International Journal of Science and Research (IJSR) ISSN: 2319-7064 ResearchGate Impact Factor (2018): 0.28 | SJIF (2018): 7.426 Volume 9 Issue 1, January 2020 www.ijsr.net Licensed Under Creative Commons Attribution CC BY Measurements Absorbed Doses to Some Organs during Head CT Imaging T. M. Taha 1, 2 , S. H. Allehyani 2 1 Physics Department, Faculty of Applied Sciences, Umm Al-Qura University, KSA, P. Box 21955 2 Radiation Protection Department, Nuclear Research Center, Atomic Energy Authority Abstract: Purpose to measure absorbed doses to brain, eyes and thyroid organs during head computed tomography imaging. Materials and Methods : Thermoluminscence dosimeters, TLD-100 placed inside the Head CT dosimetry phantom at four peripheral and central holes and irradiated to reference dose of CT beam. Results : The weighted dose conversion factor for TLD(s) at four peripheral holes and central hole of head phantom, at 120 kV and 400 mAs is generated. The mean absorbed doses to brain, eyes and thyroid during head CT protocol using phantom of human are found to be 1.02 mGy, 1.17 mGy and 0.99 mGy respectively. Conclusion : The mean absorbed doses to brain, eyes and thyroid organs are lower than International Commission of Radiological Protection recommendations. Keywords: Computed tomography (CT), Head and body CT phantom, CTDI, Ray safe 1. Introduction Radiation doses delivered to patients during CT imaging represents 4-5 % of all radiologic examinations and contributes about 34-35% of the annual collective dose from all medical X‐ray examinations to the population from radiological examinations (Nagel et al, 2000 and UNSCER, 2000). In large U.S. hospitals, CT represents 10% of diagnostic procedures and accounts for approximately 65% of the effective radiation dose for all medical examinations (National Cancer Institute,2006). Thyroid and the eye lens are organs considered sensitive to ionizing radiation and brain is one organ insensitive to ionizing radiation (Maylase, 2018). the International Commission on Radiological Protection (ICRP-103, 2007) lowered radiation weighting factors for some organs, thyroid gland gonads from 0.02 to 0.08 and thyroid from 0.05 to 0.04 and others organs does not change such as bone surface, skin, brain, salivary glands, 0.01. The mentioned weighted tissue factors express the stochastic radiation effect. it is the largest source of radiation dose. This study is aimed to generate procedure for measurement absorbed doses to eyes, brain and thyroid organs during Head CT Imaging. 2. Material and Method Calibration of TLD(s) placed inside the Head CT dosimetry phantom. Group sorting and identification of golden and master chips After irradiation all the 120 thermoluminscence dosimeters, TLDs were assigned into ten sorted group based on their ‘sensitivity’; according to their collected charge in nC in various ranges. The chips having approximately the equal sensitivity were grouped together. TLDs of LiF: Ti (TLD- 100) with 3.5mm diameter and 0.15mm thickness were used to evaluate organ doses. TLDs were read out in a manual TLD reader (Harshaw 3500, Thermo Fisher Scientific) with a hot planchet heating mechanism using the WinREMS Software. The read-out cycle for TLD-100 preheating at 50◦C for 10s, and a linear ramp rate of 5◦C/s to 300 ◦C for 25s. The annealing of TLDs was adjusted to 400◦C for 1 hour and to 100 ◦C to two hours in a microprocessor - controlled oven (PTW manufacture), followed by cooling to room temperature on an aluminum block. Element correction coefficient (ECC) improved the relative sensitivity of TLDs to correct the variation of individual sensitivity in the batch and calculated using the following equation Where ECC: Element correction coefficient TLD response: Reading for each TLD chips, nC X: mean of the TLD chips, nC Five groups of master TLD(s) chips were selected, placed inside the center and peripherals of head CT dosimetry phantom as a procedure mentioned by (M. Zelikman et al, 2012). Four groups of the selected TLD-chips, placed inside peripheral of head phantom. Each group is consisting of 10 TLD chips. One group of the selected TLD-chips placed inside center of head phantom. these TLD-chips calibrated in the CT beam on an axial scan obtained with acquisition parameters of 400 mAs and 120 for the head imaging. In calibration, the reference air kerma was measured with a 100 mm CT pencil ionization chamber that inserting at hole center and four peripherals CT head phantom connected with Ray safe electrometer by the same physical operating parameters similar to that irradiated the master TLD(s) chips. Computed tomography dose index, CTDIw represents the average dose over the central plane of a 100-mm scan length is calculated using the following equation. The quantity C PMMA , 100,c is measured at the center of the standard CT dosimetry phantom and C PMMA , 100p is the average of values measured at four positions around the periphery of the phantom.. Element correction coefficient for individual TLD calibrated have repeatability lower than 3.5%.Ten head human phantoms containing TLD(s) chips in position of the brain, eyes and thyroid organ irradiated to the Paper ID: ART20203958 DOI: 10.21275/ART20203958 350