ViewRay - MR Linacs

Question 1

Downside of MR compared to CT

Answer 1

Expensive
No electron density correlation, unsuitable for planning RT
Magnetic safety
Small bore size (70cm) - could limit some patients due to bariatric or positioning reasons
Geometric accuracy
Specific absorption rate (SAR)
Time for image acquisition
Electron streaming
Electron return effect

Question 2

What is shimming

Answer 2

Process of adjusting the magnetic field to make it homogenous

Only achievable within a spherical volume quoted as a diameter of spherical volume (DSV)

Question 3

Two types of shimming

Answer 3

Active and passive shimming

Question 4

Active shimming

Answer 4

Uses corrector coils inside the bore, these coils dynamically adjust to the current field inhomogeneity and correct for this

Question 5

Passive shimming

Answer 5

This involves inserting physical pieces of metal or other ferromagnetic material into the bore to address inhomogeneity by mapping the field.

Question 6

Gradient coils

Answer 6

Modifies the homogeneity of the B0 field so that the scanner is able to locate the position of the emitted RF signal in 3D

The quality of the gradient coils is responsible for the spatial accuracy of the image and imaging speed

Question 7

What is slew rate and how to affects imaging

Answer 7

The speed at which the gradient reaches its maximum amplitude

Increase in SR allows for
- faster sequences, less motion artefact
- increased image quality
- faster real time imaging

Question 8

Trade-offs for increasing field strength

Answer 8

Increasing field length = increased quality

However, it also

Increases SNR
Higher Lorenz forces distort dose distribution
Image artefacts from chemical shift and susceptibility
SAR heating of patient tissues
Increased electron return effect and streaming effect

Question 9

Electron return effect

Answer 9

Happens inside the patient

Question 10

Electron streaming effect

Answer 10

Affects electrons coming outside the patient

Question 11

How do we overcome Lorenz force

Answer 11

Use Co60
Place as many linac components at a large distance from isocentre as possible
Shield linac and sub components
Use Monte Carlo calculations to account for patients

Question 12

What is SAR

Answer 12

Measure of energy delivered to the patient during a scan
Sequence dependent but increases with B0
Energy is absorbed per uni mass
Basically an MRI is a microwave, with high B0 the patient will heat up

Question 13

What can you do with an MR-Linac

Answer 13

Better define anatomy and OARs
Image during treatment delivery
Automatically contour targets and OARs
Track targets and OARs and assess if safe to treat
Adapt treatment based on actual anatomy and predicted dose
Gate treatment based on actual target position

Shrink margins
- tighter treatment margins
- treat the tumour and spare healthy tissue
- reduce dose to OARs by reducing treatment margins

Reoptimise and escalate dose
- dose escalation in multiple tumour sites
- maintenance of normal organ dose limits or constraints

Reduce fractions
- 5 fractions pancreas
- 1 to 10 fraction breast
- 5 fraction prostate
- 1 to 5 fraction lung