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Theoretical and experimental results are presented that establish the value of parallel excitation with a transmit coil array in accelerating excitation and managing RF power deposition. While a 2D or 3D excitation pulse can be used to induce a multidimensional transverse magnetization pattern for a variety of applications (e.g., a 2D localized pattern for accelerating spatial encoding during signal acquisition), it often involves the use of prolonged RF and gradient pulses. Given a parallel system that is composed of multiple transmit coils with corresponding RF pulse synthesizers and amplifiers, the results suggest that by exploiting the localization characteristics of the coils, an orchestrated play of shorter RF pulses can achieve desired excitation profiles faster without adding strains to gradients. A closed-form design for accelerated multidimensional excitations is described for the small-tip-angle regime, and its suppression of interfering aliasing lobes from coarse excitation k-space sampling is interpreted based on an analogy to sensitivity encoding (SENSE). With or without acceleration, the results also suggest that by taking advantage of the extra degrees of freedom inherent in a parallel system, parallel excitation provides better management of RF power deposition while facilitating the faithful production of desired excitation profiles. Sample accelerated and specific absorption rate (SAR)-reduced excitation pulses were designed in this study, and evaluated in experiments. Magn Reson Med 51:775-784, 2004.

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Understanding the complexities associated with contact line dynamics on chemically heterogeneous and superhydrophobic surfaces is important for a wide variety of engineering problems. Despite significant efforts to capture the behavior of a droplet on these surfaces over the past few decades, modeling of the complex dynamics at the three-phase contact line is needed. In this work, we demonstrate that contact line distortion on heterogeneous and superhydrophobic surfaces is the key aspect that needs to be accounted for in the dynamic droplet models. Contact line distortions were visualized and modeled using a thermodynamic approach to develop a unified model for contact angle hysteresis on chemically heterogeneous and superhydrophobic surfaces. On a surface comprised of discrete wetting defects on an interconnected less wetting area, the advancing contact angle was determined to be independent of the defects, while the relative fraction of the distorted contact line with respect to the baseline surface was shown to govern the receding contact angle. This behavior reversed when the relative wettability of the discrete defects and interconnected area was inverted. The developed model showed good agreement with the experimental advancing and receding contact angles, both at low and high solid fractions. The thermodynamic model was further extended to demonstrate its capability to capture droplet shape evolution during liquid addition and removal in our experiments and those in literature. This study offers new insight extending the fundamental understanding of solid-liquid interactions required for design of advanced functional coatings for microfluidics, biological, manufacturing, and heat transfer applications.

We previously proposed a physiologically realistic model for stereo vision based on the quantitative binocular receptive field profiles mapped by Freeman and coworkers. Here we present several new results about the model that shed light on the physiological processes involved in disparity computation. First, we show that our model can be extended to a much more general class of receptive field profiles than the commonly used Gabor functions. Second, we demonstrate that there is, however, an advantage of using the Gabor filters: similar to our perception, the stereo algorithm with the Gabor filters has a small bias towards zero disparity. Third, we prove that the complex cells as described by Freeman et al. compute disparity by effectively summing up two related cross products between the band-pass filtered left and right retinal image patches. This operation is related to cross-correlation but it overcomes some major problems with the standard correlator. Fourth, we demonstrate that as few as two complex cells at each spatial location are sufficient for a reasonable estimation of binocular disparity. Fifth, we find that our model can be significantly improved by considering the fact that complex cell receptive field are, on average, larger than those of simple cells. This fact is incorporated into the model by averaging over several quadrature pairs of simple cells with nearby and overlapping receptive fields to construct a model complex cell. The disparity tuning curve of the resulting complex cell is much more reliable than the constructed from a single quadrature pair of simple cells used previously, and the computed disparity maps for random dot stereograms with the new algorithm are very similar to human perception, with sharp transitions at disparity boundaries. Finally, we show that under most circumstances our algorithm works equally well with either of the two well-known receptive field models in the literature.

The promise of increased signal-to-noise ratio and spatial/ spectral resolution continues to drive MR technology toward higher magnetic field strengths. SAR management and B 1 inhomogeneity correction become critical issues at the high frequencies associated with high field MR. In recent years, multiple coil excitation techniques have been recognized as potentially powerful tools for controlling specific absorption rate (SAR) while simultaneously compensating for B 1 inhomogeneities. This work explores electrodynamic constraints on transmit homogeneity and SAR, for both fully parallel transmission and its time-independent special case known as radiofrequency shimming. Ultimate intrinsic SAR-the lowest possible SAR consistent with electrodynamics for a particular excitation profile but independent of transmit coil design-is studied for different field strengths, object sizes, and pulse acceleration factors. The approach to the ultimate intrinsic limit with increasing numbers of finite transmit coils is also studied, and the tradeoff between homogeneity and SAR is explored for various excitation strategies. The advantages of using high magnetic field strengths for MR imaging and spectroscopy are well known: they include the promise of improved signal-to-noise ratio (SNR) and spatial or spectral resolution, as well as the potential for improvements in certain useful forms of contrast. The challenges of high field strength are also well known, including a variety of difficulties associated with reduced homogeneity in both static and radiofrequency (RF) fields. For RF fields in particular, the operating wavelength decreases as field strength and Larmor frequency increase, becoming ever smaller as compared to the dimensions of the human body, and resulting in ever larger interactions between electromagnetic fields and dielectric tissues. These interactions lead to local focusing of the RF magnetic field B 1 , both in excitation and in reception. The focusing of B 1 field results in interference patterns (1), which compromise the underlying SNR increases associated with high field strength and which would in many cases markedly impede clinical diagnosis. B 1 focusing also results in subject-specific spatial variations of flip angle, which can diminish the reliability of image contrast. Meanwhile, the focusing of RF electric fields in dielectric tissues at high frequency results in increasingly inhomogeneous and subject-dependent specific absorption rate (SAR). Furthermore, the magnitude of electric fields produced for a given strength of the transmit magnetic field (i.e., per unit flip angle of RF excitation) is generally expected to grow with frequency. These electric fields induce eddy currents and dissipate heat in the body, causing an overall SAR increase which has (in admittedly casual approximations based on low-to moderate-frequency behavior) been taken to increase approximately with the square of the frequency.Compensation of B 1 inhomogeneities and management of SAR are indeed among the most difficult challeng...