Tao, Shengzhen

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.

Ultrahigh-field 7-T MRI offers superior resolution and contrast, which enable improved detection of subtle neuroanatomic abnormalities and disease-specific imaging biomarkers, with established clinical applications across a broad range of neurologic disorders.

Deep brain stimulation (DBS) is an established therapeutic intervention for a range of neurological disorders, including Parkinson’s disease, essential tremor, and epilepsy. The efficacy of DBS is predicated on the modulation of dysfunctional neural circuits through the application of electrical pulses. Recent advancements in MRI technology have facilitated the visualization of brain nuclei and their associated networks, which is critical for proper patient-specific targeting of electrodes in surgical procedures. This advancement aims to personalize surgical planning and postoperative programming by integrating patient-specific anatomical and connectivity models. The advent of clinical ultrahigh-field MRI, particularly 7T MRI, significantly enhances the targeting of DBS by improving spatial resolution, signal-to-noise ratio, and tissue contrast. Compared with lower field strengths, 7T MRI provides superior visualization of deeply situated brain nuclei and their extensive cortical projections, including the subthalamic nucleus, globus pallidus internus, and thalamus. This review synthesizes a multi-institutional consensus regarding the technical and clinical applications of 7T MRI, drawing on experience from more than 1000 procedures using 7T DBS. It encompasses both imaging acquisition and postprocessing techniques aimed at optimizing image quality. Our extensive clinical experience informs best practices for correcting distortions, mitigating image artifacts, and employing specific imaging sequences that enhance the visualization of common DBS targets. The insights and recommendations presented are intended to promote the safe and effective utilization of 7T MRI in DBS.

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.

AbstractMulti‐energy computed tomography (MECT) offers the opportunity for advanced visualization, detection, and quantification of select elements (e.g., iodine) or materials (e.g., fat) beyond the capability of standard single‐energy computed tomography (CT). However, the use of MECT requires careful consideration as substantially different hardware and software approaches have been used by manufacturers, including different sets of user‐selected or hidden parameters that affect the performance and radiation dose of MECT. Another important consideration when designing MECT protocols is appreciation of the specific tasks being performed; for instance, differentiating between two different materials or quantifying a specific element. For a given task, it is imperative to consider both the radiation dose and task‐specific image quality requirements. Development of a quality control (QC) program is essential to ensure the accuracy and reproducibility of these MECT applications. Although standard QC procedures have been well established for conventional single‐energy CT, the substantial differences between single‐energy CT and MECT in terms of system implementations, imaging protocols, and clinical tasks warrant QC tests specific to MECT. This task group was therefore charged with developing a systematic QC program designed to meet the needs of MECT applications. In this report, we review the various MECT approaches that are commercially available, including information about hardware implementation, MECT image types, image reconstruction, and postprocessing techniques that are unique to MECT. We address the requirements for MECT phantoms, review representative commercial MECT phantoms, and offer guidance regarding homemade MECT phantoms. We discuss the development of MECT protocols, which must be designed carefully with proper consideration of MECT technology, imaging task, and radiation dose. We then outline specific recommended QC tests in terms of general image quality, radiation dose, differentiation and quantification tasks, and diagnostic and therapeutic applications.

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.

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.