Image quality for Cone Beam Computed Tomography

1. Image Quality of
Cone Beam Computed Tomography
( CBCT)
Sa’ed AL-Atawneh
Senior Medical Physicist

2. Contents
* Introduction
* System Safety
– check interlocks on tube,
panel and arms
* Geometrical Accuracy
– verify registration and alignment
process
* Image quality
– monitor image quality parameters
– verify isocenter 3D agreement

3. • IGRT is assuring patient positioning through
image guidance.
• Clinical use of IGRT started 1995
• A full CT scan of the patient on the treatment
couch is obtained before radiation delivery.
• Reconstruction takes less than 2 minutes.
• Scan can then be automatically or manually
registered to the CT-sim scan.
Introduction

4. Which Patients Should be
Imaged with CB?
 Tumors adjacent to critical structures
 Tumors prone to inter-fractional motion
Tumors with intra-fractional motion-
(Preference: 4-D planning)
Tumors prone to deformation
Compliant patients

5. CBCT: Clinical Responsibilities
Physician:
1. Determine clinical indication(s)
2. Order CBCT and frequency
3. Define primary ROIs for
physicist/therapist
4. Review images (day 1, twice/weeks,
daily??)
5. Approve repositioning shifts.

6. Physicist :-
Available to review images & shifts
QA, calibration, output, image quality…etc
Therapist:-
Daily safety and functional checks.
Daily geometrical Accuracy test by Penta-
Guide phantom
Review images
Make appropriate table shifts
CBCT: Clinical Responsibilities

7. Quality Assurance Program
Daily Daily/Monthly Monthly
QA Program
for the image
system
Safety and
functionality
checks
Geometry
checks
Image quality
checks

8. Daily Check: Safety and
Functionality
“ RTT responsibility ”
1. Interlocks: door interlock, kV source arm interlock,
terminate key. and of all the system touch guards
(accelerator head, kV imaging panel arm, MV imaging
panel arm).
The touch guards at the various locations are tested by
attempting to move the couch while triggering each guard
separately.

9. Penta Guide™
• QA phantom
• 2 sets of cross hairs
• Use To verify that the KV
source and KV detector
have maintained their
geometric accuracy and
stability
Daily QA: Geometrical Accuracy
“ RTT responsibility ”

10. QA workflow
• Setup cube using beam
cross hair at gantry 0º
and 270º (or 90º) 􀃆 use
offcenter cross hair on
cube
• Acquire CBCT
• Align with reference image
• Adjust couch position
• Acquire CBCT and verify
alignment with reference
• Verify alignment with isocenter
cross hair on cube
• Check lasers

11. QA workflow
Image registration with reference image from pinnacle

12. Monthly QA: Image Quality
“Physics responsibilities”
Figure 1 Catphan-504
1. To monitor the quality of
radiographic and CBCT
images over time
2. Not comparable to
conventional CT scanners
3. QA tests adopted from
diagnostic CT scanners using
Catphan 504 phantom
4.Based on establishing a
baseline value

13. Image Quality
Tests include:
a) 3D-Uniformity
- Cupping Artifact
- Ringing Artifact
b) 3D&2D-Low contrast visibility
c) 3D&2D-Spatial resolution
d) 3D-Registration

14. Nonuniformities and artifacts can be easily detected
during a visual inspection of a volumetric image of a
uniform.
Cupping artifact
3D-Uniformity: Cupping Artifact

15. 3D-Uniformity: Ringing Artifact
* MALFUNCTION OF A
DETECTOR IN A THIRD
GENERATION SCANNER
• Correction
• Detector Calibration
• Detector Replacement
• Reduce FOV
Figure 19. Formation of a ring artifact when a
detector is out of calibration.
Barrett J F , and Keat N Radiographics 2004;24:1679-1691
©2004 by Radiological Society of North America

16. Brain-Ringing artifact
3D-Uniformity: Ringing Artifact
Abdomen-Ringing artifact

17. 3D-Uniformity: QA Tools
• Density phantom, such as
a water bath or water-
equivalent object.
• Recommendation
<=1.5% deviations
• Using Catphan phantom.

18.  Is the ability to image two
separate objects and visually
distinguish one from the other.
 Spatial resolution is the ability
to image small objects that
have high subject contrast.
Ex: bone-soft tissue interface,
breast calcifications, calcified
lung nodule
 Conventional radiography has
excellent spatial resolution
3D&2D-Spatial Resolution
Bad Resolution Good Resolution

19. 3D&2D-Spatial Resolution: QA Tools
􀃆 CTP 504 insert
contains a spatial
resolution rule with bar
patterns between 1 to 21
lp/cm
􀃆Recommendation
10 lp/cm
􀃆TOR 18 FG Phantom,
where (1.4)lp/mm and a
minimum number of
apparent group is 10th

20. 3D&2D-Spatial Resolution
More line
More detail
High spatial
resolution

21. The contrast difference between
the prostate and the rectum is
typically 2% while that between
the normal breast tissue and a
seroma cavity is 10–15%
3D&2D-Low Contrast Visibility
Low contrast Is the ability to
distinguish anatomic structures
of similar subject contrast. Ex:
liver-spleen, gray matter-white
matter

22. 3D&2D-Low Contrast Visibility: QA Tools
• Contrast detectability
depends on phantom size,
object size, reconstructed
voxel size.
• keep parameters constant
for quality control checks
• Low contrast detectability is
tested by scanning a
phantom containing objects
with a variety of linear
attenuation coefficients like
Catphan and TOR F18
• Recommendation <= 1.5% TOR F18-phantom
Catphan504-phantom

23. 3D&2D-Low Contrast Visibility

24. 3D-Registration
A ball-bearing phantom
supplied with the CBCT
installation. The phantom
consists of a steel ball
(diameter: 8 mm) located at
the tip of a long plastic tube,
which is connected to a base
plate locked to the couch with
a set of vernier adjustments
that allow the position of the
steel ball to be adjusted in
0.01 mm
ball-bearing phantom
Ball-bearing Phantom
Linac(4) , King Hussein cancer center

25. Fig. Axial (a), coronal (b), and sagittal (c) CBCT images of the BB phantom (high
intensity area). Red crosshair represents the center of CBCT volume or the nominal
radiation isocenter
3D-Registration
(a) (b) (c)

27. Uniformity section Position coordinate Mean
value
(Mean)
Toleranc
e
Notes &
ActionX(cm) Y(cm) Z(cm)
Mean pixel value
(center)
0 0 0
<=
1.5%
Position (1) -4.5 0 0
Position (2) 0 0 -4.5
Position (3) 4.5 0 0
Position (4) 0 0 4.5
Percentage
difference
3D- Low Contrast Visibility
section
Mean value
(Mean)
Tolerance SD HU
polystyrene
<= 1.5%
-35
LDPE -100
Value of low contract
visibility
3D-Special Resolution Section Tolerance Notes & Action
Slice number (up)
10th lpSlice number (down)
Number of visible line pairs

28. 3D- Transverse Vertical and Horizontals
scale
Tolerance Notes
&Action
Distance between two air insert
+/-
1.04mmDistance between LPDE & Delrin
3D- Sagittal Scale Check Tolerance Notes
&Action
Distance between 1st & 3rd
Marker
+/-
1.04mm
3D-Registration Accuracy Tolerance Notes
&Action
Axes
< 1 mm
Longitudinal (Y)
Lateral (X)
Highest (Z)

29. CONCLUSION
Determine the frequently of CBCT should
be taken for each patient
“ New studies”
Give’s a proper training to RTT’S to
perform daily QA.
Image Quality should be done monthly.

30. References
 Acceptance test booklets
 TG142 protocol for RTOG
 Radiological Society of North America

31. Thank You