MRI

Question 1

Energy of wave with frequency w

Answer 1

E_w = hw/2 pi

Question 2

Energy difference between levels

Answer 2

delta E = gamma h B0 / 2 pi

Question 3

Larmor frequency

Answer 3

w = gamma B0

Question 4

T1 relaxation

Answer 4

Spin lattice relaxation - longitudinal.
Magnetization vector returns to equilibrium, caused by loss of energy of nuclei to surroundings - given as heat not signal

Question 5

T1 relaxation equation

Answer 5

Mz(t) = M0[1-exp(-t/T1)]
(after 180 degree pulse Mz(t) = M0[1-2exp(-t/T1)])

Question 6

Is T1 or T2 shorter

Answer 6

T2 is shorter
In solution T2 can approach T1, in vivo T2 can be 5-10 times shorter

Question 7

T2 relaxation

Answer 7

Spin-spin relaxation - transverse.
Decay of observable transverse magnetization, nuclei randomly moving together or further apart means magnetic moment affects other nuclei and experienced B field - causes dephasing.

Question 8

T2 relaxation equation

Answer 8

Mxy(t)=Mxy(0)exp(-t/T2)

Question 9

T2* relaxation

Answer 9

Caused by spatial inhomogeneities in the field - causes addition dephasing. Constant, not temporal or random.

Question 10

Is T2* or T2 shorter?

Answer 10

T2* is shorter

Question 11

Spin echo

Answer 11

Rephases with 2 pulses, 90 degree and then 180 degree. Rephases T2*, can’t help T2.

Question 12

Gradient echo

Answer 12

Rephases using gradients - not rephased T2 star, gradient echo is weighted by T2*.

Question 13

3 gradients

Answer 13

Phase encoding
Readout (frequency encoding)
Slice selection

Question 14

Readout gradient

Answer 14

Frequency encoding
Magnetic field gradient across scanner so w changes dependent on location. Spatially encoded spins. Frequency tells you where you are - amplitude tells you how many.

Question 15

Phase encoding gradient

Answer 15

Orthogonal to readout gradient. First time most negative gradient up to most positive. Changes phase dependent on y position then switch off gradient, phase shift remains.

Question 16

Effect of PE gradient strength on signal

Answer 16

No gradient, no dephasing, large signal
Small gradient, smaller signal
Large gradient, larger dephasing, zero signal.
When structures match gradient - high signal.
Testing different frequencies.

Question 17

Image formation process steps

Answer 17

Polarise
Excite
Encode
Detect
Transform

Question 18

What causes the noise hazard?

Answer 18

Vibration of the gradient coils due to rapidly switching electrical currents

Question 19

What causes peripheral nerve stimulation

Answer 19

High current, voltage and switching rate
Bodily tissues are electrically conductive
Resultant electric and induced fields stimulate nerves and muscles.
Gradient direction and body position alter sites of PNS.

Question 20

How can we prevent PNS and noise hazard

Answer 20

Reduce magnitude and frequency of coil vibration and lower gradient switching
Reduce dB/dt.

Question 21

RF field hazard, how is it measured and what raises the risk

Answer 21

A fraction of energy deposited in body as heat - main risk is thermal heating leading to burns.
Measured in terms of specific absorption rate.
Raised by induced currents in: coiled cables, implanted cables, implanted medical devices, tissue loops.

Question 22

Why do RF burns occur

Answer 22

As a result of excessive heat deposition

Question 23

Polarise step

Answer 23

Separate energy levels of nucleus leading to thermal polarisation of nuclear spins between energy levels

Question 24

Excite and detect

Answer 24

RF transmit coil transmits excitation pulse and range of local RF receiver coils detect MR signal

Question 25

Encode

Answer 25

gradient coils, which are switchbale, impose linear gradients on field such that resonant frequency becomes linearly dependent on location in scanner bore

Question 26

Transform

Answer 26

computer reconstruction and image storage system used to perform FT of data giving final image

Question 27

Centre/outside of k-space

Answer 27

Centre: low frequencies, contrast
Edges: high frequency, edges