Ultra-high Field Magnetic Resonance Imaging Research Articles

Application of 7 tesla magnetic resonance imaging for pediatric neurological disorders: Early clinical experience.

Ultra-high field magnetic resonance imaging (MRI) has been introduced for use in pediatric developmental neurology. While higher magnetic fields have certain advantages, optimized techniques with specific considerations are required to ensure rational and safe use in children and those with pediatric neurological disorders (PNDs). Here, we summarize our initial experience with clinical translational studies that utilized 7 tesla (T)-MRI in the fields of developmental neurology. T2-reversed images and three-dimensional anisotropy contrast imaging enabled the depiction of targeted pathological brain structures with better spatial resolution. Diffusion imaging and susceptibility-weighted imaging enabled visualization of intracortical, subcortical, and intratumoral microstructures in vivo within highly limited scan times appropriate for patients with PNDs. 7T-MRI appears to have significant potential to enhance the depiction of the structural and functional properties of the brain, particularly those associated with atypical brain development.

A nine-channel transmit/receive array for spine imaging at 10.5 T: Introduction to a nonuniform dielectric substrate antenna.

The purpose of this study is to introduce a new antenna element with improved transmit performance, named the nonuniform dielectric substrate (NODES) antenna, for building transmit arrays at ultrahigh-field. We optimized a dipole antenna at 10.5 Tesla by maximizing the -SAR efficiency in a phantom for a human spine target. The optimization parameters included permittivity variation in the substrate, substrate thickness, antenna length, and conductor geometry. We conducted electromagnetic simulations as well as phantom experiments to compare the transmit/receive performance of the proposed NODES antenna design with existing coil elements from the literature. Single NODES element showed up to 18% and 30% higher -SAR efficiency than the fractionated dipole and loop elements, respectively. The new element is substantially shorter than a commonly used dipole, which enables z-stacked array formation; it is additionally capable of providing a relatively uniform current distribution along its conductors. The nine-channel transmit/receive NODES array achieved 7.5% higher homogeneity than a loop array with the same number of elements. Excitation with the NODES array resulted in 33% lower peak 10g-averaged SAR and required 34% lower input power than the loop array for the target anatomy of the spine. In this study, we introduced a new RF coil element: the NODES antenna. NODES antenna outperformed the widely used loop and dipole elements and may provide improved transmit/receive performance for future ultrahigh field MRI applications.

MeTiS: A Modular Pipeline for Extracting 3D-Printable Brain-Surface Models from Conventional and Ultra-High Field MRI

Aim: To develop a modular software pipeline for robustly extracting 3D brain-surface models from MRIs for visualization or printing. No other end-to-end pipeline specialized for neuroimaging does this directly with an interchangeable combination of methods. Materials & methods: A software application was developed to dynamically generate Nipype workflows using interfaces from the Analysis of Functional NeuroImages, Advanced Normalization Tools, FreeSurfer, BrainSuite, Nighres and the FMRIB Software Library suites. The application was deployed for public use via the LONI pipeline environment. Results: In a small, head-to-head comparison test, a pipeline using FreeSurfer for both the skull stripping and cortical-mesh extraction stages earned the highest subjective quality scores. Conclusion: We have deployed a publicly available and modular software tool for extracting 3D models from brain MRIs to use in medical education.

Detection of Hippocampal Subfield Asymmetry at 7T With Automated Segmentation in Epilepsy Patients With Normal Clinical Strength MRIs.

While the etiology of hippocampal sclerosis (HS) in epilepsy patients remains unknown, distinct phenotypes of hippocampal subfield atrophy have been associated with different clinical presentations and surgical outcomes. The advent of novel techniques including ultra-high field 7T magnetic resonance imaging (MRI) and automated subfield volumetry have further enabled detection of hippocampal pathology in patients with epilepsy, however, studies combining both 7T MRI and automated segmentation in epilepsy patients with normal-appearing clinical MRI are limited. In this study, we present a novel application of the automated segmentation of hippocampal subfields (ASHS) software to determine subfield volumes of the CA1, CA2/3, CA4/DG, and the subiculum using ultra high-field 7T MRI scans, including T1-weighted MP2RAGE and T2-TSE sequences, in 27 patients with either mesial temporal lobe epilepsy (mTLE) or neocortical epilepsy (NE) compared to age and gender matched healthy controls. We found that 7T improved visualization of structural abnormalities not otherwise seen on clinical strength MRIs in patients with unilateral mTLE. Additionally, our automated segmentation algorithm was able to detect structural differences in volume and asymmetry across hippocampal subfields in unilateral mTLE patients compared to controls. Specifically, amongst unilateral mTLE patients with longer disease durations, volume loss was observed in the ipsilateral CA1 and CA2/3 subfields and contralateral CA1. There were no differences in subfield volumes in patients with NE compared to controls. We report the first application of 7T with automated segmentation to characterize the relationship between disease duration burden and asymmetry across specific hippocampal subfields in this population. Disease duration was found to have a statistically significant positive relationship with subfield asymmetry within the unilateral mTLE cohort. These findings highlight the ability of 7T MRI and automated segmentation to provide novel qualitative and quantitative information in epilepsy patients who are otherwise MRI-negative at clinical field strengths.

An interpretable machine learning model to explain the interplay between brain lesions and cortical atrophy in multiple sclerosis.

Multiple Sclerosis (MS) is the most common cause, (after trauma) of neurological disability in young adults in Western countries. While several Magnetic Resonance Imaging (MRI) studies have demonstrated a strong association between the presence of cortical grey matter atrophy and the progression of neurological impairment in MS patients, the neurobiological substrates of cortical atrophy in MS, and in particular its relationship with white matter (WM) and cortical lesions, remain unknown. The aim of this study was to investigate the interplay between cortical atrophy and different types of lesions at Ultra-High Field (UHF) 7 T MRI, including cortical lesions and lesions with a susceptibility rim (a feature which histopathological studies have associated with impaired remyelination and progressive tissue destruction). We combined lesion characterization with a recent machine learning (ML) framework which includes explainability, and we were able to predict cortical atrophy in MS from a handful of lesion-related features extracted from 7 T MR imaging. This highlights not only the importance of UHF MRI for accurately evaluating intracortical and rim lesion load, but also the differential contributions that these types of lesions may bring to determine disease evolution and severity. Also, we found that a small subset of features [WM lesion volume (not considering rim lesions), patient age and WM lesion count (not considering rim lesions), intracortical lesion volume] carried most of the prediction power. Interestingly, an almost opposite pattern emerged when contrasting cortical with WM lesion load: WM lesion load is most important when it is small, whereas cortical lesion load behaves in the opposite way.Clinical Relevance- Our results suggest that disconnection and axonal degeneration due to WM lesions and local cortical demyelination are the main factors determining cortical thinning. These findings further elucidate the complexity of MS pathology across the whole brain and the need for both statistical and mechanistic approaches to understanding the etiopathogenesis of lesions.

A 16-Channel Dipole Antenna Array for Human Head Magnetic Resonance Imaging at 10.5 Tesla.

For ultra-high field and frequency (UHF) magnetic resonance imaging (MRI), the associated short wavelengths in biological tissues leads to penetration and homogeneity issues at 10.5 tesla (T) and require antenna transmit arrays for efficiently generated 447 MHz B1+ fields (defined as the transmit radiofrequency (RF) magnetic field generated by RF coils). Previously, we evaluated a 16-channel combined loop + dipole antenna (LD) 10.5 T head array. While the LD array configuration did not achieve the desired B1+ efficiency, it showed an improvement of the specific absorption rate (SAR) efficiency compared to the separate 8-channel loop and separate 8-channel dipole antenna arrays at 10.5 T. Here we compare a 16-channel dipole antenna array with a 16-channel LD array of the same dimensions to evaluate B1+ efficiency, 10 g SAR, and SAR efficiency. The 16-channel dipole antenna array achieved a 24% increase in B1+ efficiency in the electromagnetic simulation and MR experiment compared to the LD array, as measured in the central region of a phantom. Based on the simulation results with a human model, we estimate that a 16-channel dipole antenna array for human brain imaging can increase B1+ efficiency by 15% with similar SAR efficiency compared to a 16-channel LD head array.

Key designs of a short-bore and cryogen-free high temperature superconducting magnet system for 14 T whole-body MRI

A 14 T whole-body magnetic resonance imaging (MRI) magnet is designed with Bi2223 high temperature superconducting (HTS) wire. Owing to the large critical current and large critical tensile strength of the wire type HT-NX at ultra-high field, the magnet is optimized in a compact shape with length of 1.9 m and outer diameter of 1.3 m. It is in the form of stacked double pancakes (DPs) with total wire consumption of 455 km. The DPs with various inner and outer radii enable the electromagnetic design to obtain highly homogeneous field of 1.5 ppm over 400 mm diameter of spherical volume (DSV). Hoopstress by winding process, thermal contraction and Lorenz force is calculated and below the wire strength specification. Considerable reduction of screening current effect in MRI magnet application is proved by a calculation example. Field homogeneity as it is influenced by various systematic tolerances and random geometric tolerances is numerically estimated, for further design optimization and design of shimming system. Iron room for passive shielding with dimension of 5 × 5 × 10 m is found to be with wall thickness of around 30 cm to effectively contain the 5-Gauss field to the wall surface. Cooling strategy for cryogen-free technique is proposed and the magnet coils temperature is calculated as below 8 K based on the estimation of joint Ohmic heating power of 1.0 W. Quench protection adopts circuit subdivision and dumping resistors to restrict the maximum voltage below 1.9 kV. No-insulation technique for the quench protection is also discussed. Some other wire candidates like REBCO and Nb3Sn for development of such ultra-high field MRI magnets are commented.

The strength of corticomotoneuronal drive underlies ALS split phenotypes and reflects early upper motor neuron dysfunction.

BackgroundSplit phenotypes, (split hand, elbow, leg, and foot), are probably unique to ALS, and are characterized by having a shared peripheral input of both affected and unaffected muscles. This implies an anatomical origin rostral to the spinal cord, primarily within the cerebral cortex. Therefore, split phenotypes are a potential marker of ALS upper motor neuron pathology. However, to date, reports documenting upper motor neuron dysfunction in split phenotypes have been limited to using transcranial magnetic stimulation and cortical threshold tracking techniques. Here, we consider several other potential methodologies that could confirm a primary upper motor neuron pathology in split phenotypes.MethodsWe review the potential of: 1. measuring the compound excitatory post‐synaptic potential recorded from a single activated motor unit, 2. cortical‐muscular coherence, and 3. new advanced modalities of neuroimaging (high‐resolution imaging protocols, ultra‐high field MRI platforms [7T], and novel Non‐Gaussian diffusion models).ConclusionsWe propose that muscles involved in split phenotypes are those functionally involved in the human motor repertoire used particularly in complex activities. Their anterior horn cells receive the strongest corticomotoneuronal input. This is also true of the weakest muscles that are the earliest to be affected in ALS. Descriptions of split hand in non‐ALS cases and proposals that peripheral nerve or muscle dysfunction may be causative are contentious. Only a few carefully controlled cases of each form of split phenotype, using upper motor neuron directed methodologies, are necessary to prove our postulate.

Interelement Decoupling Strategies at UHF MRI

Moving to the ultrahigh field magnetic resonance imaging (UHF MRI) brought many benefits such as potentially higher signal-to-noise ratio, contrast-to-noise ratio, and improved spectral resolution. The UHF MRI regime also introduced some challenges which could prevent full exploitation of mentioned advantages. A higher static magnetic field means increase in Larmor frequency, which further implies the shorter wavelength in a tissue. The shorter wavelength causes interferences of the RF signal and inhomogeneous excitation, which can be partially resolved by the introduction of the multichannel coil arrays. The biggest problem in UHF multichannel densely populated arrays is the existence of the interelement coupling, which should be minimized as much as possible. This article presents the nonconventional, recently developed decoupling techniques used in UHF MRI.

Equine flexor tendon imaging part 2: Current status and future directions in advanced diagnostic imaging, with focus on the deep digital flexor tendon

Flexor tendon injuries are a common cause of lameness and early retirement in equine athletes. While ultrasonography is most frequently utilised, advanced diagnostic imaging modalities are becoming more widely available for detection and monitoring of flexor tendon lesions. Part two of this literature review details current experience with low- and high-field magnetic resonance imaging (MRI) and computed tomography (CT) for the diagnosis of equine flexor tendinopathy with a focus on the deep digital flexor tendon. Implications of the ‘magic angle’ artefact as well as injection techniques and the use of contrast media are discussed. Future developments in tendon imaging aim to gain enhanced structural information about the tendon architecture with the prospect to prevent injury. Techniques as described for the assessment of the human Achilles tendon including ultra-high field MRI and positron emission tomography are highlighted.

Dynamically switchable magnetic resonance imaging contrast agents.

Contrast agents can improve the sensitivity and resolution of magnetic resonance imaging (MRI) by accelerating the relaxation times of surrounding water protons. The MRI performances of contrast agents are closely related to their structural characteristics, including size, shape, surface modification, and so on. Recently, dynamically switchable MRI contrast agents that can undergo structural changes and imaging functional activations upon reaching the disease microenvironment have been developed for high performance MRI. This perspective highlights the ingenious design, controllable structural transformation, and tunable imaging property of dynamic MRI contrast agents. Additionally, the current challenges of the dynamic MRI contrast agents for medical diagnosis are discussed. Furthermore, the future integration of high-resolution ultra-high field MRI technology and cutting-edge dynamic MRI contrast agents for non-invasive histopathological level accurate detection of microscopic lesions are commented.

Leveraging high-resolution 7-tesla MRI to derive quantitative metrics for the trigeminal nerve and subnuclei of limbic structures in trigeminal neuralgia

BackgroundTrigeminal Neuralgia (TN) is a chronic neurological disease that is strongly associated with neurovascular compression (NVC) of the trigeminal nerve near its root entry zone. The trigeminal nerve at the site of NVC has been extensively studied but limbic structures that are potentially involved in TN have not been adequately characterized. Specifically, the hippocampus is a stress-sensitive region which may be structurally impacted by chronic TN pain. As the center of the emotion-related network, the amygdala is closely related to stress regulation and may be associated with TN pain as well. The thalamus, which is involved in the trigeminal sensory pathway and nociception, may play a role in pain processing of TN. The objective of this study was to assess structural alterations in the trigeminal nerve and subregions of the hippocampus, amygdala, and thalamus in TN patients using ultra-high field MRI and examine quantitative differences in these structures compared with healthy controls.MethodsThirteen TN patients and 13 matched controls were scanned at 7-Tesla MRI with high resolution, T1-weighted imaging. Nerve cross sectional area (CSA) was measured and an automated algorithm was used to segment hippocampal, amygdaloid, and thalamic subregions. Nerve CSA and limbic structure subnuclei volumes were compared between TN patients and controls.ResultsCSA of the posterior cisternal nerve on the symptomatic side was smaller in patients (3.75 mm2) compared with side-matched controls (5.77 mm2, p = 0.006). In TN patients, basal subnucleus amygdala volume (0.347 mm3) was reduced on the symptomatic side compared with controls (0.401 mm3, p = 0.025) and the paralaminar subnucleus volume (0.04 mm3) was also reduced on the symptomatic side compared with controls (0.05 mm3, p = 0.009). The central lateral thalamic subnucleus was larger in TN patients on both the symptomatic side (0.033 mm3) and asymptomatic side (0.035 mm3), compared with the corresponding sides in controls (0.025 mm3 on both sides, p = 0.048 and p = 0.003 respectively). The inferior and lateral pulvinar thalamic subnuclei were both reduced in TN patients on the symptomatic side (0.2 mm3 and 0.17 mm3 respectively) compared to controls (0.23 mm3, p = 0.04 and 0.18 mm3, p = 0.04 respectively). No significant findings were found in the hippocampal subfields analyzed.ConclusionsThese findings, generated through a highly sensitive 7 T MRI protocol, provide compelling support for the theory that TN neurobiology is a complex amalgamation of local structural changes within the trigeminal nerve and structural alterations in subnuclei of limbic structures directly and indirectly involved in nociception and pain processing.

Increased Sodium Concentration in Substantia Nigra in Early Parkinson's Disease: A Preliminary Study With Ultra-High Field (7T) MRI.

Pathophysiology of idiopathic Parkinson's disease (iPD) is complex and still misunderstood. At a time when treatments with disease-modifying potential are being developed, identification of early markers of neurodegeneration is essential. Intracerebral sodium accumulation could be one of them. Indeed, it may be in relation to the mitochondrial dysfunction that early exists in iPD. For the first time, we used brain sodium (23Na) MRI to explore sodium concentration changes that have already been reported to be related to neurodegeneration in other diseases. We prospectively included 10 iPD patients (mean age 52.2 ± 5.9 years-old) with motor symptoms that started <36 months before inclusion and 12 healthy subjects (mean age 53 ± 6.4 years-old). Patients were scanned in OFF medication state by using proton (1H) and 23Na MRI at 7T. We then extracted quantitative Total Sodium Concentration (TSC) from five regions of interest known to be early impaired in iPD [substantia nigra (SN), putamen, caudate nucleus, pallidum, thalamus] and in one region supposed to be relatively spared in the first stages of the disease [cortical gray matter (neocortex)]. Potential atrophy in these structures was also investigated with 1H MRI. Relative to healthy subjects, iPD patients showed higher TSC in the SN (43.73 ± 4.64 vs. 37.72 ± 5.62, p = 0.006 after Bonferroni correction). A trend of increase in sodium concentrations was found within the pallidum (45.80 ± 4.19 vs. 41.07 ± 4.94, p = 0.017), putamen (48.65 ± 4.58 vs. 43.66 ± 5.04, p = 0.041) and the cortical gray matter (56.34 ± 3.92 vs. 50.81 ± 5.50, p = 0.021). No significant brain atrophy was found in patients compared to controls. Thus, alteration of sodium homeostasis in the SN in the absence of atrophy could be considered as a potential early marker of cellular dysfunction in iPD.

Safety and image quality at 7T MRI for deep brain stimulation systems: Ex vivo study with lead-only and full-systems.

Ultra-high field MRI at 7 T can produce much better visualization of sub-cortical structures compared to lower field, which can greatly help target verification as well as overall treatment monitoring for patients with deep brain stimulation (DBS) implants. However, use of 7 T MRI for such patients is currently contra-indicated by guidelines from the device manufacturers due to the safety issues. The aim of this study was to provide an assessment of safety and image quality of ultra-high field magnetic resonance imaging at 7 T in patients with deep brain stimulation implants. We performed experiments with both lead-only and complete DBS systems implanted in anthropomorphic phantoms. RF heating was measured for 43 unique patient-derived device configurations. Magnetic force measurements were performed according to ASTM F2052 test method, and device integrity was assessed before and after experiments. Finally, we assessed electrode artifact in a cadaveric brain implanted with an isolated DBS lead. RF heating remained below 2°C, similar to a fever, with the 95% confidence interval between 0.38°C-0.52°C. Magnetic forces were well below forces imposed by gravity, and thus not a source of concern. No device malfunctioning was observed due to interference from MRI fields. Electrode artifact was most noticeable on MPRAGE and T2*GRE sequences, while it was minimized on T2-TSE images. Our work provides the safety assessment of ultra-high field MRI at 7 T in patients with DBS implants. Our results suggest that 7 T MRI may be performed safely in patients with DBS implants for specific implant models and MRI hardware.

Shape and volume changes of the superior lateral ventricle after electroconvulsive therapy measured with ultra-high field MRI

The subventricular zone (SVZ) of the lateral ventricles harbors neuronal stem cells in adult mammals. Rodent studies report neurogenic effects in the SVZ of electroconvulsive stimulation. We hypothesize that if this finding translates to depressed patients undergoing electroconvulsive therapy (ECT), this would be reflected in shape changes at the SVZ.Using T1-weighted MR images acquired at ultra-high field strength (7T), the shape and volume of the ventricles were compared from pre to post ECT after 10 ECT sessions (in patients twice weekly) or 5 weeks apart (controls) using linear mixed models with age and gender as covariates.Ventricle shape significantly changed and volume significantly decreased over time in patients for the left ventricle, but not in controls. The decrease in volume of the ventricles was associated to a decrease in depression scores, and an increase in the left dentate gyrus,However, the shape changes of the ventricles were not restricted to the neurogenic niche in the lateral walls of the ventricles, providing no clear evidence for neurogenesis as sole explanation of volume changes in the ventricles after ECT.

Validation and Optimization of Proximal Femurs Microstructure Analysis Using High Field and Ultra-High Field MRI.

Trabecular bone could be assessed non-invasively using MRI. However, MRI does not yet provide resolutions lower than trabecular thickness and a comparative analysis between different MRI sequences at different field strengths and X-ray microtomography (μCT) is still missing. In this study, we compared bone microstructure parameters and bone mineral density (BMD) computed using various MRI approaches, i.e., turbo spin echo (TSE) and gradient recalled echo (GRE) images used at different magnetic fields, i.e., 7T and 3T. The corresponding parameters computed from μCT images and BMD derived from dual-energy X-ray absorptiometry (DXA) were used as the ground truth. The correlation between morphological parameters, BMD and fracture load assessed by mechanical compression tests was evaluated. Histomorphometric parameters showed a good agreement between 7T TSE and μCT, with 8% error for trabecular thickness with no significative statistical difference and a good intraclass correlation coefficient (ICC > 0.5) for all the extrapolated parameters. No correlation was found between DXA-BMD and all morphological parameters, except for trabecular interconnectivity (R2 > 0.69). Good correlation (p-value < 0.05) was found between failure load and trabecular interconnectivity (R2 > 0.79). These results suggest that MRI could be of interest for bone microstructure assessment. Moreover, the combination of morphological parameters and BMD could provide a more comprehensive view of bone quality.

Radiologically isolated syndrome: A single-center, retrospective cohort study

BackgroundRadiologically Isolated Syndrome (RIS) likely represents the earliest detectable form of multiple sclerosis (MS). There are recognized risk factors for conversion of RIS to clinically definite central nervous system (CNS) demyelinating disease. We aim to characterize a new clinical cohort with RIS and to analyze previously established risk factors for conversion to clinically definite disease. MethodsA medical records search was performed for patients who were diagnosed with RIS by their treating neurologist at our institution in Boston, USA, from January 2005 to April 2020. Demographic data, clinical outcomes, and treatment courses were analyzed. The time to first clinical event representing a demyelinating disease attack or last follow up without clinically definite disease was calculated for each person. Hazard ratios (HRs) for known risk factors for the conversion of RIS to clinically definite disease were calculated using Cox proportional hazards models. ResultsOf 89 patients, the median age at RIS diagnosis was 41.0 years (76% female, 8% with a family history of MS and 16% of any autoimmune disorder, 66% never smokers, 40% BMI >30 kg/m2, 45% with spinal cord MRI lesions). Clinically definite disease was observed in 16 patients (18%) during follow-up (median time to first event 3.4 years; median follow-up duration of full cohort 3.8 years). Median EDSS for those who developed clinically definite disease was 1.25 (range: 0–4) at most recent follow up. Of 84 patients with longitudinal brain imaging, 42 (50%) had new demyelinating lesions. Gadolinium-enhancing lesions were seen in 36 patients (43%) at either baseline (n=24) or follow-up (n=12). Most patients had at least one T1-hypointense lesion (n=70, 83%). Five patients underwent ultra-high field MRI (7 Tesla); all were positive for central vein sign, two demonstrated leptomeningeal enhancement, and one was found to have cortical lesions. Out of 30 patients with susceptibility-weighted imaging acquired during routine clinical care, 8 had at least one paramagnetic rim positive lesion. Previously reported risk factors for conversion to MS were not significant: age ≤37 years HR 1.3 (95% confidence interval (CI), 0.47–3.5), male sex 1.5 (95% CI, 0.41–5.2), and spinal cord lesions 1.3 (0.47–3.4). Nearly one-third of RIS patients (n=26) took a disease modifying therapy (DMT) for MS (median total treatment duration on any DMT=2.7 years). The sub-cohort treated with a DMT had a statistically significantly greater number of recognized risk factors for conversion to clinically definite disease compared with the untreated group (p=0.028). Most patients took a DMT for MRI changes demonstrating new demyelinating disease activity (n=16). Dimethyl fumarate (n=9) and glatiramer acetate (n=7) were the most frequently prescribed DMTs. A second-line DMT was started in 10 patients. ConclusionWe characterize a new cohort of RIS patients, demonstrating time to clinically evident demyelinating disease from RIS diagnosis of approximately 3.4 years. Our data suggest that early use of a DMT in RIS may mitigate the impact of recognized risk factors on the occurrence of clinically evident disease and reduce the likelihood of conversion to clinically definite CNS demyelinating disease in high-risk individuals.

Ultra-High Field 7-Tesla Magnetic Resonance Imaging and Electroencephalography Findings in Epilepsy

Assessment of patients for temporal lobe epilepsy (TLE) surgery requires multimodality input, including EEG recordings to ensure optimal surgical planning. Often EEG demonstrates abnormal foci not detected on 1.5T MRI. Ultra-high field MRI at 7T provides improved resolution of the brain. We investigated the utility of 7T MRI to detect potential anatomical abnormalities associated with EEG changes. Ultra-high field data were acquired on a 7T MRI scanner for 13 patients with history of drug resistant TLE who had had EEG telemetry recordings. Qualitative evaluation of 7T imaging for presence of focal abnormalities detected on EEG was performed. Correlation of 7T MRI findings with EEG recordings of focal slowing or interictal epileptic spikes (IEDs), and seizures was performed. Assessment of 7T MRI demonstrated concordance with TLE as determined by the multidisciplinary team in 61.5% of cases (n = 8). Among these, 3 patients exhibited supportive abnormal 7T MRI abnormalities not detected by 1.5T MRI. In patients who underwent surgery, 72.7% had concordant histopathology findings with 7T MRI findings (n = 8). However, qualitative assessment of 7T images revealed focal anatomical abnormalities to account for EEG findings in only 15.4% of patients (n = 2). Other regions that were found to have localized IEDs in addition to the lesional temporal lobe, included the contralateral temporal lobe (n = 5), frontal lobe (n = 3), and parieto-occipital lobe (n = 2). Ultra-high field 7T MRI findings show concordance with clinical data. However, 7T MRI did not reveal anatomical findings to account for abnormalities detected by EEG.

Improving homogeneity in abdominal imaging at 3 T with light, flexible, and compact metasurface.

Radiofrequency field inhomogeneity is a significant issue in imaging large fields of view in high- and ultrahigh-field MRI. Passive shimming with coupled coils or dielectric pads is the most common approach at 3 T. We introduce and test light and compact metasurface, providing the same homogeneity improvement in clinical abdominal imaging at 3 T as a conventional dielectric pad. The metasurface comprising a periodic structure of copper strips and parallel-plate capacitive elements printed on a flexible polyimide substrate supports propagation of slow electromagnetic waves similar to a high-permittivity slab. We compare the metasurface operating inside a transmit body birdcage coil to the state-of-the-art pad by numerical simulations and in vivo study on healthy volunteers. Numerical simulations with different body models show that the local minimum of causing a dark void in the abdominal domain is removed by the metasurface with comparable resulting homogeneity as for the pad with decreasing maximum and whole-body SAR values. In vivo results confirm similar homogeneity improvement and demonstrate the stability to body mass index. The light, flexible, and inexpensive metasurface can replace a relatively heavy and expensive pad based on the aqueous suspension of barium titanate in abdominal imaging at 3 T.

Can 7T MPRAGE match MP2RAGE for gray-white matter contrast?

Ultra-High Field (UHF) MRI provides a significant increase in Signal-to-Noise Ratio (SNR) and gains in contrast weighting in several functional and structural acquisitions. Unfortunately, an increase in field strength also induces non-uniformities in the transmit field (B1+) that can compromise image contrast non-uniformly. The MPRAGE is one of the most common T1 weighted (T1w) image acquisitions for structural imaging. It provides excellent contrast between gray and white matter and is widely used for brain segmentation. At 7T, the signal non-uniformities tend to complicate this and therefore, the self-bias-field corrected MP2RAGE is often used there. In both MPRAGE and MP2RAGE, more homogeneous image contrast can be achieved with adiabatic pulses, like the TR-FOCI inversion pulse, or special pulse design on parallel transmission systems, like Universal Pulses (UP). In the present study, we investigate different strategies to improve the bias-field for MPRAGE at 7T, comparing the contrast and GM/WM segmentability against MP2RAGE. The higher temporal efficiency of MPRAGE combined with the potential of the user-friendly UPs was the primary motivation for this MPRAGE-MP2RAGE comparison. We acquired MPRAGE data in six volunteers, adding a k-space shutter to reduce scan time, a kt-point UP approach for homogeneous signal excitation, and a TR-FOCI pulse for homogeneous inversion. Our results show remarkable signal contrast improvement throughout the brain, including regions of low B1+ such as the cerebellum. The improvements in the MPRAGE were largest following the introduction of the UPs. In addition to the CNR, both SNR and GM/WM segmentability were also assessed. Among the MPRAGEs, the combined strategy (UP + TR-FOCI) yielded highest SNR and showed highest spatial similarity between GM segments to the MP2RAGE. Interestingly, the distance between gray and white matter peaks in the intensity histograms did not increase, as better pulses and higher SNR especially benefitted the (cerebellar) gray matter. Overall, the gray-white matter contrast from MP2RAGE is higher, with higher CNR and higher intensity peak distances, even when scaled to scan time. Hence, the extra acquisition time for MP2RAGE is justified by the improved segmentability.