The future of MRI in radiation therapy: Challenges and opportunities for the MR community.

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.

Multiparametric Renal MRI: An Intrasubject Test-Retest Repeatability Study.

BACKGROUND: Renal multiparametric magnetic resonance imaging (MRI) is a promising tool for diagnosis, prognosis, and treatment monitoring in kidney disease. PURPOSE: To determine intrasubject test-retest repeatability of renal MRI measurements. STUDY TYPE: Prospective. POPULATION: Nineteen healthy subjects aged over 40 years. FIELD STRENGTH/SEQUENCES: T1 and T2 mapping, R2 * mapping or blood oxygenation level-dependent (BOLD) MRI, diffusion tensor imaging (DTI), and intravoxel incoherent motion (IVIM) diffusion-weighted imaging (DWI), 2D phase contrast, arterial spin labelling (ASL), dynamic contrast enhanced (DCE) MRI, and quantitative Dixon for fat quantification at 3T. ASSESSMENT: Subjects were scanned twice with ~1 week between visits. Total scan time was ~1 hour. Postprocessing included motion correction, semiautomated segmentation of cortex and medulla, and fitting of the appropriate signal model. STATISTICAL TEST: To assess the repeatability, a Bland-Altman analysis was performed and coefficients of variation (CoVs), repeatability coefficients, and intraclass correlation coefficients were calculated. RESULTS: CoVs for relaxometry (T1 , T2 , R2 */BOLD) were below 6.1%, with the lowest CoVs for T2 maps and highest for R2 */BOLD. CoVs for all diffusion analyses were below 7.2%, except for perfusion fraction (FP ), with CoVs ranging from 18-24%. The CoV for renal sinus fat volume and percentage were both around 9%. Perfusion measurements were most repeatable with ASL (cortical perfusion only) and 2D phase contrast with CoVs of 10% and 13%, respectively. DCE perfusion had a CoV of 16%, while single kidney glomerular filtration rate (GFR) had a CoV of 13%. Repeatability coefficients (RCs) ranged from 7.7-87% (lowest/highest values for medullary mean diffusivity and cortical FP , respectively) and intraclass correlation coefficients (ICCs) ranged from -0.01 to 0.98 (lowest/highest values for cortical FP and renal sinus fat volume, respectively). DATA CONCLUSION: CoVs of most MRI measures of renal function and structure (with the exception of FP and perfusion as measured by DCE) were below 13%, which is comparable to standard clinical tests in nephrology. LEVEL OF EVIDENCE: 2 TECHNICAL EFFICACY: Stage 1.

Introduction of the snake antenna array: Geometry optimization of a sinusoidal dipole antenna for 10.5T body imaging with lower peak SAR.

PURPOSE: To improve imaging performance for body MRI with a local transmit array at 10.5T, the geometry of a dipole antenna was optimized to achieve lower peak specific absorption rate (SAR) levels and a more uniform transmit profile. METHODS: Electromagnetic simulations on a phantom were used to evaluate the SAR and B 1 + -performance of different dipole antenna geometries. The best performing antenna (the snake antenna) was simulated on human models in a 12-channel array configuration for safety assessment and for comparison to a previous antenna design. This 12-channel array was constructed after which electromagnetic simulations were validated by B 1 + -maps and temperature measurements. After obtaining approval by the Food and Drug Administration to scan with the snake antenna array, in vivo imaging was performed on 2 volunteers. RESULTS: Simulation results on a phantom indicate a lower SAR and a higher transmit efficiency for the snake antenna compared to the fractionated dipole array. Similar results are found on a human body model: when comparing the trade-off between uniformity and peak SAR, the snake antenna performs better for all imaging targets. Simulations and measurements are in good agreement. Preliminary imaging result were acquired in 2 volunteers with the 12-channel snake antenna array. CONCLUSION: By optimizing the geometry of a dipole antenna, peak SAR levels were lowered while achieving a more uniform transmit field as demonstrated in simulations on a phantom and a human body model. The array was constructed, validated, and successfully used to image 2 individuals at 10.5T.

Understanding the biophysical effects of transcranial magnetic stimulation on brain tissue: the bridge between brain stimulation and cognition.

Transcranial magnetic stimulation (TMS) is rapidly being adopted in neuroscience, medicine, psychology, and biology, for basic research purposes, diagnosis, and therapy. However, a coherent picture of how TMS affects neuronal processing, and especially how this in turn influences behavior, is still largely unavailable despite several studies that investigated aspects of the underlying neurophysiological effects of TMS. Perhaps as a result from this "black box approach," TMS studies show a large interindividual variability in applied paradigms and TMS treatment outcome can be quite variable, hampering its general efficacy and introduction into the clinic. A better insight into the biophysical, neuronal, and cognitive mechanisms underlying TMS is crucial in order to apply it effectively in the clinic and to increase our understanding of brain-behavior relationship. Therefore, computational and experimental efforts have been started recently to understand and control the effect TMS has on neuronal functioning. Especially, how the brain shapes magnetic fields induced by a TMS coil, how currents are generated locally in the cortical surface, and how they interact with complex functional neuronal circuits within and between brain areas are crucial to understand the observed behavioral changes and potential therapeutic effects resulting from TMS. Here, we review the current knowledge about the biophysical underpinnings of single-pulse TMS and argue how to move forward to fully understand and exploit the powerful technique that TMS can be.

Plaque sealing and passivation with a mechanical self-expanding low outward force nitinol vShield device for the treatment of IVUS and OCT-derived thin cap fibroatheromas (TCFAs) in native coronary arteries: report of the pilot study vShield Evaluated at Cardiac hospital in Rotterdam for Investigation and Treatment of TCFA (SECRITT).

AIMS: The aim of the pilot SECRITT trial was to evaluate the safety and feasibility of sealing the high risk IVUS and optical coherence tomography-derived thin cap fibroatheroma (TCFA), with a dedicated nitinol self-expanding vShield device. METHODS AND RESULTS: After screening with angiography, fractional flow reserve (FFR), intravascular ultrasound virtual histology (IVUS-VH) and optical coherence tomography (OCT), 23 patients met enrolment criteria (presence of non-obstructive VH-derived TCFA lesion with thin cap on OCT) and were randomised to vShield (n=13) versus medical therapy (n=10). In the shielded group, baseline percent diameter stenosis was 33.2±13.5%, FFR was 0.93±0.06. At six-month follow-up in shielded patients percent diameter stenosis further decreased to 18.7±16.9% and FFR remained the same 0.93±0.05. Average late loss was 0.24±0.13 mm. Average baseline fibrous cap thickness was 48±12 µm. After shield placement at six-month follow-up neo-cap formation was observed with average cap thickness of 201±168 µm. There were no dissections after shield placement and no plaque ruptures. In addition, mean stent area of 8.76±2.16 mm2 increased to 9.45±2.30 mm2, that is by 9% at six-month follow-up. The number of malapposed struts decreased from 10.7% to 7.6% and the number of uncovered struts at six months was 8.1%. There were no device-related major adverse cardiovascular events (MACE) events at six-month follow-up. CONCLUSIONS: High risk plaque passivation and sealing with a vShield self-expanding nitinol device appears feasible and safe. A long-term larger randomised study with streamlined screening criteria is needed to evaluate the efficacy of this approach over medical therapy.