Quantitative magnetic resonance imaging on hybrid magnetic resonance linear accelerators: Perspective on technical and clinical validation

Abstract

Many preclinical and clinical observations support that functional magnetic resonance imaging (MRI), such as diffusion weighted (DW) and dynamic contrast enhanced (DCE) MRI, might have a predictive value for radiotherapy. The aim of this review was to assess the current status of quantitative MRI on hybrid MR-Linacs. In a literature research, four publications were identified, investigating technical feasibility, accuracy, repeatability and reproducibility of DW and DCE-MRI in phantoms and first patients. Accuracy and short term repeatability was < 5% for DW-MRI in current MR-Linac systems. Consequently, quantitative imaging providing accurate and reproducible functional information seems possible in MR-Linacs.

Keywords: Diffusion-weighted MRI; MR-Linac; MR-guided radiotherapy; Quantitative MRI.

Fig. 1

Assessment of accuracy and repeatability with respect to ADC determination based on DW MRI at the 1.5 T MR-Linac using a dedicated diffusion phantom. (A) Diffusion Phantom (Model 128, Qalibre MD, Boulder, USA) containing inserts prepared with different known diffusion coefficients. (B) Regions of interest used to analyze ADC values in the different probe areas, acquired with an EPI-based DW MRI sequence (b = 0, 200, 500 s/mm²; TE/TR = 117/6683 ms; voxel size: 1.53 × 1.53 × 4 mm³). (C,D) Bland-Altman plots showing accuracy and short term repeatability of ADC assessed in four measurements on two different days, including bias (full line) and LoA (limits of agreement, dashed line). (C) Accuracy of ADC assessment with a bias of −2.5·10-6 mm²/s and LoA ± 26.8·10-6 mm²/s. (D) Repeatability analysis presents a bias of 0.6%, LoA = ±7.6%. Median accuracy and reproducibility were −0.31% and 0.31%, respectively.

Fig. 2

Sequential DWI measurements acquired on the 1.5 T MR-Linac (Unity, Elekta AB, Stockholm, Sweden) in a HNC patient. This patient was treated in the context of a phase 2 feasibility trial (NCT04172753), which had been approved by the ethics committee of the University Hospital Tübingen, and gave written informed consent. (A) Anatomical T1-weighted MRI acquired on the 1.5 T MR-Linac after two weeks of RT (fraction 10) with gross tumor volume (GTV) delineated. (B) ADC map derived from EPI-based DW MRI (b = 200, 500, 800 s/mm², TE/TR = 107/10392 ms, voxel size: 3 × 3 × 4.8 mm³). (C) Mean ADC values in the tumor region sequentially assessed during the course of fractionated RT, in comparison to two DW-MRI scans acquired at a diagnostic 3 T MR scanner (Vida, Siemens Healthineers, Erlangen, Germany). The error bars visualize the standard deviation of the ADC distribution inside the GTV. ADC values are continuously increasing during RT, which may be a sign of individual treatment response.