Magnetization transfer weighted laminar fMRI with multi-echo FLASH.

Laminar functional magnetic resonance imaging (fMRI) using the gradient echo (GRE) blood oxygenation level dependent (BOLD) contrast is prone to signal changes arising from large unspecific venous vessels. Alternatives based on changes of cerebral blood volume (CBV) become more popular since it is expected that this hemodynamic response is dominant in microvasculature. One approach to sensitize the signal toward changes in CBV, and to simultaneously reduce unwanted extravascular (EV) BOLD blurring, is to selectively reduce gray matter (GM) signal via magnetization transfer (MT).

Relating neural oscillations to laminar fMRI connectivity in visual cortex.

Laminar functional magnetic resonance imaging (fMRI) holds the potential to study connectivity at the laminar level in humans. Here we analyze simultaneously recorded electroencephalography (EEG) and high-resolution fMRI data to investigate how EEG power modulations, induced by a task with an attentional component, relate to changes in fMRI laminar connectivity between and within brain regions in visual cortex. Our results indicate that our task-induced decrease in beta power relates to an increase in deep-to-deep layer coupling between regions and to an increase in deep/middle-to-superficial layer connectivity within brain regions.

Laminar fMRI using T-prepared multi-echo FLASH.

Functional magnetic resonance imaging (fMRI) using blood oxygenation level dependent (BOLD) contrast at a sub-millimeter scale is a promising technique to probe neural activity at the level of cortical layers. While gradient echo (GRE) BOLD sequences exhibit the highest sensitivity, their signal is confounded by unspecific extravascular (EV) and intravascular (IV) effects of large intracortical ascending veins and pial veins leading to a downstream blurring effect of local signal changes. In contrast, spin echo (SE) fMRI promises higher specificity towards signal changes near the microvascular compartment.

Enhanced POCS reconstruction for partial Fourier imaging in multi-echo and time-series acquisitions.

Purpose: To improve partial Fourier (PF) imaging reconstruction in time-series or multi-echo acquisitions.

Methods: Many PF methods use a phase estimate to restore Hermitian symmetry before filling missing k-space entries with measured data from the opposite half. This estimate is obtained from the symmetrically sampled, central part of k-space and its low-resolution results in artifacts near high-frequency phase effects (eg, tissue boundaries, vessels), limiting PF undersampling.

Improved cortical boundary registration for locally distorted fMRI scans.

With continuing advances in MRI techniques and the emergence of higher static field strengths, submillimetre spatial resolution is now possible in human functional imaging experiments. This has opened up the way for more specific types of analysis, for example investigation of the cortical layers of the brain. With this increased specificity, it is important to correct for the geometrical distortions that are inherent to echo planar imaging (EPI).

Strategies and prospects for cortical depth dependent T2 and T2* weighted BOLD fMRI studies.

Technological advancements in fMRI have afforded the opportunity to conduct submillimeter investigations into human brain function. The ability to do cortical depth dependent (or layer-specific) fMRI could allow probing intrinsic neuronal organizations and inter-connections, including the directionality of interregional information flow, while ultimately shedding light on uniquely human behaviors. The methodological development and applications of cortical depth dependent fMRI has been ongoing for nearly a decade, yet a consensus on protocols, analysis pipelines and interpretations of data has yet to be reached.

Diffusion Acceleration with Gaussian process Estimated Reconstruction (DAGER).

Purpose: Image acceleration provides multiple benefits to diffusion MRI, with in-plane acceleration reducing distortion and slice-wise acceleration increasing the number of directions that can be acquired in a given scan time. However, as acceleration factors increase, the reconstruction problem becomes ill-conditioned, particularly when using both in-plane acceleration and simultaneous multislice imaging. In this work, we develop a novel reconstruction method for in vivo MRI acquisition that provides acceleration beyond what conventional techniques can achieve.

Laminar Organization of Working Memory Signals in Human Visual Cortex.

The human primary visual cortex (V1) is not only activated by incoming visual information but is also engaged by top-down cognitive processes, such as visual working memory, even in the absence of visual input [1-3]. This feedback may be critical to our ability to visualize specific visual features, as higher-order regions lack the selectivity to represent such information [4]. Clearly, such internally generated signals do not trigger genuine perception of the remembered stimulus, meaning they must be organized in a manner that is different to bottom-up-driven signals.

High-resolution diffusion MRI at 7T using a three-dimensional multi-slab acquisition.

High-resolution diffusion MRI can provide the ability to resolve small brain structures, enabling investigations of detailed white matter architecture. A major challenge for in vivo high-resolution diffusion MRI is the low signal-to-noise ratio. In this work, we combine two highly compatible methods, ultra-high field and three-dimensional multi-slab acquisition to improve the SNR of high-resolution diffusion MRI.

The relationship between oscillatory EEG activity and the laminar-specific BOLD signal.

Electrophysiological recordings in animals have indicated that visual cortex γ-band oscillatory activity is predominantly observed in superficial cortical layers, whereas α- and β-band activity is stronger in deep layers. These rhythms, as well as the different cortical layers, have also been closely related to feedforward and feedback streams of information. Recently, it has become possible to measure laminar activity in humans with high-resolution functional MRI (fMRI).

Two-dimensional-NGC-SENSE-GRAPPA for fast, ghosting-robust reconstruction of in-plane and slice-accelerated blipped-CAIPI echo planar imaging.

Purpose: Ghosting-robust reconstruction of blipped-CAIPI echo planar imaging simultaneous multislice data with low computational load.

Methods: To date, Slice-GRAPPA, with "odd-even" kernels that improve ghosting performance, has been the framework of choice for such reconstructions due to its predecessor SENSE-GRAPPA being deemed unsuitable for blipped-CAIPI data. Modifications to SENSE-GRAPPA are used to restore CAIPI compatibility and to make it robust against ghosting.

Reducing slab boundary artifacts in three-dimensional multislab diffusion MRI using nonlinear inversion for slab profile encoding (NPEN).

Purpose: To propose a method to reduce the slab boundary artifacts in three-dimensional multislab diffusion MRI.

Methods: Bloch simulation is used to investigate the effects of multiple factors on slab boundary artifacts, including characterization of residual errors on diffusion quantification. A nonlinear inversion method is proposed to simultaneously estimate the slab profile and the underlying (corrected) image.

A hemodynamic model for layered BOLD signals.

High-resolution blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI) at the sub-millimeter scale has become feasible with recent advances in MR technology. In principle, this would enable the study of layered cortical circuits, one of the fundaments of cortical computation. However, the spatial layout of cortical blood supply may become an important confound at such high resolution.

Simultaneous multislice (SMS) imaging techniques.

Simultaneous multislice imaging (SMS) using parallel image reconstruction has rapidly advanced to become a major imaging technique. The primary benefit is an acceleration in data acquisition that is equal to the number of simultaneously excited slices. Unlike in-plane parallel imaging this can have only a marginal intrinsic signal-to-noise ratio penalty, and the full acceleration is attainable at fixed echo time, as is required for many echo planar imaging applications.

Large dynamic range relative B1+ mapping.

Purpose: Parallel transmission (PTx) requires knowledge of the B1+ produced by each element. However, B1+ mapping can be challenging when transmit fields exhibit large dynamic range. This study presents a method to produce high quality relative B1+ maps when this is the case.

Improved sensitivity and specificity for resting state and task fMRI with multiband multi-echo EPI compared to multi-echo EPI at 7 T.

A multiband multi-echo (MBME) sequence is implemented and compared to a matched standard multi-echo (ME) protocol to investigate the potential improvement in sensitivity and spatial specificity at 7 T for resting state and task fMRI. ME acquisition is attractive because BOLD sensitivity is less affected by variation in T2*, and because of the potential for separating BOLD and non-BOLD signal components. MBME further reduces TR thus increasing the potential reduction in physiological noise.

k-t FASTER: Acceleration of functional MRI data acquisition using low rank constraints.

Purpose: In functional MRI (fMRI), faster sampling of data can provide richer temporal information and increase temporal degrees of freedom. However, acceleration is generally performed on a volume-by-volume basis, without consideration of the intrinsic spatio-temporal data structure. We present a novel method for accelerating fMRI data acquisition, k-t FASTER (FMRI Accelerated in Space-time via Truncation of Effective Rank), which exploits the low-rank structure of fMRI data.

Whole brain, high resolution multiband spin-echo EPI fMRI at 7 T: a comparison with gradient-echo EPI using a color-word Stroop task.

A whole brain, multiband spin-echo (SE) echo planar imaging (EPI) sequence employing a high spatial (1.5 mm isotropic) and temporal (TR of 2 s) resolution was implemented at 7 T. Its overall performance (tSNR, sensitivity and CNR) was assessed and compared to a geometrically matched gradient-echo (GE) EPI multiband sequence (TR of 1.

BOLD fMRI signal characteristics of S1- and S2-SSFP at 7 Tesla.

Object: To compare the BOLD fMRI signal characteristics at in the cortex and on the pial surface for a non-balanced steady-state free precession sequence (nb-SSFP) at 7 T.

Materials And Methods: A multi-echo nb-SSFP sequence was used for high resolution fMRI at 7 T. Two S1 (S(+)) echoes at different echo times were acquired together with an S2 (S(-)) echo.

Accelerated human cardiac diffusion tensor imaging using simultaneous multislice imaging.

Purpose: To demonstrate the feasibility of accelerating measurements of cardiac fiber structure using simultaneous multislice (SMS) imaging.

Methods: SMS excitation with a blipped controlled aliasing (CAIPI) readout was incorporated into a diffusion-encoded stimulated echo pulse sequence to obtain diffusion measurements in three separate slices of the heart (8-mm thickness, 12-mm gap). A novel image entropy-based method for removing image ghosts in blipped CAIPI acquisitions is also introduced that enables SMS imaging of closely spaced slices in the heart.

Application of PINS radiofrequency pulses to reduce power deposition in RARE/turbo spin echo imaging of the human head.

Purpose: To explore the use of PINS radiofrequency (RF) pulses to reduce RF power deposition in multiband/simultaneous multislice imaging with the RARE/turbo spin echo (TSE) sequence at 3T and 7T.

Methods: A PINS-TSE sequence was implemented and combined with blipped CAIPI to improve the reconstruction of superposed slices. Whole brain imaging of healthy volunteers was performed at both 3T and 7T using a 32-channel coil for signal reception.

Slice accelerated diffusion-weighted imaging at ultra-high field strength.

Purpose: Diffusion magnetic resonance imaging (dMRI) data with very high isotropic resolution can be obtained at 7T. However, for extensive brain coverage, a large number of slices is required, resulting in long acquisition times (TAs). Recording multiple slices simultaneously (SMS) promises to reduce the TA.

Simultaneous multislice inversion contrast imaging using power independent of the number of slices (PINS) and delays alternating with nutation for tailored excitation (DANTE) radio frequency pulses.

A method for simultaneous multislice (SMS) inversion contrast imaging is presented using a combination of the delays alternating with nutation for tailored excitation (DANTE) and the power independent of the number of slices (PINS) techniques. In SMS imaging, simultaneously excited slices result in an aliased image that is disentangled using parallel imaging reconstruction techniques. At high-magnetic field strengths, the peak amplitude and specific absorption rate of conventional (summed) SMS radio frequency pulses can be prohibitively high.

Whole brain, high resolution spin-echo resting state fMRI using PINS multiplexing at 7 T.

This article demonstrates the application of spin-echo EPI for resting state fMRI at 7 T. A short repetition time of 1860 ms was made possible by the use of slice multiplexing which permitted whole brain coverage at high spatial resolution (84 slices of 1.6 mm thickness).