ObjectiveTo compare by 7 Tesla (7T) magnetic resonance imaging (MRI) in patients with focal epilepsy who have non-lesional clinical MRI scans with healthy controls.Methods37 patients with focal epilepsy, based on clinical and electroencephalogram (EEG) data, with non-lesional MRIs at clinical field strengths and 21 healthy controls were recruited for the 7T imaging study. The MRI protocol consisted of high resolution T1-weighted, T2-weighted and susceptibility weighted imaging sequences of the entire cortex. The images were read by two neuroradiologists, who were initially blind to clinical data, and then reviewed a second time with knowledge of the seizure onset zone.ResultsA total of 25 patients had findings with epileptogenic potential. In five patients these were definitely related to their epilepsy, confirmed through surgical intervention, in three they co-localized to the suspected seizure onset zone and likely caused the seizures. In seven patients the imaging findings co-localized to the suspected seizure onset zone but were not the definitive cause, and ten had cortical lesions with epileptogenic potential that did not localize to the suspected seizure onset zone. There were multiple other findings of uncertain significance found in both epilepsy patients and healthy controls. The susceptibility weighted imaging sequence was instrumental in guiding more targeted inspection of the other structural images and aiding in the identification of cortical lesions.SignificanceInformation revealed by the improved resolution and enhanced contrast provided by 7T imaging is valuable in noninvasive identification of lesions in epilepsy patients who are non-lesional at clinical field strengths.

These findings suggest that epilepsy may be associated with significantly asymmetric distribution of PVSs in the brain. Furthermore, the region of maximal asymmetry of the PVSs may help provide localization or confirmation of the seizure onset zone.

Subcortical volumetric changes in major depressive disorder (MDD) have been purported to underlie depressive symptomology, however, the evidence to date remains inconsistent. Here, we investigated limbic volumes in MDD, utilizing high-resolution structural images to allow segmentation of the hippocampus and amygdala into their constituent substructures. Twenty-four MDD patients and twenty matched controls underwent structural MRI at 7T field strength. All participants completed the Montgomery-Asberg Depression Rating Scale (MADRS) to quantify depressive symptomology. For the MDD group, volumes of the amygdala right lateral nucleus ( p = 0.05, r 2 = 0.24), left cortical nucleus ( p = 0.032, r 2 = 0.35), left accessory basal nucleus ( p = 0.04, r 2 = 0.28) and bilateral corticoamygdaloid transition area (right hemisphere p = 0.032, r 2 = 0.38, left hemisphere p = 0.032, r 2 = 0.35) each displayed significant negative associations with MDD severity. The bilateral centrocortical (right hemisphere p = 0.032, r 2 = 0.31, left hemisphere p = 0.032, r 2 = 0.32) and right basolateral complexes ( p = 0.05, r 2 = 0.24) also displayed significant negative relationships with depressive symptoms. Using high-field strength MRI, we report the novel finding that MDD severity is consistently negatively associated with amygdala nuclei, linking volumetric reductions with worsening depressive symptoms.

We present a method to calculate the electric (E)-fields within and surrounding a human body in a gradient coil, including E-fields induced by the changing magnetic fields and "conservative" E-fields originating with the scalar electrical potential in the coil windings. In agreement with previous numerical calculations, it is shown that magnetically-induced E-fields within the human body show no real concentration near the surface of the body, where nerve stimulation most often occurs. Both the magnetically-induced and conservative E-fields are shown to be considerably stronger just outside the human body than inside it, and under some circumstances the conservative Efields just outside the body can be much larger than the magnetically-induced E-fields there. With the growing realization of the significance of peripheral nerve stimulation (PNS) as a limiting factor in the development and application of MRI (1-9), increasing attention has been devoted to estimating electrical (E)-fields induced by time-varying magnetic fields generated by gradient coils (10 -20). Theoretically, the E-field is due not only to time-varying magnetic fields (or magnetic vector potential), but also to the electric charge distribution or scalar potential. It was previously shown that the field due to the scalar potential throughout the body is of critical importance for calculating the E-field distribution in loaded gradient coils (10,11). Another source of E-fields in gradient coils that has not yet been formally considered is the scalar potential on the windings of the coil.To create rapid current changes in gradient coils, a very strong electromotive force is required to overcome the inductance of the gradient winding. To create the voltage drop and drive the current, a voltage of up to thousands of volts is applied to the coil windings. The scalar potential is then a function of position along the winding, which is the source of a "conservative" E-field throughout space.Here we present a numerical method to calculate the scalar potential along the gradient coil windings and the resulting conservative E-field within and surrounding a human body in a gradient coil. We compare these fields with those induced by the time-varying magnetic field of the same coil, and examine the effects of the order of winding the quadrants and the presence of a passive RF shield on the conservative E-field. Finally, we discuss the possibility that these fields are important for PNS. MATERIALS AND METHODS Gradient Coil Model and Human Body ModelA digital voxel-based model of a human male body was obtained from the products of procedures performed in previous studies (21). The model has a resolution of 5 mm and 836,518 voxels in total. Each voxel was treated as a homogeneous conductor with a uniform conductivity corresponding to the tissue at its location and the permittivity of free space.An unshielded single-axis (x-axis) gradient coil was designed using constrained length/minimum inductance methods (22). The coil had a radius of 0.4 m and a length of 1...

In MRI, strong, rapidly switched gradient fields are desirable because they can be used to reduce imaging time, obtain images with better resolution, or improve image signal-to-noise ratios. Improvements in gradient strength can be made by either increasing the gradient amplifier strength or by enhancing gradient efficiency. Unfortunately, many MRI pulse sequences, in combination with high-performance amplifiers and existing gradient hardware, can cause peripheral nerve stimulation (PNS). This makes improvements in gradient amplifiers ineffective at increasing safely usable gradient strength. Customized gradient coils are one way to achieve significant improvements in gradient performance. One specific gradient configuration, a planar gradient system, promises improved gradient strength and switching time for cardiac imaging. The PNS thresholds for planar gradients were characterized through human stimulation experiments on all three gradient axes.

In this methods development we present an ultra-high-field, diffusion-weighted MRI method to quantitatively assess u-fibers and use it to compare u-fiber counts in non-lesional epilepsy patients to controls. Emerging evidence implicates white matter abnormalities in non-lesional epilepsy, including the short-range, cortical-cortical connections or u-fibers. Eight patients with non-lesional epilepsy and 8 demographically matched controls underwent 7 Tesla MRI consisting of a T1-weighted sequence (0.7 mm isotropic resolution) and high-angular-resolved diffusion-weighted MRI (1.05 mm isotropic resolution, 68 directions). MRI data were used to quantify u-fiber counts in known u-fiber populations based on an atlas and fiber tractography. From tractography, connectivity matrices summarizing the u-fiber counts were computed. Quantitative group comparisons were performed on the connectivity matrices. U-fiber counts were found to be lower on average in subjects with epilepsy than in healthy controls. The results indicate that the density or number of u-fibers is reduced in patients with non-lesional epilepsy. Future work will be focused histological validation and determining whether differences in u-fiber counts can be used clinically to non-invasively identify seizure onset zones.

Purpose Magnetic resonance spectroscopic imaging (MRSI) benefits from operation at 7 Tesla (7T) due to increased signal-to-noise ratio (SNR) and spectral separation. The 180° radiofrequency (RF) pulses used in the conventional MRSI sequences are particularly susceptible to the variation in the transmitted RF (B1) field and severe chemical shift localization errors at 7T. RF power deposition, as measured by specific absorption rate (SAR), also increases with field strength. Adiabatic 180° RF pulses may mitigate the effects of B1 variation. We designed and implemented a Semi-Adiabatic Spectral-spatial Spectroscopic Imaging (SASSI) pulse sequence to provide more uniform spectral data at 7T with reduced SAR. Method The adiabatic Shinnar Le-Roux algorithm was used to generate a high bandwidth 180° adiabatic SPSP pulse that captured a spectral range containing the main metabolites of interest. A pair of 180° SPSP pulses was used to refocus the signal excited by a 90° SPSP pulse in order to select a 3D volume of interest in the SASSI sequence Results The SASSI pulse sequence produced spectra with more uniform brain metabolite SNR when compared to the conventional non-adiabatic MRSI sequence. Conclusion SASSI achieved comparable SNR to the current adiabatic alternative, semi-LASER, but with 1/3 of the SAR.

A finite difference method was used to simulate the electric fields induced in the model by a gradient wire pattern. The pattern simulated corresponded to a design used to perform peripheral nerves stimulation experiments. The size (187.8, 169.02, and 150.24 cm tall) and position (brain and neck mode) of the model, relative to the magnet, as well as the voxel dimensions (3, 6, and 9 mm) of the model were varied to assess the effect on the simulation. The locations of stimulation reported from an experiment were classified according to nerve branch and compared with the peak-simulated electric fields. Model size and location affected the magnitude of the electric field, but not the position. Model resolution affected the location of the peak field. For the smallest resolution investigated, the nerves affected by the locations of peak stimulations in the model correlated to the frequency of stimulation in experiments. Although adequate resolution is required in order to assess the electric fields induced by gradient coil operation, the simulation of electric fields may be useful in evaluating gradient coil design prior to construction.