Lecture 9 MRI Guided Radiotherapy

Question 1

What are the advantages of MRI and their respective clinical values?

Answer 1

1. Non ionizing radiation
   —> frequent daily use, long time motion monitoring without=t extra dose
2. Superior soft tissue mage contrast
   —> better target localisation, delineation and motion tracking
3. Simultaneous MR acquisition and dose delivery
   —> motion monitoring, gating and tracking
4. Arbitrary acquisition orientation
   —> better visualisation anatomy and motion
5. Rich functional imaging capability
   —>potentials for target delineation, treatment adaptation and response evaluation

Question 2

MR Sim features?

Answer 2

• MR compatible external 3D lasers
• radio frequency coil
• 70cm diameter magnetic bore
• flat table couch

Question 3

What are the major problems of perpendicular and in-line hybrid MRI-LINAC?

Answer 3

Perpendicular: magnetic field bends charged particles, affecting beam generation and dose in patient

In line: specialised hardware design are required
Either magnet or patient must be rotated to maintain configuration alignment

Question 4

Technical hurdles of MR linac?

Answer 4

• magnetic interference between MRI and LINAC
• beam transmission through MRI
• radio frequency interference
• electron return effect

Question 5

Solutions to hurdles of MR linac?

Answer 5

• active magnetic shielding —> magnetically decouple MRI and LINAC
• additional magnetic sources —> create combined magnetic effect independent to LINAC position
• put-aside superconducting coil to allow beam passage
• RF shielding

Question 6

What are benefits if MR only simulation

Answer 6

• one scan, two images
• eliminate errors or MR-CT image registration
• MRI superior contrast for contouring
• Synthetic CT for dose calculations
• simplify planning workflows, reduce patient cost and burden

Question 7

What are the 2 adaptation strategies?

Answer 7

• adapt to shape (ATS) (deformable )
• adapt to position (ATP) (rigid translational )

Question 8

When do we need adapt to shape instead of adapt to position?

Answer 8

1. Anatomical change (eg. Target not covered, OAR too close to high dose)
2. Dosimetric criteria violation (eg. Dose not OK)
3. Online QA criteria (eg.translational shift out of tolerance, online QA failed)

Question 9

How online motion management can be achieved in MRI LINAC?

Answer 9

Orthogonal CINE MRI

Question 10

Pros and cons of 4D MRI

Answer 10

Pros:
- non ionizaing
- better visualisation of targets and OARs
- truly represent tumor motion and its variability

Cons:
- long acquisition time
- image with motion
- image distortion, not standardized