Optimization of pseudo-continuous arterial spin labeling at 7T with parallel transmission B1 shimming

PURPOSE

To optimize pseudo-continuous arterial spin labeling (pCASL) for 7 T, and to further improve the labeling efficiency with parallel RF transmission transmit B1 ( B 1 + ) shimming.

METHODS

pCASL parameters were optimized based on B 1 + / B 0 field distributions at 7 T with simulation. To increase labeling efficiency, the B 1 + amplitude at inflowing arteries was increased with parallel RF transmission B 1 + shimming. The "indv-shim" with shimming weights calculated for each individual subject, and the "univ-shim" with universal weights calculated on a group of 12 subjects, were compared with circular polarized (CP) shim. The optimized pCASL sequences with three B 1 + shimming modes (indv-shim, univ-shim, and CP-shim) were evaluated in 6 subjects who underwent two repeated scans 24 hours apart, along with a pulsed ASL sequence. Quantitative metrics including mean B 1 + amplitude, perfusion, and intraclass correlation coefficient were calculated. The optimized 7T pCASL was compared with standard 3T pCASL on 5 subjects, using spatial SNR and temporal SNR.

RESULTS

The optimal pCASL parameter set (RF duration/gap = 300/250 us, G ave = 0.6 mT / m , g R a t i o = 10 ) achieved robust perfusion measurement in the presence of B 1 + / B 0 inhomogeneities. Both indv-shim and univ-shim significantly increased B 1 + amplitude compared with CP-shim in simulation and in vivo experiment (P < .01). Compared with CP-shim, perfusion signal was increased by 9.5% with indv-shim (P < .05) and by 5.3% with univ-shim (P = .35). All three pCASL sequences achieved fair to good repeatability (intraclass correlation coefficient ≥ 0.5). Compared with 3T pCASL, the optimized 7T pCASL achieved 78.3% higher spatial SNR and 200% higher temporal SNR.

CONCLUSION

The optimized pCASL achieved robust perfusion imaging at 7 T, while both indv-shim and univ-shim further increased labeling efficiency.

Advances in resting state fMRI acquisitions for functional connectomics

Resting state functional magnetic resonance imaging (rs-fMRI) is based on spontaneous fluctuations in the blood oxygen level dependent (BOLD) signal, which occur simultaneously in different brain regions, without the subject performing an explicit task. The low-frequency oscillations of the rs-fMRI signal demonstrate an intrinsic spatiotemporal organization in the brain (brain networks) that may relate to the underlying neural activity. In this review article, we briefly describe the current acquisition techniques for rs-fMRI data, from the most common approaches for resting state acquisition strategies, to more recent investigations with dedicated hardware and ultra-high fields. Specific sequences that allow very fast acquisitions, or multiple echoes, are discussed next. We then consider how acquisition methods weighted towards specific parts of the BOLD signal, like the Cerebral Blood Flow (CBF) or Volume (CBV), can provide more spatially specific network information. These approaches are being developed alongside the commonly used BOLD-weighted acquisitions. Finally, specific applications of rs-fMRI to challenging regions such as the laminae in the neocortex, and the networks within the large areas of subcortical white matter regions are discussed. We finish the review with recommendations for acquisition strategies for a range of typical applications of resting state fMRI.

Regional and depth-dependence of cortical blood-flow assessed with high-resolution Arterial Spin Labeling (ASL)

Methods for imaging of cerebral blood flow do not typically resolve the cortex and thus underestimate flow. However, recent work with high-resolution MRI has emphasized the regional and depth-dependent structural, functional and relaxation times variations within the cortex. Using high-resolution Arterial Spin Labeling (ASL) and T₁ mapping acquisitions, we sought to probe the effects of spatial resolution and tissue heterogeneity on cortical cerebral blood flow (CBF) measurements with ASL. We acquired high-resolution (1.6mm)³ whole brain ASL data in a cohort of 10 volunteers at 3T, along with T₁ and transit-time (ATT) mapping, followed by group cortical surface-based analysis using FreeSurfer of the different measured parameters. Fully resolved regional analysis showed higher than average mid-thickness CBF in primary motor areas (+15%,p<0.002), frontal regions (+17%,p<0.01) and auditory cortex, while occipital regions had lower average CBF (-20%,p<10^-5). ASL signal was higher towards the pial surface but correction for the shorter T₁ near the white matter surface reverses this gradient, at least when using the low-resolution ATT map. Similar to structural measures, fully-resolved ASL CBF measures show significant differences across cortical regions. Depth-dependent variation of T₁ in the cortex complicates interpretation of depth-dependent ASL signal and may have implications for the accurate CBF quantification at lower resolutions.

Linking cortical circuit models to human cognition with laminar fMRI

Laboratory animal research has provided significant knowledge into the function of cortical circuits at the laminar level, which has yet to be fully leveraged towards insights about human brain function on a similar spatiotemporal scale. The use of functional magnetic resonance imaging (fMRI) in conjunction with neural models provides new opportunities to gain important insights from current knowledge. During the last five years, human studies have demonstrated the value of high-resolution fMRI to study laminar-specific activity in the human brain. This is mostly performed at ultra-high-field strengths (≥ 7 T) and is known as laminar fMRI. Advancements in laminar fMRI are beginning to open new possibilities for studying questions in basic cognitive neuroscience. In this paper, we first review recent methodological advances in laminar fMRI and describe recent human laminar fMRI studies. Then, we discuss how the use of laminar fMRI can help bridge the gap between cortical circuit models and human cognition.

Partial volume correction in arterial spin labeling perfusion MRI: A method to disentangle anatomy from physiology or an analysis step too far?

The mismatch in the spatial resolution of Arterial Spin Labeling (ASL) MRI perfusion images and the anatomy of functionally distinct tissues in the brain leads to a partial volume effect (PVE), which in turn confounds the estimation of perfusion into a specific tissue of interest such as gray or white matter. This confound occurs because the image voxels contain a mixture of tissues with disparate perfusion properties, leading to estimated perfusion values that reflect primarily the volume proportions of tissues in the voxel rather than the perfusion of any particular tissue of interest within that volume. It is already recognized that PVE influences studies of brain perfusion, and that its effect might be even more evident in studies where changes in perfusion are co-incident with alterations in brain structure, such as studies involving a comparison between an atrophic patient population vs control subjects, or studies comparing subjects over a wide range of ages. However, the application of PVE correction (PVEc) is currently limited and the employed methodologies remain inconsistent. In this article, we outline the influence of PVE in ASL measurements of perfusion, explain the main principles of PVEc, and provide a critique of the current state of the art for the use of such methods. Furthermore, we examine the current use of PVEc in perfusion studies and whether there is evidence to support its wider adoption. We conclude that there is sound theoretical motivation for the use of PVEc alongside conventional, 'uncorrected', images, and encourage such combined reporting. Methods for PVEc are now available within standard neuroimaging toolboxes, which makes our recommendation straightforward to implement. However, there is still more work to be done to establish the value of PVEc as well as the efficacy and robustness of existing PVEc methods.

An upward 9.4 T static magnetic field inhibits DNA synthesis and increases ROS-P53 to suppress lung cancer growth

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Upward 9.4 T SMF exposure for 88 h significantly inhibited A549 tumor growth in mice.

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9.4 T SMF treatment for 88 h had no severe impairment to the mice key organs or blood cell count.

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Upward 9.4 T SMF treatment for 24 h caused A549 DNA synthesis inhibition.

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Upward 9.4 T SMF treatment for 24 h significantly increased ROS and P53 levels, and caused G2 cell cycle arrest.

Studies have shown that 9.4 Tesla (9.4 T) high-field magnetic resonance imaging (MRI) has obvious advantages in improving image resolution and capacity, but their safety issues need to be further validated before their clinical approval. Meanwhile, emerging experimental evidences show that moderate to high intensity Static Magnetic Fields (SMFs) have some anti-cancer effects.

We examined the effects of two opposite SMF directions on lung cancer bearing mice and found when the lung cancer cell-bearing mice were treated with 9.4 T SMFs for 88 h in total, the upward 9.4 T SMF significantly inhibited A549 tumor growth (tumor growth inhibition=41%), but not the downward 9.4 T SMF. In vitro cellular analysis shows that 9.4 T upward SMF treatment for 24 h not only inhibited A549 DNA synthesis, but also significantly increased ROS and P53 levels, and arrested G2 cell cycle. Moreover, the 9.4 T SMF-treatments for 88 h had no severe impairment to the key organs or blood cell count of the mice.

Our findings demonstrated the safety of 9.4 T SMF long-term exposure for their future applications in MRI, and revealed the anti-cancer potential of the upward direction 9.4 T SMF.

Optimization of adiabatic pulses for pulsed arterial spin labeling at 7 tesla: Comparison with pseudo-continuous arterial spin labeling

PURPOSE

To optimize and evaluate adiabatic pulses for pulsed arterial spin labeling at ultrahigh field 7 tesla.

METHODS

Four common adiabatic inversion pulses, including hyperbolic secant, wideband uniform rate smooth truncation, frequency offset corrected inversion, and time-resampled frequency offset corrected inversion pulses, were optimized based on a custom-defined loss function that included labeling efficiency and inversion band uniformity. The optimized pulses were implemented in flow-sensitive alternating inversion recovery sequences and tested on phantom and 11 healthy volunteers with 2 constraints: 1) specific absorption rate normalized; and 2) equal peak RF amplitude, respectively. A pseudo-continuous arterial spin labeling sequence was implemented for comparison. Quantitative metrics such as perfusion and relative labeling efficiency versus residual tissue signal were calculated.

RESULTS

Among the 4 pulses, the wideband uniform rate smooth truncation pulse yielded the lowest loss in simulation and achieved a good balance between labeling efficiency and residual tissue signal from both phantom and in vivo experiments. Wideband uniform rate smooth truncation-pulsed arterial spin labeling showed significantly higher relative labeling efficiency compared to the other sequences (P < .01), whereas the perfusion signal was increased by 40% when the highest B 1 + amplitude was used. The 4 pulsed arterial spin labeling sequences yielded comparable perfusion signals compared to pseudo-continuous arterial spin labeling but with less than half the specific absorption rate.

CONCLUSION

Optimized wideband uniform rate smooth truncation pulse with the highest B 1 + amplitude allowed was recommended for 7 tesla pulsed arterial spin labeling.

New acquisition techniques and their prospects for the achievable resolution of fMRI

This work reviews recent advances in technologies for functional magnetic resonance imaging (fMRI) of the human brain and highlights the push for higher functional specificity based on increased spatial resolution and specific MR contrasts to reveal previously undetectable functional properties of small-scale cortical structures. We discuss how the combination of MR hardware, advanced acquisition techniques and various MR contrast mechanisms have enabled recent progress in functional neuroimaging. However, these advanced fMRI practices have only been applied to a handful of neuroscience questions to date, with the majority of the neuroscience community still using conventional imaging techniques. We thus discuss upcoming challenges and possibilities for fMRI technology development in human neuroscience. We hope that readers interested in functional brain imaging acquire an understanding of current and novel developments and potential future applications, even if they don't have a background in MR physics or engineering. We summarize the capabilities of standard fMRI acquisition schemes with pointers to relevant literature and comprehensive reviews and introduce more recent developments.

Using variable-rate selective excitation (VERSE) radiofrequency pulses to reduce power deposition in pulsed arterial spin labeling sequence at 7 Tesla

PURPOSE

Arterial spin labeling (ASL) is an established noninvasive MRI technique used for cerebral blood flow measurement, which generally suffers from a low signal-to-noise ratio (SNR). The use of ultra-high fields to enhance sensitivity inevitably results in an increase in TR because of specific absorption rate (SAR) constraints, causing inadequate sampling of hemodynamic response in functional MRI, and adversely affecting concurrent measurement such as blood oxygen level dependent. To address this problem, variable-rate selective excitation (VERSE) radiofrequency (RF) pulses were used.

METHODS

The conventional (sinc) selective RF pulses of the Q2TIPS block in the PICORE pulsed ASL (PASL) sequence used for blood saturation were replaced by their equivalent VERSE RF waveforms. Nine healthy volunteers were scanned using the conventional and VERSE PASL sequences on a head-only 7T scanner operating in parallel transmit mode.

RESULTS

VERSE PASL sequence provides perfusion images similar to the conventional version, with comparable perfusion SNR (conventional, 3.33 ± 0.48; VERSE, 3.26 ± 0.55) and temporal SNR (conventional, 1.02 ± 0.20; VERSE, 1.05 ± 0.12) for TR = 3.5 seconds and 70 measurements. With shorter acquisition time (TR = 2.5 seconds), VERSE PASL sequence still provides similar results to those acquired using the conventional PASL sequence with longer TR = 3.5 seconds.

CONCLUSION

The use of VERSE RF pulses in the Q2TIPS block of a PASL sequence allowed for the reduction of RF power deposition and, consequently, an increase in the temporal resolution and/or perfusion SNR.

High resolution continuous arterial spin labeling of human cerebral perfusion using a separate neck tagging RF coil

For standard clinical applications, ASL images are typically acquired with 4–8 mm thick slices and 3–4 mm in-plane resolution. However, in this paper we demonstrate that high-resolution continuous arterial spin labeling (CASL) perfusion images can be acquired in a clinically relevant scan time using current MRI technology. CASL was implemented with a separate neck coil for labeling the arterial blood on a 4.7T MRI using standard axial 2D GE-EPI. Typical-resolution to high-resolution (voxels of 95, 60, 45, 27, or 7 mm³) images were compared for qualitative and quantitative cerebral blood flow analysis (CBF) in nine healthy volunteers (ages: 24–32 years). The highest resolution (1.5x1.5x3 = 7 mm³) CASL implementation yielded perfusion images with improved cortex depiction and increased cortical CBF measurements (53 ± 8 ml/100g/min), consistent with reduced partial volume averaging. The 7 mm³ voxel images were acquired with 6 cm brain coverage in a clinically relevant scan of 6 minutes. Improved spatial resolution facilitates CBF measurement with reduced partial volume averaging and may be valuable for the detection of perfusion deficits in small lesions and perfusion measurement in small brain regions.

Ultra-high resolution blood volume fMRI and BOLD fMRI in humans at 9.4 T: Capabilities and challenges

Functional mapping of cerebral blood volume (CBV) changes has the potential to reveal brain activity with high localization specificity at the level of cortical layers and columns. Non-invasive CBV imaging using Vascular Space Occupancy (VASO) at ultra-high magnetic field strengths promises high spatial specificity but poses unique challenges in human applications. As such, 9.4 T B1+ and B0 inhomogeneities limit efficient blood tagging, while the specific absorption rate (SAR) constraints limit the application of VASO-specific RF pulses. Moreover, short T2* values at 9.4 T require short readout duration, and long T1 values at 9.4 T can cause blood-inflow contaminations. In this study, we investigated the applicability of layer-dependent CBV-fMRI at 9.4 T in humans. We addressed the aforementioned challenges by combining multiple technical advancements: temporally alternating pTx B1+ shimming parameters, advanced adiabatic RF-pulses, 3D-EPI signal readout, optimized GRAPPA acquisition and reconstruction, and stability-optimized RF channel combination. We found that a combination of suitable advanced methodology alleviates the challenges and potential artifacts, and that VASO fMRI provides reliable measures of CBV change across cortical layers in humans at 9.4 T. The localization specificity of CBV-fMRI, combined with the high sensitivity of 9.4 T, makes this method an important tool for future studies investigating cortical micro-circuitry in humans.

Cellular ATP levels are affected by moderate and strong static magnetic fields

Mitochondrion is the major cellular energy producing organelle that is at the boundary between chemical reactions and physical processes. Although mitochondria have been shown to be affected by physical methods such as nonthermal plasma, whether static magnetic field (SMF) could also affect them is still unclear. Here we used rat adrenal PC12 cells to compare SMFs of different intensities for their effects on ATP (adenosine-5'-triphosphate), the major energy source produced by mitochondria, which is essential for various cellular processes. Our results show that although 0.26 or 0.50 T SMFs did not affect ATP, 1 T and 9 T SMFs affected ATP level differently and time-dependently. Moreover, SMF-induced ATP level fluctuations are correlated with mitochondrial membrane potential changes. Our study provides insights not only into understanding various cellular effects of SMFs, but also the potential clinical applications of SMFs. Bioelectromagnetics. 39:352-360, 2018. © 2018 Wiley Periodicals, Inc.

Pulse sequences and parallel imaging for high spatiotemporal resolution MRI at ultra-high field

The SNR and CNR benefits of ultra-high field (UHF) have helped push the envelope of achievable spatial resolution in MRI. For applications based on susceptibility contrast where there is a large CNR gain, high quality sub-millimeter resolution imaging is now being routinely performed, particularly in fMRI and phase imaging/QSM. This has enabled the study of structure and function of very fine-scale structures in the brain. UHF has also helped push the spatial resolution of many other MRI applications as will be outlined in this review. However, this push in resolution comes at a cost of a large encoding burden leading to very lengthy scans. Developments in parallel imaging with controlled aliasing and the move away from 2D slice-by-slice imaging to much more SNR-efficient simultaneous multi-slice (SMS) and 3D acquisitions have helped address this issue. In particular, these developments have revolutionized the efficiency of UHF MRI to enable high spatiotemporal resolution imaging at an order of magnitude faster acquisition. In addition to describing the main approaches to these techniques, this review will also outline important key practical considerations in using these methods in practice. Furthermore, new RF pulse design to tackle the B1+ and SAR issues of UHF and the increased SAR and power requirement of SMS RF pulses will also be touched upon. Finally, an outlook into new developments of smart encoding in more dimensions, particularly through using better temporal/across-contrast encoding and reconstruction will be described. Just as controlled aliasing fully exploits spatial encoding in parallel imaging to provide large multiplicative gains in accelerations, the complimentary use of these new approaches in temporal and across-contrast encoding are expected to provide exciting opportunities for further large gains in efficiency to further push the spatiotemporal resolution of MRI.

Spatio-temporal TGV denoising for ASL perfusion imaging

In arterial spin labeling (ASL) a perfusion weighted image is achieved by subtracting a label image from a control image. This perfusion weighted image has an intrinsically low signal to noise ratio and numerous measurements are required to achieve reliable image quality, especially at higher spatial resolutions. To overcome this limitation various denoising approaches have been published using the perfusion weighted image as input for denoising. In this study we propose a new spatio-temporal filtering approach based on total generalized variation (TGV) regularization which exploits the inherent information of control and label pairs simultaneously. In this way, the temporal and spatial similarities of all images are used to jointly denoise the control and label images. To assess the effect of denoising, virtual ground truth data were produced at different SNR levels. Furthermore, high-resolution in-vivo pulsed ASL data sets were acquired and processed. The results show improved image quality, quantitative accuracy and robustness against outliers compared to seven state of the art denoising approaches.

Optimization of simultaneous multislice EPI for concurrent functional perfusion and BOLD signal measurements at 7T

Purpose

To overcome limitations of previous ultra‐high‐field arterial spin labeling (ASL) techniques concerning temporal resolution and brain coverage by utilizing the simultaneous multi‐slice (SMS) approach.

Methods

An optimized, flow‐alternating inversion recovery quantitative imaging of perfusion using a single subtraction II scheme was developed that tackles the challenges of 7 tesla (T) ASL. The implementation of tailored labeling radiofrequency pulses reduced the effect of transmit field ( B1+) inhomogeneities. The proposed approach utilizes an SMS echo‐planar imaging (EPI) readout to efficiently achieve large brain coverage.

Results

A pulsed ASL (PASL) technique with large brain coverage is described and optimized that can be applied at temporal resolutions below 2.5 s, similar to those achievable at 1.5 and 3T magnetic field strength. The influences of within‐ and through‐slice acceleration factors and reconstruction parameters on perfusion and blood‐oxygenation‐level‐dependent (BOLD)‐signal image and temporal signal‐to‐noise ratio (SNR) are presented. The proposed approach yielded twice the brain coverage as compared to conventional PASL at 7T, without notable loss in image quality.

Conclusion

The presented SMS EPI PASL at 7T overcomes current limitations in SNR, temporal resolution, and spatial coverage for functional perfusion and BOLD signal as well as baseline perfusion measurements. Magn Reson Med 78:121–129, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution‐NonCommercial‐NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non‐commercial and no modifications or adaptations are made.

Probing neuronal activation by functional quantitative susceptibility mapping under a visual paradigm: A group level comparison with BOLD fMRI and PET

Dynamic changes of brain-tissue magnetic susceptibility provide the basis for functional MR imaging (fMRI) via T2*-weighted signal-intensity modulations. Promising initial work on a detection of neuronal activity via quantitative susceptibility mapping (fQSM) has been published but consistently reported on ill-understood positive and negative activation patterns (Balla et al., 2014; Chen and Calhoun, 2015a). We set out to (i) demonstrate that fQSM can exploit established fMRI data acquisition and processing methods and to (ii) better describe aspects of the apparent activation patterns using fMRI and PET as standards of reference. Under a standardized visual-stimulation paradigm PET and 3-T gradient-echo EPI-based fQSM, fMRI data from 9 healthy volunteers were acquired and analyzed by means of Independent Component Analysis (ICA) at subject level and, for the first time, at group level. Numbers of activated (z-score>2.0) voxels were counted and their mean z-scores calculated in volumes of interest (occipital lobe (Nocc_lobe), segmented occipital gray-matter (NGM_occ_lobe), large veins (Nveins)), and in occipital-lobe voxels commonly activated in fQSM and fMRI component maps. Common but not entirely congruent regions of apparent activation were found in the occipital lobe in z-score maps from all modalities, fQSM, fMRI and PET, with distinct BOLD-negatively correlated regions in fQSM data. At subject-level, Nocc_lobe, NGM_occ_lobe and their mean z-scores were significantly smaller in fQSM than in fMRI, but their ratio, NGM_occ_lobe/Nocc_lobe, was comparable. Nveins did not statistically differ and the ratio Nveins/NGM_occ_lobe as well as the mean z-scores were higher for fQSM than for fMRI. In veins and immediate vicinity, z-score maps derived from both phase and fQSM-data showed positive and negative lobes resembling dipole shapes in simulated field and phase maps with no correlate in fMRI or PET data. Our results show that standard fMRI tools can directly be used for fQSM processing, and suggest that fQSM may have the potential to detect gray-matter activation distant from large veins, to improve detection of veins with stimulus-induced venous oxygen saturation (SvO2) variations, and to better localize areas of activation. However, our results seem to clearly expose issues that phenomenologically resemble an incomplete dipolar inversion and that need to be subject to further investigation.

Pulsed arterial spin labelling at ultra-high field with a B 1 (+) -optimised adiabatic labelling pulse

OBJECTIVE

Arterial spin labelling (ASL) techniques benefit from the increased signal-to-noise ratio and the longer T 1 relaxation times available at ultra-high field. Previous pulsed ASL studies at 7 T concentrated on the superior regions of the brain because of the larger transmit radiofrequency inhomogeneity experienced at ultra-high field that hinders an adequate inversion of the blood bolus when labelling in the neck. Recently, researchers have proposed to overcome this problem with either the use of dielectric pads, through dedicated transmit labelling coils, or special adiabatic inversion pulses.

MATERIALS AND METHODS

We investigate the performance of an optimised time-resampled frequency-offset corrected inversion (TR-FOCI) pulse designed to cause inversion at much lower peak B 1 (+) . In combination with a PICORE labelling, the perfusion signal obtained with this pulse is compared against that obtained with a FOCI pulse, with and without dielectric pads.

RESULTS

Mean grey matter perfusion with the TR-FOCI was 52.5 ± 10.3 mL/100 g/min, being significantly higher than the 34.6 ± 2.6 mL/100 g/min obtained with the FOCI pulse. No significant effect of the dielectric pads was observed.

CONCLUSION

The usage of the B 1 (+) -optimised TR-FOCI pulse results in a significantly higher perfusion signal. PICORE-ASL is feasible at ultra-high field with no changes to operating conditions.