To investigate the effects of high dielectric material padding on RF field distribution in the human head at 7.0 T, and demonstrate the feasibility and effectiveness of RF passive shimming and focusing with such an approach.
The intensity distribution changes of gradient-recalled-echo (GRE) and spin-echo (SE) images of a human head acquired with water pads (dielectric constant = 78) placed in specified configurations around the head at 7.0 T were evaluated and compared with computer simulation results using the finite difference time domain (FDTD) method. The contributions to the B(1) field distribution change from the displacement current and conductive current of a given configuration of dielectric padding were determined with computer simulations.
MR image intensity distribution in the human head with an RF coil at 7.0 T can be changed drastically by placing water pads around the head. Computer simulations reveal that the high permittivity of water pads results in a strong displacement current that enhances image intensity in the nearby region and alters the intensity distribution of the entire brain.
The image intensity distribution in the human head at ultra-high field strengths can be effectively manipulated with high permittivity padding. Utilizing this effect, the B(1) field inside the human head of a given RF coil can be adjusted to reduce the B(1) field inhomogeneity artifact associated with the wave behavior (RF passive shimming) or to locally enhance the signal-to-noise ratio (SNR) in targeted regions of interest (ROIs; RF field focusing).
"However, given the overall size as well as complicated geometry of the post-mortem brain surface this approach becomes less effective. Multiple efforts were made on both phantoms and brain samples to reproducibly implement this method using varying configurations of bags filled with multiple differing high dielectric constant materials, including water (Yang et al., 2006), calcium titanate, and barium titanate (Teeuwisse et al., 2012) with limited success. Dynamic shimming using parallel transmit channels (pTx) is another technologically promising approach (Katscher and Bornert, 2006); this would overcome the geometric variability between samples and allow for precise and unique sample dependent B 1 shimming. "
[Show abstract][Hide abstract] ABSTRACT: Post-mortem diffusion imaging of whole, human brains has potential to provide data for validation or high-resolution anatomical investigations. Previous work has demonstrated improvements in data acquired with diffusion-weighted steady-state free precession (DW-SSFP) compared with conventional diffusion-weighted spin echo at 3T. This is due to the ability of DW-SSFP to overcome signal-to-noise and diffusion contrast losses brought about by tissue fixation related decreases in T2 and ADC. In this work, data of four post-mortem human brains were acquired at 3T and 7T, using DW-SSFP with similar effective b-values (beff~5150s/mm(2)) for inter-field strength comparisons; in addition, DW-SSFP data were acquired at 7T with higher beff (~8550s/mm(2)) for intra-field strength comparisons. Results demonstrate both datasets acquired at 7T had higher SNR and diffusion contrast than data acquired at 3T, and data acquired at higher beff had improved diffusion contrast than at lower beff at 7T. These results translate to improved estimates of secondary fiber orientations leading to higher fidelity tractography results compared with data acquired at 3T. Specifically, tractography streamlines of cortical projections originating from the corpus callosum, corticospinal tract, and superior longitudinal fasciculus were more successful at crossing the centrum semiovale and projected closer to the cortex. Results suggest that DW-SSFP at 7T is a preferential method for acquiring diffusion-weighted data of post-mortem human brain, specifically where the primary region of interest involves crossing white matter tracts.
"Several techniques have been implemented to correct for image distortions (Chen and Wyrwicz, 2001, Cordes et al., 2000, Liu and Ogawa, 2006, Ojemann et al., 1997). In addition, improved local B 0 -shimming approaches reduce geometric distortions of the image (Constable and Spencer, 1999, Cusack et al., 2005, Du et al., 2007, Glover, 1999, Gu et al., 2002, Heberlein and Hu, 2004, Juchem et al., 2006, Koch et al., 2006, Takahashi et al., 2001, Wilson et al., 2003, Wilson and Jezzard, 2003, Yang et al., 2006). Most of these techniques have been shown to be successful to some degree but not fully, and require significant effort and additional imaging time for careful calibration and optimization. "
[Show abstract][Hide abstract] ABSTRACT: Although functional imaging of neuronal activity by magnetic resonance imaging (fMRI) has become the primary methodology employed in studying the brain, significant portions of the brain are inaccessible by this methodology due to its sensitivity to macroscopic magnetic field inhomogeneities induced near air-filled cavities in the head. In this paper, we demonstrate that this sensitivity is eliminated by a novel pulse sequence, RASER (rapid acquisition by sequential excitation and refocusing) (Chamberlain et al., 2007), that can generate functional maps. This is accomplished because RASER acquired signals are purely and perfectly T(2) weighted, without any T(2)*-effects that are inherent in the other image acquisition schemes employed to date. T(2)-weighted fMRI sequences are also more specific to the site of neuronal activity at ultrahigh magnetic fields than T(2)*-variations since they are dominated by signal components originating from the tissue in the capillary bed. The RASER based fMRI response is quantified; it is shown to have an inherently less noisy time series and to provide fMRI in brain regions, such as the orbitofrontal cortex, which are challenging to image with conventional techniques.
"Therefore, the ability to homogenize transmit profile is important for full realization of the potential of high-field MRI. Recently, several methods have been proposed in RF coil design to homogenize profiles, which include inserting high-permittivity materials (or passive RF shimming) , inverse coils , active RF shimming , and transmit SENSE . "
[Show abstract][Hide abstract] ABSTRACT: Transmit B(1)(+) field homogenization in high-field ( > 3.0 T) human magnetic resonance imaging (MRI) is challenging due to radio-frequency wavelength effects. An approach based on appropriately coupling surface coils to a volume coil was investigated. Electromagnetic simulation results demonstrated the feasibility and effectiveness of this method in proton MRI of the human head at 7.0 T.
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