The number of computed tomography examinations in Malaysia is increased every year. This is because all radiologist and the public itself believes CT examinations provide a good quality image for diagnostic need instead of the advanced CT technology development that they have. It is important to stress that the CT scan also using ionizing radiation and it release radiation dose to the patient.
For the past several years, there has been an emphasis on reducing the radiation dose associated with pediatric CT within the radiology community . This is because usually there is a lack of attention to pediatric CT protocols within the radiology community. Although the risk of carcinogenesis above the baseline cancer rate associated with pediatric CT scan is low, however there is growing evidence that minimizing dose in children is very important . This is because children literally have more rapidly dividing cells than adults and a longer expectancy. Accurate and precise patient radiation dose measurements are required to help to define the significance of the increasing CT examinations number especially pediatric patient population.
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In the early beginning of CT production, the industry standard technique for scanner output determination has been the computed tomography dose index (CTDI) and it has been accepted as an index of dose for single slice axial CT technology to correlate CT radiation exposure for different CT scanners. However, CTDI assessment does not measure absorbed dose for particular organ of the patient.
In this study, Gafchromic XR-CT 2 will be used as dosimeter for measuring absorbed dose during CT examination because it has relatively high sensitivity towards dose and a film emulsion. It is suitable for measuring low dose assessment for pediatric examination. The Gafchromic XR-CT 2 will be clinically applied in Biomedical Imaging Department, UMMC for pediatric patient aged between 0-15 years old. The absorbed dose for the pediatric abdomen CT scan procedure will be measured.
Gafchromic film is a based on polydiacetylene manufactured by GAFCHROMIC, International Specialty Products Wayne, NJ. It is designed to solve some problems associated with conventional 2D radiation detectors. High spatial resolutions, weak energy dependence in a broad range of beam qualities used for radiation therapy and near tissue equivalence makes them suitable for dose measurement in radiation fields with high dose gradients . Gafhchromic films are relatively insensitive to visible lights and this characteristic is useful for handling and preparation for a film type product under normal room light. Usually, the Gafchromic film is yellowish color and it will darken if expose to the radiation. They undergo a color change directly and do not require chemical processing with image formation occurring by a dye creation through polymerization process during which energy is transferred from an energetic particle to the receptive part of the colorless photo monomer molecule .
Initially, the manufacturer indicates that the color producing elements of film emulsion (chromospheres) are roughly spherical and approximately 0.75 µm in diameter. The mechanism for Gafchromic film to be used as dosimeter is when initially colorless chromosphere may become colored under irradiation and may become colored through a chemical reaction with an incident ionizing photon or electron. In principal for radiation dosimeter, the degree of color formation in the emulsion should be proportional to the energy absorbed in the emulsion .
While the new advanced product, Gafchromic XR-CT2 is a film for measuring radiation beam slice width on CT scanners in real time. The manufacturer has stated that the sensitivity for Gafchromic is relatively high which has dose range between 0.1 cGy to 20 cGy. It also calibrates the beam slice with high accuracy and it can develop itself in a processor-less environment. The energy range is between ~20 kVp to 200 kVp. The size for Gafchromic XR-CT2 is 3/4 inches- 5 inches. In addition, it has 3-layer laminate substrate-active layer-substrate. The substrate thickness for white polyester and yellow polyester are 97 micron while the active layer thickness is 25 microns.
SIGNIFICANCE OF THE PROJECT
Concerning about the absorbed dose received by the pediatric patient during CT examination especially in Malaysia. The radiation risks for pediatric examination have been reported a lot recently worldwide.
The well-established calibrated Gafchromic film CT-2 could be used especially for pediatric examination in Malaysia. This will make new trends using radiochromic film instead of using TLD to measure absorbed dose for hospital in Malaysia
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Calibration of Gafchromic XR-CT 2 film to be used in clinical CT examination.
Comparison dose results between three dosimeter system; standard CTDI dose measurement, calibrated Gafchromic XR-CT 2 film and Optically Stimulated Luminescence Dosimeter (OSLD).
Measurement of absorbed dose to organs for several pediatrics CT examinations using calibrated Gafchromic XR-CT 2 film
Gafchromic XR-CT 2 film, CTDI phantom (PMMA), Optically Stimulated Luminescence Dosimeter (OSLD).
Siemens CT scanner, 100mm-pencil type ionization chamber (Model DTC 10 RS), reflective type scanner (Microtek Scanmaker i800)
Dose response and energy dependence
Firstly, the calibration using reference dosimeter will be made using 100-mm pencil type ionization chamber. Ionisation chamber will be placed at the isocenter of the Siemens CT scanner and will be irradiated in air to 8 doses levels of different exposure parameters (125kVp, 1s, and 100-300 mA) that will be produced doses ranging from 10 cGy to 100 cGy. Next, calibration of Gafchromic films using dose response will be made using 3 - 3 of 3 strips of Gafchromic film that will irradiated using same procedure for reference dosimeter.
Secondly, for energy dependence evaluation, the films will be place in air and in CTDI phantom using same clinical tube current at tube voltages of 80,100,120 and 140 kVp. The film position in phantoms comprised the central (12 o'clock) holes of the head phantom those at body phantom. All radiation doses in the axial scanning mode will be measured by 100-mm pencil type ionization chamber.
All of the Gafchromic XR-CT2 films will be digitized with a reflective type scanner after irradiation. Before the scanner machine will scan, it must be warmed up for at least 30 min to minimize variations in scanner response. Film strips will be scanned with RGB mode (16 bit per color), at 300 dpi, and without color correction factors or filters . The red channel values will be selected for the experimental analysis. The images obtained from film digitization will be analyzed with MATLAB software. The average pixel values in a region of interest (ROI) of 3 mm - 9 mm obtain in each film strip before () and after () irradiation will be used for dose calculations as formula given.
Comparison absorbed dose between 2 different dosimeter systems in phantom
Gafchromic XR-CT2 strips films and OSLD unit separately will be placed in CTDI phantom and will be irradiated using same clinical tube voltage (125 kVp) but different tube current ranged from 10-300 mAs. The Gafchromic films will be digitized according to the scanning procedure using reflective type scanner and the dose will be calculated using calibration function. Simultaneously, the 100-mm pencil type ionization chamber will measure the dose for every increasing mAs. The dose from ionization chamber will be used as reference dose. The relationship of dose measured by both dosimeter systems and mAs then will be compared.
Implementation of Gafchromic film to pediatric abdomen examination
There will be steps to be performed for patient examination; 1) scanogram (planar image of the abdomen, 2) routine abdomen CT, 3) patient positioning, 4) pediatric abdomen CT scan procedure. Routine abdomen CT provides images to radiologist and makes possible determination of the position where scanning of abdomen study needs to be done.
After the patient table will move into that position, lasers available at the CT gantry will show the center of the X-ray beam . 3 - 3 of Gafchromic film strips will be attached at top of the patient and one at the lateral side of an abdomen pediatric patient during CT examination. The absorbed dose for abdomen examination will be calculated using calibration function. The dose will be measured only during pediatric abdomen CT scan procedure. Similarly, all the CT examination also will be measured by a 100-mm pencil type ionization chamber. The data will be collected for pediatric patient aged between 0-15 years old from early January 2013 until end of April 2013.
Submission and proposal presentation
17th October 2012
Discussion calibration data with SV
4th December 2012
Discussion comparison dosimeter system
21st January 2013
Final analysis CT exam with SV
30th April 2013
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Calibration curve for Gafchromic film CT 2 dose response and correction factor will be determined.
High regression graph on Gafchromic film dose data compared to OSLD.
The Gafchromic film assessment could be implemented for all pediatric CT examination in Universiti Malaya Medical Centre, UMMC and generally, for all hospitals in Malaysia.
This project will be published in at least in one journal