A 15-Channel receive array and 16 channel detunable transmit coil for human brain imaging at 9.4T

The radio frequency (RF) magnetic field (B1) distribution becomes more complex in MR experiments employing higher static magnetic field (B0) due to shorter wavelength in tissue. The B1 + inhomogeneities from a predefined volume of interest is reduced by influencing the amplitude and phase of the transmit current on a transceiver array coil [1, 2]. Significant gains in SNR was achieved at 7T using 32 channel receive arrays assembled on close fitting formers [3, 4]. In this study, we combine the benefits of these two methods for human brain MRI at 9.4T (400MHz). Our imaging setup consists of a 15-element receive array together with a 16-element actively detunable transmit array and hence the additional flexibility to employ RF shimming methods

Patient-specific RF safety assessment in MRI: Progress in creating surface-based human head and shoulder models

The interaction of electromagnetic (EM) fields with the human body during magnetic resonance imaging (MRI) is complex and subject specific. MRI radiofrequency (RF) coil performance and safety assessment typically includes numerical EM simulations with a set of human body models. The dimensions of mesh elements used for discretization of the EM simulation domain must be adequate for correct representation of the MRI coil elements, different types of human tissue, and wires and electrodes of additional devices. Examples of such devices include those used during electroencephalography, transcranial magnetic stimulation, and transcranial direct current stimulation, which record complementary information or manipulate brain states during MRI measurement. The electrical contact within and between tissues, as well as between an electrode and the skin, must also be preserved. These requirements can be fulfilled with anatomically correct surface-based human models and EM solvers based on unstructured meshes. Here, we report (i) our workflow used to generate the surface meshes of a head and torso model from the segmented AustinMan dataset, (ii) head and torso model mesh optimization for three-dimensional EM simulation in ANSYS HFSS, and (iii) several case studies of MRI RF coil performance and safety assessment

Thermal Decomposition of Co-Doped Calcium Tartrate and Use of the Products for Catalytic Chemical Vapor Deposition Synthesis of Carbon Nanotubes.

Thermal decomposition of Co-doped calcium tartrate in an inert atmosphere or air was studied using thermogravimetric analysis and X-ray absorption fine structure (XAFS) spectroscopy. It was shown that the powder substance containing 4 at.% of cobalt completely decomposes within 650-730 °C, depending on the environment, and the formation of Co clusters does not proceed before 470 °C. The products of decomposition were characterized by transmission electron microscopy, XAFS, and X-ray photoelectron spectroscopy. Surfaceoxidized Co metal nanoparticles as large as ∼5.6 ( 1.2 nm were found to form in an inert atmosphere, while the annealing in air led to a wide distribution of diameters of the nanoparticles, with the largest nanoparticles (30-50 nm) mainly present as a Co3O4 phase. It was found that the former nanoparticles catalyze the growth of CNTs from alcohol while a reducing atmosphere is required for activation of the latter nanoparticles. We propose the scheme of formation of CaO-supported catalyst from Co-doped tartrate, depending on the thermal decomposition conditions

Design of RF Coils for Ultra-high Field MRI

A handful of groups worldwide are equipped with magnets stronger than 7T. The excellence of MRI results from these unique scanners depends ultimately on the RF instrumentation, techniques to control the RF field distribution and acquisition methods. Two state-of the art RF coil setups were developed for 1H and 23Na imaging at 9.4T. At high Larmor frequencies, the wavelength in tissue is comparable or even smaller than the sample dimensions and the RF field distribution exhibits traveling wave behavior, resulting in an inhomogeneous image. Arranging transmit array elements in multiple rows provides additional degree of freedom to correct the inhomogeneities and to achieve whole-brain excitation. Receive arrays shaped to the contours of the anatomy increase the image signal-to-noise. A 16-element dual-row transmit array and a 31-element receive array is combined to achieve whole brain excitation and high-SNR at 9.4T. Multi-nuclei imaging also benefits from higher field strength. However, such setups should have 1H imaging capability for B0 shimming purposes. A novel coil arrangement, consisting of a combination of three coil arrays was designed to achieve high SNR, efficient transmit excitation and B0 shimming capability for 23Na MRI at 9.4T. Design requirements, coil design, phantom studies and invivo results from these two set-ups will be presented

Design, performance and safety evaluation of RF coil arrays for brain imaging at 9.4T

A number of groups are involved in ultra-high field MRI (≥7T) research because of its promise in increasing signal to noise ratio and improved contrast. RF coils, being the front-end of the imaging chain, play a significant role in determining the quality of the MR experiment. Custom built coils are used quite often at higher field strengths due to lack of suitable commercial coils. At this frequency, the RF field distribution across the sample is inhomogeneous due to the shorter wavelength in tissue and increases the risk of localized areas with elevated power deposition. Hence it is important that the RF coil meets the safety requirements before it is approved for human use. At the Max Planck Institute in Tuebingen, we developed a procedure for performance and safety evaluation of our home-built RF coils. In this presentation, I will talk about the design methods, performance evaluation and safety evaluation of an RF coil array built for human brain imaging at 9.4T

Fast Sodium Imaging at 9.4 Tesla

Fast imaging sequences are commonly used for proton imaging because of their high acquisition efficiency. In this work three spiral imaging sequences which used either RF spoiling, gradient spoiling or balanced gradients were adapted for sodium imaging at 9.4 T and compared to one another based on achieved image quality and SNR. Balanced steady-state free precession imaging provided the highest SNR while producing only negligible image artefacts. Due to the efficient acquisition with a sensitive 27-channel receiver array images with a nominal spatial resolution of 1.5x1.5x4.0 mm3 and an acceptable SNR could be acquired in 10 min

Effects of Tuning Condition, Head Size and Position on the SAR of a 9.4T Dual Row Array

For an already constructed 9.4T dual row array, excited in CP and other transmit modes, we investigated peak SAR averaged over 10 grams (SAR10g) for different tuning conditions, and different head positions with three scaling factors. For a given array, the tuning condition significantly affected SAR10g, while the influence on SAR10g of head position and scaling factor was relatively small

3D Radial GRE-EPI with Up to 8-Fold Acceleration for Functional Imaging at 9.4T

Increasing acquisition speed is beneficial in fMRI since it increases statistical power and allows the separation of physiological noise from the time series. An effective means to achieve this speed is to combine EPI with parallel imaging. 3D EPI allows very high parallel imaging factors since acceleration can be performed in both phase-encoding directions. Non-Cartesian parallel imaging can potentially allow even higher acceleration by exploiting coil sensitivities in all three dimensions. To this end, a 3D radial EPI sequence was developed and first results with up to 8-fold radial GRAPPA acceleration are presented from finger tapping experiments at 9.4T