Faculty Bibliography

We investigated how high permittivity materials affect the S-parameters and transmit

Compressed sensing is a powerful rapid imaging approach for Magnetic Resonance Imaging

Purpose: Despite the extensive opportunities offered by current state-of-the-art PET-MR systems [1], their use is still far from routine clinical practice. While it is feasible to acquire PET data from a single bed position in about 5 minutes, collecting the clinically relevant variety of traditional MR contrasts requires substantially more time. This bottleneck formed by the traditional MR paradigm leads to a relatively inefficient use of the PET component and is particularly prohibitive for multiplebed-position PET protocols. This work proposes a one-step procedure that merges the MR fingerprinting (MRF) framework [2] with the PET acquisition, and employs a dedicated multi-modality reconstruction exploiting joint information among multiple contrast weightings to enable a 6 minute comprehensive PET-MR exam, which can provide the majority of clinical MR contrasts alongside quantitative parametric maps of the relaxation parameters (T1, T2) together with improved PET images. Theory & Methods: Although MRF is inherently robust against incoherent undersampling artifacts, there is a limit beyond which the final image quality will suffer. Instead of relaying purely on incoherence between undersampling artifacts and simulated signal evolutions (standard MRF reconstruction), we propose an extension of a recently proposed nonlinear joint multimodality reconstruction [3] to simultaneously reconstruct the series of MRF images and the PET image by enforcing joint sparsity, thereby reducing residual undersampling artifacts in MR while at the same time improving PET reconstruction quality. The joint MRF-PET reconstruction is performed by minimizing the …

Abnormality in the "fear circuitry" has been known as a major neural characteristic of posttraumatic stress disorder (PTSD). Recent studies also revealed decreased functional connectivity in the default mode network in PTSD. The present study aims to investigate, in war-zone-related PTSD, the spontaneous activity and functional connectivity of the amygdala and the precuneus, which are two representative brain regions of the two networks, respectively. Two groups of 52 male US Operation Iraqi Freedom (OIF) and Operation Enduring Freedom (OEF) veterans (PTSD vs. controls), well matched on age and ethnicity, were clinically assessed and then studied in a resting state functional magnetic resonance imaging (fMRI) procedure. Functional connectivity analysis was conducted on the resting state fMRI data with the amygdala and precuneus as seeds. Compared with controls, veterans with PTSD had lower functional connectivity in the default mode network, as well as lower amygdala-frontal functional connectivity. Both the PTSD and the control group had a significant positive precuneal-amygdala functional connectivity without a significant group difference. The magnitudes of spontaneous activity of the amygdala and the precuneus were negatively correlated in the PTSD group and showed significant quadratic relationships with the amount of emotional abuse in early life trauma. These findings may improve our understanding about the relationships between fear circuitry and the default mode network in the context of war-zone-related PTSD.

PURPOSE: In this work, a generic recipe for an inexpensive and nontoxic phantom was developed within a range of biologically relevant dielectric properties from 150 MHz to 4.5 GHz. METHODS: The recipe includes deionized water as the solvent, NaCl to primarily control conductivity, sucrose to primarily control permittivity, agar-agar to gel the solution and reduce heat diffusivity, and benzoic acid to preserve the gel. Two hundred and seventeen samples were prepared to cover the feasible range of NaCl and sucrose concentrations. Their dielectric properties were measured using a commercial dielectric probe and were fitted to a 3D polynomial to generate a recipe describing the properties as a function of NaCl concentration, sucrose concentration, and frequency. RESULTS: Results indicated that the intuitive linear and independent relationships between NaCl and conductivity and between sucrose and permittivity are not valid. A generic polynomial recipe was developed to characterize the complex relationship between the solutes and the resulting dielectric values and has been made publicly available as a web application. In representative mixtures developed to mimic brain and muscle tissue, less than 2% difference was observed between the predicted and measured conductivity and permittivity values. CONCLUSIONS: It is expected that the recipe will be useful for generating dielectric phantoms for general magnetic resonance imaging (MRI) coil development at high magnetic field strength, including coil safety evaluation as well as pulse sequence evaluation (including B1 (+) mapping, B1 (+) shimming, and selective excitation pulse design), and other non-MRI applications which require biologically equivalent dielectric properties.

PURPOSE: To develop a fast and flexible free-breathing dynamic volumetric MRI technique, iterative Golden-angle RAdial Sparse Parallel MRI (iGRASP), that combines compressed sensing, parallel imaging, and golden-angle radial sampling. METHODS: Radial k-space data are acquired continuously using the golden-angle scheme and sorted into time series by grouping an arbitrary number of consecutive spokes into temporal frames. An iterative reconstruction procedure is then performed on the undersampled time series where joint multicoil sparsity is enforced by applying a total-variation constraint along the temporal dimension. Required coil-sensitivity profiles are obtained from the time-averaged data. RESULTS: iGRASP achieved higher acceleration capability than either parallel imaging or coil-by-coil compressed sensing alone. It enabled dynamic volumetric imaging with high spatial and temporal resolution for various clinical applications, including free-breathing dynamic contrast-enhanced imaging in the abdomen of both adult and pediatric patients, and in the breast and neck of adult patients. CONCLUSION: The high performance and flexibility provided by iGRASP can improve clinical studies that require robustness to motion and simultaneous high spatial and temporal resolution. Magn Reson Med, 2013. (c) 2013 Wiley Periodicals, Inc.

PURPOSE: Spatiotemporally Encoded (SPEN) MRI is based on progressive point-by-point refocusing of the image in the spatial rather than the k-space domain through the use of frequency-swept radiofrequency pulses and quadratic phase profiles. This technique provides high robustness against frequency-offsets including B0 inhomogeneities and chemical-shift (e.g., fat/water) distortions, and can consequently perform fMRI at challenging regions such as the orbitofrontal cortex and the olfactory bulb, as well as to improve imaging near metallic implants. This work aims to establish a comprehensive framework for the implementation and super-resolved reconstruction of SPEN-based imaging, and to accurately quantify this method's spatial-resolution and signal-to-noise ratio (SNR). THEORY AND METHODS: A stepwise formalism was laid-out for calculating the optimal experimental parameters for SPEN, followed by analytical analysis of the ensuing SNR and spatial-resolution versus conventional k-space encoding. Predictions were then confirmed using computer simulations and experimentally. RESULTS: Our findings show that SPEN is governed by the same fundamental signal-processing principles as k-space encoding, leading to similar averaging properties, and ultimately similar spatial-resolution and SNR levels as k-space encoding. CONCLUSION: Presented analysis is applicable to general multidimensional SPEN designs and provides a unified framework for the analysis of future SPEN and similar approaches based on quadratic phase encoding. Magn Reson Med, 2013. (c) 2013 Wiley Periodicals, Inc.