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Dosimetry > In-Room Imaging > Flashcards

In-Room Imaging Flashcards

1
Q

What is IGRT and why is it used?

A

Image at pre-treatment and delivery stage, leading to an action to improve or verify the accuracy of the treatment.
It prevents geographical misses.

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2
Q

What recommends IGRT?

A

Cancer Reform Strategy 2007, Improving Outcomes: A Strategy for Cancer 2011.

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3
Q

How does MV portal imaging work?

A

Use treatment beam onto EPID, 1-2MU.

Creates a single or pair of 2D images, or continuous imaging can be performed

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4
Q

How does an EPID work?

A

Copper conversion layer attenuates beam and produces electrons.
Scintillator produces light.
Photodiode array converts light to charge which is read out.

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5
Q

What are the advantages of MV portal imaging?

A

Uses treatment beam so simple geometry
Continuous imaging possible
Can do picket fence QA

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6
Q

What are the disadvantages of MV portal imaging?

A

Poor image contrast, 10-20x lower than DR
Only see bony anatomy
Can’t use antiscatter grid

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7
Q

How does MVCT work?

A

Take multiple 2D images, reconstructing using filtered back projection. This can be compared to the planning CT.

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8
Q

What are the advantages of MVCT?

A

3D data

Compare to planning CT

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9
Q

What are the disadvantages of MVCT?

A

Low contrast

Noisy due to scatter as it’s a conebeam.

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10
Q

How does Tomotherapy Helical MVCT work?

A

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.

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11
Q

What are the advantages of Tomo helical MVCT?

A

3D imaging
Lower MV beam so better contrast
Fan beam so less scatter and noise
Less artefact fro prosthetics

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12
Q

What are the disadvantages of Tomo helical MVCT?

A

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

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13
Q

How does Linac kV imaging work?

A

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

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14
Q

What are the components of the kV tube?

A

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

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15
Q

What are the advantages of kV imaging?

A

Good soft tissue contrast

kV, MV pair saves time

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16
Q

What are the disadvantages of kV imaging?

A

CBCT takes 1 min
2 isocentres: imaging and treatment
Limited range of couch angles available without crashing

17
Q

What QA is needed on kV imaging?

A

Image quality: resolution, SNR

Geometric: Affect image quality and treatment accuracy

18
Q

How does CT on rails work?

A

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

19
Q

What are the advantages of CT on rails?

A

Diagnostic quality images
Easy comparison to planning CT
Soft tissue matching
CT scanner geometry stable

20
Q

What are the disadvantages of CT on rails?

A

Couch spins 180, not necessarily stable

Takes a long time

21
Q

How does Exactrac work?

A

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

22
Q

What are the advantages of Exactrac?

A 
Independent of linac
3D shifts calculated with 6 degrees of freedom
Quick
Monitor patient position
Use any couch angle
23
Q

What are the disadvantages of Exactrac?

A

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

24
Q

How does optical scanning work?

A

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

25
Q

What are the advantages of optical scanning?

A

Non-ionising
Intrafraction motion monitoring
4D/gating possible

26
Q

What are the disadvantages of optical scanning?

A

Viewing problems with couch angles and undulating surfaces
Only applicable to some sites
Surface used as surrogate for internal anatomy

27
Q

How is Ultrasound used?

A

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

28
Q

What are the advantages of US?

A

Non-ionising

Good soft tissue contrast

29
Q

What are the disadvantages of US?

A

Limited field of view

Limited to specific sites

30
Q

How are fiducials used?

A

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

31
Q

What are the advantages of fiducials?

A

Non-ionising
Soft tissue target position
intrafraction motion monitoring

32
Q

How is MRI used?

A

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

33
Q

What are the advantages of using MR?

A

Great soft tissue contrast
Can track motion
Functional imaging
Adaptive planning

34
Q

What are the disadvantages of using MR?

A 
Uses Co-60
Under development
Expensive
Issues with MR planning
Image distortion due to rotating gantry

Dosimetry (11 decks)

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