Imaging Modalities

Question 1

What are the advantages of CT?

Answer 1

Quick
Cheap
Widely available
Can produce DRRs
Can be fused with MRI/PET
Contrast aids visualisation
Determines extent of bone involvement
Provides accurate patient contour
Can distinguish structures
Compatible with RT planning system - electron density data present
Good visualisation of bone

Question 2

What are the disadvantages of CT?

Answer 2

Inefficient at highlighting nodal involvement
Gives ionising radiation
Over-estimates tumour dimensions

Question 3

What are the advantages of MRI?

Answer 3

Can be weighted either T1 or T2
Can image in any plane
No radiation is delivered
Better contrast resolution than CT
Good soft tissue differentiation
Clear margins between tumour and OAR

Question 4

What are the disadvantages of MRI?

Answer 4

Can't produce DRRs
Dental fillings cause artefacts
Small bore
Claustrophobic
Noisy
Errors can occur when fused with CT
Extra time needed for fusion with CT
Can't use RT immobilisation devices
Can't scan patients with metal inside them
Not compatible with RT planning system

Question 5

What are DRRs?

Answer 5

Digitally Rendered Radiographs
Field shown from beam’s eye view to show shielding - gives a 2D image of a 3D plan
Thinner CT slides produce a better quality DRR image. Have to consider the type and size of structures that need to be localised. I.e. thinner slices are used for a brain than a prostate as it is a smaller volume.

Question 6

What do T1 MRI images look like?

Answer 6

Water is dark and fat/soft tissue is bright. Used for looking at anatomy and localising spinal levels.

Question 7

What do T2 MRI images look like?

Answer 7

Fat/soft tissue is dark and water is bright - useful for imaging oedema and pathology as blood vessels show up

Question 8

What anatomical sites would MRI localisation be useful for?

Answer 8

Brain: displays oedema better than CT; artefact is produced by bone in CT, so MRI is good for the posterior fossa
Spinal cord: provides good detail of pathological fractures and spinal cord compression
Head and neck: good for soft tissue tumours, although CT is also useful as lots of bony structures
Can be used for chest/pelvis/abdomen but movement causes artefacts

Question 9

What are the advantages of PET?

Answer 9

Functional imaging demonstrates metabolic activity
Vital tool for localisation and when combined with CT can be used for planning
Can be fused

Question 10

What are the disadvantages of PET?

Answer 10

No electron density data unless fused with CT
Increased dose to the patient - large scans and radiopharmaceuticals
Cannot produce DRRs
No soft tissue visualisation