Westerhold, Erin M

ABSTRACT Purpose Layer fMRI is an increasingly utilized technique that provides insights into the laminar organization of brain activity. However, both blood‐oxygen‐level‐dependent (BOLD) fMRI and vascular space occupancy data (VASO) have certain limitations, such as bias towards larger cortical veins in BOLD fMRI and high specific absorption rate in VASO. This study aims to explore the feasibility of whole‐brain laminar functional quantitative susceptibility mapping (fQSM) compared to laminar BOLD fMRI and VASO at ultra‐high field. Methods Data were acquired using 3D EPI techniques. Complex data were denoised with NORDIC and susceptibility maps were computed using 3D path‐based unwrapping, the variable‐kernel sophisticated harmonic artifact reduction, as well as the streaking artifact reduction for QSM algorithms. To assess layer‐specific activation, 20 layers were segmented in the somatosensory and motor cortices, obtained from a finger tapping paradigm, and further averaged into six anatomical cortical layers. The magnitude of signal change and z ‐scores were compared across layers for each technique. Results fQSM showed the largest activation‐dependent mean susceptibility decrease in Layers II/III in M1 and Layers I/II in S1 with up to −1.3 ppb while BOLD showed the strongest mean signal increase in Layer I. Our data suggest that fQSM demonstrates less bias towards activation in superficial layers compared to BOLD. Moreover, activation‐based susceptibility change was comparable to VASO data. Conclusion Studying whole‐brain, layer‐dependent activation with submillimeter fQSM is feasible and reduces bias towards venous drainage effects on the cortical surface compared to BOLD, thereby enabling better localization of laminar activation.

AbstractQuantitative susceptibility mapping (QSM) is a tool for mapping tissue susceptibility. Using QSM for functional brain mapping, it is possible to directly quantify blood‐oxygen‐level‐dependent (BOLD) susceptibility changes. This study presents a submillimeter functional QSM (fQSM) approach compared to BOLD fMRI from data acquired with 3D gradient‐echo echo planar imaging (EPI) at ultra‐high field. Complex EPI data were acquired in nine healthy subjects with varying temporal and spatial resolutions and used for BOLD fMRI and for fQSM. Right‐hand finger tapping experiments were performed as well as one measurement with intentional subject movement. Susceptibility maps were computed using 3D path‐based unwrapping, the variable‐kernel sophisticated harmonic artifact reduction for phase data, and the streaking artifact reduction for QSM algorithm. Functional data analysis included general linear modeling and computation of z‐scores. Submillimeter data were denoised using NOise reduction with DIstribution Corrected (NORDIC), which improved z‐scores in the motor cortex for fQSM and fMRI. An expected increase in BOLD fMRI signal and corresponding decrease in magnetic susceptibility was observed in sensorimotor areas during active periods. For all experiments, fQSM showed smaller activation regions compared with fMRI. The percentage of high negative t‐values localized in the cortex was higher for fQSM (52%) than for positive or negative t‐values for fMRI (45%). For the scans with intentional motion, movement exceeded the size of a voxel, but paradigm dependent signal evolution could be recovered using motion correction. In conclusion, this study demonstrates the feasibility of submillimeter whole‐brain fQSM with voxel volume of 0.53 μL. In comparison to traditional BOLD fMRI, fQSM provided improved localization of brain activation within the cortex, especially in submillimeter 3D EPI sequences.

Transient ischemic attack (TIA) has gained significant attention recently due to the increased incidence of subsequent stroke. However, there are many nonvascular clinical mimics of TIA, creating a need for improved biomarkers to identify a vascular origin. Following the recent approval of ultra-high field (UHF) 7T MRI in clinical practice, several clinical studies have highlighted its added utility in neuroimaging compared to lower-field 1.5T and 3T MRI, particularly in epilepsy and multiple sclerosis. Our case series of three patients with TIA illustrates that 7T MRI can depict small areas of intracortical microhemorrhages and microinfarctions, which could not be resolved with 3T or 1.5T MRI. There are currently no reports of intracortical localization of microhemorrhages in patients with TIA. This discovery may enhance our understanding and characterization of cerebrovascular abnormalities in TIAs. In addition, UHF imaging could potentially be utilized to distinguish transient neurological episodes secondary to cerebrovascular events from other differential considerations. Our cases highlight the underestimation of imaging abnormalities in cases of TIA and support the potential expanded application of clinical 7T to assess patients with TIA. Future studies are necessary at 7T redundant to determine the true incidence of such lesions in TIA and to examine the correlation between cortical microhemorrhages and subsequent ischemic stroke, hemorrhagic events, and neurocognitive impairment.

3D time-of-flight (TOF) MR angiography (MRA) benefits from ultra-high-field MRI (≥7 T) due to improved contrast and increased signal-to-noise ratio. However, high-resolution TOF MRA at 7T usually requires longer acquisition times. In addition, relatively higher specific absorption rate (SAR) at 7T limits the choice of optimal pulse sequence parameters, especially if venous saturation is employed. Here, we illustrate the clinical application of ultra-high resolution cerebral 7T TOF MRA using compressed sensing in cases of artery of Percheron and lacunar infarcts, which showed superior resolution and exquisite details pertinent to the clinical diagnosis. The technical challenges associated with high-resolution 7T imaging were alleviated by optimization of sequence parameters and utilization of compressed sensing acceleration.

Abstract BACKGROUND Deep brain stimulation (DBS) of the anterior nucleus of the thalamus (ANT) is an increasingly utilized treatment of drug-resistant epilepsy. To date, the effect of high-frequency stimulation (HFS) vs low-frequency stimulation (LFS) in ANT DBS is poorly understood. OBJECTIVE To assess differences in the acute effect of LFS vs HFS in ANT DBS utilizing blood-oxygen-level-dependent (BOLD) functional magnetic resonance imaging (fMRI). METHODS In this prospective study of 5 patients with ANT DBS for epilepsy, BOLD activation and deactivation were modeled for 145-Hz and 30-Hz ANT stimulation using an fMRI block design. Data were analyzed with a general linear model and combined via 2-stage mixed-effects analysis. Z-score difference maps were nonparametrically thresholded using cluster threshold of z > 3.1 and a (corrected) cluster significance threshold of P = .05. RESULTS HFS produced significantly greater activation within multiple regions, in particular the limbic and default mode network (DMN). LFS produced minimal activation and failed to produce significant activation within these same networks. HFS produced widespread cortical and subcortical deactivation sparing most of the limbic and DMN regions. Meanwhile, LFS produced deactivation in most DMN and limbic structures. CONCLUSION Our results show that HFS and LFS produce substantial variability in both local and downstream network effects. In particular, largely opposing effects were identified within the limbic network and DMN. These findings may serve as a mechanistic basis for understanding the potential of HFS vs LFS in various epilepsy syndromes.