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PURPOSE: To characterize the acceleration capabilities of a silent head insert gradient axis that operates at the inaudible frequency of 20 kHz and a maximum gradient amplitude of 40 mT/m without inducing peripheral nerve stimulation. METHODS: The silent gradient axis' acquisitions feature an oscillating gradient in the phase-encoding direction that is played out on top of a cartesian readout, similarly as done in Wave-CAIPI. The additional spatial encoding fills k-space in readout lanes allowing for the acquisition of fewer phase-encoding steps without increasing aliasing artifacts. Fully sampled 2D gradient echo datasets were acquired both with and without the silent readout. All scans were retrospectively undersampled (acceleration factors R = 1 to 12) to compare conventional SENSE acceleration and acceleration using the silent gradient. The silent gradient amplitude and the readout bandwidth were varied to investigate the effect on artifacts and g-factor. RESULTS: The silent readout reduced the g-factor for all acceleration factors when compared to SENSE acceleration. Increasing the silent gradient amplitude from 31.5 mT/m to 40 mT/m at an acceleration factor of 10 yielded a reduction in the average g-factor (gavg ) from 1.3 ± 0.14 (gmax = 1.9) to 1.1 ± 0.09 (gmax = 1.6). Furthermore, reducing the number of cycles increased the readout bandwidth and the g-factor that reached gavg = 1.5 ± 0.16 for a readout bandwidth of 651 Hz/pixel and an acceleration factor of R = 8. CONCLUSION: A silent gradient axis enables high acceleration factors up to R = 10 while maintaining a g-factor close to unity (gavg = 1.1 and gmax = 1.6) and can be acquired with clinically relevant readout bandwidths.

PURPOSE: A novel silent imaging method is proposed that combines a gradient insert oscillating at the inaudible frequency 20 kHz with slew rate-limited gradient waveforms to form a silent gradient axis that enable quiet and fast imaging. METHODS: The gradient insert consisted of a plug-and-play (45 kg) single axis z-gradient, which operated as an additional fourth gradient axis. This insert was made resonant using capacitors and combined with an audio amplifier to allow for operation at 20 kHz. The gradient field was characterized using field measurements and the physiological effects of operating a gradient field at 20 kHz were explored using peripheral nerve stimulation experiments, tissue heating simulations and sound measurements. The imaging sequence consisted of a modified gradient-echo sequence which fills k-space in readout lanes with a width proportional to the oscillating gradient amplitude. The feasibility of the method was demonstrated in-vivo using 2D and 3D gradient echo (GRE) sequences which were reconstructed using a conjugate-gradient SENSE reconstruction. RESULTS: Field measurements yielded a maximum gradient amplitude and slew rate of 40.8 mT/m and 5178T/m/s at 20 kHz. Physiological effects such as peripheral nerve stimulation and tissue heating were found not to be limiting at this amplitude and slew rate. For a 3D GRE sequence, a maximum sound level of 85 db(A) was measured during scanning. Imaging experiments using the silent gradient axis produced artifact free images while also featuring a 5.3-fold shorter scan time than a fully sampled acquisition. CONCLUSION: A silent gradient axis provides a novel pathway to fast and quiet brain imaging.

The goal of this study was to introduce and evaluate the performance of a lightweight, high‐performance, single‐axis (z‐axis) gradient insert design primarily intended for high‐resolution functional magnetic resonance imaging, and aimed at providing both ease of use and a boost in spatiotemporal resolution. The optimal winding positions of the coil were obtained using a genetic algorithm with a cost function that balanced gradient performance (minimum 0.30 mT/m/A) and field linearity (≥16 cm linear region). These parameters were verified using field distribution measurements by B0‐mapping. The correction of geometrical distortions was performed using theoretical field distribution of the coil. Simulations and measurements were performed to investigate the echo planar imaging echo‐spacing reduction due to the improved gradient performance. The resulting coil featured a 16‐cm linear region, a weight of 45 kg, an installation time of 15 min, and a maximum gradient strength and slew rate of 200 mT/m and 1300 T/m/s, respectively, when paired with a commercially available gradient amplifier (940 V/630 A). The field distribution measurements matched the theoretically expected field. By utilizing the theoretical field distribution, geometrical distortions were corrected to within 6% of the whole‐body gradient reference image in the target region. Compared with a whole‐body gradient set, a maximum reduction in echo‐spacing of a factor of 2.3 was found, translating to a 344 μs echo‐spacing, for a field of view of 192 mm, a receiver bandwidth of 920 kHz and a gradient amplitude of 112 mT/m. We present a lightweight, single‐axis gradient insert design that can provide high gradient performance and an increase in spatiotemporal resolution with correctable geometrical distortions while also offering a short installation time of less than 15 min and minimal system modifications., A single‐axis gradient insert, designed to be plug‐and‐play while providing a significant boost in gradient performance (gradient strength = 200 mT/m and slew rate = 1300 T/m/s), was presented. This coil was characterized in terms of linearity, image distortions and reduction in echo‐spacing for echo planar imaging. Distortions were found to be correctable and echo‐spacing could be reduced by a factor of 2.3 compared with a whole‐body gradient set (40 mT/m and 200 T/m/s).

Physics & Imaging in Radiation Oncology : phiRO 17, 13-19 (2021). doi:10.1016/j.phro.2020.12.002 special issue: "Magnetic Resonance Imaging in Radiotherapy / Edited by Oliver Jäkel (Guest Editor) ; Jürgen Debus (Guest Editor) ; Ludvig Muren (Editor-in-Chief); Daniela Thorwarth (Editor-in-Chief)", Published by Elsevier Science, Amsterdam [u. a.]

BACKGROUND AND PURPOSE: Cerebral MR imaging in infants is usually performed with a field strength of up to 3T. In adults, a growing number of studies have shown added diagnostic value of 7T MR imaging. 7T MR imaging might be of additional value in infants with unexplained seizures, for example. The aim of this study was to investigate the feasibility of 7T MR imaging in infants. We provide information about the safety preparations and show the first MR images of infants at 7T. MATERIALS AND METHODS: Specific absorption rate levels during 7T were simulated in Sim4life using infant and adult models. A newly developed acoustic hood was used to guarantee hearing protection. Acoustic noise damping of this hood was measured and compared with the 3T Nordell hood and no hood. In this prospective pilot study, clinically stable infants, between term-equivalent age and the corrected age of 3 months, underwent 7T MR imaging immediately after their standard 3T MR imaging. The 7T scan protocols were developed and optimized while scanning this cohort. RESULTS: Global and peak specific absorption rate levels in the infant model in the centered position and 50-mm feet direction did not exceed the levels in the adult model. Hearing protection was guaranteed with the new hood. Twelve infants were scanned. No MR imaging–related adverse events occurred. It was feasible to obtain good-quality imaging at 7T for MRA, MRV, SWI, single-shot T2WI, and MR spectroscopy. T1WI had lower quality at 7T. CONCLUSIONS: 7T MR imaging is feasible in infants, and good-quality scans could be obtained.

For high-resolution diffusion-MRI, strong gradients combined with fast readout are desired. In this study, we propose a gradient system architecture for such strong gradients and fast readouts without nerve stimulation by implementing a filter topology that allows for both low and high frequency gradient waveforms. Two gradient inserts with different specifications are realized and two proposed modulation strategies are implemented in the amplifier firmware. Field measurements demonstrated the desired dual-mode capability of the system. With one gradient insert, a gradient amplitude of 300 mT/m and slew rates well above 10.000 T/m/s can be reached with only 500A&330V as amplifier output.