Examinando por Autor "Williams, Sydney N."

Mostrando 1 - 4 de 4

A nested eight-channel transmit array with open-face concept for human brain imaging at 7 tesla
(Higher Education Press, 2021-07-21) Williams, Sydney N.; Allwood-Spiers, Sarah; McElhinney, Paul; Paterson, Gavin; Herrler, Jürgen; Liebig, Patrick; Nagel, Armin M.; Foster, John E.; Porter, David A.; Gunamony, Shajan
Purpose: Parallel transmit technology for MRI at 7 tesla will significantly benefit from high performance transmit arrays that offer high transmit efficiency and low mutual coupling between the individual array elements. A novel dual-mode transmit array with nested array elements has been developed to support imaging the human brain in both the single-channel (sTx) and parallel-transmit (pTx) excitation modes of a 7 tesla MRI scanner. In this work, the design, implementation, validation, specific absorption rate (SAR) management, and performance of the head coil is presented. Methods: The transmit array consisted of a nested arrangement to improve decoupling between the second-neighboring elements. Two large cut-outs were introduced in the RF shield for an open-face design to reduce claustrophobia and to allow patient monitoring. A hardware interface allows the coil to be used in both the sTx and pTx modes. SAR monitoring is done with virtual observation points (VOP) derived from human body models. The transmit efficiency and coverage is compared with the commercial single-channel and parallel-transmit head coils. Results: Decoupling inductors between the second-neighboring coil elements reduced the coupling to less than −20 dB. Local SAR estimates from the electromagnetic (EM) simulations were always less than the EM-based VOPs, which in turn were always less than scanner predictions and measurements for static and dynamic pTx waveforms. In sTx mode, we demonstrate improved coverage of the brain compared to the commercial sTx coil. The transmit efficiency is within 10% of the commercial pTx coil despite the two large cut-outs in the RF shield. In pTx mode, improved signal homogeneity was shown when the Universal Pulse was used for acquisition in vivo. Conclusion: A novel head coil which includes a nested eight-channel transmit array has been presented. The large cut-outs improve patient monitoring and reduce claustrophobia. For pTx mode, the EM simulation and VOP-based SAR management provided greater flexibility to apply pTx methods without the limitations of SAR constraints. For scanning in vivo, the coil was shown to provide an improved coverage in sTx mode compared to a standard commercial head coil.
Design of spectral-spatial phase prewinding pulses and their use in small-tip fast recovery steady-state imaging
(Magnetic Resonance in Medicine, 2017-07-04) Williams, Sydney N.; Nielsen, Jon-Fredrik; Fessler, Jeffrey A.; Noll, Douglas C.
Purpose: Spectrally selective “prewinding” radiofrequency pulses can counteract B0 inhomogeneity in steady-state sequences, but can only prephase a limited range of off- resonance. We propose spectral-spatial small-tip angle prewinding pulses that increase the off-resonance bandwidth thatcan be successfully prephased by incorporating spatially tailored excitation patterns. Theory and Methods: We present a feasibility study to compare spectral and spectral-spatial prewinding pulses. These pulses add a prephasing term to the target magnetization pattern that aims to recover an assigned off-resonance bandwidth at the echo time. For spectral-spatial pulses, the design bandwidth is centered at the off-resonance frequency for each spatial location in a field map. We use these pulses in the small-tip fast recovery steady-state sequence, which is similar to balanced steady-state free precession. We investigate improvement of spectral-spatial pulses over spectral pulses using simulations and small-tip fast recovery scans of a gel phantom and human brain. Results: In simulation, spectral-spatial pulses yielded lower normalized root mean squared excitation error than spectral pulses. For both experiments, the spectral-spatial pulse images are also qualitatively better (more uniform, less signal loss) than the spectral pulse images. Conclusion: Spectral-spatial prewinding pulses can prephase over a larger range of off-resonance than their purely spectral counterparts.
Simultaneous whole-brain and cervical spine imaging at 7 T using a neurovascular head and neck coil with 8-channel transceiver array and 56-channel receiver array
(Magnetic Resonance in Medicine, 2025-01-29) Baskaran, Divya; Ding, Belinda; Chu, Son; McElhinney, Paul; Allwood-Spiers, Sarah; Williams, Sydney N.; Muir, Keith; Fullerton, Natasha Eileen; Porter, David Andrew; Gunamony, Shajan
Purpose: To develop a 7T neurovascular head and neck (NVHN) coil with an extended longitudinal coverage of the brain and cervical spine, with eight transceiver (TxRx) channels and 56 receive (Rx) channels for dynamic parallel-transmit (pTx) applications. Methods: A dual-row transceiver array with six elements in the upper row and two elements in the lower row was designed using combined electromagnetic and circuit optimization and constructed. A 56Rx array covering the brain and cervical spine was designed and combined with the transceiver array. The performance of the 8TxRx56Rx NVHN coil such as, signal-to-noise ratio, and g-factor were validated in phantom and in vivo studies and compared with an in-house 8Tx64Rx head coil. High-resolution in vivo images were acquired with the NVHN and head coil. Results: The average in phantom while exciting the upper six channels and all eight channels are 43.45 nT/V and 45.80 nT/V, respectively, demonstrating that the available field is seamlessly distributed in the brain and/or cervical spine, depending on the chosen excitation. The 8TxRx56Rx NVHN coil increases the SNR in the cervical spine and central brain by a factor of 2.18 and 1.16, respectively, compared with the 8Tx64Rx head coil. Furthermore, it demonstrates similar 1/g-factor performance for acceleration factors up to 5 × 5 compared with the head coil and provides diagnostic-quality images of the brain and spinal cord in a single acquisition. Conclusion: The extended longitudinal coverage of the NVHN coil promises to improve the clinical application of the current generation of pTx 7T MRI systems with 8Tx channels.
The effects of RF coils and SAR supervision strategies for clinically applicable nonselective parallel-transmit pulses at 7 T
(Wiley, 2022-12-14) Herrler, Jürgen; Williams, Sydney N.; Liebig, Patrick; Ding, Belinda; McElhinney, Paul; Allwood-Spiers, Sarah; Meixner, Christian R.; Gunamony, Shajan; Maier, Andreas; Arnd, Dörfler; Gumbrecht, Rene; Porter, David A.; Nagel, Armin M.
Purpose: To investigate the effects of using different parallel-transmit (pTx) head coils and specific absorption rate (SAR) supervision strategies on pTx pulse design for ultrahigh-field MRI using a 3D-MPRAGE sequence. Methods: The PTx universal pulses (UPs) and fast online-customized (FOCUS) pulses were designed with pre-acquired data sets (B0 , B1 + maps, specific absorption rate [SAR] supervision data) from two different 8 transmit/32 receive head coils on two 7T whole-body MR systems. For one coil, the SAR supervision model consisted of per-channel RF power limits. In the other coil, SAR estimations were done with both per-channel RF power limits as well as virtual observation points (VOPs) derived from electromagnetic field (EMF) simulations using three virtual human body models at three different positions. All pulses were made for nonselective excitation and inversion and evaluated on 132 B0 , B1 + , and SAR supervision datasets obtained with one coil and 12 from the other. At both sites, 3 subjects were examined using MPRAGE sequences that used UP/FOCUS pulses generated for both coils. Results: For some subjects, the UPs underperformed when simulated on a different coil from which they were derived, whereas FOCUS pulses still showed acceptable performance in that case. FOCUS inversion pulses outperformed adiabatic pulses when scaled to the same local SAR level. For the self-built coil, the use of VOPs showed reliable overestimation compared with the ground-truth EMF simulations, predicting about 52% lower local SAR for inversion pulses compared with per-channel power limits. Conclusion: FOCUS inversion pulses offer a low-SAR alternative to adiabatic pulses and benefit from using EMF-based VOPs for SAR estimation.