Free-Breathing Ungated Cine Cardiac MRI in Seconds via a Cross-Slice Transformer

Cine cardiac MRI is the clinical gold standard for measuring heart function, but a full exam typically takes an hour or more, since it depends on repeated breath-holds and electrocardiogram (ECG) gating that are difficult or unreliable for patients with dyspnea or arrhythmia. Real-time MRI captures each 2D frame within a fraction of a heartbeat, needing neither, but slices are acquired sequentially, each capturing the heart at a different respiratory position and cardiac state. Reassembling them into a coherent 3D volume, known as slice-to-volume reconstruction (SVR), still relies on breath-holds, ECG, or self-gating in existing methods, which also take minutes per subject. We present $\text{CardioStitch}$, to our knowledge the first cardiac SVR method that is simultaneously $\text{free-breathing}$, $\text{gating- and synchronization-free}$, and $\text{ultra-fast}$. Rather than synchronizing slices, we $\text{condition}$ on a reference slice that defines the target cardiac state: a cross-slice transformer reasons jointly over the entire stack and, in a single forward pass, predicts dense 2D-to-3D deformations that correct respiratory and cardiac motion together, and varying the reference yields a 3D cine. Trained on 628 subjects from five public datasets, CardioStitch reconstructs a full cine in 2 s, over $35\times$ faster than optimization-based methods. It outperforms baselines given the full real-time series of every slice plus ECG or self-gating on all image metrics, cutting respiratory error from 4.2 to 0.9 mm, and on all functional metrics under simulated atrial fibrillation. CardioStitch also transfers without retraining to real free-breathing, ungated scans, including from a patient with atrial fibrillation.