Assessment Of Effective Doses For Patients During Diagnostic CT Examinations

Abstract

Computerized tomography (CT) is defined as radiography in which a three dimensional image of a body structure is constructed by computer from a series of plan cross-sectional images made along axis. CT represents about 9% of all diagnostic x-ray examinations and almost 47% of the total radiation dose from medical diagnostic examination. Because of this, there is a strong need to establish diagnostic reference dose level and relative radiation level for each examination and minimize the radiation dose to radiosensitive organs. The current thesis is aimed to assess effective doses for patients during diagnostic CT examinations and minimize a dose delivered to eyes during head CT scan. The study divided into five stages: First stage was to estimate the diagnostic reference dose levels (DRLs), the volume computed tomography dose index (CTDIvol), the dose length product (DLP) and the effective dose (E) for head, chest and chest-abdomen-pelvic CT examinations in King Abdullah Medical City (KAMC), Saudi Arabia using normalized effective per dose length product. Demographic parameters (such as age, gender, weight, height) for a sample of 535 CT patients and physical scan parameters (such as patient tube voltage (kV), reference and effective mill ampere-second (mAs), scan length, number of slices, and pitch factor) were collected from a picture archiving and communication system (PACS). CTDIvol and DLP were studied for different patients’ weights (40 -60 kg, 60-80 kg, 80-100 kg and 100-120 kg). Mean CTDIvol, DLP and effective dose for a standard weight (70 kg ±10%) was evaluated according to the three CT protocols considered. The results were, as follows: for head examinations 61.8 mGy, 1212 mGy.cm and 2.55 mSv, respectively; for chest examinations; 13.1 mGy, 657.5 mGy.cm and 9.21 mSv, respectively; for chest-abdomen-pelvic examinations 13.9 mGy, 782.5 mGy.cm and 11.73 mSv, respectively. The effective dose for the chest examinations was lower than the corresponding value communicated by the European study group under European Commission. The effective dose for head examinations, was slightly higher than the one communicated by the European Commission study. Second stage was to study some factors affecting on CT dose. Third stage was to create a method for measurement entrance skin doses for eyes, chest, abdomen and chest-abdomen-pelvis (CAP) of Rando phantom during CT examinations . The ESD is measured by distribution of the standardized thermoluminscence dosimeters on specific sections of RT Rando phantom during different CT Protocol . The entrance skin dose was, as follows: for eyes during head examinations, 53 ± 9.83 mGy ; for chest examinations 20.7 ± 8.7 mGy ; for abdomen examinations 17.1 ± 3.7 mGy and for chest-abdomen-pelvic examinations 24.1 ± 1.59 mGy respectively. Fourth stage was to calculate the equivalent dose to eye and effective skin dose for chest, abdomen and chest-abdomen-pelvic. The mean equivalent skin dose of eyes was found to be 0.53 mSv and the effective skin dose was, as follows: for chest examinations 0.21 ± 0.01 mSv ; for abdomen examination 0.17 ± 0.01 mSv and for chest-abdomen-pelvic examinations 0.23 ± 0.02 mSv respectively. Fifth stage was to generate a method to minimize a dose delivered to eyes during head CT scan using thermoluminscence dosimeters. The study head Rando phantom demonstrated that the use of bismuth coated shielding over the eyes decreased the radiation dosage by 47 % without compromising the diagnostic findings The scientist study recommends to review the CT protocol of chest and total body to minimize a dose to apply concept as low as reasonable achievable (ALARA) and cover eyes during head CT scan by bismuth latex coated material to minimize a dose.