Advanced Tech and Imaging - MR and MR Linacs

Question 1

Advantage of the use of MR at Pre-treatment?

Answer 1

• Better delineation of soft tissue and better definition than with CT
• Interoperator variability decreases as delineation is better
• Smaller margins?Smaller GTV?
• Reduces normal tissue complication
• CT-MR, outline using MR.
• Artefacts present more notably within MR

Question 2

Disadvantage of the use of MR at Pre-treatment?

Answer 2

• Margin reduction is only useful if continue to use MR on-set
• Does not increase dose to the tumour (?)
• CT is enough to calculate hounsfield units as it is spacially correct, MR is not necessarily because it is not using radiation
• Distortion with MR. Size of patient within the bore impacts image
• Require MR safe immobilisation
• Motion is not resolved using MR, MR takes longer, blurred?

Question 3

Why are there difficulties with implementing MR within RT?

Answer 3

• Only possible within developments in format and software (however this is starting to be solved)
• Diagnostic machine often required
• Distortion, although algorithms are possible to help solve the problem
• Registration algorithms, how well can this be done
• Cost
• RF- Coils - flip the H nuclei, images are bettter the closer to the patient they are (distorts the patient as they are heavy).
• Poor reference for fiducial markers?

Question 4

What are the advanatages of MR-Linacs?

Answer 4

• Excellent imaging quality
• Fast sequencing and high spacial resolution now possible
• Volumetric imaging, true interfractional
• No rad dose

Question 5

What are the disadvanatages of MR-Linacs?

Answer 5

• Giant Magnet, impact the linac?
• Does the linac impact MR images (radiofrequency)
• Distortion
• Radiotherapy delivery pair production etc produced electrons, in magnetic field will change direction, wil this impact the dose??
• Cost
• Calibration

Question 6

What interactions impact the MR Linac?

Answer 6

• Ferromagnetic materials and magnetic fields, such as in the waveguide and MLCs
• The effect of magnetic and RF fields on the electron gun and the waveguide to steer and focus
• Practical limitations with regards to size and energy, e.g field sizes

Question 7

What are the advanatages of modern MR-Linacs?

Answer 7

• Improved imaging quality
• Fast image sequencing in real time
• Therapy and imaging effectively simultaneous
• No extra radiation
• No implanted markers needed
• Ease of set up and delivery is similar to tomotherapy
• ICRT, IMRT and ART all feasible
• Realtime image based gating is a possibility

Question 8

What problems are being solved regarding MR Linacs?

Answer 8

• Active magnetic and optimised shielding - reduces the field around MR and therefore less intergerence with the linac
• Synchronised puling to reduce RF Linac interference with MR imaging sequences
• Dosimetry (linac ion chamber calibration) can now be accounted for
• Distortion correction strategies are now implemented
• IMRT compensates for megnetic effects of high energy in tissue