This evaluation of the accuracy and precision of cardiac T mapping characterizes the major vulnerabilities of the technique and will help guide protocol definition of studies that include T mapping. Magn Reson Med 77:159-169, 2017. © 2016 Wiley Periodicals, Inc.

Radiation therapy is a major component of cancer treatment pathways worldwide. The main aim of this treatment is to achieve tumor control through the delivery of ionizing radiation while preserving healthy tissues for minimal radiation toxicity. Because radiation therapy relies on accurate localization of the target and surrounding tissues, imaging plays a crucial role throughout the treatment chain. In the treatment planning phase, radiological images are essential for defining target volumes and organs-at-risk, as well as providing elemental composition (e.g., electron density) information for radiation dose calculations. At treatment, onboard imaging informs patient setup and could be used to guide radiation dose placement for sites affected by motion. Imaging is also an important tool for treatment response assessment and treatment plan adaptation. MRI, with its excellent soft tissue contrast and capacity to probe functional tissue properties, holds great untapped potential for transforming treatment paradigms in radiation therapy. The MR in Radiation Therapy ISMRM Study Group was established to provide a forum within the MR community to discuss the unmet needs and fuel opportunities for further advancement of MRI for radiation therapy applications. During the summer of 2021, the study group organized its first virtual workshop, attended by a diverse international group of clinicians, scientists, and clinical physicists, to explore our predictions for the future of MRI in radiation therapy for the next 25 years. This article reviews the main findings from the event and considers the opportunities and challenges of reaching our vision for the future in this expanding field.

Purpose: High-resolution isotropic T 2 mapping of the human brain with multi-echo spin-echo (MESE) acquisitions is challenging. When using a 2D sequence, the resolution is limited by the slice thickness. If used as a 3D acquisition, specific absorption rate limits are easily exceeded due to the high power deposition of nonselective refocusing pulses. A method to reconstruct 1-mm 3 isotropic T 2 maps is proposed based on multiple 2D MESE acquisitions. Data were undersampled (10-fold) to compensate for the prolonged scan time stemming from the super-resolution acquisition. Theory and Methods: The proposed method integrates a classical super-resolution with an iterative model-based approach to reconstruct quantitative maps from a set of undersampled low-resolution data. The method was tested on numerical and multipurpose phantoms, and in vivo data. T 2 values were assessed with a region-of-interest analysis using a single-slice spin-echo and a fully sampled MESE acquisition in a phantom, and a MESE acquisition in healthy volunteers. Results: Numerical simulations showed that the best trade-off between acceleration and number of low-resolution datasets is 10-fold acceleration with 4 acquisitions (acquisition time = 18 min). The proposed approach showed improved resolution over low-resolution images for both phantom and brain. Region-of-interest analysis of the phantom compartments revealed that at shorter T 2 , the proposed method was comparable with the fully sampled MESE. For the volunteer data, the T 2 values found in the brain structures were consistent across subjects (8.5-13.1 ms standard deviation). Conclusion: The proposed method addresses the inherent limitations associated with high-resolution T 2 mapping and enables the reconstruction of 1 mm 3 isotropic relaxation maps with a 10 times faster acquisition. K E Y W O R D Smodel-based reconstruction, parallel Imaging, super-resolution, T 2 mapping

T2* mapping can be a potential biomarker to characterise hypoxia and monitor treatment response throughout the course of MR-guided radiotherapy on an MR-Linac. This study explores the feasibility of integrating T2* mapping in daily prostate MR-Linac workflow using a cohort of patients with prostate cancer. We compared mean T2* values within the prostate with repeated measures acquired twice weekly during radiotherapy. T2* values at the end of treatment were higher than at the first fraction but didn’t show a consistent trend throughout treatment. Integrating T2* mapping with other functional measurements can aid in response based treatment adaptation.