In Vivo and Phantom Repeatability of Diffusion-Weighted MRI Sequences on 1.5T MRI-Linear Accelerator (MR-Linac) and MR Simulator Devices for Head and Neck Cancers: Results from a Prospective R-IDEAL Stage 2a Evaluation of Tumor and Normal Tissue Apparent Diffusion Coefficients as Quantitative Imaging Biomarkers

Introduction Diffusion-weighted imaging (DWI) on MRI-linear accelerator (MR-linac) systems can potentially be used for monitoring treatment response and adaptive radiotherapy in head and neck cancers (HNC) but requires extensive validation. We perform technical validation to compare six total DWI sequences on an MR-linac and MR simulator (MR sim) in patients, volunteers, and phantoms. Methods Ten human papillomavirus-positive oropharyngeal cancer patients and ten volunteers underwent DWI on a 1.5T MR-linac with three DWI sequences: echo planar imaging (EPI), split acquisition of fast spin echo signals (SPLICE), and turbo spin echo (TSE). Volunteers were also imaged on a 1.5T MR sim with three sequences: EPI, BLADE, and RESOLVE. Participants underwent two scan sessions per device and two repeats of each sequence per session. Repeatability and reproducibility within-subject coefficient of variation (wCV) of mean ADC were calculated for tumors and lymph nodes (patients) and parotid glands (volunteers). Differences in measured ADC values between sequences were quantified using Bland-Altman analysis. ADC bias, repeatability/reproducibility metrics, and SNR were quantified using a phantom. Results In vivo repeatability/reproducibility wCV of mean ADC for parotids were 5.41%/6.72%, 3.83%/8.80%, 5.66%/10.03%, 3.44%/5.70%, 5.04%/5.66%, 4.23%/7.36% for EPI MR-linac , SPLICE, TSE, EPI MR sim , BLADE, RESOLVE. Repeatability/reproducibility wCV for EPI MR-linac , SPLICE, TSE were 9.64%/10.28%, 7.84%/8.96%, 7.60%/11.68% for tumors and 7.80%/9.95%, 7.23%/8.48%, 10.82%/10.44% for nodes. Bland-Altman analysis revealed significant differences between all sequence pairs except BLADE-EPI MR-linac and RESOLVE-SPLICE. All sequences except TSE had phantom ADC biases within ±0.1×10 −3 mm 2 /s for most vials. MR-linac sequences had inconsistent ADC values between different vials with the same known ADC value, indicating spatial inhomogeneities. SNR of b=0 images was 87.3, 180.5, 161.3, 171.0, 171.9, 130.2 for EPI MR-linac , SPLICE, TSE, EPI MR sim , BLADE, RESOLVE. Conclusion MR-linac DWI sequences demonstrate near-comparable performance to MR sim sequences and warrant further clinical validation for treatment response assessment in HNC.