ESTRO 2022 - Abstract Book

S789

Abstract book

ESTRO 2022

sequences were tested prospectively on a subset of four patients at the end of a treatment fraction. To compare the contrast between the clinically used and optimized 4D-MRI sequences, the absolute ratio between them was calculated (i.e. |C opt / C clin |). Results The measured T1 and T2 values are shown in Fig. 1 for all metastases. The median (range) T1 value of the liver and lesions across all patients were 659 (450-892) and 1105 (578-2466) ms, respectively. T2 values of 58 (40-82) ms for liver and 85 (46-242) ms for metastases were found.

The absolute contrast ratio, and thus lesion visibility, improved for all patients. A minimum and maximum absolute contrast ratio of 1.1 and 4.4 for T2 TSE and 1.02 and 5.9 balanced GRE were found. Fig. 2 shows the lesion visibility improvement of the optimized compared to the clinically used 4D-MR images for the prospective test. Note brighter rim around the lesion in patient 2 and heterogeneous MR signal intensity within the lesion in patient 3 and 4. This makes delineation and contrast estimation more difficult.