21739268
Transverse acoustic trapping using a gaussian focused ultrasound.
The optical tweezer has become a popular device to manipulate particles in nanometer scales and to study the underlying principles of many cellular or molecular interactions. Theoretical analysis was previously carried out at the authors' laboratory, to show that similar acoustic trapping of microparticles may be possible with a single beam ultrasound. This article experimentally presents the transverse trapping of 125 microm lipid droplets under an acoustically transparent mylar film, which is an intermediate step toward achieving acoustic tweezers in three-dimension. Despite the lack of axial trapping capability in the current experimental arrangement, it was found that a 30 MHz focused beam could be used to laterally direct the droplets toward the focus. The spatial range within which acoustic traps may guide droplet motion was in the range of hundreds of micrometers, much greater than that of optical traps. This suggests that this acoustic device may offer an alternative for manipulating microparticles in a wider spatial range.
39975301
Effect of ultrasonic attenuation on the feasibility of acoustic tweezers.
A modified mathematical formulation for the calculation of axial radiation force was developed to incorporate the effect of ultrasonic attenuation. Axial forces, Fresnel coefficients, average internal attenuation factors and effective internal reflection coefficients were calculated. Thermal and mechanical indices were also computed to address the safety issues in the implementation of acoustic tweezers and were found to be negligible. The results show that the overall distribution of axial forces is barely affected by attenuation. Furthermore, it is found that attenuation actually works against the scattering force and may therefore reinforce the axial trapping force. For a particle size of 180 microm, the maximum trapping force increases from 29.8 x 10(-11) N to 30.3 x 10(-11) N by 1.7% when attenuation is included. In light of these results, it appears that acoustic tweezers may still be feasible beyond the focal point even under the influence of attenuation.
35716654
Elastomeric microwire-based optical gas flowmeter with stretching-enabled tunability in measurement range.
We report the utilization of transparent poly(dimethylsiloxane) (PDMS) microwires as the transducer for optical gas flowmetry. The elasticity of the PDMS microwire was exploited not only to miniaturize and simplify the flowmeter but also to widen and tune the measurement range through mechanical stretching. Using a 9 mm long microwire, we achieved 2.8~9.8 dB/SLM sensitivity. A 500 μm stretching of the microwire also shifted the measurement range from 1 to 4 SLM. The experimental results agreed well with predictions based on the fluid dynamic/optical model.
31281743
Respiration-induced B0 field fluctuation compensation in balanced SSFP: real-time approach for transition-band SSFP fMRI.
In functional MRI (fMRI) the resonance frequency shift induced from respiration is a major source of physiological noise. In transition-band SSFP fMRI the respiration-induced resonance offset not only increases noise interference, it also shifts the activation band. This leads to a reduction in the contrast-to-noise ratio (CNR) and the potential for varying contrast levels during the experiment. A novel real-time method that compensates for the respiration-induced resonance offset frequency is presented. This method utilizes free induction decay (FID) phase information to measure the resonance offset. For compensation, one can update the resonant frequency in real time by changing the transmit RF pulse and receiver phases to track the measured offset. The results show decreased signal power in the respiration frequency band and increased numbers of activated voxels with higher Z-scores compared to uncompensated experiments.
26415491
Sensitivity of MRI resonance frequency to the orientation of brain tissue microstructure
Recent advances in high-field (≥7 T) MRI have made it possible to study the fine structure of the human brain at the level of fiber bundles and cortical layers. In particular, techniques aimed at detecting MRI resonance frequency shifts originating from local variation in magnetic susceptibility and other sources have greatly improved the visualization of these structures. A recent theoretical study [He X, Yablonskiy DA (2009) Proc Natl Acad Sci USA 106:13558–13563] suggests that MRI resonance frequency may report not only on tissue composition, but also on microscopic compartmentalization of susceptibility inclusions and their orientation relative to the magnetic field. The proposed sensitivity to tissue structure may greatly expand the information available with conventional MRI techniques. To investigate this possibility, we studied postmortem tissue samples from human corpus callosum with an experimental design that allowed separation of microstructural effects from confounding macrostructural effects. The results show that MRI resonance frequency does depend on microstructural orientation. Furthermore, the spatial distribution of the resonance frequency shift suggests an origin related to anisotropic susceptibility effects rather than microscopic compartmentalization. This anisotropy, which has been shown to depend on molecular ordering, may provide valuable information about tissue molecular structure.
6998054
Microcanonical origin of the maximum entropy principle for open systems.
There are two distinct approaches for deriving the canonical ensemble. The canonical ensemble either follows as a special limit of the microcanonical ensemble or alternatively follows from the maximum entropy principle. We show the equivalence of these two approaches by applying the maximum entropy formulation to a closed universe consisting of an open system plus bath. We show that the target function for deriving the canonical distribution emerges as a natural consequence of partial maximization of the entropy over the bath degrees of freedom alone. By extending this mathematical formalism to dynamical paths rather than equilibrium ensembles, the result provides an alternative justification for the principle of path entropy maximization as well.
19449141
Quantitative measures of facial expression for patients with head and neck cancer
In addition to changes in facial morphology, head and neck cancer treatment can impact the facial expression. Quantification of changes in facial expression, particularly of smiling, would enable a deeper understanding of the relationship between physical changes and psychosocial adjustment in patients being treated for facial cancer. This study proposes 48 quantitative facial expression measures, which consist of the length normalized distances and slopes between 27 manually annotated fiducial points on standard 2D clinical photographs of patients with head and neck cancer. Using the proposed measures, the maximum intensity of smiling for patients with head and neck cancer prior to their treatments was compared to that of a sample from a healthy population. A total of 7 facial expression measures captured statistically significant differences between patients with head and neck cancer and healthy individuals. These measures suggest that patients with head and neck cancer are less expressive than people without head and neck cancer. This study is the first attempt to quantify the facial expression of patients with head and neck cancer and to build a foundation for studying how surgical interventions may affect their facial expressions. Moreover, this study lays the groundwork for future investigation of the relationship between facial expression and psychosocial adjustment in cancer patients.