Spatial resolution in medical ultrasound images is a key component in image quality and an important factor for clinical diagnosis. In early systems, the lateral resolution was optimal in the focus but rapidly decreased outside the focal region. Improvements have been found in, e.g., dynamic-receive beamforming, in which the entire image is focused in receive, but this requires complex processing of element data and is not applicable for mechanical scanning of single-element images. This paper exploits the concept of two-stage beamforming based on virtual source-receivers, which reduces the front-end computational load while maintaining a similar data rate and frame rate compared to dynamic-receive beamforming. We introduce frequency-wavenumber domain data processing to obtain fast second-stage data processing while having similarly high lateral resolution as dynamic-receive beamforming and processing in time-space domain. The technique is very suitable in combination with emerging technologies such as application-specific integrated circuits (ASICs), hand-held devices, and wireless data transfer. The suggested method consists of three steps. In the first step, single-focused RF line data are shifted in time to relocate the focal point to a new origin t' = 0, z' = 0. This new origin is considered as an array of virtual source/receiver pairs, as has been suggested previously in literature. In the second step, the dataset is efficiently processed in the wavenumber-frequency domain to form an image that is in focus throughout its entire depth. In the third step, the data shift is undone to obtain a correct depth axis in the image. The method has been tested first with a single-element scanning system and second in a tissue-mimicking phantom using a linear array. In both setups, the method resulted in a -6-dB lateral point spread function (PSF) which was constant over the entire depth range, and similar to dynamic-receive beamforming and synthetic aperture sequential be@en

Asphalt materials generally exhibit temperature-dependent properties, which can influence the performance of fatigue crack inspection and monitoring systems for bridge deck structures. For a non-intrusive fatigue crack sizing methodology applied to steel decks using ultrasonic guided waves, the effect of asphalt has been investigated. A higher-order spectral finite element model has been implemented to capture the propagation characteristics of guided waves in the multi-layer waveguide. Experimental evaluation has been performed on an aluminum plate covered with bitumen. The results reveal that the extent of the effect of asphalt on propagation of guided waves depends on the type and layout of the asphalt. For a particular asphalt type of interest, i.e. open asphalt surfacing, however, this effect appeared to have limited influence on the performance of the crack sizing methodology, i.e. in the order of a few percents. In the analysis, temperature variation, and uncertainties in the material properties and asphalt bonding condition have been taken into account.

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