Abstract
According to the UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation) Report 2000, the largest man made source of ionizing radiation exposure to the world population is from diagnostic medical x-rays. The increasing use of computed tomography (CT) in clinical practice and the relatively high dose delivered to the patient make CT a significant contributor to the population dose from diagnostic medical x-rays. CT images with excellent resolution and high contrast of soft tissue have led to significant advances in medical radiology. Recently, much attention has been shifted to pediatric CT dosimetry due to the increased patient dose and the extra health detriment. Therefore, there is a need for the assessments of radiation dose and health detriment to pediatric CT patients. To study pediatric CT dosimetry, age-dependent cylindrical phantoms are required for the measurements of CTDI (Computed tomography dose index). In this project, we propose to fabricate several age-dependent cylindrical PMMA phantoms based on the weight- and height-data published by the Department of Health and other references. Age-dependent CT phantoms with diameters in 10, 16, 20, 24 and 32 cm are represented patients with ages in newborn, 1-5y, 5-10y, 10-15y and adult. CTDI in various locations and beam quality are measured by a CT pencil-type ionization chamber and make using traditional and modern CT scanners, i.e. the GE Prospeed Plus, the Simens Somatom Sensation 64, the Philips BrillianceTM 40, and the GE Lightspeed VCT. GE series have the highest nCTDI (18.81(head), 9.39(body) mGy/100mAs) and Siemens Somatom Sensation 64 has the lowest (10.67(head), 5.41(body) mGy/100mAs). More importantly, parameters should be adjusted based on children size. For scanning the pediatric body, 80kVp usually reduced radiation dose for ~69% without sacrificing diagnostic images quality instead of using the same protocol of adults. Furthermore, the contributions from scattered beam to CTDIC are 39%, 63%, 77%, 81%, 85% and 89% with phantom diameters in 5, 10, 16, 20, 24 and 32cm. Also, CT source model and object models with Siemens Somatom Sensation 64 to calculate CT dose have been established for MCNP and the difference between simulated and measured CTDI is within 16%. With these dose data available, a single-scan dose analysis will be applied to derive the pediatric dose guidance levels. The results of this study are very valuable to understand the pediatric CT radiation dose in Taiwan.