MRI Flashcards

1
Q

Energy of wave with frequency w

A

E_w = hw/2 pi

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Energy difference between levels

A

delta E = gamma h B0 / 2 pi

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Larmor frequency

A

w = gamma B0

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

T1 relaxation

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

T1 relaxation equation

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

Is T1 or T2 shorter

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

T2 relaxation

A

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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

T2 relaxation equation

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

T2* relaxation

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

Is T2* or T2 shorter?

A

T2* is shorter

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Spin echo

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Gradient echo

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

3 gradients

A

Phase encoding
Readout (frequency encoding)
Slice selection

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Readout gradient

A

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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Phase encoding gradient

A

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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Effect of PE gradient strength on signal

A

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.

17
Q

Image formation process steps

A

Polarise
Excite
Encode
Detect
Transform

18
Q

What causes the noise hazard?

A

Vibration of the gradient coils due to rapidly switching electrical currents

19
Q

What causes peripheral nerve stimulation

A

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.

20
Q

How can we prevent PNS and noise hazard

A

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

21
Q

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

A

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.

22
Q

Why do RF burns occur

A

As a result of excessive heat deposition

23
Q

Polarise step

A

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

24
Q

Excite and detect

A

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

25
Encode
gradient coils, which are switchbale, impose linear gradients on field such that resonant frequency becomes linearly dependent on location in scanner bore
26
Transform
computer reconstruction and image storage system used to perform FT of data giving final image
27
Centre/outside of k-space
Centre: low frequencies, contrast Edges: high frequency, edges