CT and MRI

Question 1

CT

Answer 1

 Multiple X-ray images are taken by a tube rotating around the patient
 A computer combines these to generate a composite image

Question 2

Advantages of CT

Answer 2

 useful in showing bony lesions i.e. calcification or fractures
 good contrast between different tissues
 cannot differentiate well between different parts of the same organ
 useful in detecting metallic Foreign body vs MRI
 rapid imaging vs MRI

Question 3

disadvantages of CT

Answer 3

 High ionizing dose
 Can miss very small lesions
 Limited contrast as cannot differentiate between tissues of similar densit

Question 4

voxel pixels

Answer 4

are expressed in Hounsfield units
- air -1000
- water 0
- bone +1000

Question 5

phases of CT

Answer 5

non/pre-contrast
arterial
venous
delayed

Question 6

x-ray doses

Answer 6

mSV = milisivert
background = 2.7 mSV per year
Chest X ray equivalent to 0.02 mSV/ 3 days background radiation
Abdo X ray 0.7mSV/ 3 months
Head CT 2msV/ 9 months
Abdo CT 8mSV/ 3 years

Question 7

MRI

Answer 7

 Nuclear dipoles (protons) are aligned with each other by a strong external magnetic
field
 In this state, the protons rotate (precess) around their own axes in different phases
 The direction of nuclear dipole alignment is then altered by a radio frequency pulse
 The protons also precess in phase with each other in response to the pulse
 The pulse then dissipates
 The protons realign along the original magnetic field (T1 relaxation)
 They begin to rotate out of phase again (T2 relaxation)

Question 8

Short tau inversion recovery

Answer 8

STIR
sequences correlate strongly with orbital
inflammation and clinical activity scores in thyroid eye disease (T2 STIR
specifically)

Question 9

Fluid-attenuated inversion recovery (FLAIR)

Answer 9

sequences employ fluid suppression
techniques to highlight paraventricular pathology: useful to identify demyelinatio

Question 10

weightings T1

Answer 10

T1 fat > white > grey > CSF

Question 11

weightings T2

Answer 11

CSF > grey > white > fate

Question 12

weightings flaire

Answer 12

fat > grey > white > CSF

Question 13

weightings stir

Answer 13

CSF = grey > white > fat

Question 14

MRI orbital protocol

Answer 14

o Targeted sequences should be requested ( radiology would arrange this)
o Not practical to have both brain and orbital protocol
o Lead to uncomfortably long examination

Question 15

MRI orbital protocol stages

Answer 15

o Precontrast axial, coronal T1
o Good for assessing marrow/ bony orbit walls
o Extraocular muscles
o Detection of intracranial masses
o Axial T2 ( after gadolinium injection)
o Best for orbital apex, optic canal
o Coronal T2 with fat suppression (after gadolinium injection)

Question 16

When to use MRI orbital

Answer 16

optic nerve, chiasm, tracts

Question 17

MRI optic nerve inflammation findings

Answer 17

o Findings most apparent in orbital segment
o Increased T2 signal
o Contrast enhancement best seen with fat suppression in coronal plane
o Enhancement seen in >90% cases if MRI performed within 3 weeks of symptom onset