MRI

Question 1

types of magnets

Answer 1

closed (mostly) - 1.5T or 3T
open systems are available (low field) - less than 0.4T

Question 2

why MRI in RT

Answer 2

• improved visualisation in MRI can help delineate treatment areas
• non invasive and better for repeated scanning
• better for paediatric patients
• some artefacts in CT (dental fillings) are better on MR
• moving organs can be visualised

Question 3

why NOT MRI in RT

Answer 3

• MRI contraindications
  
  • certain implants are not compatible with MRI (pacemakers, aneurysm clips)
  • claustrophobia
  • certain implants, while safe for the patient, cause problems in the image
  • Gd contrast agent - allergic reactions, NSF
  • patient needs to be changed into scrubs and remove external metallic devices (jewellery, belt)

Question 4

if MR and CT are both required prior to treatment, which one first?

Answer 4

MR bore is more restrictive - if done first patient positioning will be compatible across both scanners
- same head tilt
- reduced the need for deformable registrations

if CT has occurred first - use body markings for positioning
- be aware of ink - some cause signal loss on MR

Question 5

MR quality - volume (too large and too small)

Answer 5

too large - distortion increases, scan time increases
too small - missed target or image registration fails

make sure target volume is in isocentre to reduce distortion

Question 6

distortions in MRI arise from

Answer 6

B0 homogeneity
magnetic susceptibility
- air tissue artefacts
- tissue bone interfaces
- metal implants
chemical shift artefacts
distance from the isocentre
pulse sequence and parameters

Question 7

application examples of MR and RT

Answer 7

brain
spine
lung
pancreas
prostate
cervix
rectum
musculoskeletal

Question 8

hybrid systems - PET/MRI

Answer 8

• simultaneous PET and MR
• high resolution, dynamic MR with molecular sensitivity of PET

Question 9

PET/MRI issues

Answer 9

attenuation compensation, motion detection and correction, partial volume correction

Question 10

PET/MRI advantages

Answer 10

• lower radiation dose compared to PET/CT
• great for follow up or paediatric scanning

Question 11

PET/MRI clinical uses

Answer 11

• hypoxia/RT treatment
• metastases - improving diagnosis and treatment
• head and neck RT planning
• neuroinflammation - repeated head injury
• dementia/Alzgiemer’s - amyloid/tau/cholinergic function
• epilepsy
• prostate cancer

Question 12

MRI/Linac

Answer 12

MRI plus linear accelerator
- radiation beams targeting tumours in real time
- see where the radiation is going as it happens
- maximise dose to tumour and protect surrounding tissue
- particularly good for moving tumours
- breathing can move the tumour 2cm
- prostate, lung, liver, pancreas and abdomen
- head and neck - swallowing

Question 13

benefits of high tesla (T)

Answer 13

more signal, better resolution = increased image quality

Question 14

Give some examples of closed receiver coils

Answer 14

head and neck coils
knee coil
foot coil

Question 15

Give some examples of flexible receiver coils

Answer 15

chest, flex, body coils and spinal array

Question 16

Explain how contrast changes an image

Answer 16

Contrast can change the degree of excitation, types of signal echo, changing time can change the contrast between different tissue types (pulse sequence)

Question 17

In T1 relaxation list in order from shortest to longest relaxation rates for grey matter, white matter, liver, spinal cord and blood

Answer 17

Liver, Spinal cord, White matter, Grey matter, Blood

Question 18

What are some MR considerations?

Answer 18

Signal is relative to distance away from the receiver coils so immobilization devices should lower signal to background ratio (SBR).

Fiducial makers such as gold will be void and have no signal.

Smaller voxels will reduce SNR (longer scan time can improve SNR for small voxel)

Question 19

What affects MR quality?

Answer 19

Volume size - too large increase distortion, scan time increases

too small cause image registration fails and missing targets, say close to isocenter. For 2D images scan with no slice gaps and 3D scan the shortest axis

Question 20

How is motion accounted for?

Answer 20

Image rapidly or trigger with the movement.
Gating - respiratory & cardiac

Question 21

benefits of MR in oncology

Answer 21

Ability to map tissue characteristics, microstructure information looking at diffusion of water, fiber tracking in the brain, tissue perfusion and angiography, temperature mapping, metabolic mapping with spectroscopy

Question 22

What does T2 images show?

Answer 22

higher water content means high signals cancer have high density = low water content = low signal

Question 23

How do diffusion images work?

Answer 23

measures the microstructure by looking at the movement of water.
Cancer has high density - image will appear brighter due to restricted water. Can be used for white matter compression in brain. Diffusion is dependent upon temperature therapy monitoring

Question 24

What is magnetic resonance spectroscopy?

Answer 24

measure of metabolites in-vivo. Choline and creatine concentrations can increase in tumours with grade; citrate concentration decrease - time consuming

Question 25

What is T1/T2 is good for?

Answer 25

Separating gliomas and metastases, separating high- and low-grade prostate cancer

Question 26

pros of MRI in RT

Answer 26

non ionising
good soft tissue delineation
contrast agent less likely to cause allergy

Question 27

cons of MRI in RT

Answer 27

expensive
contraindications
time consuming
noisy and large magnetic field
MR planning not efficient

Question 28

the purpose of flex bridge

Answer 28

plastic bridges to positioning the flex coils
placing on the patient directly can cause the tissue to deform due to the weight –> we want accurate soft tissue for RT planning

as signal is relative to distance, the image quality wont be as good compared to coils being on the patient
further away from the coil –> low SNR