4: CT/MRI

Question 1

Tomography principles

Answer 1

• X-rays transmitted through the patient
• Radiation detector positioned opposite
• Each material has its own attenuation (ability to reduce the source intensity)
• Radiation detector determines emerging intensity

Question 2

CT grey scale

Answer 2

• Imaging system assigns grey levels to different ranges of attenuation coefficients
• Grey-scale image can be produced which contrasts between organs
• This forms the CT image
• Current systems utilise a spinning source surrounded by 700+ detectors to enable accumulation times as short as 1s

Question 3

CT future developments

Answer 3

• Utilise a scanning electron beam
• X-rays produced where electrons hit anodes- xray source rotates without moving parts
• Projection much faster- fast enough to image beating heart

Question 4

Basic CT system

Answer 4

•	Gantry consisting of:
o	X-ray source
o	X-ray detectors
o	Data-acquisition system (DAS)
•	Patient table
•	Control console
•	Computer- receive data, reconstruct data, produce image

Question 5

MRI system constituents

Answer 5

• Primary magnet- generates magnetic field
• Gradient magnets- allow magnetic field to be altered precisely- image slices can be created
• Coil- emits radiofrequency pulse to align protons

Question 6

MRI Principles 1

Answer 6

• Atomic nuclei contain a natural spin, which corresponds to an individual magnetic field
• Magnetic field is generated in the MRI machine
• Magnetic field causes these nuclei to align their fields with it (like a compass needle)
• Energy is applied to the realigned nuclei via a radio frequency
• RF causes nuclei to follow a cone-shaped path in field direction
• Possible to tune in to specific nuclear species with these RF’s
• After RF pulse is switched off, the nuclei continue to rotate freely
• Known as relaxation time

Question 7

MRI Principles 2

Answer 7

• Nuclei generate an alternating voltage which can be detected
• Signal detected only lasts for a brief time (T2)and is dependent on the nucei’s state of motion
• T2= spin-spin relaxation time
• Nuclei in solids = more static = very short T2
• Nuclei in liquids = more freedom = much longer T2
• Nuclei then return to normal vectors = spin-lattice relaxation = T1
• Differences in T1 and T2 enables contrast to be observed in MRI scans

Question 8

Advantages of MRI

Answer 8

• No radiation/ no long term effects demonstrated
• Variable thickness in any plane
• Better contrast and resolution than CT
• Easier to characterise abnormal tissue
• Many details without IV contrast

Question 9

Disadvantages

Answer 9

• Time consuming
• Loud
• Tunnel
• Not easily available
• Limitations with bone, air, resolution

Question 10

MRI best uses

Answer 10

• Soft tissue contrast
• Muscles
• Ligaments
• Inflammation
• Spine imaging- although disc lesion may not show up- request MRI to answer a specific question
• Before neurosurgery

Question 11

CT best uses

Answer 11

• Haemorrhage
• Calcifications
• Bone details

Question 12

Structural MRI

Answer 12

measures hydrogen atoms in water, contrasts tissue based on proportions of water

Question 13

Functional MRI

Answer 13

measures haemoglobin in blood. Can detect changes