Faculty Bibliography

In ultra-high-field magnetic resonance imaging, parallel radiofrequency (RF) transmission presents both opportunities and challenges for specific absorption rate management. On one hand, parallel transmission provides flexibility in tailoring electric fields in the body while facilitating magnetization profile control. On the other hand, it increases the complexity of energy deposition as well as possibly exacerbating local specific absorption rate by improper design or delivery of RF pulses. This study shows that the information needed to characterize RF heating in parallel transmission is contained within a local power correlation matrix. Building upon a calibration scheme involving a finite number of magnetic resonance thermometry measurements, this work establishes a way of estimating the local power correlation matrix. Determination of this matrix allows prediction of temperature change for an arbitrary parallel transmit RF pulse. In the case of a three transmit coil MR experiment in a phantom, determination and validation of the power correlation matrix were conducted in less than 200 min with induced temperature changes of <4 degrees C. Further optimization and adaptation are possible, and simulations evaluating potential feasibility for in vivo use are presented. The method allows general characteristics indicative of RF coil/pulse safety determined in situ. Magn Reson Med, 2012. (c) 2012 Wiley Periodicals, Inc.

Radiofrequency shimming with multiple channel excitation has been proposed to increase the transverse magnetic field uniformity and reduce specific absorption rate at high magnetic field strengths (>/=7 T) where high-frequency effects can make traditional single channel volume coils unsuitable for transmission. In the case of deep anatomic regions and power-demanding pulse sequences, optimization of transmit efficiency may be a more critical requirement than homogeneity per se. This work introduces a novel method to maximize transmit efficiency using multiple channel excitation and radiofrequency shimming. Shimming weights are calculated in order to obtain the lowest possible net radiofrequency power deposition into the subject for a given transverse magnetic field strength. The method was demonstrated in imaging studies of articular cartilage of the hip joint at 7 T. We show that the new radiofrequency shimming method can enable reduction in power deposition while maintaining an average flip angle or adiabatic condition in the hip cartilage. Building upon the improved shimming, we further show that the signal-to-noise ratio in hip cartilage at 7 T can be substantially greater than that at 3 T, illustrating the potential benefits of high field hip imaging. Magn Reson Med, 2012. (c) 2012 Wiley Periodicals, Inc.

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

The critical design aim for a sodium/proton coil is to maximize sodium sensitivity and transmit field homogeneity while simultaneously providing adequate proton sensitivity and homogeneity. While most dual-frequency coils use lossy high-impedance trap circuits or PIN diodes to allow dual-resonance, we explored a nested-coil design for sodium/proton knee imaging at 7 T. A stand-alone eight-channel sodium receive array was implemented without standard dual-resonance circuitry to provide improved sodium signal-to-noise ratio. A detunable sodium birdcage was added for homogeneous sodium excitation and a four-channel proton transmit-receive array was added to provide anatomical reference imaging and B(0) shimming capabilities. Both additional modules were implemented with minimal disturbance to the eight-channel sodium array by managing their respective resonances and geometrical arrangement. In vivo sodium signal-to-noise ratio was 1.2-1.7 times greater in the developed eight-channel array than in a mononuclear sodium birdcage coil, whereas the developed four-channel proton array provided signal-to-noise ratio similar to that of a commercial mononuclear proton birdcage coil. Magn Reson Med, 2012. (c) 2012 Wiley Periodicals, Inc.

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

Specific absorption rate management and excitation fidelity are key aspects of radiofrequency pulse design for parallel transmission at ultra-high magnetic field strength. The design of radiofrequency pulses for multiple channels is often based on the solution of regularized least-squares optimization problems for which a regularization term is typically selected to control the integrated or peak pulse waveform amplitude. Unlike single-channel transmission, the specific absorption rate of parallel transmission is significantly influenced by interferences between the electric fields associated with the individual transmission elements, which a conventional regularization term does not take into account. This work explores the effects upon specific absorption rate of incorporating experimentally measurable electric field interactions into parallel transmission pulse design. Results of numerical simulations and phantom experiments show that the global specific absorption rate during parallel transmission decreases when electric field interactions are incorporated into pulse design optimization. The results also show that knowledge of electric field interactions enables robust prediction of the net power delivered to the sample or subject by parallel radiofrequency pulses before they are played out on a scanner. Magn Reson Med, 2011. (c) 2011 Wiley-Liss, Inc

PURPOSE: To image the femoral arteries in peripheral arterial disease (PAD) patients using a bilateral receive coil. MATERIALS AND METHODS: An eight-channel surface coil array for bilateral MRI of the femoral arteries at 3T was constructed and evaluated. RESULTS: The bilateral array enabled imaging of a 25-cm segment of the superficial femoral arteries (SFA) from the profunda to the popliteal. The array provided improved the signal-to-noise ratio (SNR) at the periphery and similar SNR in the middle of a phantom compared to three other commercially available coils (4-channel torso, quadrature head, whole body). Multicontrast bilateral images of the in vivo SFA with 1 mm in-plane resolution made it possible to directly compare lesions in the index SFA to the corresponding anatomical site in the contralateral vessel without repositioning the patient or coil. A set of bilateral time-of-flight, T1-weighted, T2-weighted, and proton density-weighted images was acquired in a clinically acceptable exam time of approximately 45 minutes. CONCLUSION: The developed bilateral coil is well suited for monitoring dimensional changes in atherosclerotic lesions of the SFA.

Blood suppression in the lower extremities using flow-reliant methods such as double inversion recovery may be problematic due to slow blood flow. T(2) mapping using fast spin echo (FSE) acquisition was utilized to quantitate the effectiveness of double inversion recovery blood suppression in 13 subjects and showed that 25 +/- 12% of perceived vessel wall pixels in the popliteal arteries contained artifactual blood signal. To overcome this problem, a flow-insensitive T(2)-prepared inversion recovery sequence was implemented and optimal timing parameters were calculated for FSE acquisition. Black blood vessel wall imaging of the popliteal and femoral arteries was performed using two-dimensional T(2)-prepared inversion recovery-FSE in the same 13 subjects. Comparison with two-dimensional double inversion recovery-FSE showed that T(2)-prepared inversion recovery-FSE reduced wall-mimicking blood artifacts that inflated double inversion recovery-FSE vessel wall area measurements in the popliteal artery.

PURPOSE: To quantify femoral-popliteal vessel deformation during thigh contraction. MATERIALS AND METHODS: Eleven subjects underwent a magnetic resonance (MR) examination of the femoral-popliteal vasculature on a 1.5 T system. A custom 3D balanced steady-state free precession (SSFP) sequence was implemented to image a 15-20-cm segment of the vasculature during relaxation and voluntary isometric thigh contraction. The arterial and venous lumina were outlined using a semiautomated method. For the artery, this outline was fit to an ellipse whose aspect ratio was used to describe arterial deformation, while venous deformation was characterized by its cross-sectional area. RESULTS: Focal compression of the femoral-popliteal artery during contraction was observed 94-143 mm superior to the condyle that corresponds to the distal adductor canal (AC) immediately superior to the adductor hiatus. This was illustrated by a significant reduction (P < or = 0.05) in aspect ratio from 0.88 +/- 0.06 during relaxation to 0.77 +/- 0.09 during contraction. A negligible change in arterial aspect ratio was observed inferior to the AC and in the proximal AC. Similarly, venous area was dramatically reduced in the distal AC region during contraction. CONCLUSION: Rapid 3D SSFP MR angiography of the femoral-popliteal vasculature during thigh contraction demonstrated focal compression of the artery in the distal AC region. This may help explain the high stent failure rate and the high likelihood of atherosclerotic disease in the AC. J. Magn. Reson.

Noise correlation is an important topic in radiofrequency (RF) coil design for magnetic resonance imaging (MRI). The original noise correlation formula derived by Redpath showed correlation depends on the real part of the inverse impedance matrix. In this work, we introduce a more generalized formula to include effects from transmission lines and preamplifier input impedance mismatch. This formula is used to calculate correlation between two-channel circuits connected to preamplifiers using transmission lines of variable electrical length. The calculated correlation matches well with that measured on the MR scanner and suggests transmission line length can be utilized as an additional degree of freedom for coil decoupling, and can be adjusted to achieve zero noise correlation.