In-Room Imaging Flashcards
What is IGRT and why is it used?
Image at pre-treatment and delivery stage, leading to an action to improve or verify the accuracy of the treatment.
It prevents geographical misses.
What recommends IGRT?
Cancer Reform Strategy 2007, Improving Outcomes: A Strategy for Cancer 2011.
How does MV portal imaging work?
Use treatment beam onto EPID, 1-2MU.
Creates a single or pair of 2D images, or continuous imaging can be performed
How does an EPID work?
Copper conversion layer attenuates beam and produces electrons.
Scintillator produces light.
Photodiode array converts light to charge which is read out.
What are the advantages of MV portal imaging?
Uses treatment beam so simple geometry
Continuous imaging possible
Can do picket fence QA
What are the disadvantages of MV portal imaging?
Poor image contrast, 10-20x lower than DR
Only see bony anatomy
Can’t use antiscatter grid
How does MVCT work?
Take multiple 2D images, reconstructing using filtered back projection. This can be compared to the planning CT.
What are the advantages of MVCT?
3D data
Compare to planning CT
What are the disadvantages of MVCT?
Low contrast
Noisy due to scatter as it’s a conebeam.
How does Tomotherapy Helical MVCT work?
Use the max injector current so the average energy increase in the waveguide is less, leading to a 1MV beam.
Take mulitple 1D projections as the couch moves through the bore using a Xe gas detector from a CT scanner.
What are the advantages of Tomo helical MVCT?
3D imaging
Lower MV beam so better contrast
Fan beam so less scatter and noise
Less artefact fro prosthetics
What are the disadvantages of Tomo helical MVCT?
Detector not designed for MV
Detector designed for different SSD so beam is more unfocused as it goes from centre of beam - get a change in detector response across detector, resulting in double peak detector profile
Worse contrast than kV
Slow acquisition times, gantry rotates at 6RPM
How does Linac kV imaging work?
kV add-ons are put on the linac for Varian and Elekta. These are a kV tube and an EPID without the copper layer.
Siemens uses either modifies the treatment beam or puts a kV tube opposite the treatment head and introduces a flat panel detector.
Can do planar kV images, CBCT, kV pair, kV/MV pair, fluoroscopy
What are the components of the kV tube?
Primary collimator - reduce scatter by restricting field size
kV collimator/blades - 2 pairs of blades to control image size
Norm chamber - photodiode measures output of tube, projections correxted for kV output before reconstruction
kV filter - hardens beam to reduce skin dose and improve HU uniformity and accuracy
Bowtie filter - reduces skin dose, prevents saturation, reduces scatter from patient
What are the advantages of kV imaging?
Good soft tissue contrast
kV, MV pair saves time
What are the disadvantages of kV imaging?
CBCT takes 1 min
2 isocentres: imaging and treatment
Limited range of couch angles available without crashing
What QA is needed on kV imaging?
Image quality: resolution, SNR
Geometric: Affect image quality and treatment accuracy
How does CT on rails work?
Have CT in room with linac
Spin bed around and perform CT by moving the CT scanner
Get multiple 1D projections and compare to planning CT
What are the advantages of CT on rails?
Diagnostic quality images
Easy comparison to planning CT
Soft tissue matching
CT scanner geometry stable
What are the disadvantages of CT on rails?
Couch spins 180, not necessarily stable
Takes a long time
How does Exactrac work?
kV tubes in floor, flat panels on ceiling
Acquire orthogonal pair of images and compare to DRRs
Use IR camera opposite gantry with reflectors on patient, or on an array, to calibrate patient position compared to isocentre. Monitor patient movement during treatment
What are the advantages of Exactrac?
Independent of linac 3D shifts calculated with 6 degrees of freedom Quick Monitor patient position Use any couch angle
What are the disadvantages of Exactrac?
External
Don’t know if better than on-board imaging
Images are post-oblique view - hard to get used to
Make images with little info
No manual adjustments possible
How does optical scanning work?
Camera units on ceiling project speckle pattern onto patient
Fixed relationship between linac isocentre and camera so calculate patient position via triangulation.
Surface map of patient created
Compare to surface rendering of patient from planning CT or reference surface map from treatment verification
What are the advantages of optical scanning?
Non-ionising
Intrafraction motion monitoring
4D/gating possible
What are the disadvantages of optical scanning?
Viewing problems with couch angles and undulating surfaces
Only applicable to some sites
Surface used as surrogate for internal anatomy
How is Ultrasound used?
Use ultrasound prior to treatment
IR markers on probe or mechanical tracking arm are tracked with respect to the isocentre
Compared to a fused US and CT scan or reference 3D US scan
What are the advantages of US?
Non-ionising
Good soft tissue contrast
What are the disadvantages of US?
Limited field of view
Limited to specific sites
How are fiducials used?
Implants which can be used with x-rays to show soft tissue and intrafraction motion
Or used with RF measuring electromagnetic array with infared camera. Transponders emit RF signal when excited, with the array tracking position. IR cameras relate position to isocentre
Compare to planning CT with transponder coordinates identified
What are the advantages of fiducials?
Non-ionising
Soft tissue target position
intrafraction motion monitoring
How is MRI used?
Combined MRI-linac/Co-60 being developed
Viewray is a 0.35T MRI with a Co-60 head. 1.17 and 1.33 MeV gamma rays used
Elekta - 1.5T Philips scanner with rotating linac.
Need to overcome dosimetry issues in magnetic field and treating through an MR scanner
What are the advantages of using MR?
Great soft tissue contrast
Can track motion
Functional imaging
Adaptive planning
What are the disadvantages of using MR?
Uses Co-60 Under development Expensive Issues with MR planning Image distortion due to rotating gantry
