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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]

Purpose: Develop a true real‐time implementation of MR signature matching (MRSIGMA) for free‐breathing 3D MRI with sub‐200 ms latency on the Elekta Unity 1.5T MR‐Linac. Methods: MRSIGMA was implemented on an external computer with a network connection to the MR‐Linac. Stack‐of‐stars with partial kz sampling was used to accelerate data acquisition and ReconSocket was employed for simultaneous data transmission. Movienet network computed the 4D MRI motion dictionary and correlation analysis was used for signature matching. A programmable 4D MRI phantom was utilized to evaluate MRSIGMA with respect to a ground‐truth translational motion reference. In vivo validation was performed on patients with pancreatic cancer, where 15 patients were employed to train Movienet and 7 patients to test the real‐time implementation of MRSIGMA. Dice coefficients between real‐time MRSIGMA and a retrospectively computed 4D reference were used to evaluate motion tracking performance. Results: Motion dictionary was computed in under 5 s. Signature acquisition and matching presented 173 ms latency on the phantom and 193 ms on patients. MRSIGMA presented a mean error of 1.3–1.6 mm for all phantom experiments, which was below the 2 mm acquisition resolution along the motion direction. The Dice coefficient over time between MRSIGMA and reference contours was 0.88 ± 0.02 (GTV), 0.87 ± 0.02(duodenum‐stomach), and 0.78 ± 0.02(small bowel), demonstrating high motion tracking performance for both tumor and organs at risk. Conclusion: The real‐time implementation of MRSIGMA enabled true real‐time free‐breathing 3D MRI with sub‐200 ms imaging latency on a clinical MR‐Linac system, which can be used for treatment monitoring, adaptive radiotherapy and dose accumulation mapping in tumors affected by respiratory motion. [ABSTRACT FROM AUTHOR]

Background: Several (online) adaptive radiotherapy procedures are available to maximize healthy tissue sparing in the presence of inter/intrafractional motion during stereotactic body radiotherapy (SBRT) on an MR‐linac. The increased treatment complexity and the motion‐delivery interplay during these treatments require MR‐compatible motion phantoms with time‐resolved dosimeters to validate end‐to‐end workflows. This is not possible with currently available phantoms. Purpose: Here, we demonstrate a new commercial hybrid film‐scintillator cassette, combining high spatial resolution radiochromic film with four time‐resolved plastic scintillator dosimeters (PSDs) in an MRI‐compatible motion phantom. Methods: First, the PSD's performance for consistency, dose linearity, and pulse repetition frequency (PRF) dependence was evaluated using an RW3 solid water slab phantom. We then demonstrated the MRI4D scintillator cassette's suitability for time‐resolved and motion‐included quality assurance for adapt‐to‐shape (ATS), trailing, gating, and multileaf collimator (MLC) tracking adaptations on a 1.5 T MR‐linac. To do this, the cassette was inserted into the Quasar MRI4D phantom, which we used statically or programmed with artificial and patient‐derived motion. Simultaneously with dose measurements, the beam‐gating latency was estimated from the time difference between the target entering/leaving the gating window and the beam‐on/off times derived from the time‐resolved dose measurements. Results: Experiments revealed excellent detector consistency (standard deviation ≤$\le$ 0.6%), dose linearity (R2 = 1), and only very low PRF dependence (≤$\le$0.4%). The dosimetry cassette demonstrated a near‐perfect agreement during an ATS workflow between the time‐resolved PSD and treatment planning system (TPS) dose (0%–2%). The high spatial resolution film measurements confirmed this with a 1%/1‐mm local gamma pass‐rate of 90%. When trailing patient‐derived prostate motion for a prostate SBRT delivery, the time‐resolved cassette measurements demonstrated how trailing mitigated the motion‐induced dose reductions from 1%–17% to 1%–2% compared to TPS dose. The cassette's simultaneously measured spatial dose distribution highlighted the dosimetric gain of trailing by improving the 3%/3‐mm local gamma pass‐rates from 80% to 97% compared to the static dose. Similarly, the cassette demonstrated the benefit of real‐time adaptations when compensating patient‐derived respiratory motion by showing how the TPS dose was restored from 2%–56% to 0%–12% (gating) and 1%–26% to 1%–7% (MLC tracking) differences. Larger differences are explainable by TPS‐PSD coregistration uncertainty combined with a steep dose gradient outside the PTV. The cassette also demonstrated how the spatial dose distributions were drastically improved by the real‐time adaptations with 1%/1‐mm local gamma pass‐rates that were increased from 8 to 79% (gating) and from 35 to 89% (MLC tracking). The cassette‐determined beam‐gating latency agreed within ≤$\le$12 ms with the ground truth latency measurement. Film and PSD dose agreed well for most cases (differences relative to TPS dose <$<$4%), while film‐PSD coregistration uncertainty caused relative differences of 5%–8%. Conclusions: This study demonstrates the excellent suitability of a new commercial hybrid film‐scintillator cassette for simultaneous spatial, temporal, and motion‐included dosimetry. [ABSTRACT FROM AUTHOR]

Purpose: Respiratory‐correlated 4D‐MRI may provide motion characteristics for radiotherapy but is susceptible to irregular breathing. This study investigated the effectiveness of visual biofeedback (VBF) guidance for breathing regularization during 4D‐MRI acquisitions on an MR‐linac. Methods: A simultaneous multislice‐accelerated 4D‐MRI sequence was interleaved with a one‐dimensional respiratory navigator (1D‐RNAV) in 10 healthy volunteers on a 1.5T Unity MR‐linac (Elekta AB, Stockholm, Sweden). Volunteer‐specific breathing amplitudes and periods were derived from the 1D‐RNAV signal obtained during unguided 4D‐MRI acquisitions. These were used for the guidance waveform, while the 1D‐RNAV positions were overlayed as VBF. VBF effectiveness was quantified by calculating the change in coefficient of variation (CVdiff$$ {\mathrm{CV}}^{\mathrm{diff}} $$) for the breathing amplitude and period, the position SD of end‐exhale, end‐inhale and midposition locations, and the agreement between the 1D‐RNAV signals and guidance waveforms. The 4D‐MRI quality was assessed by quantifying amounts of missing data. Results: VBF had an average latency of 520 ± 2 ms. VBF reduced median breathing variations by 18% to 35% (amplitude) and 29% to 57% (period). Median position SD reductions ranged from −3% to 35% (end‐exhale), 29% to 38% (end‐inhale), and 25% to 37% (midposition). Average differences between guidance waveforms and 1D‐RNAV signals were 0.0 s (period) and +1.7 mm (amplitude). VBF also decreased the median amount of missing data by 11% and 29%. Conclusion: A VBF system was successfully implemented, and all volunteers were able to adapt to the guidance waveform. VBF during 4D‐MRI acquisitions drastically reduced breathing variability but had limited effect on missing data in respiratory‐correlated 4D‐MRI. [ABSTRACT FROM AUTHOR]

Purpose: To evaluate the feasibility of spatio‐temporal encoding (SPEN) readout for pseudo‐continuous ASL (pCASL) in brain, and its robustness to susceptibility artifacts as introduced by aneurysm clips. Methods: A 2D self‐refocused T2*‐compensated hybrid SPEN scheme, with super‐resolution reconstruction was implemented on a 1.5T Philips system. Q (=BWchirp*Tchirp) was varied and, the aneurysm clip‐induced artifact was evaluated in phantom (label‐images) as well as in vivo (perfusion‐weighted signal (PWS)‐maps and temporal SNR (tSNR)). In vivo results were compared to gradient‐echo EPI (GE‐EPI) and spin‐echo EPI (SE‐EPI). The dependence of tSNR on TR was evaluated separately for SPEN and SE‐EPI. SPEN with Q ˜ 75 encodes with the same off‐resonance robustness as EPI. Results: The clip‐induced artifact with SPEN decreased with increase in Q, and was smaller compared to SE‐EPI and GE‐EPI in vivo. tSNR decreased with Q and the tSNR of GE‐EPI and SE‐EPI corresponded to SPEN with a Q‐value of approximately ˜85 and ˜108, respectively. In addition, SPEN perfusion images showed a higher tSNR (p < 0.05) for TR = 4000 ms compared to TR = 2100 ms, while SE‐EPI did not. tSNR remained relatively stable when the time between SPEN‐excitation and start of the next labeling‐module was more than ˜1000 ms. Conclusion: Feasibility of combining SPEN with pCASL imaging was demonstrated, enabling cerebral perfusion measurements with a higher robustness to field inhomogeneity (Q > 75) compared to SE‐EPI and GE‐EPI. However, the SPEN chirp‐pulse saturates incoming blood, thereby reducing pCASL labeling efficiency of the next acquisition for short TRs. Future developments are needed to enable 3D scanning. [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.)