Kamil Ugurbil

Motivation Third order shim coils can impact gradient-magnet interactions with consequences on image quality and magnet safety. Goal (s): To demonstrate the influence of third order shim coils on the SC72 gradient coil using field and vibration measurements. Approach The gradient transfer function was measured at 11.7 T (Iseult) and at 7T (Terra) with and without connection of the 3rd order shim coils. Vibration measurements were carried out on Iseult from 0 to 11.7 T in the two configurations as well. Results The data demonstrate a drastic influence of the 3rd order shim coils and their circuits. Impact The work suggests caution when using third order shim coils at 7T and above. It also paves the way for further investigations to improve gradient waveform fidelity.

Development of diffusion MRI (dMRI) denoising approaches has experienced considerable growth over the last years. As noise can inherently reduce accuracy and precision in measurements, its effects have been well characterised both in terms of uncertainty increase in dMRI-derived features and in terms of biases caused by the noise floor, the smallest measurable signal given the noise level. However, gaps in our knowledge still exist in objectively characterising dMRI denoising approaches in terms of both of these effects and assessing their efficacy. In this work, we reconsider what a denoising method should and should not do and we accordingly define criteria to characterise the performance. We propose a comprehensive set of evaluations, including i) benefits in improving signal quality and reducing noise variance, ii) gains in reducing biases and the noise floor and improving, iii) preservation of …

A method for generating a perfusion weighted image using arterial spin labeling (ASL) with segmented acquisitions includes dividing an anatomical area of interest into a plurality of slices and performing a multi-band (MB) echo planar imaging (EPI) acquisition process using a magnetic resonance imaging (MRI) system to acquire a control image dataset representative of the plurality of slices using a central-to-peripheral or peripheral-to-central slice acquisition order. An ASL preparation process is performed using the MRI system to magnetically label protons in arterial blood water in an area upstream from the anatomical area of interest. Following a post-labeling delay time period, the MB EPI acquisition process is performed to a labeled image dataset corresponding to the slices using the central-to-peripheral or peripheral-to-central slice acquisition order. A perfusion weighted image of the anatomical area is …

This paper provides comparisons between microstructure and two-dimensional fiber orientations measured optically using polarization-sensitive optical coherence tomography (PS-OCT) and those estimated from ultra-high-field diffusion MRI (dMRI) at 10.5T in the macaque brain. The PS-OCT imaging is done at an in-plane resolution of ~10 microns in and around the thalamus. Whole brain dMRI is acquired at an isotropic resolution of 0.75 mm. We provide comparisons between cross-polarization and optical orientation from PS-OCT with the fractional anisotropy and two-dimensional orientations extracted from dMRI using a diffusion tensor model. The orientations from PS-OCT are also extracted computationally using a structure tensor. Additionally, we demonstrate the utility of mesoscale, PS-OCT imaging in improving the MRI resolution by learning the mapping between these contrasts using a super-resolution Generative Adversarial Network.

Purpose We examined magnetic field dependent SNR gains and ability to capture them with multichannel receive arrays for human head imaging in going from 7 T, the most commonly used ultrahigh magnetic field (UHF) platform at the present, to 10.5 T, which represents the emerging new frontier of >10 T in UHFs. Methods Electromagnetic (EM) models of 31‐channel and 63‐channel multichannel arrays built for 10.5 T were developed for 10.5 T and 7 T simulations. A 7 T version of the 63‐channel array with an identical coil layout was also built. Array performance was evaluated in the EM model using a phantom mimicking the size and electrical properties of the human head and a digital human head model. Experimental data was obtained at 7 T and 10.5 T with the 63‐channel array. Ultimate intrinsic SNR (uiSNR) was calculated for the two field strengths using a voxelized cloud of dipoles enclosing …

Purpose To mitigate B1+$$ {B}_1^{+} $$ inhomogeneity at 7T for multi‐channel transmit arrays using unsupervised deep learning with convolutional neural networks (CNNs). Methods Deep learning parallel transmit (pTx) pulse design has received attention, but such methods have relied on supervised training and did not use CNNs for multi‐channel B1+$$ {B}_1^{+} $$ maps. In this work, we introduce an alternative approach that facilitates the use of CNNs with multi‐channel B1+$$ {B}_1^{+} $$ maps while performing unsupervised training. The multi‐channel B1+$$ {B}_1^{+} $$ maps are concatenated along the spatial dimension to enable shift‐equivariant processing amenable to CNNs. Training is performed in an unsupervised manner using a physics‐driven loss function that minimizes the discrepancy of the Bloch simulation with the target magnetization, which eliminates the calculation of reference …

ObjectiveTo assess the possible influence of third-order shim coils on the behavior of the gradient field and in gradient–magnet interactions at 7 T and above.Materials and methodsGradient impulse response function measurements were performed at 5 sites spanning field strengths from 7 to 11.7 T, all of them sharing the same exact whole-body gradient coil design. Mechanical fixation and boundary conditions of the gradient coil were altered in several ways at one site to study the impact of mechanical coupling with the magnet on the field perturbations. Vibrations, power deposition in the He bath, and field dynamics were characterized at 11.7 T with the third-order shim coils connected and disconnected inside the Faraday cage.ResultsFor the same whole-body gradient coil design, all measurements differed greatly based on the third-order shim coil configuration (connected or not). Vibrations and gradient transfer …

MethodThe resonant elements of the proposed 10.5 T 8-channel transceive array are built using the 50-Ohm semi-rigid coaxial transmission line RG405 with a diameter of 0.08 inch. In the CTL resonator, the outer conductor is cut in the middle. This cut creates a small capacitance, next to which a tunable capacitor (C1) is connected in parallel to adjust the resonant frequency as shown in Fig 1. At the two ends of the CTL, the inner conductors are connected by a capacitor (C2) while the outer conductors are shorted, where a virtual ground is generated in this symmetric circuit topology. Before the coil construction, the individual CTL resonator and 8-channel CTL array were modeled and analyzed numerically using full wave electromagnetic analysis provided in commercial software (Ansys/HFSS, Canonsburg, PA). The resonant elements are fabricated based on the simulation results and mounted on thin Teflon …