Homogeneous non-selective and slice-selective parallel-transmit excitations at 7 Tesla with universal pulses: A validation study on two commercial RF coils (original) (raw)
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Zenodo (CERN European Organization for Nuclear Research), 2023
We present a versatile high-resolution whole-brain multiparametric/susceptibility mapping protocol at 7T, introducing interleaved fly-back skipped-CAIPI 3D-EPI. To overcome the impeding limitations of B 1 + inhomogeneity, universal pTX pulses for excitation and magnetization transfer saturation are employed. Within 17 minutes scan time, 0.6mm isotropic whole-brain MT-/PD-/T1-weighted magnitude and phase images at four echo times and corresponding quantitative parameter maps are obtained. Supplementary output, like SWI and MPRAGElike images can be generated as well. Scan efficiency is high and geometric distortions are negligible. Optionally, the interleaved fly-back acquisition allows a separate analysis of two monopolar readout images.
Magnetic Resonance in Medicine, 2008
A novel radio-frequency (RF) pulse design algorithm is presented that generates fast slice-selective excitation pulses that mitigate inhomogeneity present in the human brain at high field. The method is provided an estimate of the field in an axial slice of the brain and then optimizes the placement of sinc-like "spokes" in k z via an L 1 -norm penalty on candidate (k x , k y ) locations; an RF pulse and gradients are then designed based on these weighted points. Mitigation pulses are designed and demonstrated at 7T in a head-shaped water phantom and the brain; in each case, the pulses mitigate a significantly non-uniform transmit profile and produce nearly uniform flip angles across the field of excitation (FOX). The main contribution of this work, the sparsity-enforced spoke placement and pulse design algorithm, is derived for conventional single-channel excitation systems and applied in the brain at 7T, but readily extends to lower field systems, nonbrain applications, and multichannel parallel excitation arrays.
2008
A novel radio-frequency (RF) pulse design algorithm is presented that generates fast slice-selective excitation pulses that mitigate inhomogeneity present in the human brain at high field. The method is provided an estimate of the field in an axial slice of the brain and then optimizes the placement of sinc-like "spokes" in k z via an L 1 -norm penalty on candidate (k x , k y ) locations; an RF pulse and gradients are then designed based on these weighted points. Mitigation pulses are designed and demonstrated at 7T in a head-shaped water phantom and the brain; in each case, the pulses mitigate a significantly non-uniform transmit profile and produce nearly uniform flip angles across the field of excitation (FOX). The main contribution of this work, the sparsity-enforced spoke placement and pulse design algorithm, is derived for conventional single-channel excitation systems and applied in the brain at 7T, but readily extends to lower field systems, nonbrain applications, and multichannel parallel excitation arrays.
Journal of magnetic resonance imaging : JMRI, 2014
To predict signal-to-noise ratio (SNR) trends and absorbed energy in magnetic resonance imaging (MRI) of the brain up to 14T. A human head in an eight-channel transmit/receive coil was simulated with Maxwell and Bloch equations to determine excitation homogeneity with radiofrequency (RF) shimming, image homogeneity, SNR, and absorbed energy in MRI from 1.5 to 14T considering realistic field distributions and relaxation properties. RF shimming alone achieved a standard deviation in excitation flip angle less than 10° in mid-brain up to 14T, but produced a small region with low excitation on a lower slice. Current reconstruction methods may produce shading artifacts at 14T. SNR increases with a greater-than-linear rate for gradient recalled echo (GRE) sequences having short (2 msec) echo time (TE) and long relaxation time (TR) (∼2.3-fold increase from 7T to 14T), but a less-than-linear rate if TE is 10 msec (∼1.6-fold increase from 7T to 14T). Depending on the sequence, SNR per square...
RF inhomogeneity compensation in structural brain imaging
Magnetic Resonance in Medicine, 2002
Three-dimensional T 1 -weighted magnetization-prepared rapid gradient-echo (MP-RAGE) sequences with centric phase encoding (PE) in the inner loop provide structural brain images with a high spatial resolution and high tissue contrast. A disadvantage of this sequence type is the susceptibility to inhomogeneities of the radiofrequency (RF) coil, which may result in poor image contrast in some peripheral regions. A special excitation pulse is presented which compensates for these effects in both the head/foot and anterior/posterior directions. This pulse has a duration of only 1.3 ms and is thus compatible with the short repetition times (TRs) required for MP-RAGE imaging. It is shown experimentally that images acquired with the compensation pulse may be segmented without using intensity correction algorithms during data postprocessing. Magn Reson Med 47:398 -402, 2002.
Journal of Magnetic Resonance, 2013
T 2 * weighted fMRI at high and ultra high field (UHF) is often hampered by susceptibilityinduced, through-plane, signal loss. Three-dimensional tailored RF (3DTRF) pulses have been shown to be an effective approach for mitigating through-plane signal loss at UHF. However, the required RF pulse lengths are too long for practical applications. Recently, parallel transmission (PTX) has emerged as a very effective means for shortening the RF pulse duration for 3DTRF without sacrificing the excitation performance. In this article, we demonstrate a RF pulse design strategy for 3DTRF based on the use of multi-slice PTX 3DTRF to simultaneously and precisely recover signal with whole-brain coverage. Phantom and human experiments are used to demonstrate the effectiveness and robustness of the proposed method on three subjects using an eight-channel whole body parallel transmission system.
Robust and Fast Whole Brain Mapping of the RF Transmit Field B1 at 7T
PLoS ONE, 2012
In-vivo whole brain mapping of the radio frequency transmit field B 1 + is a key aspect of recent method developments in ultra high field MRI. We present an optimized method for fast and robust in-vivo whole-brain B 1 + mapping at 7T. The method is based on the acquisition of stimulated and spin echo 3D EPI images and was originally developed at 3T. We further optimized the method for use at 7T. Our optimization significantly improved the robustness of the method against large B 1 + deviations and off-resonance effects present at 7T. The mean accuracy and precision of the optimized method across the brain was high with a bias less than 2.6 percent unit (p.u.) and random error less than 0.7 p.u. respectively.
Comparison of three commercially available radio frequency coils for human brain imaging at 3 Tesla
Magnetic Resonance Materials in Physics Biology and Medicine, 2008
Objective To evaluate a transverse electromagnetic (TEM), a circularly polarized (CP) (birdcage), and a 12-channel phased array head coil at the clinical field strength of B 0 = 3T in terms of signal-to-noise ratio (SNR), signal homogeneity, and maps of the effective flip angle α. Materials and methods SNR measurements were performed on low flip angle gradient echo images. In addition, flip angle maps were generated for αnominal = 30° using the double angle method. These evaluation steps were performed on phantom and human brain data acquired with each coil. Moreover, the signal intensity variation was computed for phantom data using five different regions of interest. Results In terms of SNR, the TEM coil performs slightly better than the CP coil, but is second to the smaller 12-channel coil for human data. As expected, both the TEM and the CP coils show superior image intensity homogeneity than the 12-channel coil, and achieve larger mean effective flip angles than the combination of body and 12-channel coil with reduced radio frequency power deposition. Conclusion At 3T the benefits of TEM coil design over conventional lumped element(s) coil design start to emerge, though the phased array coil retains an advantage with respect to SNR performance.
Magnetic Resonance in Medicine, 2004
The performance of a 16-channel receive-only RF coil for brain imaging at 3.0 Tesla was investigated using a custom-built 16-channel receiver. Both the image signal-to-noise ratio (SNR) and the noise amplification (g-factor) in sensitivity-encoding (SENSE) parallel imaging applications were quantitatively evaluated. Furthermore, the performance was compared with that of hypothetical coils with one, two, four, and eight elements (n) by combining channels in software during image reconstruction. As expected, both the g-factor and SNR improved substantially with n. Compared to an equivalent (simulated) singleelement coil, the 16-channel coil showed a 1.87-fold average increase in brain SNR. This was mainly due to an increase in SNR in the peripheral brain (an up to threefold SNR increase), whereas the SNR increase in the center of the brain was 4%. The incremental SNR gains became relatively small at large n, with a 9% gain observed when n was increased from 8 to 16. Compared to the (larger) product birdcage head coil, SNR increased by close to a factor of 2 in the center, and by up to a factor of 6 in the periphery of the brain. For low SENSE acceleration (rate-2), g-factors leveled off for n > 4, and improved only slightly (1.4% averaged over brain) going from n ؍ 8 to n ؍ 16. Improvements in g for n > 8 were larger for higher acceleration rates, with the improvement for rate-3 averaging 12.0%.
NeuroImage, 2019
We investigate the utility of RF parallel transmission (pTx) for whole-brain resting-state functional MRI (rfMRI) acquisition at 7 Tesla (7T). To this end, Human Connectome Project (HCP)-style data acquisitions were chosen as a showcase example. Five healthy subjects were scanned in pTx and single-channel transmit (1Tx) modes. The pTx data were acquired using a prototype 16channel transmit system and a commercially available Nova 8-channel transmit 32-channel receive RF head coil. Additionally, pTx single-spoke multiband (MB) pulses were designed to image sagittal slices. HCP-style 7T rfMRI data (1.6-mm isotropic resolution, 5-fold slice and 2-fold inplane acceleration, 3600 volumes and ~ 1-hour scan) were acquired with pTx and the results were compared to those acquired with the original 7T HCP rfMRI protocol. The use of pTx significantly improved flip-angle uniformity across the brain, with coefficient of variation (i.e., std/ mean) of whole-brain flip-angle distribution reduced on average by ~39%. This in turn yielded ~17% increase in group temporal SNR (tSNR) as averaged across the entire brain and ~10% increase in group functional contrast-to-noise ratio (fCNR) as averaged across the grayordinate space (including cortical surfaces and subcortical voxels). Furthermore, when placing a seed in either the posterior parietal lobe or putamen estimate seed-based dense connectome, the increase in fCNR was observed to translate into stronger correlation of the seed with the rest of the grayordinate space. We have demonstrated the utility of pTx for slice-accelerated highresolution whole-brain rfMRI at 7T; as compared to current state-of-the-art, the use of pTx improves flipangle uniformity, increases tSNR, enhances fCNR and strengthens functional connectivity estimation.