Radiotherapy Imaging Flashcards
Types of Imaging which use the standard linac equipment
MV:
Portal
Fluoro
MV cone-beam CT
KV:
CT on rails
kV Pair
Fluoro
kV Cone-beam CT
Less common imaging
Tomotherapy (MV fan beam ct) Exactrac Vision RT / Catalyst Ultrasound Fiducials MR linac
MV portal acquisition
Uses MV treatment beam and electronic portal imaging detector
Can acquire single or pair of 2D planar images as well as fluoro
Cu layer: MV beam -> 2nd electrons
Scintillator: electron -> visible light
A-Si photodiode: vis light -> charge
TFT in pixels store charge till read out
Benefits and Lims of MV portal
AD:
Simples geometry
Continuous imaging
Good for QA
DIS
Poor contrast
Need surogate position markers
Scatter issues - can’t use a grid
Summarise MV cone beam CT
3D data collected by rotating gantry whilst capturing 2D images
Can compare to planning CT
Noisy due to MV and Fan geometry
Describe tomotherapy imaging
Helical treatment device - can create helical CT, from multiple 1D projection images whilst couch moves through the bore
Advantages:
3D imaging
Low MV beam therefore better contrast than MV CBCT
Fan beam therefore less scatter-induced noise than MV CBCT
Higher energy than kV therefore reduced artefacts from prostheses (e.g. hips)
Disadvantages:
Detector not designed for MV
Higher dose than kV imaging
Slow acquisition time (minutes cf. seconds for diagnostic CT)
Advantages and Disadvantage of linac kV imaging
Advantages:
kV images therefore ‘good’ soft tissue contrast (i.e. especially in CBCT)
kV/MV pair could offer time savings?
Bow tie filter can be used to reduce cbct periphery dose
Disadvantages:
CBCT take 1 min to acquire
2 ‘isocentres’: imaging and treatment!
Limited to a range of couch angles (depending on approach- CBCT has to be done near zero degrees, kV pair allows much bigger couch angles but still not the whole range clinically used)
QA issues: geometry could effect image quality and treatment accuracy
Describe CT on rails
A standard CT unit positioned opposite a linac
1D helical proj CT
Advantages Diagnostic quality imaging Direct comparison with planning CT Soft tissue matching possible CT scanner geometry stable
Disadvantages
Couch spins 180° - QC must demonstrate stability of couch!
Time?
Describe exactrac
kV tubes recessed into floor either side of gantry and
Flat panels suspended from ceiling opposite. The system acquires an orthogonal pair of DRR reference images
Can use IR positional tracking
Advantages:
Fixed geometry independent of linac
3D shifts and rotations calculated (6DoF)
Quick to acquire (in principle)
Can monitor patient position during treatment (x-ray and/or infrared in some circumstances)
Imaging at different couch angles possible
Disadvantages:
External system
Independent imaging system can give different results to on-board imaging-
Post-oblique view of anatomy
Can produce little information depending on tumour location
Little manual adjustment possible
Describe Vision RT/C-rad
Projector / stereoscopic camera units suspended from ceiling
Creates a surface map of the patient by using its fixed geometry and imaging a speckle pattern projected on to the patient.
Advantages:
Non-ionising
Intrafraction-motion monitoring
4D / gating possible
Disadvantages:
Viewing problems with couch angles and/or undulating surfaces
Only applicable to some sites
Surface used as surrogate for internal anatomy
Describe the use of ultrasound in rad-therapy imaging
Images the patient and tracks object position by imaging a transducer mounted on an arm, tracked using IR markers. Can fuse results with CT
Advantages:
Non-ionising
Good soft tissue contrast
Disadvantages:
Limited FOV
Limited to specific sites
Describe the use of fiducials
Implanted makers used with kV or MV planar imaging
Compare to ref ct im with the markers
Advantages:
Soft tissue target now visible
Fast image acquisition & marker match
Intrafraction motion monitoring possible (with some systems)
Disadvantages Invasive procedure Markers can migrate Limited to specific sites (e.g. prostate) Don’t get 3D data
What is the calypso system?
A fiducials marker system that as uses RF transponders
The transponders emit RF signals when excited and can be tracked via IR cameras. Can ref transponder co-ords to a CT image
Advantages:
Non-ionising
Soft tissue target position
Intrafraction motion monitoring
Disadvantages Invasive procedure Limited to specific sites (e.g. prostate) Array in treatment beam Complicated QA?
Describe the use of MRI in radiotherapy imaging
Co and Linac systems are available
Treat around the magnet array - use MRI for ref images
Advantages: Gold standard soft tissue imaging! Potential for motion tracking Functional imaging Adaptive planning
Disadvantages: Obvious Cobalt issues for ViewRay Linac still under development… Expensive MR planning- electron density
