Faculty Bibliography

In vivo sodium magnetic resonance imaging (MRI) measures tissue sodium content in living human brain but current methods do not allow noninvasive quantitative assessment of intracellular sodium concentration (ISC) - the most useful marker of tissue viability. In this study, we report the first noninvasive quantitative in vivo measurement of ISC and intracellular sodium volume fraction (ISVF) in healthy human brain, made possible by measuring tissue sodium concentration (TSC) and intracellular sodium molar fraction (ISMF) at ultra-high field MRI. The method uses single-quantum (SQ) and triple-quantum filtered (TQF) imaging at 7 Tesla to separate intra- and extracellular sodium signals and provide quantification of ISMF, ISC and ISVF. This novel method allows noninvasive quantitative measurement of ISC and ISVF, opening many possibilities for structural and functional metabolic studies in healthy and diseased brains

Radiofrequency (RF) emitting wireless devices such as mobile phones are required to undergo standardized safety testing prior to entering the consumer market. Strict regulations are imposed on the amount of RF energy these devices are allowed to emit to prevent excessive deposition of RF energy into the body. In this work, a novel safety evaluation test for wireless devices using magnetic resonance thermometry is proposed.

Purpose: To investigate the integrity of the default-mode network (DMN) by using independent component analysis (ICA) methods in patients shortly after mild traumatic brain injury (MTBI) and healthy control subjects, and to correlate DMN connectivity changes with neurocognitive tests and clinical symptoms. Materials and Methods: This study was approved by the institutional review board and complied with HIPAA regulations. Twenty-three patients with MTBI who had posttraumatic symptoms shortly after injury (<2 months) and 18 age-matched healthy control subjects were included in this study. Resting-state functional magnetic resonance imaging was performed at 3 T to characterize the DMN by using ICA methods, including a single-participant ICA on the basis of a comprehensive template from core seeds in the posterior cingulate cortex (PCC) and medial prefrontal cortex (MPFC) nodes. ICA z images of DMN components were compared between the two groups and correlated with neurocognitive tests and clinical performance in patients by using Pearson and Spearman rank correlation. Results: When compared with the control subjects, there was significantly reduced connectivity in the PCC and parietal regions and increased frontal connectivity around the MPFC in patients with MTBI (P < .01). These frontoposterior opposing changes within the DMN were significantly correlated (r = -0.44, P = .03). The reduced posterior connectivity correlated positively with neurocognitive dysfunction (eg, cognitive flexibility), while the increased frontal connectivity correlated negatively with posttraumatic symptoms (ie, depression, anxiety, fatigue, and postconcussion syndrome). Conclusion: These results showed abnormal DMN connectivity patterns in patients with MTBI, which may provide insight into how neuronal communication and information integration are disrupted among DMN key structures after mild head injury. (c) RSNA, 2012.

At high and ultra-high magnetic field strengths, understanding interactions between tissues and the electromagnetic fields generated by radiofrequency coils becomes crucial for safe and effective coil design as well as for insight into limits of performance. In this work, we present a rigorous electrodynamic modeling framework, using dyadic Green's functions, to derive the electromagnetic field in homogeneous spherical and cylindrical samples resulting from arbitrary surface currents in the presence or absence of a surrounding radiofrequency shield. We show how to calculate ideal current patterns that result in the highest possible signal-to-noise ratio (ultimate intrinsic signal-to-noise ratio) or the lowest possible radiofrequency power deposition (ultimate intrinsic specific absorption rate) compatible with electrodynamic principles. We identify familiar coil designs within optimal current patterns at low to moderate field strength, thereby establishing and explaining graphically the near-optimality of traditional surface and volume quadrature designs. We also document the emergence of less familiar patterns, e.g., involving substantial electric- as well as magnetic-dipole contributions, at high field strength. Performance comparisons with particular coil array configurations demonstrate that optimal performance may be approached with finite arrays if ideal current patterns are used as a guide for coil design. Magn Reson Med, 2011. (c) 2011 Wiley Periodicals, Inc

We present high-resolution anatomical imaging of the cervical spinal cord in healthy volunteers at the ultrahigh field of 7 T with a prototype four-channel radiofrequency coil array, in comparison with 3-T imaging of the same subjects. Signal-to-noise ratios at both field strengths were estimated using the rigorous Kellman method. Spinal cord cross-sectional area measurements were performed, including whole-cord measurements at both fields and gray matter segmentation at 7 T. The 7-T array coil showed reduced sagittal coverage, comparable axial coverage and the expected significantly higher signal-to-noise ratio compared with equivalent 3-T protocols. In the cervical spinal cord, the signal-to-noise ratio was found by the Kellman method to be higher by a factor of 3.5 with the 7-T coil than with standard 3-T coils. Cervical spine imaging in healthy volunteers at 7 T revealed not only detailed white/gray matter differentiation, but also structures not visualized at lower fields, such as denticulate ligaments, nerve roots and rostral-caudal blood vessels. Whole-cord cross-sectional area measurements showed good agreement at both field strengths. The measurable gray/white matter cross-sectional areas at 7 T were found to be comparable with reports from histology. These pilot data demonstrate the use of higher signal-to-noise ratios at the ultrahigh field of 7 T for significant improvement in anatomical resolution of the cervical spinal cord, allowing the visualization of structures not seen at lower field strength, particularly for axial imaging

Although there have been many advancements in cancer research, much is still unknown about the heterogeneous tumor microenvironment. Diffusion-weighted MRI has proven to be a viable and versatile microstructural probe. Diffusion-weighted sequences specifically sensitive to intravoxel incoherent motion (IVIM) have seen a recent resurgence of interest as they promise to provide a valuable window on the vascular microenvironment. To understand, test, and optimize IVIM-sensitive approaches, a complex flow phantom was constructed to mimic certain characteristics of the tumor microenvironment such as tortuous microvasculature, heterogeneous vascular permeability, and interstitial fluid pressure buildup. Results using this phantom on a clinical scanner platform confirmed IVIM sensitivity to microscopic flow effects. Biexponential fitting of signal decay curves enabled quantitative extraction of perfusion fraction, IVIM-related pseudodiffusivity, and tissue diffusivity. Parametric maps were also generated, illustrating the potential utility of IVIM-sensitive imaging in clinical settings. The flow phantom proved to be an effective test-bed for validating and optimizing the IVIM-MRI technique to provide surrogate markers for microvascular properties. Magn Reson Med, 2011. (c) 2011 Wiley Periodicals, Inc

The effective delivery of a therapeutic drug to the core of a tumor is often impeded by physiological barriers, such as the interstitial fluid pressure (IFP). There are a number of therapies that can decrease IFP and induce tumor vascular normalization. However, a lack of a noninvasive means to measure IFP hinders the utilization of such a window of opportunity for the maximization of the treatment response. Thus, the purpose of this study was to investigate the feasibility of using intravoxel incoherent motion (IVIM) diffusion parameters as noninvasive imaging biomarkers for IFP. Mice bearing the 4T1 mammary carcinoma model were studied using diffusion-weighted imaging (DWI), immediately followed by wick-in-needle IFP measurement. Voxelwise analysis was conducted with a conventional monoexponential diffusion model, as well as a biexponential model taking IVIM into account. There was no significant correlation of IFP with either the median apparent diffusion coefficient from the monoexponential model (r = 0.11, p = 0.78) or the median tissue diffusivity from the biexponential model (r = 0.30, p = 0.44). However, IFP was correlated with the median pseudo-diffusivity (D(p) ) of apparent vascular voxels (r = 0.76, p = 0.02) and with the median product of the perfusion fraction and pseudo-diffusivity (f(p) D(p) ) of apparent vascular voxels (r = 0.77, p = 0.02). Although the effect of IVIM in tumors has been reported previously, to our knowledge, this study represents the first direct comparison of IVIM metrics with IFP, with the results supporting the feasibility of the use of IVIM DWI metrics as noninvasive biomarkers for tumor IFP

The markedly increased degrees of freedom introduced by parallel radiofrequency transmission presents both opportunities and challenges for specific absorption rate (SAR) management. On one hand they enable E-field tailoring and SAR reduction while facilitating excitation profile control. On other hand they increase the complexity of SAR behavior and the risk of inadvertently exacerbating SAR by improper design or playout of radiofrequency pulses. The substantial subject-dependency of SAR in high field magnetic resonance can be a compounding factor. Building upon a linear system concept and a calibration scheme involving a finite number of in situ measurements, this work establishes a clinically applicable method for characterizing global SAR behavior as well as channel-by-channel power transmission. The method offers a unique capability of predicting, for any excitation, the SAR and power consequences that are specific to the subject to be scanned and the MRI hardware. The method was validated in simulation and experimental studies, showing promise as the foundation to a prospective paradigm where power and SAR are not only monitored but, through prediction-guided optimization, proactively managed. Magn Reson Med, 2011. (c) 2011 Wiley Periodicals, Inc

Noncontrast techniques for peripheral MR angiography are receiving renewed interest because of safety concerns about the use of gadolinium in patients with renal insufficiency. One class of techniques involves subtraction of dark-blood images acquired during fast systolic flow from bright-blood images obtained during slow diastolic flow. The goal of this work was to determine whether the inherent sparsity of the difference images could be exploited to achieve greater acceleration without loss of image quality in the context of generalized autocalibrating partially parallel acquisition (GRAPPA). It is shown that noise amplification at high acceleration factors can be reduced by performing subtraction on the raw data, before calculation of the GRAPPA weights, rather than on the final magnitude images. Use of the difference data to calculate the GRAPPA weights decreases the geometry factor (g-factor), because the difference data represent a sparse image set. This demonstrates an inherent property of GRAPPA and does not require the use of compressed sensing. Application of this approach to highly accelerated data from healthy volunteers resulted in similar depiction of large arteries to that obtained with low acceleration and standard reconstruction. However, visualization of very small vessels and arterial branches was compromised. Magn Reson Med, 2011. (c) 2011 Wiley-Liss, Inc