Accelerated MRI at 9.4 T with electronically modulated time-varying receive sensitivities

PURPOSE To investigate how electronically modulated time-varying receive sensitivities can improve parallel imaging reconstruction at ultra-high field. METHODS Receive sensitivity modulation was achieved by introducing PIN diodes in the receive loops, which allow rapid switching of capacitances in both arms of each loop coil and by that alter B$_{1}$ $^{-}$ profiles, resulting in two distinct receive sensitivity configurations. A prototype 8-channel reconfigurable receive coil for human head imaging at 9.4T was built, and MR measurements were performed in both phantom and human subject. A modified SENSE reconstruction for time-varying sensitivities was formulated, and g-factor calculations were performed to investigate how modulation of receive sensitivity profiles during image encoding can improve parallel imaging reconstruction. The optimized modulation pattern was realized experimentally, and reconstructions with the time-varying sensitivities were compared with conventional static SENSE reconstructions. RESULTS The g-factor calculations showed that fast modulation of receive sensitivities in the order of the ADC dwell time during k-space acquisition can improve parallel imaging performance, as this effectively makes spatial information of both configurations simultaneously available for image encoding. This was confirmed by in vivo measurements, for which lower reconstruction errors (SSIM = 0.81 for acceleration R = 4) and g-factors (max g = 2.4; R = 4) were observed for the case of rapidly switched sensitivities compared to conventional reconstruction with static sensitivities (SSIM = 0.74 and max g = 3.2; R = 4). As the method relies on the short RF wavelength at ultra-high field, it does not yield significant benefits at 3T and below. CONCLUSIONS Time-varying receive sensitivities can be achieved by inserting PIN diodes in the receive loop coils, which allow modulation of B$_{1}$ $^{-}$ patterns. This offers an additional degree of freedom for image encoding, with the potential for improved parallel imaging performance at ultra-high field

In vivo characterization of the downfield part of 1 H MR spectra of human brain at 9.4 T: Magnetization exchange with water and relation to conventionally determined metabolite content

PURPOSE: To perform exchange-rate measurements on the in vivo human brain downfield spectrum (5-10 ppm) at 9.4 T and to compare the variation in concentrations of the downfield resonances and of known upfield metabolites to determine potential peak labels. METHODS: Non-water-suppressed metabolite cycling was used in combination with an inversion transfer technique in two brain locations in healthy volunteers to measure the exchange rates and T1 values of exchanging peaks. Spectra were fitted with a heuristic model of a series of 13 or 14 Voigt lines, and a Bloch-McConnell model was used to fit the exchange rate curves. Concentrations from non-water-inverted spectra upfield and downfield were compared. RESULTS: Mean T1 values ranged from 0.40 to 0.77 s, and exchange rates from 0.74 to 13.8 s-1 . There were no significant correlations between downfield and upfield concentrations, except for N-acetylaspartate, with a correlation coefficient of 0.63 and P < 0.01. CONCLUSIONS: Using ultrahigh field allowed improved separation of peaks in the 8.2 to 8.5 ppm amide proton region, and the exchange rates of multiple downfield resonances including the 5.8-ppm peak, previously tentatively assigned to urea, were measured in vivo in human brain. Downfield peaks consisted of overlapping components, and largely missing correlations between upfield and downfield resonances-although not conclusive-indicate limited contributions from metabolites present upfield to the downfield spectrum. Magn Reson Med, 2017. © 2017 International Society for Magnetic Resonance in Medicine

Unilateral RF sensors based on parallel-plate architecture for improved surface-scan MRI analysis of commercial pouch cells

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Novel splittable N-Tx/2N-Rx transceiver phased array to optimize both signal-to-noise ratio and transmit efficiency at 9.4T

PURPOSE: The goal of this study was to optimize signal-to-noise ratio (SNR) and parallel receive (Rx) performance of ultrahigh field (UHF) (≥7T) transceiver arrays without compromising their transmit (Tx) efficiency. UHF transceiver head phased arrays with a tight fit improve Tx efficiency in comparison with Tx-only arrays, which are usually larger so that Rx-only arrays can fit inside. However, having ≥16 elements inside a head transceiver array presents decoupling problems. Furthermore, the available number of Tx channels is limited. METHODS: A prototype of a splittable transceiver phased array was constructed. The array consisted of four flat surface Tx loops positioned in two rows. Each loop could be split into two smaller overlapped Rx loops during reception. RESULTS: Experimental data demonstrated that both SNR and parallel reception performance improved substantially by doubling the number of Rx elements from four to eight. CONCLUSION: As a proof of concept, we developed and constructed a novel splittable transceiver phased array that allows doubling of the number of Rx elements while keeping both Tx and Rx elements at the same distance from the subject. Both Tx and Rx performance can be optimized at the same time using this method. Magn Reson Med, 2015. © 2015 Wiley Periodicals, Inc

Operando Magnetic Resonance Imaging Reveals Phase Transitions Driven by Nonuniform Cathode Lithiation in Li-Ion Pouch Cells

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Open birdcage coil for head imaging at 7T

International audienceTo theoretically describe, design and test an open head birdcage coil, called opencage, that facilitates access to the patient under examination. This access improves the patient's comfort, but also may be suitable for many tasks, for example fMRI or motion correction. Theory and Methods: Using transfer matrix approach, the birdcagelike coil having non-periodical distribution of rungs is constructed with optimized currents in the coil's rungs. Subsequently, the the coil was adjusted in full-wave simulations. Eventually, these results were confirmed on phantom and in-vivo imaging. Results: Indeed, the high enough computed isolation coefficient between the feeding ports of the coil as well as a birdcage-like B + 1 pattern showed that the coil was properly optimized. After the numerical optimization, the coil was assembled and fine tuned and matched on the bench. Experimental assessment of the developed coil showed slightly lower B + 1 homogeneity, competitive transmit efficiency and coverage to the birdcage coil of comparable size. Conclusion: It was shown that the proposed open birdcage coil can be designed without dramatic drop of performance by means of B1 field homogeneity and efficiency, SAR