Fast functional and molecular photoacoustic microscopy requires pulsed laser excitations at multiple wavelengths with enough pulse energy and short wavelength-switching time. Recent development of stimulated Raman scattering in optical fiber offers a low-cost laser source for multiwavelength photoacoustic imaging. In this approach, long fibers temporally separate different wavelengths via optical delay. The time delay between adjacent wavelengths may eventually limits the highest A-line rate. In addition, a long-time delay in fiber may limit the highest pulse energy, leading to poor image quality. In order to achieve high pulse energy and ultrafast dual-wavelength excitation, we present opticalresolution photoacoustic microscopy with ultrafast dual-wavelength excitation and a signal separation method. The signal separation method is validated in numerical simulation and phantom experiments. We show that when two photoacoustic signals are partially overlapped with a 50-ns delay, they can be recovered with 98% accuracy. We apply this ultrafast dual-wavelength excitation technique to in vivo OR-PAM. Results demonstrate that A-lines at two wavelengths can be successfully separated, and sO 2 values can be reliably computed from the separated data. The ultrafast dual-wavelength excitation enables fast functional photoacoustic microscopy with negligible misalignment among different wavelengths and high pulse energy, which is important for in vivo imaging of microvascular dynamics.
K E Y W O R D Sfast dual-wavelength excitation, functional photoacoustic imaging, multiwavelength, signal separation
A design for a transmission x-ray microscope with 20 nm transverse spatial resolution is presented. The microscope, which is based on the electron-optical imaging of the photoemitted electrons from an x-ray shadowgraph, consists of a transmission x-ray photocathode coupled to a photoelectron emission microscope (PEEM—also called a PEM for photoelectron microscope). Unlike the conventional PEEM, which produces a surface map of photoelectron yield, this microscope can provide information on the subsurface properties of thin samples. The analysis of the microscope’s electron-optical performance is based on the evaluation of Gaussian focusing properties and third-order aberration coefficients computed using several complementary methods. The electron optical properties of the microscope are examined with an emphasis on issues affecting overall performance and achieving the best possible resolution. Preliminary experimental results using a cesium iodide photocathode are shown.
The walking characteristics and collision avoidance strategies in bidirectional pedestrian flow with both individuals and groups under three different pedestrian densities were investigated experimentally in the present study. The results showed that pedestrians formed lanes in bidirectional flow, and the lane number basically increased with pedestrian density. In addition, the lane number was positively related to the number of groups in the first row of the two opposite crowds. Compared with the no-group experiments, the walking speed of pedestrians showed less difference in the experiments with groups. The fundamental diagram of the bidirectional pedestrian flow with groups under different pedestrian density conditions was calculated. Three types of collision avoidance strategies, detouring around conflicting pedestrians, passing through the conflicting group in the middle, and group splitting, were summarised based on the experimental results. The statistical results of the three collision avoidance strategies adopted by different size groups under the low, medium, and high pedestrian density conditions were obtained. It was found that the large group was more likely to split into subgroups when they encountered conflicting pedestrians, while the small group was more likely to detour around the conflicting pedestrians. For the same group size, the larger the pedestrian density, the higher the proportion of groups that would choose to split into subgroups during the collision avoidance process.
We present dual-band photoacoustic and ultrasound microscopy that can maintain high resolution and improve the imaging penetration. The harmonic US imaging on the mouse eye reveals finer lens and iris boundary structures, which offers a high-resolution anatomical reference for co-registered PA imaging.
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