2D Images
KV CBCT Imaging
Part I
Rabih Hammoud, MS, DABR
Henry Ford Health System, Detroit, MI
Hamad Medical Corp. Doha, Qatar
Learning objective:
Detroit vs. Doha
Outline
1.
The objective of this educational session
is to review KV-CBCT and MV-CBCT
2.
imaging systems for daily localization
4.
3.
5.
6.
Commissioning, image quality, dose,
registration process, and acquisition
modes
Clinical integration
QA, stability over time, and downtime
Standard clinical applications
Novel clinical applications
Technology evolution and future directions
1
3D kV Imaging
Siemens Artiste™
KVision
Varian Trilogy OBI®
Elekta Synergy™
VolumeView
What’s CBCT?
Novalis Tx™
On Board Imager CBCT acquires projections of
a patient,
Send them to a reconstruction application, and
then returns a 3D image
In the 3D/3D Match workspace, the CBCT
images can be registered to the reference images,
either manually or automatically, using a 3D
mutual information algorithm.
Elekta Axesse™
Why CBCT?
Advances in treatment planning and
delivery systems allow for higher doses to
target and lower dose to normal tissue
With the resulting steep dose gradients,
motion management ( inter and intrafraction) becomes more critical
Tumors often not visible in 2D images
Role of CBCT is to help reduce
interfractional motion and try to assess
patient status ( tumor evaluation, adaptive
planning, ..)
Volume = 4/3 r3
Small margin reduction
(5mm) decreases the
volume of an orange by
1/2
2
CBCT work Flow
Intra-fraction Prostate Motion
What can we do to
decrease intra-fraction
motion?
Data Preparation
Planning CT with structure set
Isocenter information
Send via Record Verify System
• Acquisition time (scan
modes)
Workflow
• Reconstruction time
Select patient
Extend arms/gantry starting point
Imaging parameters
Acquire/reconstruct CBCT
Align (bony, soft tissue, VOI)
Apply shifts and record
Post shifts?
Treat
• Matching (ROI, Intensity
range, automatic/manual)
• Treatment time
(RapidArc?)
Langen et al. IJROBP 71(4):1084-1090, 2008
Tolerated difference between the planning CT and CBCT depends on image quality, image
registration, internal organ motion, margin definitions, user experience which are clinic and
anatomical site dependent
Elekta Synergy™
Half Scan
vs.
conventional x-ray tube mounted on a
retractable arm extends from the
accelerator gantry’s drum structure
A 41×41 cm2 flat-panel x-ray detector is
mounted opposite the kV tube at a gantry
position of 590° in IEC coordinates
The X-ray tube is powered by a highfrequency generator, operates ( 60-150
kVp)
Full Scan
Varian Trilogy OBI®
Novalis Tx™
Full Fan Scan
Half Fan Scan
3
Commissioning
Commissioning of the OnOn-Board
Imaging system is divided into 2 parts:
Mechanical Calibration
Imaging System Calibrations
Mechanical Calibration
Indexing the Arms:
to establish a
reference point for
the motion of the
joints of arms for KV
source, KV and MV
detectors using
encoders with no
absolute positions
Indexing the Exact ArmArm-MV
Detector Arm
KV Source Mechanical
Calibration
ResultsResults-KV Source Mechanical
Calibration
Before
After
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Imaging System Calibration
CBCT Mode
Calibration
To correct for non-uniform response of the image receptor
and non-uniform intensity of the x-ray source,
Reinsures the uniformity of receptor electronic readouts
and updates pixel defect map (PDM), procedure includes:
Dark field image (DF image): acquired without radiation & reflects any
imperfection
Flood field image (FF image): several images are acquired using radiation,
it reflects field homogeneity pixel sensitivity and amplifier gain
Geometry
Calibration
Imaging
Calibration
HU
Calibration
Dual Gain
Calibration
I0
Calibration
Normalization
Defective pixels locations are stored in PDM. When images
are acquired the PDM is loaded and defective pixels are
replaced with an average of neighboring pixels
Geometry Calibration
• Corrects for machine geo instability
during scans
Dual Gain Calibration
Used to enhance the dynamic range of the imager for CBCT imaging
Each projection is read twice ( at high sensitivity, and at low sensitivity)
Determines the linearity of each pixel and the threshold above which the
pixel values from the high sensitivity image are replaced by scaled pixels
from the low sensitivity range image
• Uses a phantom of known geometry
(five needles) placed at the isocenter
to estimate the kV/CBCT isocenter.
• Many images of the phantom are
acquired at different angles.
• Since phantom/needles dimensions
are known, one can estimate the
location of the OBI isocenter
• 3D/3D match of the CBCT uses the
results from geo Cal to estimate the
location of the kV isocenter
Imaging Calibration
I0 Calibration
Needle Phantom
It reduces ring artifacts in scans, stored in the reconstructor
It’s repeated multiple times resulting in charge trapping similar to what
occurs during CBCT acquiition
5
HU Calibration
Quality Assurance Program
Uses Catphan Phantom
Calibrate HU for pixels
in the reconstructed
image based for known
inserts
Beam Hardening Correction:
• Corrects for increase in energy as x-rays passes through the patient
• Due to the polychromatic nature of the X-rays, lower energy components are
attenuated as goes through the patient
• Will increase the effective x-ray energy which causes inconsistency in the
reconstruction that assumes constant energy
Safety and Functionality Check
To check that the safety features are
functioning properly and that the entire
system is ready for clinical operations
Daily
Daily/Weekly Monthly or Quarterly
Geometry Check
To verify that the KV source and KV detector have
maintained their geometric accuracy and stability
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weekly geometric QA
Image Quality
To monitor the quality of
radiographic and CBCT
images over time
Not comparable to
conventional CT scanners
QA tests adopted from
diagnostic CT scanners using
Catphan 504 phantom
Based on establishing a
baseline value
Image Quality
Tests include:
a)
b)
c)
d)
Low contrast resolution
Spatial resolution
HU accuracy
HU uniformity
Some References for OBI QA…
S. Yoo, G. Y. Kim, R. Hammoud, E. Elder, T.
Pawlicki, H. Guan, T. Fox, G. Luxton, F. F. Yin
and P. Munro, Med Phys 33 (11), 4431-4447
(2006)
J. P. Bissonnette, D. Moseley, E. White, M.
Sharpe, T. Purdie and D. A. Jaffray, Int J Radiat
Oncol Biol Phys 71 (1 Suppl), S57-61 (2008)
J. P. Bissonnette, D. J. Moseley and D. A.
Jaffray, Med Phys 35 (5), 1807-1815 (2008)
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Patient Dose from CBCT
System Dependent
kV/ mA
Number of projections
kV system properties (bow-tie)
kV system field size
Patient Dependent
Size/shape of patient
Body part
What is our interest?
Some Approaches of Characterizing CBCT Dose…
Taking in vivo dose measurements in a Rando phantom
and using the data as a predictor of patient dose
Taking dose measurements directly on patients
undergoing CBCT using TLD
CT Dose Index (CTDI). Well defined conditions.
Defining a dose ,metric for the cone beam dose index
(CBDI) and taking dose measurements with a standard
cylindrical
risk/ benefit ratio needs to be considered
Some Approaches of Characterizing CBCT Dose
Taking in vivo dose measurements in a
Rando phantom and using the data as a
predictor of patient dose
Taking dose measurements directly on
patients undergoing CBCT using TLD
Technique used: 125 kVp, 80 mA, 25
ms (2 mA s)
AP skin doses ranged from 3-6 cGy
for 20-23 separation
Central dose was ~3.0 cGy
The left hip received 10-11 cGy while
the right received 6-7 cGy
Wen N, Guan H, Hammoud R et. al. Dose delivered from Varian’s CBCT
to patients receiving IMRT for prostate 2007 Phys. Med. Biol. 53(11):
2897-909
Another Approach
Defining a dose metric for the cone beam dose index (CBDI)
and taking dose measurements with a standard cylindrical CT
phantom using both a 100 mm ion chamber and 0.6 cc farmer
chamber to predict patient dose
CBDIfW= 1/3 CBDIfCentre+ 2/3 CBDIfPeriphery (cGy)
Adopting the same area averaging approximation used in
conventional CT called CTDIW in which the CAX dose D0 (r =
0, where r is the distance from the centre of the CT phantom to
the point of measurement) is weighted by 1/3 and peripheral
axis doses Dp (r = R 5 1 cm, where R is the phantom radius) are
weighted by 2/3
E. K. Osei et al., J. Radiol. Prot. 29 (1), 37-50 (2009)
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CBCT Dose-OBI Advanced Imaging
16 cm phantom
40 mA, 25 ms (1 mA s)
Full Fan
No
Bowtie
• Amer et al.2007, reported
0.38 cGy/ 100 mAs for the
Elekta Synergy System using
32 cm diameter phantom
• Song et al. 2008, reported
0.43 cGy/ 100 mAs using 30
cm diameter phantom
Full Fan
with
Bowtie
32 cm phantom
40 mA, 25 ms (1 mA s)
Half Fan
No
Bowtie
Half Fan
with
Bowtie
CBDIfCentre
(cGy)
7.47
4.65
2.85
2.01
CBDIfPeriphery
(cGy)
9.34
5.06
6.34
2.97
CBDIfw
(cGy)
8.72
4.93
5.18
2.65
CBDIfCentre
(cGy/ 100 mAs)
1.15
0.72
0.44
0.31
CBDIfPeriphery
(cGy/ 100 mAs)
1.44
0.78
0.98
0.46
CBDIfw
(cGy/ 100 mAs)
1.34
0.76
0.80
0.41
AAPM TG for Imaging Dose
Murphy, M.J., et al., The management of imaging
dose during image-guided radiotherapy: report of the
AAPM Task Group 75. Med Phys, 2007. 34(10):
p. 4041-63.
Two assumptions are made in the
current MLC bases IMRT process
1. Geometric sizes, shapes, and
locations of the targets and organs
are the same at the time of planning
CT pCT
2. Delivered fluence maps are the
same as the planned ones
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This can trigger a replan
for specific patients when
warranted by significant
changes
Summary
Commissioning of kV CBCT systems involves the characterization of the
alignment of the kV CBCT system components with the linear accelerator
isocentre
No image
Guidance
Bony/Soft
Tissue
Commissioning work then typically focuses on establishing baseline values
for image quality. Parameters assessed include spatial linearity (i.e., distance
and scale), image uniformity, high and low contrast spatial resolution, and
accuracy of CT numbers
Finally, commissioning work involves characterization of the radiation dose
to be used for IGRT. The literature reports point dose measurements
involving Farmer chambers inserted in cylindrical water or acrylic
phantoms
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Recently, a parameter analogous to the commonly accepted
CTDI has been adapted for the kV CBCT geometry, leading
to the introduction of CBDI
Though dose delivered from CBCT is small compared to the
treatment dose, according to ICRP-60, the dose delivered to
patients in medical imaging should be justified and optimized
Justification is positional and target localization which may
allow for dose escalation and potentially higher rates of tumor
control and lower rates of complications
Thank You
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