A silent echo-planar spectroscopic imaging readout with high spectral bandwidth MRSI using an ultrasonic gradient axis.

PURPOSE: We present a novel silent echo-planar spectroscopic imaging (EPSI) readout, which uses an ultrasonic gradient insert to accelerate MRSI while producing a high spectral bandwidth (20 kHz) and a low sound level. METHODS: The ultrasonic gradient insert consisted of a single-axis (z-direction) plug-and-play gradient coil, powered by an audio amplifier, and produced 40 mT/m at 20 kHz. The silent EPSI readout was implemented in a phase-encoded MRSI acquisition. Here, the additional spatial encoding provided by this silent EPSI readout was used to reduce the number of phase-encoding steps. Spectroscopic acquisitions using phase-encoded MRSI, a conventional EPSI-readout, and the silent EPSI readout were performed on a phantom containing metabolites with resonance frequencies in the ppm range of brain metabolites (0-4 ppm). These acquisitions were used to determine sound levels, showcase the high spectral bandwidth of the silent EPSI readout, and determine the SNR efficiency and the scan efficiency. RESULTS: The silent EPSI readout featured a 19-dB lower sound level than a conventional EPSI readout while featuring a high spectral bandwidth of 20 kHz without spectral ghosting artifacts. Compared with phase-encoded MRSI, the silent EPSI readout provided a 4.5-fold reduction in scan time. In addition, the scan efficiency of the silent EPSI readout was higher (82.5% vs. 51.5%) than the conventional EPSI readout. CONCLUSIONS: We have for the first time demonstrated a silent spectroscopic imaging readout with a high spectral bandwidth and low sound level. This sound reduction provided by the silent readout is expected to have applications in sound-sensitive patient groups, whereas the high spectral bandwidth could benefit ultrahigh-field MR systems.

Accelerating Brain Imaging Using a Silent Spatial Encoding Axis.

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.

A silent gradient axis for soundless spatial encoding to enable fast and quiet brain imaging.

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.

A "flared-end" gradient coil with outer-wall direct cooling for human brain imaging: A feasibility study.

Optimal gradient performance is arguably a pre-requisite to realize the full potential of ultrahigh field magnetic resonance imaging (MRI). The values of using tailored gradient coils for brain imaging have been well acknowledged. Unfortunately, conventional head-only gradient coils have two major technical limitations, i.e. limited shoulder clearance and limited cooling capacity. A design, coined "flared-end" gradient coil, combined with a cooling method, named "outer-wall direct cooling", is proposed to address these problems. The "flared-end" design permits brain access to the center of gradient coil. The "flared end" structure is 3D-printed. It has electrical winding patterns (grooves) on one side and evenly spaced cooling channels on the opposite side. Electrical conductor (copper wire) is fixed into the grooves; coolant is in direct contact with the outer surface of the electrical conductor above each cooling channel, eliminating interfacial thermal resistance between coolant and copper wires. Heat transfer area is thus determined by the size and the number of cooling channels. This approach allows high electric current density for high gradient field strength while maintaining high cooling efficiency. Additionally, the symmetric coil geometry guarantees intrinsic torque balance. As a proof of concept, we have made a gradient coil prototype without active shielding. This coil has an inner diameter of 0.3 m, and is capable of generating 0.337, 0.225 and 0.485 mT/m/A along X, Y and Z, respectively. Active shielding was designed theoretically, but not pursued in the construction of this coil prototype. The new coil geometry and cooling method offer a novel avenue for new gradient coils tailored for human brain imaging at ultrahigh field.