Straub, Sina

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.

OBJECTIVE The aim of this study was to compare outcomes of direct targeting in deep brain stimulation (DBS) for essential tremor using 7T MRI versus 3T MRI. The authors hypothesized that 7T MRI direct targeting would be noninferior to 3T MRI in early tremor outcomes. METHODS A retrospective study was conducted on patients undergoing unilateral thalamic DBS for essential tremor between 2021 and 2023. Two matched cohorts were assessed, one using 7T MRI and the other using 3T MRI for surgical planning. The primary endpoint was the percentage improvement in the Fahn-Tolosa-Marin Tremor Rating Scale (TRS) scores. Additionally, the authors assessed optimized programming settings and variance in electrode position on postoperative imaging. Demographic and clinical data were compared using the nonparametric Mann-Whitney U-test. The squared Euclidean distance of each contact from the group mean centroid was calculated and averaged across the entire cohort to provide the variance (i.e., the mean squared distance) of electrode contact position. RESULTS A total of 34 patients were analyzed, with 17 in each cohort. There were no significant differences in demographic information or mean surgical dates between the groups. There were no differences in intraoperative target repositioning or adverse events. The 7T group had a significantly greater TRS improvement than the 3T group (64.9% ± 11.4% vs 50.9% ± 16.4%, p = 0.004). Patients in the 7T cohort also had a lower mean stimulation current compared with those in the 3T cohort (2.0 ± 0.8 mA vs 2.7 ± 0.9 mA, p = 0.01). Image evaluation revealed that although the mean electrode position was comparable between 7T and 3T, the 7T electrode positioning was more clustered, indicating a lower variance in the final electrode location. The mean Euclidean distance between the individual electrode tips and the group centroid was significantly less at 7T than at 3T (1.82 ± 0.68 mm vs 2.75 ± 0.81 mm, p = 0.001). CONCLUSIONS Despite concerns for increased artifacts and distortions at 7T, the authors show that these effects can be mitigated with an appropriate workflow, leading to improved surgical outcomes with direct targeting using 7T MRI. Their results suggest similar accuracy but greater precision in targeting with 7T MRI compared with 3T MRI, resulting in lower stimulation currents and improved tremor reduction. Future studies are needed to assess outcomes related to 7T MRI in targeting other subcortical structures.

AbstractThis article provides recommendations for implementing QSM for clinical brain research. It is a consensus of the International Society of Magnetic Resonance in Medicine, Electro‐Magnetic Tissue Properties Study Group. While QSM technical development continues to advance rapidly, the current QSM methods have been demonstrated to be repeatable and reproducible for generating quantitative tissue magnetic susceptibility maps in the brain. However, the many QSM approaches available have generated a need in the neuroimaging community for guidelines on implementation. This article outlines considerations and implementation recommendations for QSM data acquisition, processing, analysis, and publication. We recommend that data be acquired using a monopolar 3D multi‐echo gradient echo (GRE) sequence and that phase images be saved and exported in Digital Imaging and Communications in Medicine (DICOM) format and unwrapped using an exact unwrapping approach. Multi‐echo images should be combined before background field removal, and a brain mask created using a brain extraction tool with the incorporation of phase‐quality‐based masking. Background fields within the brain mask should be removed using a technique based on SHARP or PDF, and the optimization approach to dipole inversion should be employed with a sparsity‐based regularization. Susceptibility values should be measured relative to a specified reference, including the common reference region of the whole brain as a region of interest in the analysis. The minimum acquisition and processing details required when reporting QSM results are also provided. These recommendations should facilitate clinical QSM research and promote harmonized data acquisition, analysis, and reporting.

Magnetic materials in tissue, such as iron, calcium, or collagen, can be studied using quantitative susceptibility mapping (QSM). To date, QSM has been overwhelmingly applied in the brain, but is increasingly utilized outside the brain. QSM relies on the effect of tissue magnetic susceptibility sources on the MR signal phase obtained with gradient echo sequence. However, in the body, the chemical shift of fat present within the region of interest contributes to the MR signal phase as well. Therefore, correcting for the chemical shift effect by means of water‐fat separation is essential for body QSM. By employing techniques to compensate for cardiac and respiratory motion artifacts, body QSM has been applied to study liver iron and fibrosis, heart chamber blood and placenta oxygenation, myocardial hemorrhage, atherosclerotic plaque, cartilage, bone, prostate, breast calcification, and kidney stone.