Modeling of nonlinear ultrasound propagation in tissue from array transducers

A computationally efficient model capable of simulating finite-amplitude ultrasound beam propagation in water and in tissue from phased linear arrays and other transducers of arbitrary quasiplanar geometry is described. It is based on a second-order operator splitting approach [Tavakkoli et al., J. Acoust. Soc. Am. 104, 2061-2072 (1998)], with a fractional step-marching scheme, whereby the effects of diffraction, attenuation, and nonlinearity can be computed independently over incremental steps. This approach is an extension to that of Christopher and Parker [J. Acoust. Soc. Am. 90, 507-521; 90, 488-499 (1991)], wherein linear and nonlinear effects are propagated separately over incremental steps, and the computation of the diffractive substeps are based on an angular spectrum technique with a modified sampling scheme for accurate and efficient implementation of diffractive propagation from nonradially symmetric sources. Results of the model are compared with published data. Predicted field profiles for nonlinear propagation in tissue from realistic array transducers using the pulse inversion method are presented.     

Label-free in vivo fiber-based optical-resolution photoacoustic microscopy

In this Letter, the capability of label-free fiber-based optical-resolution photoacoustic microscopy is demonstrated. This real-time imaging system takes advantage of image-guide fibers and a unique fiber laser. The 800?μm image-guide consists of 30,000 individual single-mode fibers in a bundle and the diode-pumped, pulsed Ytterbium fiber laser is utilized to perform up to 600?kHz repetition rate. Phantom studies indicate 7?μm resolution. The proposed setup keeps many of the powerful properties of previous tabletop OR-PAM systems, but also offers a submillimeter probe footprint and high flexibility due to the nature of the image-guide. This system could have significant clinical impact for endoscopic applications where the thin fiber can be inserted into the body.     

Imaging with unfocused regions of focused ultrasound beams

This article gives an analytical, computational, and experimental treatment of the spatial resolution encoded in unfocused regions of focused ultrasound beams. This topic is important in diagnostic ultrasound since ultrasound array systems are limited to a single transmit focal point per acoustic transmission, hence there is a loss of spatial resolution away from the transmit focus, even with the use of dynamic receive focusing. It is demonstrated that the spatial bandwidth of a Gaussian-apodized beam is approximately constant with depth, which means that there is just as much encoded spatial resolution away from the transmit focus as there is in the focal region. The practical application of this principle is discussed, an algorithm for retrospectively focusing signals from unfocused regions of fixed-focus beams is presented, and a quantitative comparison between the authors' methods and dynamic-receive beamforming is provided.