# Apparent diffusion coefficient as a quantitative biomarker for prostate cancer treatment response on a 1.5 Tesla magnetic resonance-linear accelerator: Impact of image registration and acquisition type

## TL;DR

This study evaluates how different MRI techniques affect the accuracy of measuring prostate cancer treatment response using a combined MRI and radiation therapy system.

## Contribution

The study introduces the impact of image registration and acquisition type on ADC repeatability in MR-Linac systems for prostate cancer monitoring.

## Key findings

- Image registration improves ADC repeatability and detection of ADC changes during treatment.
- Fast spin echo diffusion MRI reduces distortion but has lower repeatability compared to echo planar imaging.
- Region of interest size significantly affects ADC measurement repeatability.

## Abstract

•We compared diffusion sequences on a magnetic resonance-linear accelerator (MR-Linac).•Region of interest size impacts apparent diffusion coefficient (ADC) repeatability.•Image registration improves ADC repeatability on 1.5 T MR-Linac.•Registration improves detecting ADC changes during MR-guided radiotherapy.•Fast spin echo diffusion MRI reduces distortion but requires further optimization.

We compared diffusion sequences on a magnetic resonance-linear accelerator (MR-Linac).

Region of interest size impacts apparent diffusion coefficient (ADC) repeatability.

Image registration improves ADC repeatability on 1.5 T MR-Linac.

Registration improves detecting ADC changes during MR-guided radiotherapy.

Fast spin echo diffusion MRI reduces distortion but requires further optimization.

Diffusion-weighted magnetic resonance imaging (DW-MRI) is a quantitative biomarker for cancer detection and treatment monitoring. On magnetic resonance-linear accelerator (MR-Linac) systems, diffusion-weighted echo planar imaging (DW-EPI) suffers from geometric distortion, reducing the repeatability of apparent diffusion coefficient (ADC) measurements. This study evaluated the effect of low-distortion split acquisition of fast spin-echo signal (SPLICE) sequences, and of image registration on the repeatability coefficient (RC) of ADC.

ADC bias, repeatability, signal-to-noise ratio (SNR) and geometric fidelity were measured in a diffusion phantom using three DW-EPI and two DW-SPLICE protocols. ADC short-term and long-term RCs were measured in healthy volunteers. In patients, the registration of DW-EPI to unweighted images (b0) was tested for its effect on RC in gross tumour volume (GTV) and non-tumour prostate (NT-P), and for its ability to detect significant ADC changes.

Phantom experiments showed strong linear correlation with ground-truth ADC (R2 > 0.99). Among EPI protocols, DW-EPI-AP offered the best balance of high SNR and low RC, while Z-direction encoded DW-EPI was the most variable. Both DW-SPLICE variants exhibited reduced distortion compared with EPI but poorer repeatability. In volunteers, long-term RCs (8.0–33.7 %) varied more than short-term RCs (8.9–15.4 %). In patients, registration improved RCs (GTV: 28.0 → 25.1 %; NT-P: 19.6 → 12.6 %) and improved detection of significant ADC change in patients (GTV: 0/6 → 1/6; NT-P: 2/6 → 5/6).

RC and accuracy of DW-EPI agrees with published literature and improves after registration. DW-SPLICE shows lower geometric distortion but would require further optimization and validation to improve repeatability.