Validation of daily 0.35 T diffusion‐weighted MRI for MRI‐guided glioblastoma radiotherapy

Abstract Background MRI‐Linac systems enable daily diffusion‐weighed imaging (DWI) MRI scans for assessing glioblastoma tumor changes with radiotherapy treatment. Purpose Our study assessed the image quality of echoplanar imaging (EPI)‐DWI scans compared with turbo spin echo (TSE)‐DWI scans at 0.35 Tesla (T) and compared the apparent diffusion coefficient (ADC) values and distortion of EPI‐DWI on 0.35 T MRI‐Linac compared to high‐field diagnostic MRI scanners. Methods The calibrated National Institute of Standards and Technology (NIST)/Quantitative Imaging Biomarkers Alliance (QIBA) Diffusion Phantom was scanned on a 0.35 T MRI‐Linac, and 1.5 T and 3 T MRI with EPI‐DWI. Five patients were scanned on a 0.35 T MRI‐Linac with a TSE‐DWI sequence, and five other patients were scanned with EPI‐DWI on a 0.35 T MRI‐Linac and a 3 T MRI. The quality of images was compared between the TSE‐DWI and EPI‐DWI on the 0.35 T MRI‐Linac assessing signal‐to‐noise ratios and presence of artifacts. EPI‐DWI ADC values and distortion magnitude were measured and compared between 0.35 T MRI‐Linac and high‐field MRI for both phantom and patient studies. Results The average ADC differences between EPI‐DWI acquired on the 0.35 T MRI‐Linac, 1.5 T and 3 T MRI scanners and published references in the phantom study were 1.7%, 0.4% and 1.0%, respectively. Comparing the ADC values based on EPI‐DWI in glioblastoma tumors, there was a 3.36% difference between 0.35 and 3 T measurements. Susceptibility‐induced distortions in the EPI‐DWI phantoms were 0.46 ± 1.51 mm for 0.35 MRI‐Linac, 0.98 ± 0.51 mm for 1.5 T MRI and 1.14 ± 1.88 mm for 3 T MRI; for patients ‐0.47 ± 0.78 mm for 0.35 T and 1.73 ± 2.11 mm for 3 T MRIs. The mean deformable registration distortion for a phantom was 1.1 ± 0.22 mm, 3.5 ± 0.39 mm and 4.7 ± 0.37 mm for the 0.35 T MRI‐Linac, 1.5 T MRI, and 3 T MRI scanners, respectively; for patients this distortion was –0.46 ± 0.57 mm for 0.35 T and 4.2 ± 0.41 mm for 3 T. EPI‐DWI 0.35 T MRI‐Linac images showed higher SNR and lack of artifacts compared with TSE‐DWI, especially at higher b ‐values up to 1000 s/mm 2 . Conclusion EPI‐DWI on a 0.35 T MRI‐Linac showed superior image quality compared with TSE‐DWI, minor and less distortions than high‐field diagnostic scanners, and comparable ADC values in phantoms and glioblastoma tumors. EPI‐DWI should be investigated on the 0.35 T MRI‐Linac for prediction of early response in patients with glioblastoma.