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

A 700MHz Receive Array using Patch Antenna for Spin Excitation

The availability of receive array coils at high field, small bore animal scanners is limited by the lack of space for classical transmit volume resonators coupled with its inability to generate homogenous transmit B1 field due to wavelength effects. We explore the possibility of the traveling wave concept for spin excitation along with the phased array technique for signal reception at 16.4T. To this effect, a 3-channel phased array coil and a patch antenna were designed and combined. Signal to noise ratio and parallel imaging techniques were studied and achieved SNR equivalent to that of a quadrature surface coil

Functional MRI in the rat at 9.4 T and 16.4 T

Functional MRI (fMRI) in animals at high magnetic fields keeps expanding our knowledge about the basics of neural processing but the specificity of the fMRI-signal is still under ongoing investigation. Yet, as the signal to noise ratio in MRI depends linearly on the magnetic field strength and calls for even stronger magnets for the detection of even smaller anatomical details, the relation between the functional MR-response and field strength can only be approximated with complex models. In this study the blood oxygenation dependent (BOLD) effect was measured and compared at 9.4 T and 16.4 T in the same animal with segmented gradient-echo (GE) and spin-echo (SE) echo planar imaging (EPI) sequence using optimal echo times for the respective field. Furthermore, high resolution fMRI acquisition at 16.4 T was performed up to a 50 µm in-plane accuracy and for an 8 s temporal resolution without the use of cryo-coils or coil-arrays

A Microstrip Resonator for Animal MRI at 16.4T

Introduction: MRI is increasingly moving towards higher magnetic field because of the inherent high sensitivity and greater spectroscopic resolution. To realize these advantages, the front end, the RF coil and the receive chain, needs to be optimized. A Quadrature Microstrip Transmission Line (MTL) resonator was built and SNR is compared with a commercially available shielded linear birdcage coil. This work also includes development of a TR switch module (consisting of T/R switch and preamplifier) and a Microstrip transmission line based Quadrature hybrid. Subjects and Methods: Experiments were performed on a 16.4T/26cm horizontal bore Magnex Magnet attached to a Bruker biospec Spectrometer. The imaging gradient system has an inner diameter of 12cms. The resonator (Figure 1) is built on Teflon cylinder with wall thickness 5mm. It consists of 8 equally spaced λ/2 MTL resonators. Electromagnetic coupling among the 8 resonators makes the entire MTL volume coil resonate at the desired frequency. In this case, each resonator is tuned to 725MHz to resonate the volume coil to 705MHz in the unloaded condition. The TR switch module consists of a diode network and a 2 stage low noise amplifier (LNA). The first stage of the LNA has PHEMT ATF35143 and the second stage has BJT BFR193. The TR switch module has a gain of 30dB and noise figure of 1.1dB at 698MHz. Results: The losses in the receive chain is reduced by connecting the TR switch module closer to the birdcage coil. An SNR gain of 1.4X is observed as shown in Table 1. The MTL resonator has double the volume of the birdcage resonator and has comparable SNR as that of the birdcage resonator (Table 1). The linear and combined phantom images are shown in Figure 2. Figure 3 shows in-vivo Axial and Sagittal rat brain RARE images with in-plane spatial resolution of 117μm. Conclusion: A Quadrature MTL resonator with good SNR and signal uniformity has been built. This paves the way for developing actively decoupled resonators and to use smaller receive only surface coils for improved Sensitivity at 16.4T

Functional MRI in the rat brain with single-shot gradient echo EPI at 16.4 T

The feasibility of gradient echo echo-planar imaging sequence (GE-EPI) for the accurate detection of stimulation-specific BOLD activation contrast in the rat brain at 16.4 T was investigated. An experimental protocol for longitudinal fMRI studies with extensive monitoring of the animal’s physiological status was employed. It was found that parameter optimized single-shot GE-EPI detects high quality images and is suitable for fMRI studies, provided motion effects during the timeseries can be compensated by data processing. The first specific BOLD activation maps at 16.4 T are presented and methodical details are discussed

Strong BOLD-effect with TurboCRAZED MRI following hyperoxia in the rat brain at 16.4 T

Introduction: MR imaging techniques based on the indirect detection of intermolecular multiple quantum (iMQC) coherences provide pronounced BOLD signal. The aim of this study was to asses the percent signal change observable via the iMQC BOLD-effect at 16.4T in the rat brain using the accelerated method TurboCRAZED (1). Subjects and Methods: Experiments were performed on a 16.4T/26cm horizontal bore Magnex magnet interfaced to a Bruker spectrometer. The imaging gradient system had an inner diameter of 12cm. MR images were acquired with homebuilt 14mm and 22mm loop coils. Healthy adult Wistar rats (255g and 603g) were anesthetized by inhalation of Isoflurane. Breathing was monitored and body temperature was kept constant using electric heating pads. Results: High resolution TurboCRAZED images were acquired in the rat brain with a nominal inplane resolution of (156μm)2 and a slice thickness of 500μm within one hour measuring time (Fig.1). Experiments with different iMQC correlation length (dc) provided different contrast, because the spatial origin of the local CRAZED signal is in a fair approximation a spherical shell, which can be shifted along the sphere radius by simply changing the area under the correlation gradient in the pulse sequence. TurboCRAZED images were also sensitive to the BOLD-effect (Fig.2). A 30 minutes time series of four images was acquired, where the animal was inhaling air in the initial and pure oxygen in the final 15 minutes. The maximum of the difference image was 8.8 from the maximum value in the same brain region of the summed air-images. Conclusion: High resolution TurboCRAZED images of the rat brain can be obtained in experimental times that allow for fMRI experiments. In combination with the pronounced sensitivity for the BOLD-effect, the tunable iMQC contrast (2) may provide additional information in small animal fMRI, which is not available with conventional methods

Regional Variations of Metabolite Concentrations in the Rat Brain Assessed with in vivo 1H MR Spectroscopy at 16.4T

Regional differences of metabolites in the rat brain were investigated by using localized in vivo 1H MR spectroscopy at 16.4T. Three regions, thalamus, striatum and hippocampus, were investigated with an ultra-short TE STEAM sequence. The results demonstrated significant variations in all metabolites except aspartate and NAA. The remarkable variation of spectra was the substantially decreased level of the Tau methylene signal at 3.25 ppm in thalamus. The significant increase of the GABA methylene signal at 1.89 ppm was also observed in thalamus

Highly Accurate Quantification of Proton MR Spectroscopy in Rat Brain in Vivo at 16.4 T

In the present study, we used a 16.4 T scanner to evaluate the quantification of metabolites from the rat brain with an ultra-short TE STEAM sequence. All metabolite resonances, previously observed in vivo and published in the literature, were detected. Additionally, a newly discernible Histidine peaks at 3.11 ppm, 3.22 ppm and 3.97 ppm were detected and quantified for the first time in vivo, with CRLB of below 20 , indicating high quantification reliability. These preliminary results demonstrate the feasibility of detecting new metabolites in combination with the advantages of a high magnetic field strength