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Cardiorespiratory motion induces position uncertainties resulting in decreased dose conformality. Electocardiograms (ECG) are typically used during cardiac MRI (CMR) to acquire images in a predefined cardiac phase, thus mitigating cardiac motion during image acquisition.<\/jats:p><\/jats:sec><jats:sec><jats:title>Purpose<\/jats:title><jats:p>We demonstrate real\u2010time cardiac physiology\u2010based radiotherapy beam gating within a preset cardiac phase on an\u00a0MR\u2010linac.<\/jats:p><\/jats:sec><jats:sec><jats:title>Methods<\/jats:title><jats:p>MR images were acquired in healthy volunteers (<jats:italic>n<\/jats:italic> = 5, mean age = 29.6 years, mean heart\u2010rate (HR) = 56.2 bpm) on the 1.5 T Unity MR\u2010linac (Elekta AB, Stockholm, Sweden) after obtaining written informed consent. The images were acquired using a single\u2010slice balance steady\u2010state free precession (bSSFP) sequence in the coronal or sagittal plane (TR\/TE = 3\/1.48 ms, flip angle = 48, SENSE = 1.5,  , voxel size =  , partial Fourier factor = 0.65, frame rate = 13.3 Hz). In parallel, a 4\u2010lead ECG\u2010signal was acquired using MR\u2010compatible equipment. The feasibility of ECG\u2010based beam gating was demonstrated with a prototype gating workflow using a Quasar MRI<jats:sup>4D<\/jats:sup> motion phantom (IBA Quasar, London, ON, Canada), which was deployed in the bore of the MR\u2010linac. Two volunteer\u2010derived combined ECG\u2010motion traces (<jats:italic>n<\/jats:italic> = 2, mean age = 26 years, mean HR = 57.4 bpm, peak\u2010to\u2010peak amplitude = 14.7 mm) were programmed into the phantom to mimic dose delivery on a cardiac target in breath\u2010hold. Clinical ECG\u2010equipment was connected to the phantom for ECG\u2010voltage\u2010streaming in real\u2010time using research software. Treatment beam gating was performed in the quiescent phase (end\u2010diastole). System latencies were compensated by delay time correction. A previously developed MRI\u2010based gating workflow was used as a benchmark in this study. A 15\u2010beam intensity\u2010modulated radiotherapy (IMRT) plan ( Gy) was delivered for different motion scenarios onto radiochromic films. Next, cardiac motion was then estimated at the basal anterolateral myocardial wall via normalized cross\u2010correlation\u2010based template matching. The estimated motion signal was temporally aligned with the ECG\u2010signal, which were then used for position\u2010 and ECG\u2010based gating simulations in the cranial\u2013caudal (CC), anterior\u2013posterior (AP), and right\u2013left (RL) directions. The effect of gating was investigated by analyzing the differences in residual motion at 30, 50, and 70% treatment beam duty cycles.<\/jats:p><\/jats:sec><jats:sec><jats:title>Results<\/jats:title><jats:p>ECG\u2010based (MRI\u2010based) beam gating was performed with effective duty cycles of 60.5% (68.8%) and 47.7% (50.4%) with residual motion reductions of 62.5% (44.7%) and 43.9% (59.3%). Local gamma analyses (1%\/1 mm) returned pass rates of 97.6% (94.1%) and 90.5% (98.3%) for gated scenarios, which exceed the pass rates of 70.3% and 82.0% for nongated scenarios, respectively. In average, the gating simulations returned maximum residual motion reductions of 88%, 74%, and 81% at 30%, 50%, and 70% duty cycles, respectively, in favor of MRI\u2010based\u00a0gating.<\/jats:p><\/jats:sec><jats:sec><jats:title>Conclusions<\/jats:title><jats:p>Real\u2010time ECG\u2010based beam gating is a feasible alternative to MRI\u2010based gating, resulting in improved dose delivery in terms of high  rates, decreased dose deposition outside the PTV and residual motion reduction, while by\u2010passing cardiac MRI\u00a0challenges.<\/jats:p><\/jats:sec>","DOI":"10.1002\/mp.17024","type":"journal-article","created":{"date-parts":[[2024,3,13]],"date-time":"2024-03-13T15:11:59Z","timestamp":1710342719000},"page":"2354-2366","update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":15,"title":["Experimental demonstration of real\u2010time cardiac physiology\u2010based radiotherapy gating for improved cardiac radioablation on an MR\u2010linac"],"prefix":"10.1002","volume":"51","author":[{"given":"Osman","family":"Akdag","sequence":"first","affiliation":[{"name":"Department of Radiotherapy University Medical Center Utrecht Utrecht The Netherlands"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Pim T. 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