ESTRO 2021 Abstract Book

S519

ESTRO 2021

Experiments were performed on a 1.5T Unity MR-linac (Elekta AB, SE) using a 4DMRI motion phantom (ModusQA, CA) and on two healthy volunteers (IRB approved). T1w and bSSFP (T2/T1w) GRE scans were performed using the dd-CASPR sampling pattern (FOV 275x450x110 mm 3 ). k- space profiles were streamed continuously using ReconSocket to a reconstruction server running open-source reconstruction software (BART), generating three different image streams: (i) 1D cranial- caudal (CC) projection navigator at 3Hz, (ii) 3D 10mm 3 at 0.5Hz, and (iii) 3D 2mm 3 at 0.15Hz. ReconSocket’s streaming latency and jitter was estimated by comparing the 1D navigator stream with the zero-delay phantom output. For the two (ii and iii) 3D streams, the resolved CC motion was estimated using rigid registration on the prostate. Volunteer 1 was instructed to move at the end of the scan to simulate bulk motion. Motion traces from the three image streams were compared to each other. Results The streaming latency of the k-space profiles via ReconSocket to the external reconstruction server was 7ms with a maximum jitter of 10ms, while the reconstruction times of (i-iii) were 25ms, 0.8s, and 7s, respectively. Fig. 1 shows that prostate motion can be tracked on the low-res images (green), but that fast movements are lost on the high-res images (orange). Bulk motion can be clearly detected on the 3Hz projection navigator.

Fig. 1: Simultaneously reconstructed 1D navigator (blue), 3D low-resolution (green), and 3D high-resolution (orange) image streams, and estimated motion traces of the prostate for the bSSFP (top, volunteer 1) and T1w (bottom, volunteer 2) scans. The 1D navigator shows the bulk motion of volunteer 1 around t=225. Conclusion The proposed 3D dd-CASPR MRI sequence enables real-time reconstructions of a single MRI acquisition at multiple different spatio-temporal resolutions, making it a suitable one-fits-all candidate for online bulk motion detection, target motion monitoring, and dose accumulation.

PH-0651 MRI-based tumour localisation after clip placement for proton beam therapy of uveal melanoma J. Beenakker 1,2,3 , M. Jaarsma-Coes 2,1 , T. Ferreira 2 , M. Marinkovic 1 , M. Rodrigues 3,4 , Y. Klaver 3,4 , K. Vu 1 , B. Verbist 2 , G. Luyten 1 , C. Rasch 3 1 Leiden University Medical Center, Ophthalmology, Leiden, The Netherlands; 2 Leiden University Medical Center, Radiology, Leiden, The Netherlands; 3 Leiden University Medical Center, Radiotherapy, Leiden, The Netherlands; 4 HollandPTC, Radiotherapy, Delft, The Netherlands Purpose or Objective Uveal melanoma (UM) is the most common primary ocular malignancy. Proton beam radiation therapy (PBT) is the preferred eye sparing treatment for large sized UM or juxtapapillary UM. Currently, 2.5mm tantalum clips