Toward magnetic resonance fingerprinting for low‐field MR‐guided radiation therapy

Abstract

Purpose The acquisition of multiparametric quantitative magnetic resonance imaging (qMRI) is becoming increasingly important for functional characterization of cancer prior to‐ and throughout the course of radiation therapy. The feasibility of a qMRI method known as magnetic resonance fingerprinting (MRF) for rapid T1 and T2 mapping was assessed on a low‐field MR‐linac system. Methods A three‐dimensional MRF sequence was implemented on a 0.35T MR‐guided radiotherapy system. MRF‐derived measurements of T1 and T2 were compared to those obtained with gold standard single spin echo methods, and the impacts of the radiofrequency field homogeneity and scan times ranging between 6 and 48 min were analyzed by acquiring between 1 and 8 spokes per time point in a standard quantitative system phantom. The short‐term repeatability of MRF was assessed over three measurements taken over a 10‐h period. To evaluate transferability, MRF measurements were acquired on two additional MR‐guided radiotherapy systems. Preliminary human volunteer studies were performed. Results The phantom benchmarking studies showed that MRF is capable of mapping T1 and T2 values within 8% and 10% of gold standard measures, respectively, at 0.35T. The coefficient of variation of T1 and T2 estimates over three repeated scans was < 5% over a broad range of relaxation times. The T1 and T2 times derived using a single‐spoke MRF acquisition across three scanners were near unity and mean percent errors in T1 and T2 estimates using the same phantom were < 3%. The mean percent differences in T1 and T2 as a result of truncating the scan time to 6 min over the large range of relaxation times in the system phantom were 0.65% and 4.05%, respectively. Conclusions The technical feasibility and accuracy of MRF on a low‐field MR‐guided radiation therapy device has been demonstrated. MRF can be used to measure accurate T1 and T2 maps in three dimensions from a brief 6‐min scan, offering strong potential for efficient and reproducible qMRI for future clinical trials in functional plan adaptation and tumor/normal tissue response assessment.

Feasibility of cardiac-synchronized quantitative T1 and T2 mapping on a hybrid 1.5 Tesla magnetic resonance imaging and linear accelerator system

“…Previous works showed the feasibility of quantitative MRI on 0.35 T [31] , [32] and 1.5 T [26] , [33] , but not in the context of cardiac imaging. These studies demonstrated good agreement between their proposed qMRI methods and either gold-standard measurements or reference values on phantom.…”

Low‐rank inversion reconstruction for through‐plane accelerated radial MR fingerprinting applied to relaxometry at 0.35 T

“…28 The dimensionality of the temporal subspace was experimentally set to K = 5. The R = 1 data sets were reconstructed using 20 iterations of a linear conjugate gradient algorithm (ie, without regularization), as this was previously demonstrated to agree well with ground-truth spin-echo measurements at 0.35 T. 14 The R = 2 reconstructions were performed using eight alternating direction method of multipliers iterations, five conjugate gradient iterations, LLR patch sizes of 8 × 8 pixels, and 𝜆 = 1 × 10 −4 . These parameters were all chosen experimentally.…”

“…An adiabatic inversion-prepared fast imaging with steady-state precession MRF sequence was implemented with stack-of-stars k-space coverage and a 512-point flip-angle pattern shown in our previous work. 14 Coronal stack-of-stars MRF data were acquired in the National Institute of Standards and Technology/International Society for Magnetic Resonance in Medicine system phantom 23 using a 12-channel torso coil array with a matrix size of 128 × 128 × 32 and spatial resolution of 1.64 × 1.64 × 5 mm. To match in vivo data, slice oversampling of 25% was used, bringing the total number of slice phase-encoding partitions to 40.…”

“…Therefore, radial k-space coverage is used to keep the TR-and thus the scan time-short for a fixed MRF pattern length. 14 Although low-rank inversion methods have proven to be exceptionally useful for recovering MRF contrast dynamics from a single k-space spoke per point along the MRF pattern, 13,16,17 no methods have been proposed to date that recover contrast-resolved images from through-plane accelerated radial MRF acquisitions. Having such a capability would enable substantially shorter scan times, thereby minimizing the chance of patient motion corrupting the multiparametric qMRI measurements, while also enabling more efficient integration into the standard radiation therapy workflow where high-frequency imaging is possible (eg, daily or weekly).…”

Technical overview of magnetic resonance fingerprinting and its applications in radiation therapy

“…In this pilot study, a deep learning-aided MRF acquisition and reconstruction framework reduced the acquisition time from 21 minutes to a clinically feasible time frame. 98 Thorough evaluations of MRF sequence design with a 0.35 T ViewRay system, which could include optimization of FA/TR combinations and optimization of feasible acquisition time against acceptable accuracy and precision of MRF T 1 /T 2 maps, will warrant future studies.…”