Fast Hadamard-Encoded 7T Spectroscopic Imaging of Human Brain.

: The increased SNR available at 7T combined with fast readout trajectories enables accelerated spectroscopic imaging acquisitions for clinical applications. In this report, we evaluate the performance of a Hadamard slice encoding strategy with a 2D rosette trajectory for multi-slice fast spectroscopic imaging at 7T. : Moderate-TE (~40 ms) spin echo and J-refocused polarization transfer sequences were acquired with simultaneous Hadamard multi-slice excitations and rosette in-plane encoding.

Adolescent maturation of dorsolateral prefrontal cortex glutamate:GABA and cognitive function is supported by dopamine-related neurobiology.

Developmental changes in prefrontal cortex (PFC) excitatory (glutamatergic, Glu) and inhibitory (gamma- aminobutryic acid, GABA) neurotransmitter balance (E:I) have been identified during human adolescence, potentially reflecting a critical period of plasticity that supports the maturation of PFC-dependent cognition. Animal models implicate increases in dopamine (DA) in regulating changes in PFC E:I during critical periods of development, however, mechanistic relationships between DA and E:I have not been studied in humans. Here, we used high field (7T) echo planar imaging (EPI) in combination with Magnetic Resonance Spectroscopic Imaging (MRSI) to assess the role of basal ganglia tissue iron-reflecting DA neurophysiology-in longitudinal trajectories of dorsolateral PFC Glu, GABA, and their relative levels (Glu:GABA) and working memory performance from adolescence to adulthood in 153 participants (ages 10-32 years old, 1-3 visits, 272 visits total).

Prefrontal excitation/inhibition balance supports adolescent enhancements in circuit signal to noise ratio.

The development and refinement of neuronal circuitry allow for stabilized and efficient neural recruitment, supporting adult-like behavioral performance. During adolescence, the maturation of PFC is proposed to be a critical period (CP) for executive function, driven by a break in balance between glutamatergic excitation and GABAergic inhibition (E/I) neurotransmission. During CPs, cortical circuitry fine-tunes to improve information processing and reliable responses to stimuli, shifting from spontaneous to evoked activity, enhancing the SNR, and promoting neural synchronization.

Leveraging ultra-high field (7T) MRI in psychiatric research.

Non-invasive brain imaging has played a critical role in establishing our understanding of the neural properties that contribute to the emergence of psychiatric disorders. However, characterizing core neurobiological mechanisms of psychiatric symptomatology requires greater structural, functional, and neurochemical specificity than is typically obtainable with standard field strength MRI acquisitions (e.g.

Prefrontal Excitation/ Inhibition Balance Supports Adolescent Enhancements in Circuit Signal to Noise Ratio.

The development and refinement of neuronal circuitry allow for stabilized and efficient neural recruitment, supporting adult-like behavioral performance. During adolescence, the maturation of PFC is proposed to be a critical period (CP) for executive function, driven by a break in balance between glutamatergic excitation and GABAergic inhibition (E/I) neurotransmission. During CPs, cortical circuitry fine-tunes to improve information processing and reliable responses to stimuli, shifting from spontaneous to evoked activity, enhancing the SNR, and promoting neural synchronization.

Aperiodic EEG and 7T MRSI evidence for maturation of E/I balance supporting the development of working memory through adolescence.

Adolescence has been hypothesized to be a critical period for the development of human association cortex and higher-order cognition. A defining feature of critical period development is a shift in the excitation: inhibition (E/I) balance of neural circuitry, however how changes in E/I may enhance cortical circuit function to support maturational improvements in cognitive capacities is not known. Harnessing ultra-high field 7 T MR spectroscopy and EEG in a large, longitudinal cohort of youth (N = 164, ages 10-32 years old, 347 neuroimaging sessions), we delineate biologically specific associations between age-related changes in excitatory glutamate and inhibitory GABA neurotransmitters and EEG-derived measures of aperiodic neural activity reflective of E/I balance in prefrontal association cortex.

Evaluation of the performance of a 7-T 8 × 2 transceiver array.

The decoupled 8 × 2 transceiver array has been shown to achieve a mean B of 11.7 uT with a coefficient of variation of ~11% over the intracranial brain volume for 7-T MR imaging. However, this array may be thought to give lower signal-to-noise ratio (SNR) and higher g-factors for parallel imaging compared with a radio frequency (RF) receive-only coil due to the latter's higher coil count and use of coil overlap to reduce the mutual impedance.

Map-based B shimming for single voxel brain spectroscopy at 7T.

While B shimming is an important requirement for in vivo brain spectroscopy, for single voxel spectroscopy (SVS), the role for advanced shim methods has been questioned. Specifically, with the small spatial dimensions of the voxel, the extent to which inhomogeneities higher than second order exist and the ability of higher order shims to correct them is controversial. To assess this, we acquired SVS from two loci of neurophysiological interest, the rostral prefrontal cortex (rPFC; 8 cc) and hippocampus (Hc; 9 cc).

N-Acetylaspartate and Choline Metabolites in Cortical and Subcortical Regions in Clinical High Risk Relative to Healthy Control Subjects: An Exploratory 7T MRSI Study.

N-acetylaspartate (NAA) and choline (Cho) are two brain metabolites implicated in several key neuronal functions. Abnormalities in these metabolites have been reported in both early course and chronic patients with schizophrenia (SCZ). It is, however, unclear whether NAA and Cho's alterations occur even before the onset of the disorder.

Dorsolateral Prefrontal Cortex Glutamate/Gamma-Aminobutyric Acid (GABA) Alterations in Clinical High Risk and First-Episode Schizophrenia: A Preliminary 7-T Magnetic Resonance Spectroscopy Imaging Study.

Converging lines of evidence suggest that an imbalance between excitation and inhibition is present in the dorsolateral prefrontal cortex (DLPFC) of schizophrenia (SCZ). Gamma-aminobutyric-acid (GABA) and, to a lesser extent, glutamate (Glu) abnormalities were reported in the DLPFC of SCZ patients, especially on the right hemisphere, by post-mortem studies. However, in vivo evidence of GABA, Glu, and Glu/GABA DLPFC abnormalities, particularly on the right side and the early stages of illness, is limited.

Development of frontal GABA and glutamate supports excitation/inhibition balance from adolescence into adulthood.

Animal and human postmortem studies provide evidence for changes in gamma-aminobutyric acid (GABA) and glutamate in prefrontal cortex (PFC) during adolescence, suggesting shifts in excitation and inhibition balance consistent with critical period plasticity. However, how GABA and glutamate change through adolescence and how the balance of these inhibitory and excitatory neurotransmitters changes is not well understood in vivo in humans. High field (7 Tesla) Magnetic Resonance Spectroscopic Imaging was used to investigate age-related changes in the balance of GABA/creatine (Cr) and glutamate/Cr in multiple developmentally-relevant regions of frontal cortex in 144 10-30-year-olds.

A longitudinal investigation of GABA, glutamate, and glutamine across the insula during antipsychotic treatment of first-episode schizophrenia.

Individuals with first-episode schizophrenia (FES) typically present with acute psychotic symptoms. Though antipsychotic drugs are the mainstay for treatment, the neurobiology underlying successful treatment remains largely elusive. Recent evidence from functional connectivity studies highlights the insula as a key structure in the neural mechanism of response.

Fast, regional three-dimensional hybrid (1D-Hadamard 2D-rosette) proton MR spectroscopic imaging in the human temporal lobes.

H-MRSI is commonly performed with gradient phase encoding, due to its simplicity and minimal radio frequency (RF) heating (specific absorption rate). Its two well-known main problems-(i) "voxel bleed" due to the intrinsic point-spread function, and (ii) chemical shift displacement error (CSDE) when slice-selective RF pulses are used, which worsens with increasing volume of interest (VOI) size-have long become accepted as unavoidable. Both problems can be mitigated with Hadamard multislice RF encoding.

MR spectroscopic imaging at 3 T and outcomes in surgical epilepsy.

For the spectroscopic assessment of brain disorders that require large-volume coverage, the requirements of RF performance and field homogeneity are high. For epilepsy, this is also challenging given the inter-patient variation in location, severity and subtlety of anatomical identification and its tendency to involve the temporal region. We apply a targeted method to examine the utility of large-volume MR spectroscopic imaging (MRSI) in surgical epilepsy patients, implementing a two-step acquisition, comprised of a 3D acquisition to cover the fronto-parietal regions, and a contiguous parallel two-slice Hadamard-encoded acquisition to cover the temporal-occipital region, both with T /T = 2000/40 ms and matched acquisition times.

Electromagnetic simulation of a 16-channel head transceiver at 7 T using circuit-spatial optimization.

Purpose: With increased interest in parallel transmission in ultrahigh-field MRI, methods are needed to correctly calculate the S-parameters and complex field maps of the parallel transmission coil. We present S-parameters paired with spatial field optimization to fully simulate a double-row 16-element transceiver array for brain MRI at 7 T.

Methods: We implemented a closed-form equation of the coil S-parameters and overall spatial field.

High resolution automated labeling of the hippocampus and amygdala using a 3D convolutional neural network trained on whole brain 700 μm isotropic 7T MP2RAGE MRI.

Image labeling using convolutional neural networks (CNNs) are a template-free alternative to traditional morphometric techniques. We trained a 3D deep CNN to label the hippocampus and amygdala on whole brain 700 μm isotropic 3D MP2RAGE MRI acquired at 7T. Manual labels of the hippocampus and amygdala were used to (i) train the predictive model and (ii) evaluate performance of the model when applied to new scans.

Water and lipid suppression techniques for advanced H MRS and MRSI of the human brain: Experts' consensus recommendations.

The neurochemical information provided by proton magnetic resonance spectroscopy (MRS) or MR spectroscopic imaging (MRSI) can be severely compromised if strong signals originating from brain water and extracranial lipids are not properly suppressed. The authors of this paper present an overview of advanced water/lipid-suppression techniques and describe their advantages and disadvantages. Moreover, they provide recommendations for choosing the most appropriate techniques for proper use.

Reduced GABA/glutamate in the thalamus of individuals at clinical high risk for psychosis.

Youth at clinical high risk (CHR) are a unique population enriched for precursors of major psychiatric disorders, especially schizophrenia (SCZ). Recent neuroimaging findings point to abnormalities in the thalamus of patients with SCZ, including chronic and early course patients, as well as in CHR individuals relative to healthy comparison groups, thus suggesting that thalamic dysfunctions are present even before illness onset. Furthermore, modeling data indicate that alteration between excitatory and inhibitory control, as reflected by alteration in GABAergic and glutamatergic balance (i.

Dynamic B shimming for multiband imaging using high order spherical harmonic shims.

Purpose: To describe and implement a strategy for dynamic slice-by-slice and multiband B shimming using spherical harmonic shims in the human brain at 7T.

Theory: For thin axial slices, spherical harmonic shims can be divided into pairs of shims (z-degenerate and non-z-degenerate) that are spatially degenerate, such that only ½ of the shims (non-z-degenerate) are required for single slice optimizations. However, when combined, the pairs of shims can be used to simultaneously generate the same in-plane symmetries but with different amplitudes as a function of their z location.

B shimming for in vivo magnetic resonance spectroscopy: Experts' consensus recommendations.

Magnetic resonance spectroscopy (MRS) and spectroscopic imaging (MRSI) allow the chemical analysis of physiological processes in vivo and provide powerful tools in the life sciences and for clinical diagnostics. Excellent homogeneity of the static B magnetic field over the object of interest is essential for achieving high-quality spectral results and quantitative metabolic measurements. The experimental minimization of B variation is performed in a process called B shimming.

Periventricular White Matter Alterations From Explosive Blast in a Large Animal Model: Mild Traumatic Brain Injury or "Subconcussive" Injury?

The neuropathology of mild traumatic brain injury in humans resulting from exposure to explosive blast is poorly understood as this condition is rarely fatal. A large animal model may better reflect the injury patterns in humans. We investigated the effect of explosive blasts on the constrained head minimizing the effects of whole head motion.

Modelling and B1 Shim Analysis of 16-Element Transceiver Array at 7 T.

The work examines the workflow of using commercially available software for electromagnetic modelling and validation of a transceiver array coil operating at 298 MHz for magnetic resonance imaging and spectroscopy at 7 T. The coneshaped, tight-fit parallel transmit head array consists of two rows with eight loop coils per row and transmits two distinct spatial distributions by means of shimming. Considerations for finite-difference time-domain simulation setup and post-processing with circuit-domain co-simulation are examined, as is the generation of virtual observation points suitable for online safety monitoring.

Methodological consensus on clinical proton MRS of the brain: Review and recommendations.

Proton MRS ( H MRS) provides noninvasive, quantitative metabolite profiles of tissue and has been shown to aid the clinical management of several brain diseases. Although most modern clinical MR scanners support MRS capabilities, routine use is largely restricted to specialized centers with good access to MR research support. Widespread adoption has been slow for several reasons, and technical challenges toward obtaining reliable good-quality results have been identified as a contributing factor.

Interslice current change constrained B shim optimization for accurate high-order dynamic shim updating with strongly reduced eddy currents.

Purpose: To overcome existing challenges in dynamic B shimming by implementing a shim optimization algorithm which limits shim current amplitudes and their temporal variation through the application of constraints and regularization terms.

Theory And Methods: Spherical harmonic dynamic B shimming is complicated by eddy currents, ill-posed optimizations, and the need for strong power supplies. Based on the fact that eddy current amplitudes are proportional to the magnitude of the shim current changes, and assuming a smoothness of the B inhomogeneity variation in the slice direction, a novel algorithm was implemented to reduce eddy current generation by limiting interslice shim current changes.

Cerebrospinal fluid-suppressed T -weighted MR imaging at 7 T for human brain.

Purpose: T -weighted lesional imaging is most commonly performed using inversion recovery turbo spin echoes. At 7 T, however, this acquisition is limited for specific absorption rate and resolution. This work describes and implements a method to generate CSF-suppressed T -weighted imaging.