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Background: Stereotactic arrhythmia radioablation (STAR) is a novel treatment approach for refractory ventricular tachycardia (VT). The risk of treatment‐induced toxicity and geographic miss can be reduced with online MRI‐guidance on an MR‐linac. However, most VT patients carry cardiac implantable electronic devices (CIED), which compromise MR images. Purpose: Robust MR‐linac imaging sequences are required for cardiac visualization and accurate motion monitoring in presence of a CIED during MRI‐guided STAR. We optimized two clinically available cine sequences for cardiorespiratory motion estimation in presence of a CIED on a 1.5 T MR‐linac. The image quality, motion estimation accuracy, and geometric fidelity using these cine sequences were evaluated. Methods: Clinically available 2D balanced steady‐state free precession (bSSFP, voxel size = 3.0 ×$\times$ 3.0 ×$\times$ 10 mm3, Tscan = 96 ms, bandwidth (BW) = 1884 Hz/px) and T1${\rm T}_{1}$‐spoiled gradient echo (T1${\rm T}_{1}$‐GRE, voxel size = 4.0 ×$ \times$ 4.0 ×$ \times$ 10 mm3, Tscan = 97 ms, BW = 500 Hz/px) sequences were adjusted for real‐time cardiac visualization and cardiorespiratory motion estimation on a 1.5 T Unity MR‐linac (Elekta AB, Stockholm, Sweden), while complying with safety guidelines for MRI in presence of CIEDs (specific absorption rate <$ <$ 2 W/kg and dBdt<$\frac{dB}{dt}<$ 80 mT/s). Cine acquisitions were performed in five healthy volunteers, with and without an implantable cardioverter– defibrillator (ICD) placed on the clavicle, and a VT patient. Generalized divergence‐curl (GDC) deformable image registration (DIR) was used for automated landmark motion estimation in the left ventricle (LV). Gaussian processes (GP), a machine‐learning technique, was trained using GDC landmarks and deployed for real‐time cardiorespiratory motion prediction. B0$B_{0}$‐mapping was performed to assess geometric image fidelity in the presence of CIEDs. Results: CIEDs introduced banding artifacts partially obscuring cardiac structures in bSSFP acquisitions. In contrast, the T1${\rm T}_{1}$‐GRE was more robust to CIED‐induced artifacts at the expense of a lower signal‐to‐noise ratio. In presence of an ICD, image‐based cardiorespiratory motion estimation was possible for 85% (100%) of the volunteers using the bSSFP (T1${\rm T}_{1}$‐GRE) sequence. The in‐plane 2D root‐mean‐squared deviation (RMSD) range between GDC‐derived landmarks and manual annotations using the bSSFP (T1‐GRE) sequence was 3.1–3.3 (3.3–4.1) mm without ICD and 4.6–4.6 (3.2–3.3) mm with ICD. Without ICD, the RMSD between the GP‐predictions and GDC‐derived landmarks ranged between 0.9 and 2.2 mm (1.3–3.0 mm) for the bSSFP (T1‐GRE) sequence. With ICD, the RMSD between the GP‐predictions and GDC‐derived landmarks ranged between 1.3 and 2.2 mm (1.2–3.2 mm) using the bSSFP (T1‐GRE) sequence resulting in an RMSD‐increase of 42%–143% (bSSFP) and −61%–142% (T1‐GRE). Lead‐induced spatial distortions ranged between −0.2 and 0.2 mm (−0.7–1.2 mm) using the bSSFP (T1${\rm T}_{1}$‐GRE) sequence. The 98th percentile range of the spatial distortions in the gross target volume of the patient was between 0.0 and 0.4 mm (0.0–1.8 mm) when using bSSFP (T1${\rm T}_{1}$‐GRE). Conclusions: Tailored bSSFP and T1${\rm T}_{1}$‐GRE sequences can facilitate real‐time cardiorespiratory estimation using GP trained with GDC‐derived landmarks in the majority of landmark locations in the LV despite the presence of CIEDs. The need for high temporal resolution noticeably reduced achievable spatial resolution of the cine MRIs. However, the effect of the CIED‐induced artifacts is device, patient and sequence dependent and requires specific assessment per case. [ABSTRACT FROM AUTHOR]

At ultrahigh field strengths images of the body are hampered by B1‐field inhomogeneities. These present themselves as inhomogeneous signal intensity and contrast, which is regarded as a "bias field" to the ideal image. Current bias field correction methods, such as the N4 algorithm, assume a low frequency bias field, which is not sufficiently valid for T2w images at 7 T. In this work we propose a deep learning based bias field correction method to address this issue for T2w prostate images at 7 T. By combining simulated B1‐field distributions of a multi‐transmit setup at 7 T with T2w prostate images at 1.5 T, we generated artificial 7 T images for which the homogeneous counterpart was available. Using these paired data, we trained a neural network to correct the bias field. We predicted either a homogeneous image (t‐Image neural network) or the bias field (t‐Biasf neural network). In addition, we experimented with the single‐channel images of the receive array and the corresponding sum of magnitudes of this array as the input image. Testing was carried out on four datasets: the test split of the synthetic training dataset, volunteer and patient images at 7 T, and patient images at 3 T. For the test split, the performance was evaluated using the structural similarity index measure, Wasserstein distance, and root mean squared error. For all other test data, the features Homogeneity and Energy derived from the gray level co‐occurrence matrix (GLCM) were used to quantify the improvement. For each test dataset, the proposed method was compared with the current gold standard: the N4 algorithm. Additionally, a questionnaire was filled out by two clinical experts to assess the homogeneity and contrast preservation of the 7 T datasets. All four proposed neural networks were able to substantially reduce the B1‐field induced inhomogeneities in T2w 7 T prostate images. By visual inspection, the images clearly look more homogeneous, which is confirmed by the increase in Homogeneity and Energy in the GLCM, and the questionnaire scores from two clinical experts. Occasionally, changes in contrast within the prostate were observed, although much less for the t‐Biasf network than for the t‐Image network. Further, results on the 3 T dataset demonstrate that the proposed learning based approach is on par with the N4 algorithm. The results demonstrate that the trained networks were capable of reducing the B1‐field induced inhomogeneities for prostate imaging at 7 T. The quantitative evaluation showed that all proposed learning based correction techniques outperformed the N4 algorithm. Of the investigated methods, the single‐channel t‐Biasf neural network proves most reliable for bias field correction. [ABSTRACT FROM AUTHOR]

Background: Novel motion-compensated treatment techniques on the MR-linac can address adverse intra-fraction motion effects. These techniques involve beam gating or intra-fraction adaptations of the treatment plan based on real-time magnetic resonance imaging (MRI) performed during treatment. For quality assurance (QA) of these workflows, a multi-purpose motion platform is desirable. This platform should accommodate various phantoms, enabling multiple QA workflows., Purpose: This study aims to evaluate the new IBA QUASAR Motion MR Platform for use in the 1.5 T MR-linac., Methods: The motion platform was assessed for several magnetic resonance (MR) characteristics, including spurious noise generation and B0&B1 homogeneity. In addition, the motion platform's motion accuracy and beam attenuation were assessed. An application was shown with a ScandiDos Delta4 Phantom+ MR demonstrating patient-specific plan QA of gated treatments using time-resolved dosimetry that includes motion based on a patient's respiratory motion trace., Results: All MR characterization measurements were within the set tolerances for MRI QA. The motion platform motion accuracy showed excellent agreement with the reference, with a standard deviation of the amplitude of 0.01  mm (20 kg load) for the motor's self-estimated positions and 0.22 mm (no load) for the images acquired with the electronic portal imager. Beam attenuation was found to be 11.8%. The combination of the motion platform and Delta4 demonstrated motion-included dosimetry at high temporal and spatial resolutions. Motion influenced the measured dose in non-gated treatments by up to -20.1%, while gated deliveries showed differences of up to -1.7% for selected diodes., Conclusion: The motion platform was found to be usable in a 1.5 T magnetic field, and for all MR characterization experiments, no influence from the motion platform was observed. This motion platform enables to perform motion-included QA, with a measurement device of choice., (© 2025 The Author(s). Medical Physics published by Wiley Periodicals LLC on behalf of American Association of Physicists in Medicine.)

Background and Purpose: Patients with cardiac implantable electronic devices (CIED patients) are often ineligible for online magnetic resonance-guided radiotherapy (MRgRT), most likely due to the absence of established guidelines. Existing radiotherapy (RT) and magnetic resonance imaging (MRI) guidelines offer an opportunity to construct MRgRT protocols, promoting equitable access. Our objective was to present such a workflow, share multi-institutional experiences treating CIED patients with MRgRT on a 1.5 T magnetic resonance-linear accelerator (MR-linac), and investigate geometric accuracy and electrocardiogram (ECG) monitoring for thoracic treatment., Materials and Methods: A risk analysis identified strategies for safe MRgRT for CIED patients. At three institutions, 21 pelvic and abdominal patients were treated. Patient records were analyzed for adverse events. Geometric accuracy was investigated using B 0 -mapping with a phantom simulating moving lung and cardiac lesions near a CIED. Volunteer measurements evaluated the effects of patient positioning and MRI sequences on ECG signal distortion., Results: MRI and RT workflows were adaptable to MRgRT. No adverse events were recorded. B 0 -maps showed a maximum mean difference between static and dynamic phantom configurations of 0.1 mm, increasing to 0.4 mm distortion in the presence of a CIED. ECG readings exhibited severe distortions during scanning, hampering heart rhythm detection for most MRI sequences., Conclusions: CIED patients can safely undergo treatment on a 1.5 T MR-linac following RT and MRI guidelines. For targets near CIEDs, a B 0 -mapping procedure was considered accurate enough to determine MRgRT eligibility. Pulse oximetry is recommended for cardiac monitoring during MRI scanning due to ECG signal distortion., Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (© 2024 The Author(s).)

Objective. A robotic needle implant device for MR-guided high-dose-rate (HDR) prostate brachytherapy was developed. This study aimed to assess the feasibility and spatial accuracy of HDR brachytherapy using the robotic device, for a single intraprostatic target point. Approach. Five patients were treated from November 2019-June 2022 with the robot. The robot fits a 1.5 T MR scanner and the needle can be shifted and angulated. An intraprocedural MR scan was fused with the diagnostic MR and one preplanned needle position was selected for robotic insertion. The needle entry point and angles were set for a needle tip target point within the intraprostatic target volume. The needle was tapped stepwise towards the target point pneumatically. Final needle position was verified with MR, followed by plan optimization and dose delivery. Any remaining planned needles were inserted manually. Needle tip to geometrical target error (NTG-error) was defined as the deviation of the actual tip position relative to the predefined geometric target point, using MR-coordinates. Needle tip to treatment target error (NTT-error) was defined as the deviation of the actual tip position relative to the treatment target point, using fused MR-images pre- and post-needle implantation taking into account prostate deformation. Difference between NTT-error and NTG-error and fiducial marker shifts indicated prostate movement. For determining prostate deformation, the Jaccard index and prostate volumes were assessed. Main results. The robotic device was able to tap the needle to the planned depth for all patients. Mean robotic procedure duration was 142 min. NTG-error was 3.2 (range 1.1-6.7) mm and NTT-error 4.5 (range 2.6-9.6) mm. Marker displacements were smaller than 3 mm. No treatment-related acute toxicity was reported. Feasibility of needle placement within the prostate was considered adequate. Significance. MR-guided robotic needle insertion is feasible with a mean geometric accuracy of 3.2 mm and <3 mm prostate movement., (Creative Commons Attribution license.)

This work investigates the use of a multi-2D cine magnetic resonance imaging-based comprehensive motion monitoring (CMM) system for the assessment of prostate intrafraction 3D drifts. The data of six healthy volunteers were analyzed and the values of a clinically-relevant registration quality factor metric exported by CMM were presented. Additionally, the CMM-derived prostate motion was compared to a 3D-based reference and the 2D-3D tracking agreement was reported. Due to the low quality of SI motion tracking (often > 2 mm tracking mismatch between anatomical planes) we conclude that further improvements are desirable prior to clinical introduction of CMM for prostate drift corrections., Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (© 2024 The Author(s).)

This work reports on the first seven patients treated with gating and baseline drift correction on the high-field MR-Linac system. Dosimetric analysis showed that the active motion management system improved congruence to the planned dose, efficiently mitigating detrimental effects of intrafraction motion in the upper abdomen., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Author(s). Published by Elsevier B.V. All rights reserved.)

Background and Purpose: The apparent diffusion coefficient (ADC), a potential imaging biomarker for radiotherapy response, needs to be reproducible before translation into clinical use. The aim of this study was to evaluate the multi-centre delineation- and calculation-related ADC variation and give recommendations to minimize it., Materials and Methods: Nine centres received identical diffusion-weighted and anatomical magnetic resonance images of different cancerous tumours (adrenal gland, pelvic oligo metastasis, pancreas, and prostate). All centres delineated the gross tumour volume (GTV), clinical target volume (CTV), and viable tumour volume (VTV), and calculated ADCs using both their local calculation methods and each of the following calculation conditions: b-values 0-500 vs. 150-500 s/mm 2 , region-of-interest (ROI)-based vs. voxel-based calculation, and mean vs. median. ADC variation was assessed using the mean coefficient of variation across delineations (CV D ) and calculation methods (CV C ). Absolute ADC differences between calculation conditions were evaluated using Friedman's test. Recommendations for ADC calculation were formulated based on observations and discussions within the Elekta MRI-linac consortium image analysis working group., Results: The median (range) CV D and CV C were 0.06 (0.02-0.32) and 0.17 (0.08-0.26), respectively. The ADC estimates differed 18% between b-value sets and 4% between ROI/voxel-based calculation (p-values < 0.01). No significant difference was observed between mean and median (p = 0.64). Aligning calculation conditions between centres reduced CV C to 0.04 (0.01-0.16). CV D was comparable between ROI types., Conclusion: Overall, calculation methods had a larger impact on ADC reproducibility compared to delineation. Based on the results, significant sources of variation were identified, which should be considered when initiating new studies, in particular multi-centre investigations., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Author(s). Published by Elsevier B.V. All rights reserved.)

Intrafraction motion during magnetic resonance (MR)-guided dose delivery of esophageal cancer tumors was retrospectively analyzed. Deformable image registration of cine-MR series resulted in gross tumor volume motion profiles in all directions, which were subsequently filtered to isolate respiratory and drift motion. A large variability in intrafraction motion patterns was observed between patients. Median 95% peak-to-peak motion was 7.7 (3.7 - 18.3) mm, 2.1 (0.7 - 5.7) mm and 2.4 (0.5 - 5.6) mm in cranio-caudal, left-right and anterior-posterior directions, relatively. Furthermore, intrafraction drift was generally modest (<5mm). A patient specific approach could lead to very small margins (<3mm) for most patients., Competing Interests: The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: The Radiation Oncology Department of the University Medical Center Utrecht receives financial and technical support under research agreements as well sponsoring for travels and scientific symposia from: Elekta AB (Stockholm, Sweden), Philips NV (Best, The Netherlands)., (© 2023 The Authors.)