Radiotherapy Cone Beam CT Imaging physics concepts

1. CT BASED IMAGE
GUIDED
RADIOTHERAPY –
PHYSICS & QA
- Sambasivaselli R

2. OUTLINE
Image Guidance
Existing Technologies
X-Ray Volumetric Imaging
Quality Assurance Issues

3. IMAGE GUIDANCE
 3D image based setup verification and
correction
 Knowledge on 3D patient anatomy for
every treatment fraction.
 Adaptive corrections of inter fractional
uncertainties of the treatment.

4. EXISTING TECNOLOGIES

5. CT-ON-RAILS
 Earliest implementation of IGRT
 Integrating a diagnostic CT scanner into a RT treatment
room, developed by Uematsu et al, in Japan.

6. KV CBCT
 At present, an important tool for setup
verification and correction
 It uses Cone beam X-ray source, which
acquires an entire volume ( 14 – 26cm in
length )
 Though not of diagnostic quality, generally
adequate for imaging bone and soft tissues

7. FAN BEAM MV CT
 Equipped with helical tomotherapy delivery
system
 Nominal energy is reduced to 3.5 MV Beam,
and it is collimated to 4mm at isocenter
 Eliminates artifacts, but inherently causes
poor subject contrast

8. Cone beam and Fan Beam

9. MV CBCT
 It consists of a Si flat panel adapted for MV imaging
attached to linac
 It uses Cone Beam X-ray
 CBCT image reconstruction and remote controlled
couch movement
 Nominal scan dose is comparatively more ( 3 to 10
cGy)

10. X-RAY VOLUMETRIC IMAGING

11. XVI Basics
 XVI is an electronic imaging device (EID) consists of a kV X-ray
source and an amorphous silicon (a-Si) radiation image
detector panel.
 The kV source generates X-rays that are projected as planar
images on to the plate of the kV detector.
 Acquired planer images are reconstructed into volume images
using reconstruction algorithm.

13. X Ray Generator Tube
 It has two focal spots
 Small 0.4mm
 Large 0.8mm
 Maximum current in small is 80 mAs and in large is 100 to 500
mAs.
 Anode rotation is 16000rpm and air cooling system is used
 Total filtration 3.6mm which includes Inherent filtration of
0.9mm Al, a cone shaped Al 0.6mm, Al disc of 2.0mm and
0.1mm Cu.

14. Imaging Panel
 It has a matrix of 1024 X 1024 pixels.
 The size of the image receptor 409.6mm ×
409.6mm.
 Radiation on each pixel is represented by a
number which is known as pixel value.
 600 – 650 frames can be taken in volume view.

15.  Gantry Arc minimum is 187.5° and maximum is 360°.
 Gantry Speed = 3.18°/sec.
 Frame rate = 5.75 frames/sec.

16. Image Acquisition
 Three different modes available
 PlanerViewTM - Radiography
 MotionViewTM - Fluoroscopic view
 VolumeViewTM - CT like X ray

17. Image Process
Raw Images
Offset and Multi level gain correction
Bad Pixel Correction
Downsize
Invert Image
Rotate 90˚
DB
Multi level Gain
Calibration
Auto Offset
Bad Pixel Map
ONLINE
Images
Reconstruction Online / Offline
1024 / 512
1024
1024
1024
Flex Map

18.  Gain is the mean ratio of signal output to the signal input of a
system.
 Here the output is the pixel value and input is the incident beam.
 Two types
 Single Level gain – Response of detectors for a single dose value.
 Multi level Gain – To eliminate the nonlinearity between dose and
detector response.
Multi level Gain Calibration

19.  It is an image that has a record of all pixels on the panel
that do not respond correctly to a dose of radiation.
 This map is then applied to an acquired image to remove
the bad pixels which are replaced with average data from
the surrounding 8 pixels.
Bad Pixel Map

20.  The mechanical system of the XVI move under gravity as
the gantry rotates.
 This has the effect of causing the radiation field centre to
move with respect to the kV detector.
 A map is created of the radiation centre against gantry
rotation and stored as a flex map.
 The flex map is used to make corrections to the images as
the data is back projected to create the 3D reconstruction.
Flex Map

21. Flex Map Calibration
Isocentre accuracy is ±0.25mm in all three axes.

22.  XVI does a reconstruction process based on Feldkemp-Davis-Kress (FDK)
algorithm.
 It is a kind of filtered back projection (FBP) algorithm widely in use.
 Reconstruction is exact for any object which is invariant for translational
in the axial direction orthogonal to plane of orbit.
 Reconstructed CBCT projections are not truncated.
 It is popular for its simplicity and ease of implementation and its good
robustness to CB artefacts.
Reconstruction

23. Data Filtering in FBP
 It is to smooth out the statistical noise
 Some typical filters used are the Hanning filter,
Butterworth filter, low pass cosine filter, Weiner
filter, etc.
 XVI reconstruction is based on Feldkemp variant
and weiner filter.
x
y

24. QUALITY ASSURANCE ISSUES

25. Geometric Accuracy
 Flex map calibration similar to Winston-Lutz test
 KV / MV / Laser Alignment
 Accuracy of couch shifts

26. Pentaguide Phantom

27.  It checks accuracy of KV and MV isocenter
coincidence
 It also evaluates the accuracy of remote controlled
couch with a known shifts
 Image registration accuracy can be determined
using Penta guide phantom

28. IMAGE QUALITY
 Scale and distance accuracy
 Uniformity
 Low Contrast Visibility
 Spatial Resolution

29. Uniformity
 To Verify the change in the average signal (pixel value) in
different portions of the homogeneous phantom(CTP 600)
image.
 Tolerance ≤ 1.5%.

30. Low Contrast Visibility
 To verify the visibility of low dense materials in XVI image.
 Using the mean value and standard deviation of the pixel
values in both polystyrene and LDPE low contrast visibility is
computed.
 Tolerance ≤ 1.5%.

31. Spatial Resolution
 To find the spatial resolution on patterns of high contrast
resolution.
 Tolerance minimum 10 lp/cm.

32. Image Registration Accuracy
 To check the accuracy of kV images to
the MV isocentre and make sure that
it is within specification for
registration.
 This test done using Ball Bearing
phantom made with long plastic tube
and a steel ball of diameter 8mm at
the tip.
 Image registration accuracy is ± 1mm.

33. Image Registration
 It is the movement of one reference point in a 3D coordinates
system.
 Three types of image registration method available in XVI.
Grey Value
Bone
Manual
 Manual: The translational and rotational movement is done
manually until a good match between CBCT and CT is
achieved.

34. Bone Registration
 It uses chamfer matching algorithm.
 It is very simple and fast.

35.  It is also based on the cross correlation method.
 The algorithm matches the edges or outlines of the bone
(high value pixel) by minimizing a generalized distance
between them.
 It uses fast simulated annealing.
 Reduces computational load
 Reasonable, insensitive to noise

36. Grey Value Registration
 It refers to pixel value
 It is the cross correlation of the pixel value on the whole
volume (clip box) of the both study sets.
 The algorithm used is ‘Grey level correlation ratio’ technique.

39.  Here the cost function is the summation of RMS difference of
the pixel value of the reference image and localized image.
 Here the variables are

40. QA for the X-Ray generator
 X-Ray output consistency
 KVp accuracy
 mA accuracy
 KVp linearity
 mA linearity

41. References
 AAPM TG 142
 AAPM TG 179
 AAPM TG 101
 AAPM TG 66
 Practical cone beam algorithm, Fred kemp et al
 New grey scale template image matching algorithm, Ryo Takei
et al
 Hierarchical Chamfer matching , parametric edge matching,
Gunilla Borgefors
 Filtered Back Projection algorithm

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