ESTRO 2021 Abstract Book

S518

ESTRO 2021

wall. Additionally, the frequency spectrum of the MR-derived navigation-position was computed to verify the presence of both cardiac and respiratory motion components. This was compared to the cardiac and respiratory frequency recorded using MR-compatible physiology monitoring (ECG and respiratory bellow) as independent reference. Results Spectral analysis showed that both respiratory and cardiac frequency components were present in the LV navigator, while only the respiratory motion was sensed by the liver navigator (figure 1). Non-navigated scans had an average motion (20-80% amplitude CC) of the liver dome and LV wall of 8.8 and 6.5 mm, liver navigated scans 1.7 and 7.6 mm and LV navigated scans 1.1 and 2.8 mm, respectively (see figure 2).

Conclusion We show that a 1D MRI navigator on the heart can detect simultaneously respiratory and cardiac motion and can be used to reduce both cardiac and respiratory motion during MRI scans. As these navigators are readily available, we envision that this principle can be used to improve pre-beam imaging as well as MR-linac gated treatment strategies in the thorax without the use of external hardware. Using the LV navigator could ensure direct coupling of the image and treatment geometry. PH-0650 Real-time multi-resolution image reconstruction for MR-guided prostate radiotherapy P. Borman 1 , T. Bruijnen 2 , T. Schakel 1 , B. Raaymakers 1 1 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands; 2 UMC Utrecht, Radiotherapy / Computational Imaging Group, Utrecht, The Netherlands Purpose or Objective Intra-fractional tumor motion is a major source of uncertainty in prostate radiotherapy. Using hybrid MR- Linac systems, 3D CINE MR images can be acquired during radiation treatment, enabling real-time motion monitoring and dose accumulation. Typically, a trade-off must be made between frame rate and resolution, rendering it unfeasible to do both simultaneously. We present a novel imaging sequence, dual- density CArtesian SPiRal (dd-CASPR), allowing for image reconstructions at multiple time and spatial resolutions. By coupling this with an in-house developed online reconstruction pipe-line (ReconSocket), we demonstrate the feasibility of three parallel image streams for real-time (i) bulk motion detection (3Hz), (ii) motion quantification (0.8Hz), and (iii) high-resolution images (0.15Hz) of the prostate. Materials and Methods