Med Phys
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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.
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Magnetic resonance guided ultrasonic therapy is a promising minimally invasive technology for constantly growing variety of clinical applications. Delivery of focused ultrasound (FUS) energy to the targeted point with optimal intensity is highly desired; however, due to tissue aberrations, optimal focal intensity is not always achieved. Especially in transcranial applications, the acoustic waves are shifted and distorted mainly by the skull. In order to verify that magnetic resonance acoustic radiation force imaging (MR-ARFI) can be used as a focusing tool in transcranial treatments, such an imaging was applied in vivo on a porcine brain via ex vivo human skull. Then, by the use of MR-ARFI technique, an improved ultrasound focusing algorithm is proposed and demonstrated for both transcranial and none brain applications. ⋯ In the present work, the authors show for the first time a result of MR-ARFI in a live brain through ex vivo human skull. They have demonstrated that aberration correction could be done using MR-ARFI by measuring the radiation force at the focal point. Aberration correction using MR-ARFI is a promising noninvasive technique for transcranial focusing, which may result in near optimal focus and more reliable and safer brain FUS treatments.