until k-space is filled.

37
Q

What are the advantages of echo planar imaging?

A
  • Total scan time is very short (<100 ms)
  • No motion artefacts
  • Dynamic processes can be scanned
  • Quantitative imaging
38
Q

What are the disadvantages of echo planar imaging?

A
  • Low intrinsic signal-to-noise ratio
  • Low spatial resolution
  • Sensitive to ‘ghost’ artefacts
  • Very noisy unless low inductance gradient coils are used
  • Sensitive to distortions
  • Sensitive to signal loss

PHYS4010: Functional Medical Imaging (8 decks)

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