MR 4

Question 1

Describe a magnetic field gradient varying along x

Answer 1

It varies from left to right

Question 2

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

Answer 2

Question 3

Describe a magnetic field gradient varying along y

Answer 3

It varies from back to nose

Question 4

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

Answer 4

Question 5

Describe a magnetic field gradient varying along z

Answer 5

It varies from toes to head

Question 6

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

Answer 6

Question 7

What is the impact of applying a magnetic field gradient?

Answer 7

Spins within the field will precess at slightly different frequencies.

Question 8

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

Answer 8

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

Question 9

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

Answer 9

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

Question 10

What is the isocentre?

Answer 10

The position at which spins precess at the Larmor frequency.

Question 11

_______ _____ are designed to produce linearly varying magnetic fields.

Answer 11

Gradient coils

Question 12

Which gradient coil produces a gradient along z?

Answer 12

Maxwell pairs

Question 13

Which gradient coil produces a gradient along x or y?

Answer 13

Straight wires

Question 14

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

Answer 14

By using a linear combination of x and y gradients.

Question 15

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

Answer 15

Question 16

Why is MRI acoustically noisy?

Answer 16

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

Question 17

How does slice select work?

Answer 17

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.

Question 18

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

Answer 18

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

Question 19

How are RF pulses created?

Answer 19

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

Question 20

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

Answer 20

Convolution in the frequency space.

Question 21

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

Answer 21

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.

Question 22

How can the thickness of an imaging slice be altered?

Answer 22

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

Question 23

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

Answer 23

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

Question 24

Give the equation that relates time to frequency

Answer 24

ω = frequency
t = time

Question 25

Give the equation that relates space to spatial frequency

Answer 25

k = spatial frequency λ = space

Question 26

What is k-space?

Answer 26

An array of numbers representing spatial frequencies in an image.

Question 27

Why is k-space important in MRI imaging?

Answer 27

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

Question 28

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

Answer 28

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.

Question 29

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

Answer 29

The signal can be localised

Question 30

What applications are best suited to back projection reconstruction?

Answer 30

Imaging things with a short T₂* like the lungs.

Question 31

What are the 3 main drawbacks of back projection reconstruction?

Answer 31

- 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.

Question 32

Define signal decay

Answer 32

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

Question 33

What is spin-warp imaging?

Answer 33

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.

Question 34

What are the advantages of spin-warp imaging?

Answer 34

- Most widely used imaging sequence - Robust - Field errors cause distortion rather than blurring

Question 35

What are the disadvantages of spin-warp imaging?

Answer 35

- Long imaging times - Motion causes artefacts (exemplified by long imaging times)

Question 36

What is echo-planar imaging?

Answer 36

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.

Question 37

What are the advantages of echo planar imaging?

Answer 37

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

Question 38

What are the disadvantages of echo planar imaging?

Answer 38

- 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