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.

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.

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