Inter-scanner super-resolution of 3D cine MRI using a transfer-learning network for MRgRT.

. Deep-learning networks for super-resolution (SR) reconstruction enhance the spatial-resolution of 3D magnetic resonance imaging (MRI) for MR-guided radiotherapy (MRgRT). However, variations between MRI scanners and patients impact the quality of SR for real-time 3D low-resolution (LR) cine MRI.

Characterization of real-time cine MR imaging distortion on 0.35 T MRgRT with concentric cine imaging QA phantom.

. Real-time MRgRT uses 2D-cine imaging for target tracking and motion evaluation. Rotation of gantry inducedoff-resonance, resulting in image artifacts and imaging isocenter-shift precluding MR-guided arc therapy.

Technical note: Minimizing CIED artifacts on a 0.35 T MRI-Linac using deep learning.

Background: Artifacts from implantable cardioverter defibrillators (ICDs) are a challenge to magnetic resonance imaging (MRI)-guided radiotherapy (MRgRT).

Purpose: This study tested an unsupervised generative adversarial network to mitigate ICD artifacts in balanced steady-state free precession (bSSFP) cine MRIs and improve image quality and tracking performance for MRgRT.

Methods: Fourteen healthy volunteers (Group A) were scanned on a 0.

Comprehensive MR imaging QA of 0.35 T MR-Linac using a multi-purpose large FOV phantom: A single-institution experience.

Purpose: Magnetic resonance-guided radiotherapy (MRgRT) is desired for the treatment of diseases in the abdominothoracic region, which has a broad imaging area and continuous motion. To ensure accurate treatment delivery, an effective image quality assurance (QA) program, with a phantom that covers the field of view (FOV) similar to a human torso, is required. However, routine image QA for a large FOV is not readily available at many MRgRT centers.

Real-time B compensation during gantry rotation in a 0.35-T MRI-Linac.

Background: Rotation of the ferromagnetic gantry of a low magnetic field MRI-Linac was previously demonstrated to cause large center frequency offsets of ±400 Hz. The B off-resonances cause image artifacts and imaging isocenter shifts that would preclude MRI-guided arc therapy.

Purpose: The purpose of this study was to measure and compensate for center frequency offsets in real time during gantry rotation on a 0.

Technical Note: Effects of rotating gantry on magnetic field and eddy currents in 0.35 T MRI-guided radiotherapy (MR-IGRT) system.

Purpose: The purpose of this study was to identify the cause of severe image artifacts that occurred during gantry rotation in a 0.35 T MRI-Linac by comparing measurements of eddy currents, center frequency, and field inhomogeneities made with the gantry in motion and stationary.

Methods: Gradient and B eddy currents were calculated from the free induction decays (FIDs) resulting from selective excitation at a temporal resolution of 200 ms/measurement.

Phase I Trial of Stereotactic MRI-Guided Online Adaptive Radiation Therapy (SMART) for the Treatment of Oligometastatic Ovarian Cancer.

Purpose: Stereotactic body radiation therapy is increasingly used to treat a variety of oligometastatic histologies, but few data exist for ovarian cancer. Ablative stereotactic body radiation therapy dosing is challenging in sites like the abdomen, pelvis, and central thorax due to proximity and motion of organs at risk. A novel radiation delivery method, stereotactic magnetic-resonance-guided online-adaptive radiation therapy (SMART), may improve the therapeutic index of stereotactic body radiation therapy through enhanced soft-tissue visualization, real-time nonionizing imaging, and ability to adapt to the anatomy-of-the-day, with the goal of producing systemic-therapy-free intervals.

Effects of B eddy currents on imaging isocenter shifts in 0.35-T MRI-guided radiotherapy (MR-IGRT) system.

Purpose: The purpose of this study was to measure gantry angle-related eddy currents in a 0.35-T MRI-Linac and determine if B (zeroth order) eddy currents are the primary cause of gantry angle-dependent imaging isocenter shifts vs other potential causes like B inhomogeneities and gradient (first order) eddy currents. For conventional Cartesian acquisitions, B eddy currents can cause imaging isocenter shifts along both phase encode and readout directions.

B field homogeneity recommendations, specifications, and measurement units for MRI in radiation therapy.

Purpose: The purpose is: (a) Relate magnetic resonance imaging (MRI) quality recommendations for radiation therapy (RT) to B field homogeneity; (b) Evaluate manufacturer specifications of B homogeneity for 34 commercial whole-body MRI systems based on the MRI quality recommendations and RT application; (c) Measure field homogeneity in five commercial MRI systems and one commercial MRI-Linac used in RT and compare the results with their B homogeneity specifications.

Methods: Magnetic resonance imaging quality recommendations for spatial integrity, image blurring, fat saturation, and null banding in RT were developed based on the literature. Guaranteed (maximum) and typical B field homogeneity specifications for various diameter spherical volumes (DSVs) were provided by GE, Philips, Siemens, and Canon.

MRI quality control for low-field MR-IGRT systems: Lessons learned.

Purpose: To present lessons learned from magnetic resonance imaging (MRI) quality control (QC) tests for low-field MRI-guided radiation therapy (MR-IGRT) systems.

Methods: MRI QC programs were established for low-field MRI- Co and MRI-Linac systems. A retrospective analysis of MRI subsystem performance covered system commissioning, operations, maintenance, and quality control.

Stereotactic MR-Guided Online Adaptive Radiation Therapy (SMART) for Ultracentral Thorax Malignancies: Results of a Phase 1 Trial.

Purpose: Stereotactic body radiation therapy (SBRT) is an effective treatment for oligometastatic or unresectable primary malignancies, although target proximity to organs at risk (OARs) within the ultracentral thorax (UCT) limits safe delivery of an ablative dose. Stereotactic magnetic resonance (MR)-guided online adaptive radiation therapy (SMART) may improve the therapeutic ratio using reoptimization to account for daily variation in target and OAR anatomy. This study assessed the feasibility of UCT SMART and characterized dosimetric and clinical outcomes in patients treated for UCT lesions on a prospective phase 1 trial.

In Silico Trial of MR-Guided Midtreatment Adaptive Planning for Hypofractionated Stereotactic Radiation Therapy in Centrally Located Thoracic Tumors.

Purpose: Hypofractionated (>5 fraction) stereotactic radiation therapy (HSRT) may allow for ablative biologically equivalent dose to tumors with a lower risk of organ-at-risk (OAR) toxicity in central thoracic tumors. Adaptive planning may further improve OAR sparing while maintaining planning target volume (PTV) coverage. We hypothesized that midtreatment adaptive replanning would offer dosimetric advantages during HSRT for central thorax malignancies using magnetic resonance imaging (MRI)-guided radiation therapy.

First clinical implementation of real-time, real anatomy tracking and radiation beam control.

Purpose: We describe the acceptance testing, commissioning, periodic quality assurance, and workflow procedures developed for the first clinically implemented magnetic resonance imaging-guided radiation therapy (MR-IGRT) system for real-time tracking and beam control.

Methods: The system utilizes real-time cine imaging capabilities at 4 frames per second for real-time tracking and beam control. Testing of the system was performed using an in-house developed motion platform and a commercially available motion phantom.

Remote Cherenkov imaging-based quality assurance of a magnetic resonance image-guided radiotherapy system.

Purpose: Tools to perform regular quality assurance of magnetic resonance image-guided radiotherapy (MRIgRT) systems should ideally be independent of interference from the magnetic fields. Remotely acquired optical Cherenkov imaging-based dosimetry measurements in water were investigated for this purpose, comparing measures of dose accuracy, temporal dynamics, and overall integrated IMRT delivery.

Methods: A 40 × 30.

Phase I trial of stereotactic MR-guided online adaptive radiation therapy (SMART) for the treatment of oligometastatic or unresectable primary malignancies of the abdomen.

Purpose/objectives: SBRT is used to treat oligometastatic or unresectable primary abdominal malignancies, although ablative dose delivery is limited by proximity of organs-at-risk (OAR). Stereotactic, magnetic resonance (MR)-guided online-adaptive radiotherapy (SMART) may improve SBRT's therapeutic ratio. This prospective Phase I trial assessed feasibility and potential advantages of SMART to treat abdominal malignancies.

Performance of a multi leaf collimator system for MR-guided radiation therapy.

Purpose: The purpose of this study was to investigate and characterize the performance of a Multi Leaf Collimator (MLC) designed for Cobalt-60 based MR-guided radiation therapy system in a 0.35 T magnetic field.

Methods: The MLC design and unique assembly features in the ViewRay MRIdian system were first reviewed.

Three-Dimensional Dosimetric Validation of a Magnetic Resonance Guided Intensity Modulated Radiation Therapy System.

Purpose: To validate the dosimetric accuracy of a commercially available magnetic resonance guided intensity modulated radiation therapy (MRgIMRT) system using a hybrid approach: 3-dimensional (3D) measurements and Monte Carlo calculations.

Methods And Materials: We used PRESAGE radiochromic plastic dosimeters with remote optical computed tomography readout to perform 3D high-resolution measurements, following a novel remote dosimetry protocol. We followed the intensity modulated radiation therapy commissioning recommendations of American Association of Physicists in Medicine Task Group 119, adapted to incorporate 3D data.

A method to reconstruct and apply 3D primary fluence for treatment delivery verification.

Motivation: In this study, a method is reported to perform IMRT and VMAT treatment delivery verification using 3D volumetric primary beam fluences reconstructed directly from planned beam parameters and treatment delivery records. The goals of this paper are to demonstrate that 1) 3D beam fluences can be reconstructed efficiently, 2) quality assurance (QA) based on the reconstructed 3D fluences is capable of detecting additional treatment delivery errors, particularly for VMAT plans, beyond those identifiable by other existing treatment delivery verification methods, and 3) QA results based on 3D fluence calculation (3DFC) are correlated with QA results based on physical phantom measurements and radiation dose recalculations.

Methods: Using beam parameters extracted from DICOM plan files and treatment delivery log files, 3D volumetric primary fluences are reconstructed by forward-projecting the beam apertures, defined by the MLC leaf positions and modulated by beam MU values, at all gantry angles using first-order ray tracing.

Technical Note: Magnetic field effects on Gafchromic-film response in MR-IGRT.

Purpose: Magnetokinetic changes may affect crystal orientation and polymerization within the active layer of radiochromic film (RCF). This effect is investigated in a magnetic resonance image-guided radiotherapy unit within the context of film dosimetry.

Methods: Gafchromic EBT2 RCF was irradiated in a 30 × 30 × 30 cm solid water phantom using a Co-60 MRI guided radiotherapy system (B = 0.

Magnetic Resonance Image Guided Radiation Therapy for External Beam Accelerated Partial-Breast Irradiation: Evaluation of Delivered Dose and Intrafractional Cavity Motion.

Purpose: To use magnetic resonance image guided radiation therapy (MR-IGRT) for accelerated partial-breast irradiation (APBI) to (1) determine intrafractional motion of the breast surgical cavity; and (2) assess delivered dose versus planned dose.

Methods And Materials: Thirty women with breast cancer (stages 0-I) who underwent breast-conserving surgery were enrolled in a prospective registry evaluating APBI using a 0.35-T MR-IGRT system.

Biomechanics of cell membrane under low-frequency time-varying magnetic field: a shell model.

Cell membrane deforms in the electromagnetic field, suggesting an interesting control of cellular physiology by the field. Previous research has focused on the biomechanical analysis of membrane deformation under electric fields that are generated by electrodes. An alternative, noninvasive method to generate an electric field is the use of electromagnetic induction with a time-varying magnetic field, such as that used for transcranial magnetic stimulation (TMS).

Vesicle biomechanics in a time-varying magnetic field.

Background: Cells exhibit distortion when exposed to a strong electric field, suggesting that the field imposes control over cellular biomechanics. Closed pure lipid bilayer membranes (vesicles) have been widely used for the experimental and theoretical studies of cellular biomechanics under this electrodeformation. An alternative method used to generate an electric field is by electromagnetic induction with a time-varying magnetic field.