Chapter 28 - MRI

Question 1

MRI key concept

Answer 1

1) Large external magnetic field generating magnetic field gradient
2) applied oscillating magnetic field (radiofrequency field)
3) makes proton in subject align parallel with external field
4) field define resonance frequency of proton
5) Fourier transform to produce MR image

Question 2

T2 weight image

Answer 2

simple fluid urine, bile, CSF = bright

other tissue = lower signal

Question 3

T1 weighted image

Answer 3

bright = fat, methemoglobin, flow effects, MRI contrast

Question 4

Spin echo pulse sequence

vs gradient echo pulse sequence

Answer 4

spin = use radiofrequency alone to produce MR signal = T2
gradient = use RF applied to gradient = T1

Question 5

Echo time and repetition time for T1 and T2

Answer 5

T2
Long echo time > 80 ms
long repetition time ~few seconds

T1
short TE (< 1 ms) and short TR (hundre ms to < 10 ms)

Question 6

Time of flight angiography

Answer 6

Use rapid T1 pulse sequence
Rapid single slice image causes protons within slice to lose magnetization
less signal from these protons and therefore not well visualized
as protons from outside the slice move, allow to visualize again

Question 7

Non-contrast MRA techniques

Answer 7

1) time of flight
2) ECG-gated steady state free precession SSFP
3) ECG-gated arterial spin labeling (ASL)
4) navigator gated SSFP
5) half-fourier fast spin echo imaging HASTE with flow spoiled gradients

Question 8

Key difference between CTA and MRA contrast

Answer 8

MRA does not image the agent itself but rather its effect on the surrounding protons

therefore a little agent can image a lot

Question 9

Effect of chelation of gadolinium

Answer 9

1) Unbound form of gadolinium is toxic
2) potentiates T1 signal
3) at high doses potentiates T2

Question 10

How to reduce metal artifact on MRI

Answer 10

1) metal artifact reduction sequence MARS
2) multiacquisition with variable resonance image combination MAVRIC
3) slice encoding metal artifact correction SEMAC

Question 11

Methemoglobin and MRI

where do we see it

Answer 11

Clots can contain high amounts of methemoglobin

signal can be higher enough to show up on T1 images

Question 12

Nephrogenic systemic fibrosis

Answer 12

aka nephrogenic fibrosing dermopathy

skin thickening, hyperpigmentation of extremity and trunk
organ fibrosis
death

Question 13

EVAR and MRI

Answer 13

Previous Zenith made of stainless steel could not be used but now ok with even up to 3T

Question 14

Time to delay before using MRI on implanted metal stent/coil/filter

Answer 14

6-8 weeks

Question 15

Benefit of 3T imaging

Answer 15

increase signal traded for decreased imaging time or increased spatial resolution

Question 16

Specific absorption rate SAR define

Answer 16

Deposition of RF energy into the patient

Question 17

Advantages of parallel imaging for MRI

Answer 17

1) faster imaging - less motion artifact
2) higher resolution in same acquisition time
3) lower SAR
4) improved image homogeneity

Question 18

Disadvantages of parallel imaging for MRI

Answer 18

1) more expensive machine
2) increase image noise
3) image artifact specific to parallel imaging

Question 19

Time-resolved MRA vs bolus chasing MRA

Answer 19

bolus chasing = moving table technique
chase contrast down the arteries

time-resolved = multiple rapid acquisition in same region
loss of spatial information for temporal resolution

Question 20

Blood pool contrast agents

Answer 20

remain intravascular for extended period which allow higher resolution imaging

Question 21

Types of blood pool contrast agents

Answer 21

1) gadolinium bound to macromolecules

2) ultrasmall superparamagnetic iron oxide particles