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Infants and toddlers are a challenging population upon which to perform magnetic resonance imaging (MRI) of the brain, both in research and clinical settings. Because of the large range in head size during the early years of development, paediatric neuro‐MRI requires a radiofrequency (RF) coil, or set of coils, that is tailored to head size to provide the highest image quality. Mitigating techniques must also be employed to reduce and correct for subject motion. This manuscript describes an RF coil with a tailored mechanical–electrical design that can adapt to the head size of 3‐month‐old infants to 3‐year‐old toddlers. The RF coil was designed with tight‐fitting coil elements to improve the signal‐to‐noise ratio (SNR) in comparison with commercially available adult head coils, while simultaneously aiding in immobilization. The coil was designed without visual obstruction to facilitate an unimpeded view of the child's face and the potential application of camera or motion‐tracking systems. Despite the lack of elements over the face, the paediatric coil produced higher SNR over most of the brain compared with adult coils, including more than twofold in the periphery. Acceleration rates of fourfold in each Cartesian direction could be achieved. High SNR allowed for short acquisition times through accelerated imaging protocols and reduced the probability of motion during a scan. Modification of the acquisition protocol, with immobilization of the head through the adjustable coil geometry, and subsequently being combined with a motion‐tracking system, provides a compelling platform for scanning paediatric populations without sedation and with improved image quality. [ABSTRACT FROM AUTHOR]

Purpose: To develop an RF coil with an integrated commercial field camera for ultrahigh field (7T) neuroimaging. The RF coil would operate within a head‐only gradient coil and be subject to the corresponding design constraints. The RF coil can thereafter be used for subject‐specific correction of k‐space trajectories—notably in gradient‐sensitive sequences such as single‐shot spiral imaging. Methods: The transmit and receive performance was evaluated before and after the integration of field probes, whereas field probes were evaluated when in an optimal configuration external to the coil and after their integration. Diffusion‐weighted EPI and single‐shot spiral acquisitions were employed to evaluate the efficacy of correcting higher order field perturbations and the consequent effect on image quality. Results: Field probes had a negligible effect on RF‐coil performance, including the transmit efficiency, transmit uniformity, and mean SNR over the brain. Modest reductions in field‐probe signal lifetimes were observed, caused primarily by nonidealities in the gradient and shim fields of the head‐only gradient coil at the probe positions. The field‐monitoring system could correct up to second‐order field perturbations in single‐shot spiral imaging. Conclusion: The integrated RF coil and field camera was capable of concurrent‐field monitoring within a 7T head‐only scanner and facilitated the subsequent correction of k‐space trajectories during spiral imaging. [ABSTRACT FROM AUTHOR]

Many neuroscience applications have adopted functional MRI as a tool to investigate the healthy and diseased brain during the completion of a task. While ultra‐high‐field MRI has allowed for improved contrast and signal‐to‐noise ratios during functional MRI studies, it remains a challenge to create local radiofrequency coils that can accommodate an unobstructed visual field and be suitable for routine use, while at the same time not compromise performance. Performance (both during transmission and reception) can be improved by using close‐fitting coils; however, maintaining sensitivity over the whole brain often requires the introduction of coil elements proximal to the eyes, thereby partially occluding the subject's visual field. This study presents a 7 T head coil, with eight transmit dipoles and 32 receive loops, that is designed to remove visual obstructions from the subject's line of sight, allowing for an unencumbered view of visual stimuli, the reduction of anxiety induced from small enclosures, and the potential for eye‐tracking measurements. The coil provides a practical solution for routine imaging, including a split design (anterior and posterior halves) that facilitates subject positioning, including those with impaired mobility, and the placement of devices required for patient comfort and motion reduction. The transmit and receive coils displayed no degradation of performance due to adaptions to the design topology (both mechanical and electrical) required to create an unobstructed visual field. All computer‐aided design files, electromagnetic simulation models, transmit field maps and local specific absorption rate matrices are provided to promote reproduction. [ABSTRACT FROM AUTHOR]

The magnetization‐prepared two‐rapid‐gradient‐echo (MP2RAGE) sequence is used for structural T1‐weighted imaging and T1 mapping of the human brain. In this sequence, adiabatic inversion RF pulses are commonly used, which require the B1+ magnitude to be above a certain threshold. Achieving this threshold in the whole brain may not be possible at ultra‐high fields because of the short RF wavelength. This results in low‐inversion regions especially in the inferior brain (eg cerebellum and temporal lobes), which is reflected as regions of bright signal in MP2RAGE images. This study aims at eliminating the low‐inversion‐efficiency induced artifacts in MP2RAGE images at 7 T. The proposed technique takes advantage of parallel RF transmission systems by splitting the brain into two overlapping slabs and calculating the complex weights of transmit channels (ie RF shims) on these slabs for excitation and inversion independently. RF shims were calculated using fast methods implemented in the standard workflow. The excitation RF pulse was designed to obtain slabs with flat plateaus and sharp edges. These slabs were joined into a single volume during the online image reconstruction. The two‐slab strategy naturally results in a signal‐to‐noise ratio loss; however, it allowed the use of independent shims to make the B1+ field exceed the adiabatic threshold in the inferior brain, eliminating regions of low inversion efficiency. Accordingly, the normalized root‐mean‐square errors in the inversion were reduced to below 2%. The two‐slab strategy was found to outperform subject‐specific kT‐point inversion RF pulses in terms of inversion error. The proposed strategy is a simple yet effective method to eliminate low‐inversion‐efficiency artifacts; consequently, MP2RAGE‐based, artifact‐free T1‐weighted structural images were obtained in the whole brain at 7 T. [ABSTRACT FROM AUTHOR]

Purpose To develop and evaluate a local frequency shift (LFS) mapping method specifically designed for multi-echo acquisitions and multi-channel receive coils. Methods The proposed method uses the pixel-by-pixel inter-echo variance (IEV) as a weighting factor during channel-combination. Five healthy volunteers were scanned at 7 T. The IEV-weighted method was quantitatively compared to established (adaptive and Hermitian product) channel-combination methods with respect to IEV of LFS over the entire brain. Results In all experiments, the IEV-weighted method generated LFS maps free of artifacts caused by unwrapping errors. Based on measurements of the inter-echo frequency variance throughout the whole brain, the IEV-weighted method produced the lowest variation and the best contrast at the edge of the brain. Conclusion The primary finding of the present study is that accurate LFS maps are achievable if the data from each channel is processed independently prior to combination followed by a weighted combination using IEV as the weighting term. The software is freely available to the scientific community. Magn Reson Med 73:1654-1661, 2015. © 2014 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Characterization of large-scale brain networks using blood-oxygenation-level-dependent functional magnetic resonance imaging is typically based on the assumption of network stationarity across the duration of scan. Recent studies in humans have questioned this assumption by showing that within-network functional connectivity fluctuates on the order of seconds to minutes. Time-varying profiles of resting-state networks (RSNs) may relate to spontaneously shifting, electrophysiological network states and are thus mechanistically of particular importance. However, because these studies acquired data from awake subjects, the fluctuating connectivity could reflect various forms of conscious brain processing such as passive mind wandering, active monitoring, memory formation, or changes in attention and arousal during image acquisition. Here, we characterize RSN dynamics of anesthetized macaques that control for these accounts, and compare them to awake human subjects. We find that functional connectivity among nodes comprising the 'oculomotor (OCM) network' strongly fluctuated over time during awake as well as anaesthetized states. For time dependent analysis with short windows (<60 s), periods of positive functional correlations alternated with prominent anticorrelations that were missed when assessed with longer time windows. Similarly, the analysis identified network nodes that transiently link to the OCM network and did not emerge in average RSN analysis. Furthermore, time-dependent analysis reliably revealed transient states of large-scale synchronization that spanned all seeds. The results illustrate that resting-state functional connectivity is not static and that RSNs can exhibit nonstationary, spontaneous relationships irrespective of conscious, cognitive processing. The findings imply that mechanistically important network information can be missed when using average functional connectivity as the single network measure. Hum Brain Mapp 34:2154-2177, 2013. © 2011 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Parallel transmission has been used to reduce the inevitable inhomogeneous radiofrequency fields produced in human high-field MRI greater than 3 T. Further improvements in the transmit homogeneity and efficiency are possible by leveraging the additional degree of freedom permitted by multislice acquisitions. Compared to simple scaling of the flip angle to compensate for B1+ falloff along the radiofrequency coil, calculation of B1+ shim solutions on a slice-by-slice basis can markedly improve homogeneity and/or reduce transmitted power and global SAR. Performance measures were acquired at 7 T with a 15-channel head-only transceive array featuring elements distributed over all three logical axes, facilitating B1+ shimming over arbitrary orientations. Compared to a circularly polarized volume mode of the same coil, shimming to maximize excitation efficiency on a slice-by-slice basis yielded improvements in mean B1+ by 12.8 ± 2.4% and a reduction in standard deviation of B1+ of 16.3 ± 6.8%, while reducing relative SAR by 6.2 ± 3.1%. When shimming for greater uniformity, the mean and standard deviation of B1+ were further improved by 15.9 ± 2.6% and 26.2 ± 10.4%, respectively, at the expense of a 135 ± 8% increase in global SAR. Robust multislice-shim solutions are demonstrated that can be quickly calculated, applied in real time, and reliably improve on volume coil modes. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

The first 16-channel transceive surface-coil array that conforms to the human head and operates at 298 MHz (7 T) is described. Individual coil elements were decoupled using circumferential shields around each element that extended orthogonally from the former. This decoupling method allowed elements to be constructed with arbitrary shape, size, and location to create a three-dimensional array. Radiofrequency shimming achieved a transmit-field uniformity of 20% over the whole brain and 14% over a single axial slice. During radiofrequency transmission, coil elements couple tightly to the head and reduce the amount of power necessary to achieve a mean 90° flip angle (660-μs and 480-μs pulse lengths were required for a 1-kW hard pulse when shimming over the whole brain and a single axial slice, respectively). During reception, the close proximity of coil elements to the head increases the signal-to-noise ratio in the periphery of the brain, most notably at the superior aspect of the head. The sensitivity profile of each element is localized beneath the respective shield. When combined with the achieved isolation between elements, this results in the capacity for low geometry factors during both transmit and receive: 1.04/1.06 (mean) and 1.25/1.54 (maximum) for 3-by-3 acceleration in the axial/sagittal plane. High cortical signal-to-noise ratio and parallel imaging performance make the conformal coil ideal for the study of high temporal and/or spatial cortical architecture and function. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

A 15-channel transmit-receive (transceive) radiofrequency (RF) coil was developed to image the human brain at 7 T. A hybrid decoupling scheme was implemented that used both capacitive decoupling and the partial geometric overlapping of adjacent coil elements. The decoupling scheme allowed coil elements to be arrayed along all three Cartesian axes; this facilitated shimming of the transmit field, B [ABSTRACT FROM AUTHOR]

Paramagnetic chemical exchange saturation transfer (PARACEST) contrast agents are under development for biological target identification by magnetic resonance imaging. Image contrast associated with PARACEST agents can be generated by radiofrequency irradiation of the chemically shifted protons bound to a PARACEST contrast agent molecule or by direct irradiation of the on-resonance bulk water protons. The observed signal change in a magnetic resonance image after the administration of a PARACEST contrast agent is due to both altered relaxation time constants and the CEST effect. Despite high sensitivity in vitro, PARACEST agents have had limited success in vivo where sensitivity is reduced by the magnetization transfer effect from endogenous macromolecules. The purpose of this study was to demonstrate the in vivo detection of a PARACEST contrast agent using the on-resonance paramagnetic chemical exchange effect (OPARACHEE) in a mouse glioblastoma multiforme tumor model and to isolate the OPARACHEE effect from the changes in relaxation induced by the PARACEST agent. Three mice with tumors were imaged on a 9.4 T MRI scanner following tail vein injection of 150 μL 50 mM Tm-DOTAM-glycine-lysine. A fast low angle shot pulse sequence with a low power radiofrequency pulse train (WALTZ-16) as the preparation pulse was used to generate OPARACHEE contrast. To study the dynamics of agent uptake, reference images (without the preparation pulse) and OPARACHEE images were acquired continuously in an alternating fashion before, during and after agent injection. Signal intensity decreased by more than 10% in tumor in the control images after agent administration. Despite these changes, a clear OPARACHEE contrast of 1-5% was also observed in brain tumors after contrast agent injection and maintained in the hour following injection. This result is the first in vivo observation of OPARACHEE contrast in brain tumors with correction of T and T relaxation effects. Magn Reson Med, 2011. © 2011 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

A novel method for decoupling coil elements of transmit/receive (transceive) arrays is reported. Each element of a coil array is shielded both concentrically and radially to reduce the magnetic flux linkage between neighboring coils; this substantially reduces the mutual inductance between coil elements and allows them to behave independently. A six-channel transceive coil was developed using this decoupling scheme and compared with two conventional decoupling schemes: the partial overlapping of adjacent elements and capacitive decoupling. The radiofrequency coils were designed to image the human head and were tested on a 7-T Varian scanner. The decoupling, transmit uniformity, transmit efficiency, signal-to-noise ratio, and geometry factors were compared between coils. The individually shielded coil achieved higher minimum isolation between elements (2.7-4.0 dB) and lower geometry factors (2-14%) than the overlapped and capacitively decoupled coils, while showing a reduction in transmit efficiency (2.8-5.9 dB) and signal-to-noise ratio (up to 34%). No difference was found in the power absorbed by the sample during a 90° radiofrequency pulse. The inset distance of coil elements within their shields was then reduced, resulting in significant improvement of the transmit efficiency (1.3 dB) and signal-to-noise ratio (28%). The greatest asset of this decoupling method lies in its versatility: transceive coils can be created with elements of arbitrary shape, size, location, and resonant frequency to produce three-dimensional conformal arrays. Magn Reson Med, 2010. © 2010 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

The purpose of the present study was to determine the cortical structures involved with integrated baroreceptor-mediated modulation of autonomic cardiovascular function in conscious humans independent of changes in arterial blood pressure. We assessed the brain regions associated with lower body negative pressure (LBNP)-induced baroreflex control using functional magnetic resonance imaging with blood oxygen level-dependent (BOLD) contrast in eight healthy male volunteer subjects. The levels of LBNP administered were 5, 15 and 35 mmHg. Heart rate (HR; representing the cardiovascular response) and LBNP (representing the baroreceptor activation level) were simultaneously monitored during the scanning period. In addition, estimated central venous pressure (CVP), arterial blood pressure (ABP) and muscle sympathetic nerve activity were recorded on a separate session. Random effects analyses (SPM2) were used to evaluate significant ( P < 0.05) BOLD signal changes that correlated separately with both LBNP and HR (15- and 35-mmHg versus 5-mmHg LBNP). Compared to baseline, steady-state LBNP at 15 and 35 mmHg decreased CVP (from 7 ± 1 to 5 ± 1 and 4 ± 1 mmHg, respectively) and increased MSNA (from 12 ± 1 to 23 ± 3 and 36 ± 4 bursts min−1, respectively, both P < 0.05 versus baseline). Furthermore, steady-state LBNP elevated HR from 54 ± 2 beats min−1 at baseline to 64 ± 2 beats min−1 at 35-mmHg suction. Both mean arterial and pulse pressure were not different between rest and any level of LBNP. Cortical regions demonstrating increased activity that correlated with higher HR and greater LBNP included the right superior posterior insula, frontoparietal cortex and the left cerebellum. Conversely, using the identical statistical paradigm, bilateral anterior insular cortices, the right anterior cingulate, orbitofrontal cortex, amygdala, midbrain and mediodorsal nucleus of the thalamus showed decreased neural activation. These data corroborate previous investigations highlighting the involved roles of the insula, anterior cingulate cortex and amygdala in central autonomic cardiovascular control. In addition, we have provided the first evidence for the identification of the cortical network involved specifically with baroreflex-mediated autonomic cardiovascular function in conscious humans. [ABSTRACT FROM AUTHOR]

We conducted two functional magnetic resonance imaging (fMRI) experiments that examined novelty responses in the human medial temporal lobe (MTL) to determine whether the hippocampus makes contributions to memory processing that differ from those of structures in the adjacent parahippocampal region. In light of proposals that such differential contributions may pertain to relational processing demands, we assessed event-related fMRI responses in the MTL for novel single objects and for novel spatial and non-spatial object relationships; subjects were asked to detect these different types of novelties among previously studied items, and they successfully performed this task during scanning. A double dissociation that emerged from the response pattern of regions in the hippocampus and perirhinal cortex provided the strongest support for functional specialization in the MTL. A region in the right middle hippocampus responded to the novelty of spatial and non-spatial relationships but not to the novelty of individual objects. By contrast, a region in right perirhinal cortex, situated in the anterior collateral sulcus, responded to the novelty of individual objects but not to that of either type of relationship. Other MTL regions that responded to novelty in the present study showed no reliable difference in their response to the various novelty types; these regions included anterior parts of the hippocampus and posterior aspects of parahippocampal cortex. Together, our findings indicate that relational processing demands are a critical determinant of functional specialization in the human MTL. They also suggest, however, that a neuroanatomical framework that only distinguishes between the hippocampus and the parahippocampal region is not sufficiently refined to account for all functional differences and similarities observed with respect to relational processes in the human MTL. ©2005 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]