Med Phys
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The presence of implanted electronic devices with conducting leads and electrodes are contraindicated for magnetic resonance imaging (MRI), denying many patients its potential benefits. The prime concern is MRI's radio frequency (RF) fields, which can cause elevated local specific absorption rates (SARs) and potential heat injury. The purpose of this article is to develop and compare a range of passive implantable "MRI-safe" lead designs. ⋯ Lead insulation and length strongly affect implanted lead safety to RF exposure during MRI. Lead designs employing impedance and reversed winding sections offer hope for the development of passive, MRI-safe, implantable conducting leads for future human use.
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The purpose of this article was to determine the suitability of the prostate and seminal vesicle volumes as factors to consider patients for treatment with image-guided 3D-conformal radiation therapy (3D-CRT) or intensity-modulated radiation therapy (IMRT), using common dosimetry parameters as comparison tools. ⋯ Volumes of prostate and seminal vesicles provide a reproducible and consistent basis for considering patients for treatment with image-guided 3D or IMRT plans. Patients with prostate and prostate+SV volumes <65 and 85 cm3, respectively, would be suitable for 3D-CRT. Patients with prostate and prostate+SV volumes >65 and 85 cm3, respectively, might get benefit from IMRT.
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A critical question in the development of combined MR/PET scanners is whether MR can provide the tissue attenuation data required for PET reconstruction. Unfortunately, MR images are often unable to encompass the entire patient. The resulting truncation in the transverse plane leads to incomplete attenuation maps, causing artifacts in the reconstructed PET image. This article describes the experiments performed to quantify these artifacts. A method to compensate the missing data was evaluated to determine whether software correction is possible or whether additional transmission hardware has to be included in the scanner. ⋯ MR FOV restrictions can indeed make the reconstructed PET data unacceptable for diagnostic purposes. Biases can be globally compensated by automatic preprocessing of the attenuation map. However, inaccuracies in the correction will result in small artifacts near the periphery of the image that could lead to false-positive findings.
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Respiratory motion adversely affects CBCT image quality and limits its localization accuracy for image-guided radiation treatment. Motion correction methods in CBCT have focused on the thorax because of its higher soft tissue contrast, whereas low-contrast tissue in abdomen remains a challenge. The authors report on a method to correct respiration-induced motion artifacts in 1 min CBCT scans that is applicable in both thorax and abdomen, using a motion model adapted to the patient from a respiration-correlated image set. ⋯ Motion correction of CBCT is feasible and yields observable improvement in the thorax and abdomen. The PCA-based model is an important component: First, by reducing deformation errors caused by the nonrigid registration and second, by relating deformation to surrogate position rather than phase, thus accommodating breathing pattern changes between imaging sessions. The accuracy of the method requires confirmation in further patient studies.
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Dose calculation during optimization of volumetric modulated arc therapy (VMAT) is necessarily simplified to keep computation time manageably low; however the approximations used in the scatter dose calculation lead to discrepancy with more accurate dose calculation following optimization. The purpose of this study is to develop a dose correction strategy in optimization that can minimize the disagreement. ⋯ When dose correction is applied during optimization, dose discrepancies between optimization and full dose calculation are reduced. Integrating dose correction in VMAT optimization allows planners to adjust the optimization constraints more easily and confidently during optimization and has the potential to improve plan quality.