TU‐E‐BRC‐04: Strategies for Real‐Time MR Imaging for Integrated MRI+Linac Systems

Purpose: Integrated MRI+linac systems can potentially yield complete spatio‐temporal knowledge of the irradiated anatomy during beam‐on — representing the ideal guidance strategy for 4D radiotherapy delivery. In this work, we investigate rapid imaging strategies for such devices to enable real‐time, MR‐guided, motion‐adaptive radiation delivery. Method and Materials: SNR in MR images is described by: SNR μ B0×(Dx×Dy×Dz)× Tacq, where 0 B is the primary field, Dx, Dy and Dz are voxel dimensions and acq T is the acquisition time. We investigated trade‐offs between SNR and two important design and operational parameters for MRI+linac systems — (i) 0 B, which impacts design complexity, and (ii) acq T, which impacts the spatio‐temporal accuracy of real‐time guidance. In the first study, SSFP and SPGR sequences were employed to acquire 3D volumes (1.2 s/volume) and 2D coronal slices (0.3 s/slice) of the thoracic region from five human subjects (1.5T scanner). To simulate lower fieldstrength, image SNR was progressively degraded by adding increasing levels of gaussian noise. A fat deposit on the diaphragm was segmented in the noise‐free and the degraded images and the error in the estimated position was computed. In the second study, faster acquisition through partial k‐space scanning was simulated. A cylindrical water‐filled phantom containing seven oilfilled cylinders was imaged and a progressively increasing number of mid‐frequency phase encode lines were zeroed prior to reconstruction. The centers of oil‐filled cylinders were auromatically segmented in the full and partial k‐space acquisitions, and the positional error was computed with respect to the full k‐space image. Results: Positional errors of the anatomic feature were within 1.5 mm for a factor‐of‐6 SNR degradation, corresponding to B0 = 0.25T. Partial k‐space acquisition could be performed to increase acquisition speed by over a factor of 5, while maintaining sub‐1 mm accuracy. Conclusion: These initial studies indicate the feasibility of low‐field, real‐time MRI for intrafraction motion management using integrated MRI+linac systems.