As multidetector computed tomography(CT) serves as an increasingly frequent diagnostic modality, radiation risks to patients became a greater concern, especially for children due to their inherently higher radiosensitivity to stochastic radiation damage. Current dose evaluation protocols include the computed tomography dose index(CTDI) or point detector measurements using anthropomorphic phantoms that do not sufficiently provide accurate information of the organ-averaged absorbed dose and corresponding effective dose to pediatric patients. In this study, organ and effective doses to pediatric patients under helical multislice computed tomography (MSCT) examinations were evaluated using an extensive series of anthropomorphic computational phantoms and Monte Carloradiation transport simulations. Ten pediatric phantoms, five stylized (equation-based) ORNL phantoms (newborn, 1-year, 5-year, 10-year, and 15-year) and five tomographic (voxel-based) UF phantoms (9-month male, 4-year female, 8-year female, 11-year male, and 14-year male) were implemented into MCNPX for simulation, where a source subroutine was written to explicitly simulate the helical motion of the CTx-ray source and the fan beam angle and collimator width. Ionization chamber measurements were performed and used to normalize the Monte Carlo simulation results. On average, for the same tube current setting, a tube potential of resulted in effective doses that were 105% higher than seen at , and 210% higher at regardless of phantom type. Overall, the ORNL phantom series was shown to yield values of effective dose that were reasonably consistent with those of the gender-specific UF phantom series for CT examinations of the head, pelvis, and torso. However, the ORNL phantoms consistently overestimated values of the effective dose as seen in the UF phantom for MSCT scans of the chest, and underestimated values of the effective dose for abdominal CT scans. These discrepancies increased with increasing kVp. Finally, absorbed doses to select radiation sensitive organs such as the gonads, red bone marrow, colon, and thyroid were evaluated and compared between phantom types. Specific anatomical problems identified in the stylized phantoms included excessive pelvic shielding of the ovaries in the female phantoms, enhanced red bone marrow dose to the arms and rib cage for chest exams, an unrealistic and constant torso thickness resulting in excessive x-ray attenuation in the regions of the abdominal organs, and incorrect positioning of the thyroid within the stylized phantom neck resulting in insufficient shielding by clavicles and scapulae for lateral beam angles. To ensure more accurate estimates of organ absorbed dose in multislice CT, it is recommended that voxel-based phantoms, potentially tailored to individual body morphometry, be utilized in any future prospective epidemiological studies of medically exposed children.
This work was supported in part by Grant No. RO1 HD38932 from the National Institute for Child and Health Development (NICHD) and Grant No. RO1 EB00267 from the National Institute for Biomedical Imaging and Bioengineering (NIBIB) with the University of Florida, and by Grant RO1 CA116743 from the National Cancer Institute (NCI) through Rensselaer Polytechnic Institute with the University of Florida. Additional support was provided by Siemens Medical Solutions of Erlangen, Germany.
II. MATERIALS AND METHODS
II.A. Voxel phantoms—UF pediatric series
II.B. Stylized phantoms—ORNL pediatric phantom series
II.C. Monte Carlo simulations
II.D. Selected technique factors for CT scanning
II.E. Organ and effective dose calculations
III. RESULTS AND DISCUSSION
III.A. Age-dependent effective dose comparisons for MSCT examinations
III.B. Age-dependent organ absorbed dose comparisons for MSCT examinations
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