ESTRO 2023 - Abstract Book

S1860

Digital Posters

ESTRO 2023

Results Results: 54/60 datasets were analysed in total. Mean time difference between Day 0 and #1 scans was 19 days (range: 7- 30 days). RC/%RC are 0.23/25% for test-retest analysis with bias and limit of agreement of 0.013± 0.17. ICC for all patients is 0.77. Conclusion Conclusion: The repeatability of ADC in cervix, bladder and rectal cancer on 1.5T MR-Linac appears acceptable. Future projects involving DWI on MR-Linac can be planned using above measurements as guidelines.

PO-2080 Time-dependent diffusion MRI for non-invasive tumor characterization on a 1.5 T MR sim

M. Jokivuolle 1,2 , H. Lundell 3 , K. Madsen 3,4 , F. Mahmood 1,2

1 Odense University Hospital, Laboratory of Radiation Physics, Department of Oncology, Odense, Denmark; 2 University of Southern Denmark, Department of Clinical Research, Odense, Denmark; 3 Danish Research Centre for Magnetic Resonance, Copenhagen University Hospital Amager and Hvidovre, Centre for Functional and Diagnostic Imaging and Research, Copenhagen, Denmark; 4 Technical University of Denmark, Department of Applied Mathematics and Computer Science, Copenhagen, Denmark Purpose or Objective Non-invasive methods for characterization of tumor microstructure are required to improve radiotherapy individualization. Time-dependent diffusion MRI (TDD-MRI) has shown potential to provide quantitative estimates of tissue microstructure, such as extracellular volume, membrane permeability, cellularity and cell size. However, so far TDD-MRI has mainly been investigated on high performance pre-clinical systems. Here, we investigated the capability of a clinical 1.5 T MR sim system to perform TDD-MRI to estimate cell sizes using asparagus as a model tissue, and demonstrated in vivo feasibility in human muscle tissue. Materials and Methods Asparagus ( asparagus officinalis ) stems (N = 9) were cut to 4 cm pieces and stabilized in water. The left calf muscle of one healthy volunteer (M, 44 yrs, 81 kg) was imaged in vivo. TDD-MRI was acquired on a 1.5 T MR sim (Ingenia, Philips) with a 45 mT/m gradient system, using spin-echo EPI readout (sequence parameters in Table 1). Measurements were repeated with increasing effective diffusion time (Td) to assess the time-dependency of the signal. For validation, Monte Carlo simulations of MRI signal were performed for varying cell radii using intrinsic diffusivity of 2.1 µ m ² /ms for asparagus and 3.0 µ m ² /ms for muscle [1]. Analytical model for diffusion in cylinders [2] was fitted to the asparagus data to estimate cell radius ( R ) and diffusivity within two regions of interest (ROIs), corresponding to the central (ROI_C) and peripheral (ROI_P) area of asparagus stems. Results The measured diffusion signals from asparagus showed a clear time-dependency (fig. 1A), in agreement with the simulations (fig. 1B). The slope of the curves increased with b-value, indicating that the time-dependency is due to restricted diffusion within the cells. Signal changes up to 7.5 %-points in ROI_C and 8.9 %-points in ROI_P were observed (fig. 2A). The calf muscle showed a weaker, yet detectable effect (fig. 1C & 2A). The analytical model estimated correctly a larger cell radius in ROI_C (16.38 µ m) and a smaller one in ROI_P (14.75 µ m) with estimated diffusivity of 1.40 µ m ² /ms in both ROIs. The difference to microscopy reference (18.52 µ m) was small (~12%) in ROI_C, while for ROI_P difference to reference (7.69 µ m) was large, likely due to partial volume effects (caused by large in-plane voxels). Table 1: MRI acquisition parameters. TR/TE (ms) Matrix/resolution b-values (s/mm ² ) Gradient duration (ms) Td (ms) Scan time (mm:ss) Asparagus 3216/117 152x70x12/1x1x5 0, 500, 1000, 1500 24.0 3216/95 64x61x10/2.5x2.5x5 0, 250, 500, 750 19.5 Muscle 32.0-62.0 01:27 20.5-50.5 03:23