Name of presentation

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NORMOXIC GEL DOSIMETRY USING MULTISLICE X-RAY CT
PRELIMINARY STUDY
N.Gopishankar
Medical Physicist
All India Institute of Medical Sciences
New Delhi
India
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Aims and Objectives

• To validate and optimize the X-Ray CT imaging
  parameters for its utility in
• polymer gel dosimetry and image enhancement
  technique using digital
• filtering.

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Introduction

• Basic principle of CT

• CT scanner performs attenuation measurements
  through the plane of finite
• thickness cross-section of the body.
• The data obtained is used to reconstruct a
  digital image of the cross-section,
• with each pixel in the image representing a
  measurement of the mean
• attenuation of a box-like element (voxel)
  that extend through the thickness of
• the section.

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Introduction

• Current trend in radiation therapy dose delivery
  is towards highly localized, conformal techniques
  such as IMRT, SRS etc.
• These techniques yield complex three dimensional
  dose distributions which require dose
  verification in 3D.
• Polymer gel dosimetry attempts to meet the
  requirements of 3D radiation dose verification.
• Most recently, X-ray computed tomography , a
  prevalent imaging modality in radiotherapy has
  been proposed as a technique for extracting dose
  information from polymer gels.
• Feasibility of the technique and protocol for CT
  imaging is outlined in this study

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Materials and Method

• CT imaging for all experiments was performed
  using a Siemens Somatom VolumeZoom CT scanner.
• It is a Non-uniform multiple-row detector CT
  capable of producing 4 slices per gantry
  rotation.

1 mm 2
1.5
1.5
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• Advantage
• High speed can be utilized for fast imaging of
  large volume of tissue with wide slices.
• Data acquired 8 times faster than with single row
  detector.
• Main advantage is the better utilization of x-ray
  tube.

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Materials and Method

• There are two types of error
• Systematic
• Error that is potentially avoidable if we are
  attentive during measurement.
• Random
• Includes all types of errors that is not possible
  to avoid.
• There are always uncertainties in the results of
  measurements.
• Values recorded in each pixel is the result of a
  measurement.
• Hence there are always uncertainties in the pixel
  values.
• From statistical point of view there is always
  random error in the pixel values.

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Dose Resolution

• Dose resolution, or minimal detectable
  difference(MDD) in dose is one of the most
  important features of gel dosimeter. (Baldock
  et.al. PMB 2001)
• Where KP is the coverage factor which is
  given by the t-distribution for the
• experimental degrees of freedom.
• 
  sD is the standard deviation or dose
  uncertainty in the measured dose.
• In CT gel dosimetry, the most significant
  factors affecting dose resolution are CT- Dose
  response sensitivity (slope of the linear plot)
  and the level of noise in CT images.
• Since the image noise varies greatly with CT
  imaging technique, phantom size etc. these
  studies require special attention and therefore
  were analyzed.

DP? KPv2 sD
 
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Materials and Method
Studies Performed

• Effect of phantom diameter
• Imaging protocol experiments
• Image averaging
• kV study
• mA study
• FOV study
• Slice thickness study
• Reconstruction Algorithm study
• Dose Response experiments
• Image processing and analysis
• Digital filtering

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Materials and Method

• CT imaging was done in a cylindrical PET
  container.
• PET(Polyethylene Terephthalate) container is a
  good oxygen barrier compared to other plastic
  materials.Ref Wheaton Science Products
• Easily available and cheap to purchase.
• Glass is a high density container which can
  produce extreme artifacts that are difficult to
  remove by background subtraction hence was not
  used.

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1. Effect of Phantom Diameter

• Effect of phantom diameter on image noise was
  investigated by imaging three water filled
  plastic bottles(PET) with different diameters
  (7.5 cm, 9 cm, 11.5 cm) selected as typical size
  for 3D verification of radiation therapy
  treatments.

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2. Imaging Protocol experiments

• Effect of CT imaging protocol on image noise were
  studied using a single cylindrical water filled
  phantom.
• Phantom was 10.5cm in diameter and 18.5cm in
  length to mimic typical size for a gel dosimetry
  phantom.
• The dependence of image noise on each available
  CT imaging was measured individually.
• Different scanning parameters were used for
  different studies.
• For each set of scan parameters two images were
  obtained in order to remove artifacts by
  background subtraction prior to making noise
  measurements.

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3. Dose Response Experiments

• CT imaging Protocol 140kV, 200mAs, 2.5mm slice
  thickness, 130mm FOV
• Matrix size of 512 x 512 and reconstruction
  algorithm B30s (Siemens) was used.

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3a. GEL Preparation

• 4500ml of PAGAT Normoxic gel was prepared. All
  the components used were purchased from Sigma
  aldrich, India.
• Gel Compostion
• 3 Acrylamide
• 3 Bis
• 6 Gelatin
• 88 distilled water
• 10mM THPC
• The gel was poured into the PET cylindrical
  containers immediately after preparation to avoid
  unnecessary exposure to the atmospheric oxygen.
• First container was used for calibration purpose
  Second used for exposing routine clinical
  beams.
• The gel containers were placed in the
  refrigerator to solidify at 50C.
• The containers covered with black sheet to avoid
  exposure to light.
• Overall preparation time was 4hrs.

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Water Bath Design

• Size 40 X 36 cm2
• prepare nearly 10L of Gel
• Overhead stirrer
• Robust than Magnetic stirrer
• high stirring capacity
• stainless steel blade
• stir for 5hrs

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3b. Gel Calibration Method

• For calibration a large volume flask of gel was
  placed in air into which numerous small fields of
  varying doses were directed. (Oldham et
  al.PMB.1998).
• Dmax was chosen as the calibration point for this
  technique.
• Though several alternative techniques are
  available , large flask method appears to be more
  accurate.(Taylor et.al. PMB.2007).

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3c. GEL Irradiation

• The prepared and stored gels were exposed
  approximately 24hrs after preparation.
• The gels were brought to the Linac room
  temperature before exposure.
• Irradiation was performed using the 6MV X-ray
  beam (CL2300CD, Varian associates, Pal Alto, CA).
• Gel was calibrated with 7 regions of uniform
  dose (from 2Gy to 14Gy).
• Four fields with field size of 4X4 cm2 were
  directed into the clinical gel to simulate a
  patient treatment.
• After exposure the containers were placed back in
  the refrigerator.

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4. Image processing and analysis

• Image averaging, background subtraction and all
  other image analyses were performed by MatLab
  Programming.
• Background subtraction was critical to ensure
  removal of artefacts that might obscure noise
  measurements.
• For studying effects of phantom diameter, CT
  imaging technique and dose response studies,
  ROIs with specific pixels were chosen.
• For performing digital filtering some functions
  were programmed and some inbuilt functions of
  Matlab were used.

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Image Processing
2 Gy
14 Gy
After background subtraction
Before background subtraction
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Clinical Beam Exposure
7Gy
5Gy
11 Gy
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Results and Discussions

• Effect of phantom diameter

Noise
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Results and Discussions

• Image average study
• Ability of gel dosimetry readout technique to
  produce spatially uniform images is critical for
  accurate gel dosimetry as non uniformity could be
  misinterpreted as inhomogeneity in the recorded
  dose distribution.
• Fig a. b and c shown in the following slide
  indicate the mean and standard deviation for 36
  spatially distinct 21x21 pixels ROIs in
  Single,16imgave, 32imgave respectively.

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Results and Discussions
Std Dev
Mean
a
b
c
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Results and Discussions

• The variation between ROIs is far less than the
  uncertainty within each ROI.
• Even with low noise present in a large no of
  averaged images (32 images averaged) the
  uniformity remains better than the intra-ROI
  variation.

 
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Results and Discussions

• kVStudy
• Increasing tube voltage decreases image noise.

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Results and Discussions

• mA Study
• Increasing mA decreases noise.

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Results and Discussions

• FOV study
• Increasing FOV decreases image noise.

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Results and Discussions

• Slice thickness study
• Increasing slice thickness decreases image noise.

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Results and Discussions
Reconstruction Algorithm Study
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Results and Discussions
Dose response study (24Hrs)
Linear fit
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Results and Discussions
Dose response study (53Hrs)
Linear fit
Monoexponential fit
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Results and Discussions
Dose response study (145Hrs)
Linear fit
Monoexponential fit
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Results and Discussions
Dose response study (288Hrs)
Monoexponential fit

• Linear fit

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Results and Discussions
Cumulative Dose response study (Intra gel
reproducibility Test)
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Results and Discussions

• Comparative study between plots

0.4232
 
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Slope Response
 
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Digital Filtering

• In general digital filtering is performed in
  either the frequency or spatial domains.
• Filters in spatial domain reduce noise
  effectively and preserve edges.
• These filters are all based on the same general
  principle a mask (m x n pixels) is centered on
  each pixel of the image, g(s,t), and a function
  is applied to the image pixel in the region of
  the mask (Sxy) so that the center pixel is
  replaced with a new value, f(x,y).

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ROI SELECTION

25 PIXELS CHOSEN FROM EXPOSED IMAGE
RADIATION EXPOSED IMAGE
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Enlarged View of ROI
Area Chosen for Filtering
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DIGITAL FILTERING
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DIGITAL FILTERING
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Conclusion

• The image noise increase with phantom size due to
  the result of photon attenuation in the phantom.
  
• Hence CT gel dosimetry phantoms should be as
  small as possible.
• Increasing tube voltage should be the top
  priority for scan technique.
• mA improves noise by 1/vmA, since N (No. of
  Photons attenuated) is linearly related to mA.
• Increasing slice thickness and pixel size
  decrease image noise.
• Effect of pixel dimension on noise is measured to
  be greater than that of slice thickness.

 
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Conclusion

• Reconstruction algorithm has a very significant
  effect on the image noise.
• Among the algorithms B30medium and B41 medium
  appears to be the better choice for noise
  reduction.
• Dose sensitivity may be affected if calibration
  and clinical treatment exposure are done in
  different containers. Hence both should be done
  in similar containers.
• Average Dose sensitivity was found to be 4.761 x
  10 -3 H Gy -1.

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Conclusion

• In our study we found the mean filters and alpha
  trimmed filter seem to provide good dose
  resolution.
• There are several other digital filters which can
  be utilized for image enhancement.
• This study is still progressing.

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• Thank You

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Acknowledgements

• Funded by Atomic Energy Regulatory Board,
  Mumbai, India, Project No N964

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Dose to NCT Response curve
216cGy/H