ESTRO 2023 - Abstract Book

S560

Sunday 14 May 2023

ESTRO 2023

Figure 1: Two examples of a 1.5T (left-top) and 0.35T (left-bottom) MRI scans and an impression of four contours on a 1.5T image in the sagittal plane (right). Each color represents the delineation of one RTO.

Figure 2: Intra-modality results showing a higher consistency between specialists' contours for the 0.35T images

PD-0668 Phantom measurements of apparent diffusion coefficient on a 0.35T MR-Linac P. Wallimann 1 , B. Pouymayou 1,2 , M. Mayinger 1 , S. Nowakowska 3 , A. Boss 3 , M. Guckenberger 1 , S. Tanadini-Lang 1 , N. Andratschke 1 1 University Hospital Zürich, Department of Radiation Oncology, Zürich, Switzerland; 2 University Hospital Zürich, Department of Neuroradiology, Zürich, Switzerland; 3 University Hospital Zürich, Department of Radiology, Zürich, Switzerland Purpose or Objective Using diffusion-weighted MR imaging (DWI) on an MR-Linac could enable new ways of early response monitoring or adapting treatment during radiotherapy. We investigated the feasibility of DWI on the MRIdian 0.35T MR-Linac (ViewRay) by analyzing the impact of different receiver coils and imaging settings on apparent diffusion coefficient (ADC) values in a phantom. Materials and Methods We used a spin echo, single shot echo planar imaging DWI sequence with the settings: TR=3200ms, TE=120ms, flip angle=90°, receiver bandwidth=1352Hz/Px, voxel size 3x3x6mm^3, 20 slices without gap, linac gantry angle 330°, three orthogonal diffusion directions and five b values (0, 200, 300, 500, 800 s/mm^2 ). Furthermore, we varied the number of signal averages (NSA) (6 or 24 per b image), the coil used (head & neck coil (HNC) or prototype head coil (PHC)) and the application of prescan normalization (PN). For each coil, a noise shot (RF transmit turned off) was acquired with NSA 6 and with PN. We calculated the ADC values with an in-house python script using a voxel-wise mono-exponential fit. The different diffusion directions were combined using the geometric average of the intensities. We analyzed three different variations of noise correction: No correction for noise, a correction for uniform Rician background noise (Dietrich et al., 2001) and a correction of the same form but using the non-uniform noise shot. We investigated a custom-built phantom (HQ imaging) containing four vials with calibrated ADC values (400, 1000, 1600 and 2020 µm^2/s), in each of which a homogeneous region of interest (ROI) was manually selected. The determined ADC values were corrected per phantom manual from the measured temperature of 20.5°C to the calibration temperature of 20°C. Results The images with NSA 6, with the HNC and using PN showed a strong inhomogeneity of noise across the image, which remained roughly constant for different b values. The same pattern could also be observed on the noise shot, but to a lesser degree on images with the PHC and not at all without PN (Figure 1).