Med Phys
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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 Carlo radiation transport simulations. ⋯ 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.
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In this paper, we present an evaluation study of a set of registration strategies for the alignment of sequences of 3D dynamic contrast-enhanced magnetic resonance breast images. The accuracy of the optimal registration strategies was determined on unseen data. The evaluation is based on the simulation of physically plausible breast deformations using finite element methods and on contrast-enhanced image pairs without visually detectable motion artifacts. ⋯ These results were obtained on average within 26 (81) min for 40 (20) mm control point spacing. The visual appearance of the difference images from 30 patients was significantly improved after 20 mm volume-preserving single-resolution nonrigid registration in comparison to no registration or rigid registration. No substantial volume changes within the region of the enhancing lesions were introduced by this nonrigid registration.
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The purpose of this study was to evaluate the effect of z overscanning on normalized effective dose for pediatric patients undergoing multidetector-computed tomography (CT) examinations. Five commercially available mathematical anthropomorphic phantoms representing newborn, 1-, 5-, 10-, and 15-year-old patients and the Monte Carlo N-Particle (MCNP, version 4C2) radiation transport code were employed in the current study to simulate pediatric CT exposures. For all phantoms, axial and helical examinations at 120 kV tube voltage were simulated. ⋯ Normalized data for the estimation of effective dose to pediatric patients undergoing standard axial and helical CT examinations on an multidetector CT system were calculated. This data was found to depend strongly on CT acquisition mode and exposure parameters as well as patient age and sex. The effective dose from a pediatric CT scan performed in axial mode was always considerably lower compared to the corresponding scan performed in helical mode, due to the additional tissue regions exposed to the primary beam in helical examinations as a result of z overscanning.
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The work presented herein describes our methods and results for predicting, measuring and correcting geometric distortions in a 3 T clinical magnetic resonance (MR) scanner for the purpose of image guidance in radiation treatment planning. Geometric inaccuracies due to both inhomogeneities in the background field and nonlinearities in the applied gradients were easily visualized on the MR images of a regularly structured three-dimensional (3D) grid phantom. From a computed tomography scan, the locations of just under 10 000 control points within the phantom were accurately determined in three dimensions using a MATLAB-based computer program. ⋯ Because the various sources of machine-related distortions can be individually characterized, distortions present in other imaging sequences (for which 3D distortion cannot accurately be measured using phantom methods) can be predicted negating the need for individual distortion calculation for a variety of other imaging sequences. Distortions were found to be primarily caused by gradient nonlinearities and maximum image distortions were reported to be less than those previously found by other researchers at 1.5 T. Finally, the image slices were corrected for distortion in order to provide geometrically accurate phantom images.