Imaging Flashcards
What is image guided radiotherapy
RT with imaging at pre-treatment and treatment delivery stage which leads to an action which can improve or verify the accuracy of RT
What is MV portal imaging & reference image
Using EPID - electronic portal imaging device
Using treatment beam / MV beam (might image with opened MLCs/jaws for imaging - don’t have to image with same field)
Reference image is DRR of planning CT
Using single or pair of 2D planar images
What is DRR
Digitally reconstructed radiograph, 2D image generated from 3D dataset
Simulated planar x-ray, might have iso or OAR marked on if generated by TPS
How does EPID work?
Copper conversion layer attenuates MV beam and generates secondary electrons
Scintillator converts photons/electrons into light
Photodiode array converts light to charge which is read out (A-Si)
Advantages of MV Portal imaging
Not treating blind
Use treatment beam - simple geometry
Good for QA images
Continuous imaging gives reassurance that target remains in beam
Disadvantages of MV portal imaging
Contrast 10-20 times lower than x-ray (compton dominates not PE)
Need surrogate for target position, eg bony anatomy
Scatter affects image quality
What is MV CBCT & reference image
Using same MV portal imaging but take multiple 2D projections and reconstruct with FBP
Reference image is planning CT
Advantages of MV CBCT
3D data, view sagittal, axial, coronal
Direct comparison with planning CT
Disadvantages of MV CBCT
Still have low contrast due to MV beam
CB means more scatter and therefore noisier images
Tomotherapy imaging and reference image?
Helical MVCT - couch moves through
Reference image is planning CT
Multiple 1D projections per axial slice, couch moves through bore
Detectors on tomotherapy
Designed for conventional CT with different SDD.
Detector cells more unfocussed moving away from centre of beam
Change in response across detector. Coupled with unflattened beam, double peaked detector response
Software corrects for this
Advantages of tomotherapy MVCT
3D imaging
Low MV beam, better contrast
Fan beam - less scatter induced noise than MVCBCT
Higher energy than kV, reduced artefacts from metal implants
Disadvantages of tomotherapy MVCT
Detector not designed for MV
Higher dose than kV imaging
Contrast not as good as kV
Slow acquisition time
Considerations in kV tube
Collimators reduce scatter by reducing field
Norm chamber (like monitor chamber) is photodiode that measures output during scanning and projections corrected for kV output
kV filter hardens beam to reduce skin dose
Bow tie filter reduces skin dose, prevents image saturation and reduces scatter
Why is reducing hardening in patient ideal?
Better dose distribution, gives better HU uniformity and accuracy
Advantages of kV imaging
3D information, direct comparison with planning CT
Better soft tissue contrast as kV
kV/MV pair time saving?
Disadvantages of kV imaging
CBCT takes 1 minute to acquire
2 isocentres, imaging and treatment
limited couch angles
Additional QA considerations
What is kVCT imaging?
Have a conventional CT scanner and couch then moves into linac.
Advantages of kVCT
Diagnostic quality imaging
Dirct comparison with planning CT
Soft tissue matching possible
CT scanner geometry stable
Disadvantages of kVCT imaging
Couch moves between imaging and treatment, patient could move
Imaging and treatment isocentres different
Time and space
What is exactrac?
External kV imaging
kV tubes recessed into floor either side of gantry
Orthogonal pair of images
Reference images are DRRs
Advantages of exactrac
Fixed geometry independent of linac should remain stable
3D shifts and rotations calculated
Quick to acquire
Can monitor patient positioning during treatment
Imaging at different couch angles possible
Disadvantages of exactrac
External system, can provide issues
Independent imaging systems can give different results to on-board, which is best?
Post-oblique view of anatomy difficult to get used to
Can produce images with little information depending on tumour location
Little manual adjustment possible.
What is surface scanning
Projector or camera suspended from ceiling
Speckle pattern projected onto patient, common points in images from each camera identified, fixed relationship between machine isocentre and camera allows position calculation
Reference image is surface rendering from planning CT
Advantages of surface scanning
Non-ionising
Intrafraction motion monitoring
4D, gating possible
Tattoo-less treatment possible
Disadvantages of surface scanning
Surface usd as surrogate for internal anatomy
Viewing problems with couch angles and or undulating surfaces
Only works for some sites
Reference images for ultrasound
3D US images fused with planning CT, or reference US scan taken at planning CT
Advantages of US
Non-ionising
Minimally invasive
Good soft tissue contrast
Disadvantages of US
Limited FoV
Limited to specific sites
User variation
Difficult matching to other modalities
What are fiducials and what are reference images?
Implanted markers, can use them with x-rays or have RF ones. reference imaging is either DRR with markers or planning CT with fiducial coordinates.
Advantages of fiducials with x-rays
Soft tissue information now available
Fast image acquisition to marker match
Intrafraction motion monitoring possible with some systems
Disadvantages of fiducials with x-rays
Invasive procedure
Markers can migrate
Limited to specific sites
Don’ get 3D anatomical information
Advantages of RF fiducials
Non-ionising
Soft tissue target position
Intrafraction motion monitoring
Disadvantages of RF fiducials
Invasive procedure
Limited to specific sites
Array in treatment beam
Complicated QA
Problems to overcome for MRI linac
Dosimetry in magnetic field and treatment through MR scanner
Image distortion due to rotation gantry
Electron return effect increases skin dose
Advantages of MRI
Gold standard soft tissue imaging
Potential for motion tracking
Functional imaging
Adaptive replanning
Disadvantages of MRI
Expensive
Need more clinical evidence
MR planning
Adaptive planning issues - long treatment times, time between imaging and treatment
How is tomotherapy imaging beam altered?
Modified to ~1MV down from unflattened 6MV
What is detector in tomotherapy?
Xe gas detector straight from CT
How does detector differ in LINAC kV than MV?
Might have anti scatter grid (not always - elekta don’t, but fix with software)
No Cu layer
Different scintillator
How is beam shaped in tomotherapy?
Binary pneumatic MLCs shape fan beam
