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radiotherapy year 2 > Imaging > Flashcards

Imaging Flashcards

1
Q

What is image guided radiotherapy

A

RT with imaging at pre-treatment and treatment delivery stage which leads to an action which can improve or verify the accuracy of RT

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2
Q

What is MV portal imaging & reference image

A

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

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3
Q

What is DRR

A

Digitally reconstructed radiograph, 2D image generated from 3D dataset

Simulated planar x-ray, might have iso or OAR marked on if generated by TPS

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4
Q

How does EPID work?

A

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)

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5
Q

Advantages of MV Portal imaging

A

Not treating blind
Use treatment beam - simple geometry
Good for QA images
Continuous imaging gives reassurance that target remains in beam

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6
Q

Disadvantages of MV portal imaging

A

Contrast 10-20 times lower than x-ray (compton dominates not PE)
Need surrogate for target position, eg bony anatomy
Scatter affects image quality

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7
Q

What is MV CBCT & reference image

A

Using same MV portal imaging but take multiple 2D projections and reconstruct with FBP
Reference image is planning CT

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8
Q

Advantages of MV CBCT

A

3D data, view sagittal, axial, coronal
Direct comparison with planning CT

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8
Q

Disadvantages of MV CBCT

A

Still have low contrast due to MV beam
CB means more scatter and therefore noisier images

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9
Q

Tomotherapy imaging and reference image?

A

Helical MVCT - couch moves through
Reference image is planning CT

Multiple 1D projections per axial slice, couch moves through bore

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10
Q

Detectors on tomotherapy

A

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

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11
Q

Advantages of tomotherapy MVCT

A

3D imaging
Low MV beam, better contrast
Fan beam - less scatter induced noise than MVCBCT
Higher energy than kV, reduced artefacts from metal implants

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12
Q

Disadvantages of tomotherapy MVCT

A

Detector not designed for MV
Higher dose than kV imaging
Contrast not as good as kV
Slow acquisition time

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13
Q

Considerations in kV tube

A

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

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14
Q

Why is reducing hardening in patient ideal?

A

Better dose distribution, gives better HU uniformity and accuracy

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15
Q

Advantages of kV imaging

A

3D information, direct comparison with planning CT
Better soft tissue contrast as kV
kV/MV pair time saving?

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16
Q

Disadvantages of kV imaging

A

CBCT takes 1 minute to acquire
2 isocentres, imaging and treatment
limited couch angles
Additional QA considerations

17
Q

What is kVCT imaging?

A

Have a conventional CT scanner and couch then moves into linac.

18
Q

Advantages of kVCT

A

Diagnostic quality imaging
Dirct comparison with planning CT
Soft tissue matching possible
CT scanner geometry stable

19
Q

Disadvantages of kVCT imaging

A

Couch moves between imaging and treatment, patient could move
Imaging and treatment isocentres different
Time and space

20
Q

What is exactrac?

A

External kV imaging
kV tubes recessed into floor either side of gantry
Orthogonal pair of images
Reference images are DRRs

21
Q

Advantages of exactrac

A

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

22
Q

Disadvantages of exactrac

A

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.

23
Q

What is surface scanning

A

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

24
Q

Advantages of surface scanning

A

Non-ionising
Intrafraction motion monitoring
4D, gating possible
Tattoo-less treatment possible

25
Q

Disadvantages of surface scanning

A

Surface usd as surrogate for internal anatomy
Viewing problems with couch angles and or undulating surfaces
Only works for some sites

26
Q

Reference images for ultrasound

A

3D US images fused with planning CT, or reference US scan taken at planning CT

27
Q

Advantages of US

A

Non-ionising
Minimally invasive
Good soft tissue contrast

28
Q

Disadvantages of US

A

Limited FoV
Limited to specific sites
User variation
Difficult matching to other modalities

29
Q

What are fiducials and what are reference images?

A

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.

30
Q

Advantages of fiducials with x-rays

A

Soft tissue information now available
Fast image acquisition to marker match
Intrafraction motion monitoring possible with some systems

31
Q

Disadvantages of fiducials with x-rays

A

Invasive procedure
Markers can migrate
Limited to specific sites
Don’ get 3D anatomical information

32
Q

Advantages of RF fiducials

A

Non-ionising
Soft tissue target position
Intrafraction motion monitoring

33
Q

Disadvantages of RF fiducials

A

Invasive procedure
Limited to specific sites
Array in treatment beam
Complicated QA

34
Q

Problems to overcome for MRI linac

A

Dosimetry in magnetic field and treatment through MR scanner
Image distortion due to rotation gantry
Electron return effect increases skin dose

35
Q

Advantages of MRI

A

Gold standard soft tissue imaging
Potential for motion tracking
Functional imaging
Adaptive replanning

36
Q

Disadvantages of MRI

A

Expensive
Need more clinical evidence
MR planning
Adaptive planning issues - long treatment times, time between imaging and treatment

37
Q

How is tomotherapy imaging beam altered?

A

Modified to ~1MV down from unflattened 6MV

38
Q

What is detector in tomotherapy?

A

Xe gas detector straight from CT

39
Q

How does detector differ in LINAC kV than MV?

A

Might have anti scatter grid (not always - elekta don’t, but fix with software)
No Cu layer
Different scintillator

40
Q

How is beam shaped in tomotherapy?

A

Binary pneumatic MLCs shape fan beam

radiotherapy year 2 (16 decks)

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