Stochastic-offset-enhanced restricted slice excitation and 180° refocusing designs with spatially non-linear ΔB0 shim array fields.

PURPOSE: Developing a general framework with a novel stochastic offset strategy for the design of optimized RF pulses and time-varying spatially non-linear ΔB0 shim array fields for restricted slice excitation and refocusing with refined magnetization profiles within the intervals of the fixed voxels. METHODS: Our framework uses the decomposition property of the Bloch equations to enable joint design of RF-pulses and shim array fields for restricted slice excitation and refocusing with auto-differentiation optimization. Bloch simulations are performed independently on orthogonal basis vectors, Mx, My, and Mz, which enables designs for arbitrary initial magnetizations. Requirements for refocusing pulse designs are derived from the extended phase graph formalism obviating time-consuming sub-voxel isochromatic simulations to model the effects of crusher gradients. To refine resultant slice-profiles because of voxelwise optimization functions, we propose an algorithm that stochastically offsets spatial points at which loss is computed during optimization. RESULTS: We first applied our proposed design framework to standard slice-selective excitation and refocusing pulses in the absence of non-linear ΔB0 shim array fields and compared them against pulses designed with Shinnar-Le Roux algorithm. Next, we demonstrated our technique in a simulated setup of fetal brain imaging in pregnancy for restricted-slice excitation and refocusing of the fetal brain. CONCLUSIONS: Our proposed framework for optimizing RF pulse and time-varying spatially non-linear ΔB0 shim array fields achieve high fidelity restricted-slice excitation and refocusing for fetal MRI, which could enable zoomed fast-spin-echo-MRI and other applications.

Velocity-selective excitation: Principles and applications.

Velocity-selective (VS) excitation is a relatively new type of excitation that can be useful for generating image contrast based on spin's motion. This review aims to explain the principles of VS excitation and their utilization for clinical applications. We first review the generalized excitation k-space formalism, which reveals a Fourier relationship between sequence parameters and excitation profiles for spins with arbitrary spatial location, off-resonance, and velocity. Based on the k-space framework, we analyze practical VS excitation pulse sequences that yield sinusoidal or sinc-shaped velocity profiles. Then we demonstrate how these two types of VS excitation can be used as magnetization preparation for clinical applications, including saturation- or inversion-based arterial spin labeling and black- or bright-blood angiography. We also discuss practical considerations and issues for each application, including the determination of design parameters and the effects of MR system errors, such as magnetic field offsets and eddy currents.

Miniaturized Wirelessly Powered and Controlled Implants for Multisite Stimulation.

This paper presents a miniaturized implant with a diameter of only 14 mm, which houses a novel System on Chip (SoC) enabling two voltage level stimulation of up to 16 implants using a single Tx coil. Each implant can operate at a distance of 80 mm in the air through the inductive resonant link. The SoC consumes only 27 µW static power and enables two channels with stimulation amplitudes of 1.8 V and 3.3 V and timing resolution of 100 µs. The SoC is implemented in the standard 180 nm complementary metal oxide semiconductor (CMOS) technology and has an area of 0.75 mm × 1.6 mm. The SoC comprises an RF rectifier, low drop-out regulator (LDO), error detection block, clock data recovery, finite state machine (FSM), and output stage. Each implant has a PCB-defined passcode, which enables the individual addressability of the implants for synchronized therapies. The implantable device weighs only 80 mg and sizes 20.1 mm3. Tolerance of up to 70° to angular misalignment was measured at a distance of 50 mm. The efficacy of bilateral stimulation was further verified by implanting two devices on two sides of a pig's neck and performing bilateral vagus nerve stimulation (VNS), while monitoring the heart rate.

Rapid, B1 -insensitive, dual-band quasi-adiabatic saturation transfer with optimal control for complete quantification of myocardial ATP flux.

PURPOSE: Phosphorus saturation-transfer experiments can quantify metabolic fluxes noninvasively. Typically, the forward flux through the creatine kinase reaction is investigated by observing the decrease in phosphocreatine (PCr) after saturation of γ-ATP. The quantification of total ATP utilization is currently underexplored, as it requires simultaneous saturation of inorganic phosphate ( Pi ) and PCr. This is challenging, as currently available saturation pulses reduce the already-low γ-ATP signal present. METHODS: Using a hybrid optimal-control and Shinnar-Le Roux method, a quasi-adiabatic RF pulse was designed for the dual saturation of PCr and Pi to enable determination of total ATP utilization. The pulses were evaluated in Bloch equation simulations, compared with a conventional hard-cosine DANTE saturation sequence, before being applied to perfused rat hearts at 11.7 T. RESULTS: The quasi-adiabatic pulse was insensitive to a >2.5-fold variation in B1 , producing equivalent saturation with a 53% reduction in delivered pulse power and a 33-fold reduction in spillover at the minimum effective B1 . This enabled the complete quantification of the synthesis and degradation fluxes for ATP in 30-45 minutes in the perfused rat heart. While the net synthesis flux (4.24 ± 0.8 mM/s, SEM) was not significantly different from degradation flux (6.88 ± 2 mM/s, P = .06) and both measures are consistent with prior work, nonlinear error analysis highlights uncertainties in the Pi -to-ATP measurement that may explain a trend suggesting a possible imbalance. CONCLUSIONS: This work demonstrates a novel quasi-adiabatic dual-saturation RF pulse with significantly improved performance that can be used to measure ATP turnover in the heart in vivo.

A comparison and evaluation of reduced-FOV methods for multi-slice 7T human imaging.

Eight different reduced field-of-view (FOV) MRI techniques suitable for high field human imaging were implemented, optimized, and evaluated at 7T. These included selective Inner-Volume Imaging (IVI) based methods, and Outer-Volume Suppression (OVS) techniques, some of which were previously unexplored at ultra-high fields. Design considerations included use of selective composite excitation and adiabatic refocusing radio-frequency (RF) pulses to address B1 inhomogeneities, twice-refocused spin echo techniques, frequency-modulated pulses to sharply define suppressed regions, and pulse sequence designs to improve SNR in multi-slice scans. The different methods were quantitatively compared in phantoms and in vivo human brain images to provide measurements of relative signal to noise ratio (SNR), power deposition (specific absorption rate, SAR), suppression of signal, artifact strength and prevalence, and general image quality. Multi-slice signal losses in out-of-slice locations were simulated for IVI methods, and then measured experimentally across a range of slice numbers. Corrections for B1 nonuniformities demonstrated an improved SNR and a reduction in artifact power in the reduced-FOV, but produced an elevated SAR. Multi-slice sequences with reordering of pulses in traditional and twice-refocused IVI techniques demonstrated an improved SNR compared to conventional methods. The combined results provide a basis for use of reduced-FOV techniques for human imaging localized to a small FOV at 7T.