High peak and high average radiofrequency power transmit/receive switch for thermal magnetic resonance.

PURPOSE: To study the role of temperature in biological systems, diagnostic contrasts and thermal therapies, RF pulses for MR spin excitation can be deliberately used to apply a thermal stimulus. This application requires dedicated transmit/receive (Tx/Rx) switches that support high peak powers for MRI and high average powers for RF heating. To meet this goal, we propose a high-performance Tx/Rx switch based on positive-intrinsic-negative diodes and quarter-wavelength (λ/4) stubs. METHODS: The λ/4 stubs in the proposed Tx/Rx switch design route the transmitted RF signal directly to the RF coil/antenna without passing through any electronic components (e.g., positive-intrinsic-negative diodes). Bench measurements, MRI, MR thermometry, and RF heating experiments were performed at f = 297 MHz (B0 = 7 T) to examine the characteristics and applicability of the switch. RESULTS: The proposed design provided an isolation of -35.7dB/-41.5dB during transmission/reception. The insertion loss was -0.41dB/-0.27dB during transmission/reception. The switch supports high peak (3.9 kW) and high average (120 W) RF powers for MRI and RF heating at f = 297 MHz. High-resolution MRI of the wrist yielded image quality competitive with that obtained with a conventional Tx/Rx switch. Radiofrequency heating in phantom monitored by MR thermometry demonstrated the switch applicability for thermal modulation. Upon these findings, thermally activated release of a model drug attached to thermoresponsive polymers was demonstrated. CONCLUSION: The high-power Tx/Rx switch enables thermal MR applications at 7 T, contributing to the study of the role of temperature in biological systems and diseases. All design files of the switch will be made available open source at www.opensourceimaging.org.

An uncertainty-aware, shareable, and transparent neural network architecture for brain-age modeling.

The deviation between chronological age and age predicted from neuroimaging data has been identified as a sensitive risk marker of cross-disorder brain changes, growing into a cornerstone of biological age research. However, machine learning models underlying the field do not consider uncertainty, thereby confounding results with training data density and variability. Also, existing models are commonly based on homogeneous training sets, often not independently validated, and cannot be shared because of data protection issues. Here, we introduce an uncertainty-aware, shareable, and transparent Monte Carlo dropout composite quantile regression (MCCQR) Neural Network trained on N = 10,691 datasets from the German National Cohort. The MCCQR model provides robust, distribution-free uncertainty quantification in high-dimensional neuroimaging data, achieving lower error rates compared with existing models. In two examples, we demonstrate that it prevents spurious associations and increases power to detect deviant brain aging. We make the pretrained model and code publicly available.

Diffusion-sensitized ophthalmic magnetic resonance imaging free of geometric distortion at 3.0 and 7.0 T: a feasibility study in healthy subjects and patients with intraocular masses.

OBJECTIVES: This study is designed to examine the feasibility of diffusion-sensitized multishot split-echo rapid acquisition with relaxation enhancement (RARE) for diffusion-weighted ophthalmic imaging free of geometric distortions at 3.0 and 7.0 T in healthy volunteers and patients with intraocular masses. MATERIALS AND METHODS: A diffusion-sensitized multishot split-echo RARE (ms-RARE) variant is proposed as an alternative imaging strategy for diffusion-weighted imaging. It is compared with standard single-shot echo planar imaging (EPI) and readout-segmented EPI in terms of geometric distortions in a structure phantom as well as in vivo at 3.0 and 7.0 T. To quantify geometric distortions, center of gravity analysis was carried out. Apparent diffusion coefficient (ADC) mapping in a diffusion phantom was performed to verify the diffusion sensitization within ms-RARE. An in vivo feasibility study in healthy volunteers (n = 10; mean age, 31 ± 7 years; mean body mass index, 22.6 ± 1.7 kg/m²) was conducted at 3.0 and 7.0 T to evaluate clinical feasibility of ms-RARE. As a precursor to a broader clinical study, patients (n = 6; mean age, 55 ± 12 years; mean body mass index, 27.5 ± 4.7 kg/m²) with an uveal melanoma and/or retinal detachment were examined at 3.0 and 7.0 T. In 1 case, the diseased eye was enucleated as part of the therapy and imaged afterward with magnetic resonance microscopy at 9.4 T. Macrophotography and histological investigation was carried out. For qualitative assessment of the image distortion, 3 independent readers reviewed and scored ms-RARE in vivo images for all subjects in a blinded reading session. Statistical significance in the difference of the scores (a) obtained for the pooled ms-RARE data with b = 0 and 300 s/mm² and (b) for the 3 readers was analyzed using the nonparametric Mann-Whitney test. RESULTS: The assessment of geometric integrity in phantom imaging revealed the ability of ms-RARE to produce distortion-free images. Unlike ms-RARE, modest displacements (2.3 ± 1.4 pixels) from the fast low angle shot imaging reference were observed for readout-segmented EPI, which were aggravated for single-shot EPI (8.3 ± 5.7 pixels). These observations were confirmed in the in vivo feasibility study including distortion-free diffusion-weighted ophthalmic images with a 0.5 × 0.5 × 5 mm³ spatial resolution at 3.0 T and as good as 0.2 × 0.2 × 2 mm³ at 7.0 T. The latter represents a factor of 40 enhancement in spatial resolution versus clinical protocols recently reported for diffusion-weighted imaging of the eye at 1.5 T. Mean ADC values within the vitreous body were (2.91 ± 0.14) × 10⁻³ mm²/s at 3.0 T and (2.93 ± 0.41) × 10⁻³ mm²/s at 7.0 T. Patient data showed severe retinal detachment in the anatomical images. Whereas the tumor remained undetected in T1-weighted and T2-weighted imaging at 3.0/7.0 T, in vivo ADC mapping using ms-RARE revealed the presence of a uveal melanoma with a significant contrast versus the surrounding subretinal hemorrhage. This observation was confirmed by high-resolution ex vivo magnetic resonance microscopy and histology. Qualitative analysis of image distortion in ms-RARE images obtained for all subjects yielded a mean ± SD image quality score of 1.06 ± 0.25 for b = 0 s/mm² and of 1.17 ± 0.49 for b = 300 s/mm². No significant interreader differences were observed for ms-RARE with a diffusion sensitization of b = 0 s/mm² and 300 s/mm². CONCLUSIONS: This work demonstrates the capability of diffusion-sensitized ms-RARE to acquire high-contrast, high-spatial resolution, distortion-free images of the eye and the orbit at 3.0 and 7.0 T. Geometric distortions that are observed for EPI-based imaging approaches even at lower field strengths are offset by fast spin-echo-based imaging techniques. The benefits of this improvement can be translated into the assessment of spatial arrangements of the eye segments and their masses with the ultimate goal to provide guidance during diagnostic treatment of ophthalmological diseases.