Edward J. Auerbach

Purpose: To enable sodium SWIFT imaging, which is a zero‐echo time imaging technique, at ultra‐high magnetic fields through the development of custom electronics hardware, and to showcase this capability with in vivo imaging results. Methods: The custom hardware developed consists of a high‐speed optical trigger with 10 ns resolution, an in‐bore PIN diode driver capable of sourcing high current, and RF switches (both an in‐line switchable attenuator and transmit/receive switch) optimized to achieve sub‐microsecond switching speeds while also producing isolation between the transmitter and receiver. Results: Practical switching speeds of 0.6 μs (receive to transmit) and 1.7 μs (transmit to receive) are achieved on the RF switches, limited by transient‐induced spurious emissions from the low noise amplifier. Transmit to receive isolation of 115 dB is achieved over a 1 MHz bandwidth, with 120 dB of isolation at the Larmor frequency. This was crucial to suppress unblanked RFPA noise. SWIFT images of the human wrist with 1.5 mm isotropic resolution were acquired in less than 5 min to demonstrate the utility of sodium SWIFT imaging at ultra‐high field. SWIFT imaging signal‐to‐noise ratio in a reference phantom with heterogeneous sodium concentrations was comparable to ultra‐short echo time imaging, which provided assurance that hardware had achieved the necessary specifications. Conclusions: The feasibility of sodium SWIFT imaging at ultra‐high field was established, and a musculoskeletal imaging application was demonstrated. The success of this work enables further development of SWIFT at high and ultra‐high fields for imaging of sodium, proton, and other nuclei.

Purpose: High-density RF receive arrays are required to realize the inherently available SNR and parallel imaging advantages at ultrahigh field strengths, which are essential for high-resolution functional and anatomical brain MRI. This study aims to systematically assess the impacts of often-overlooked parasitic losses associated with various RF coil components, as these losses can degrade the realized SNR and cause significant deviation from the ultimate intrinsic SNR (uiSNR; the theoretical upper bound of available SNR). In addition, we seek to detail engineering solutions to each of these loss mechanisms in pursuit of achieving a higher fraction of the uiSNR limit. Methods: A 16-channel loop-folded dipole transceiver array was developed for 10.5T human head applications and paired with a fully-updated 64-channel receive-only loop array. The optimization of the receive array considered several factors, including (but not limited to) coil dimensions to accommodate a larger population, the size and number of loops to enhance SNR and parallel imaging performance, and circuit design strategies to minimize parasitic losses. The SNR and parallel imaging performance of the receive array were quantitatively assessed by comparison with the uiSNR, as well as existing high-channel-count receive arrays at 7T and 10.5T. Finally, the complete 16-channel transmit, 80-channel receive coil array was safety validated for human use and employed for high-resolution functional and anatomical MRI at 10.5T. Results: Initial results show that the 80-channel array, featuring larger loops in an overlapped layout with optimized circuitry, significantly improves the SNR and approaches the uiSNR limit in a large fraction of the head, while maintaining or enhancing the parallel imaging performance compared to previously used non-overlap layout. Conclusion: This study suggests that, although the traditionally used high-channel-count loop receive array technology can approach the uiSNR limit in the >10T regime, meticulous design optimization—including systematic assessment and minimization of parasitic losses—has become increasingly critical for achieving this goal in this new field-strength territory.

Purpose: To demonstrate mesoscale whole‐brain T ₂ *‐weighted ( T ₂ *w) MRI at 10.5 T, quantify R ₂ * relaxation rate and magnetic susceptibility ( χ ), and evaluate T ₂ *w contrast at such high field strength. Methods: Multi‐echo GRE (ME‐GRE) data were collected in healthy adults at 0.5 mm isotropic resolution at 10.5 T. Whole‐brain images were reconstructed with navigator‐guided joint motion and field correction and were used for quantitative R ₂ * and χ mapping. Regional R ₂ * and χ values and R ₂ * contrast were analyzed in volumetric regions of interest (ROIs) and intra‐cortical surface‐based ROIs. For comparison, ME‐GRE data from the same subjects were acquired using a similar protocol at 7 T. Results: High‐quality whole‐brain T ₂ *w images were obtained, enabling R ₂ * and χ mapping with delineation of fine‐scale brain structures. Regional R ₂ * analysis revealed a linear relationship between 10.5 T and 7 T R ₂ * values with a slope of 1.52, in agreement with previously reported linear field dependency of R ₂ *. Estimated χ values were field‐independent in most brain regions under consideration except for the basal ganglia where χ was observed to be lower at 10.5 T than at 7 T. The normalized R ₂ * contrast that is, the R ₂ * difference normalized by the mean R ₂ *, increased by about 3% between brain regions and 12% between cortical depths from 7 to 10.5 T. Conclusion: It is feasible to achieve high‐quality mesoscale whole‐brain T ₂ *w MRI at 10.5 T and associated quantitative R ₂ * and χ mapping. Our results may aid future optimization of anatomic T ₂ *w brain MRI at ultrahigh field beyond 7 T.

Purpose: This study proposes a numerical technique to estimate peak local specific absorption rate (SAR) uncertainty of multi‐channel RF coils during the process of safety validation as an alternative to experimental temperature and electric‐field measurements, and demonstrates its use to enable human studies at 10.5 T. Methods: To ensure patient safety, SAR limits established under international guidelines must not be exceeded. Predicting SAR on state‐of‐the‐art parallel transmit systems relies on electromagnetic simulations, which require extensive experimental validation. Despite a well‐established validation workflow, SAR prediction errors are unavoidable and must be quantified as a safety margin. While MRT tests are commonly used for this purpose, their technical challenges necessitate an alternative. The proposed technique propagates the error between experimentally and numerically acquired distributions to the uncertainty in simulated peak local SAR using Monte‐Carlo simulations without the need for MRT. This method was validated using a 16‐channel transceiver 10.5 T torso coil, as well as an 8‐channel 10.5 T head coil. Results: The proposed numerical technique proved more conservative than existing MRT‐based SAR error quantification methods across all tested scenarios. Its application to validate three state‐of‐the‐art head coils (16Tx/32Rx, 16Tx/80Rx, and 16Tx/128Rx) led to regulatory approval for human head imaging and high‐quality diffusion and functional MRI results at 10.5 T. Conclusion: The proposed technique requires only the experimental acquisition of maps for comparison with simulations, enabling the estimation of SAR prediction uncertainty. This technique was applied to three 16‐channel transmit arrays, each used in conjunction with high‐channel‐count receive arrays for in vivo imaging.

Mapping brain connectivity in primates remains a major challenge due to difficulties in resolving microscopic white matter architecture, while maintaining whole-brain coverage. Increasing imaging spatial resolution is key for disambiguating fiber configurations within smaller anatomical volumes. Here, we present developments that allow high-resolution diffusion MRI of the macaque brain, both in vivo and ex vivo, using one of the world’s highest-field human MRI scanners operating at 10.5 Tesla. Our approach achieves data of highest reported resolutions for this field strength and scanner type, (750 μm ) ³ in vivo and (400 μm ) ³ ex vivo , with diffusion weighting up to b = 6000 s/mm ² . We detail methodological advances in data acquisition, image reconstruction, processing and whole-brain tractography that overcome critical challenges associated with ultra-high-field imaging. This work establishes a new benchmark for high-resolution neuroimaging at 10.5T, paving the way for comparable human studies and enabling analyses of brain connectivity across species and tissue states at unprecedented detail. The dataset, along with all processing pipelines, containerised workflows, and reusable web services, is openly shared to support reproducibility and future integration with microscopy for studying white matter microstructure and connections at the mesoscale.

Diffusion MRI provides a non-invasive probe of local fibre bundles and long-range anatomical connections to characterise the structural connectome. One way to achieve very high spatial resolution diffusion MRI data for connectivity investigations is to scan ex-vivo brains over many hours or days, ideally at ultra-high field strength to boost signal levels. However, conventional diffusion MRI acquisition techniques do not generally deliver good data quality for the challenging conditions of ex-vivo tissue, characterised by reduced diffusivities and relaxation times when compared to in vivo. In this work, we investigate the potential of the diffusion-weighted steady-state free precession (DW-SSFP) sequence for ex vivo diffusion imaging of the macaque brain using a 10.5 T human MRI scanner with a conventional (G_max=70 mT/m) gradient set. SNR-efficiency optimisations incorporating experimental relaxation times demonstrate that the DW-SSFP sequence is predicted to achieve improved or similar SNR efficiency compared to a diffusion-weighted spin- and stimulated-echo sequence. Importantly, DW-SSFP can achieve this with the additional benefit of negligible geometric distortions, unlike conventional diffusion MRI using an echo-planar imaging readout. Using optimised DW-SSFP sequence parameters, we propose a protocol at 0.4 mm isotropic resolution using a two-shell multi-orientation protocol (effective b-values of 3200 s/mm^2 and 5600 s/mm^2). We fit the data using Tensor, Ball and 3-Sticks and Constrained Spherical Deconvolution signal representations. The results demonstrate high-quality diffusivity estimates across the entire brain with the ability to resolve multiple fibre populations in challenging crossing-fibre regions. The data will be made fully open source and multimodal as part of the Center for Mesoscale Connectomics, providing a resource for future connectivity investigations.

Personalized functional brain developmental trajectories can be studied with Precision Functional Mapping (PFM). Our previous work has demonstrated that PFM can be achieved in infants despite rapid brain growth. However, even with extensive data collection (up to 1 hour of fMRI), the reliability and precision of these maps remain lower than those observed in youth and adults - particularly within subcortical structures. In this work we demonstrate the utility of high-field 7T MRI compared to 3T MRI for facilitating PFM in infants. We showcase data from multi-echo fMRI acquisitions in the same infants at both 7T and 3T and demonstrate that 7T imaging in infants is safe and feasible with our subject-specific safety workflow. Moreover, we demonstrate that the use of a higher magnetic field strength affords a spatial resolution more appropriately matched to infants’ smaller head and brain sizes, yielding notable improvements in data quality, especially for PFM. The increase in both spatial precision and reliability also suggests that 7T MRI can reduce the amount of data required for PFM. Last, we show how ultra-high field imaging can help us study the development of subcortical-to-cortical connectivity patterns, crucial for understanding brain development during this developmental window. 7T MRI is a promising new avenue for developmental cognitive neuroscience.

Purpose: To propose a new method for characterizing sequences with higher resolution or readout length than allowed by standard field monitoring approaches. Methods: Our proposed method was devised to characterize entire readout gradients by stitching multiple segment‐specific dynamic field measurements obtained across a matched number of consecutive TRs corresponding to a certain segmentation of the readout gradient. The utility of our proposed stitching method for high‐order dynamic field measurements was illustrated using 2D spiral sequences. It was first demonstrated at 10.5 T and then at 7 T using both simulated and experimental MRI data. At 10.5 T, two extreme spiral readout scenarios were considered where standard field monitoring did not work. Our method was then validated in humans at 7 T, where spiral sequences were employed for brain imaging. At 7 T, our method was further compared against gradient impulse response function (GIRF)‐based trajectory prediction. Results: At 10.5 T, our stitching method outperformed the standard approach, producing plausible dynamic field measurements throughout entire readouts. At 7 T, it produced nearly identical measurements for one readout where standard field monitoring also worked; for another readout when standard field monitoring did not work, it still produced sensible field measurements, improving image reconstruction for both simulated and experimental MRI data. Conclusion: Our proposed stitching method provides an effective means to characterize challenging imaging gradients using commercially available hardware and without assuming a linear gradient system, thereby having utility for many applications especially those aiming for ultrahigh‐resolution MRI at ultrahigh field.

Purpose: To demonstrate the feasibility and performance of whole‐brain blood oxygen level–dependent functional MRI (fMRI) in humans at 10.5 T by combining motion‐robust three‐dimensional gradient‐echo echo‐planar imaging, parallel transmission, and high‐density radiofrequency (RF) receive coils. Methods: Resting‐state fMRI time series were collected in healthy adults at 1.58 mm and approximately 2‐s spatiotemporal resolution using a custom‐built 16‐channel transmit/80‐channel receive RF array. Individualized parallel‐transmission, spatial‐spectral RF pulses were designed to achieve uniform water‐selective excitation across the entire brain without the need for additional fat saturation. Images were reconstructed with navigator‐guided joint motion and field correction. Reconstructed images were preprocessed using fMRIPrep and postprocessed using XCP‐D pipelines. Relevant resting‐state fMRI metrics were evaluated including temporal SNR (tSNR), amplitude of low‐frequency fluctuation, and regional homogeneity. The results were compared with those obtained using uncorrected reconstruction (i.e., using same raw data but without motion or field correction). Results: Our motion‐corrected reconstruction largely improved image quality for fMRI time series, reducing motion confounds when compared with uncorrected reconstruction. The reduction in motion confounds translated into an improvement in tSNR, with tSNR averaged across all volunteers being increased by about 11%. Our motion‐corrected reconstruction also improved both amplitude of low‐frequency fluctuation and regional homogeneity in most cortical surfaces and subcortical regions. Conclusion: It is feasible to perform quality three‐dimensional whole‐brain blood oxygen level–dependent fMRI in humans at 10.5 T using a new comprehensive motion‐robust imaging method. This work paves the way for promising future applications at 10.5 T aimed at studying brain function and networks with high spatiotemporal resolution.

Purpose: To develop and characterize a 128‐channel head array for brain imaging at 10.5 T, evaluate signal‐to‐noise ratio (SNR) relative to ultimate intrinsic SNR (uiSNR) and lower field strengths, and demonstrate human brain anatomical and functional imaging with this unique magnetic field and high‐channel‐count array. Methods: The coil consists of a 16‐channel self‐decoupled loop transmit/receive (16Tx/Rx) array with a 112‐loop receive‐only (Rx) insert. Interactions between the 16Tx/Rx array and the 112Rx insert were mitigated using coaxial cable traps placed every 1/16 of a wavelength on each feed cable, locating most preamplifier boards outside the transmitter field, and miniaturizing those placed directly on individual coils. Results: The effect of the 112Rx insert on the circumscribing 16Tx/Rx array was minimized, leading to similar transmit field maps obtained experimentally with and without the 112Rx array in place and by electromagnetic simulations of the 16Tx/Rx array alone. The 128‐channel array captured 77% of uiSNR centrally. Significantly higher 1/g‐factor values across the whole brain was achieved compared with 7 T. Excellent SNR, high parallel‐imaging performance, and minimal Tx‐Rx interactions collectively facilitated acquisition of high‐quality, high‐resolution, proof‐of‐concept functional and anatomical images, including with power‐demanding sequences in the human brain. Conclusions: Counterintuitive to expectations based on magnetic fields less than or equal to 7 T, the higher channel counts provided SNR gains centrally, capturing about 80% uiSNR. The fraction of uiSNR achieved centrally in 64Rx, 80Rx, and 128Rx arrays suggested that a plateau was being reached at 80%. At this plateau, B ₀ ‐dependent SNR gains for 10.5 T relative to 7 T were approximately linear to quadratic for the periphery and the center, respectively.