Session J04: Ultrasonics

Backscatter techniques for ultrasonic bone assessment at the hip

There is interest in developing ultrasonic backscatter techniques to detect changes in bone caused by osteoporosis. Backscatter measurements are performed by propagating ultrasonic pulses into bone and receiving signals returned from the porous microstructure. The goal of this study was to test the in vivo performance of two backscatter parameters developed for ultrasonic bone assessment: apparent integrated backscatter (AIB) and the normalized mean of the backscatter difference (nMBD). Measurements were performed at the hip (femoral neck) of 80 healthy volunteers. Results were analyzed by evaluating the correlation between measurements made at the left and right hip. AIB and nMBD both demonstrated significant correlations indicating sensitivity to naturally occurring variations in bone density. These results suggest that AIB and nMBD may be sensitive to changes in bone caused by osteoporosis.   

Two-dimensional mapping of the reflection coefficient and acoustic impedance of brain

Brain is inhomogeneous due to its composition of different tissue types (gray and white matter), anatomical structures (e.g. thalamus and cerebellum), and cavities in the brain (ventricles).  These inhomogeneities lead to spatial variations in the ultrasonic properties of the organ.  1-cm-thick slices of tissue from the coronal, sagittal and transverse cardinal planes were prepared from 12 brains.  Pulse-echo measurements were performed using broadband transducers with center frequencies of 3.5, 5.0, 7.5 and 10 MHz.  By mechanically scanning the transducers over the specimens, detailed, two-dimensional maps of acoustic impedance and reflection coefficient were produced, providing a clear visualization of the spatial variation of these ultrasonic properties of normal mammalian brain.

    

Ultrasonic testing

Ultrasonic testing is a conventional method to detect defects non-destructively. The prevailing technique is to use the ultrasonic wave in the pulsed or burst mode and locate and sometimes visualize a defect. The difference in the acoustic impedance of the anomaly reflects the incident wave for analysis. We use a single frequency and analyze the transmitted signal. In this fashion, we can evaluate the phase change and thereby characterize the elastic property of anomalies. In this presentation, we will discuss various factors that affect this method, including the wavefront of the ultrasonic wave incident to defects and the diffraction due to anomalies.   

Predictive Models of Plane-wave-based Point Spread Function Angulation

Ultrasound localization microscopy (ULM) methods ascribe precise spatial coordinates to individual circulating microbubbles whose trajectories are superimposed to trace out a super-resolution image of the vasculature. Conventional localization algorithms use data from coarsely-pixelated B-mode images. In prior work, a novel localization algorithm was developed based on raw complex RF data from point spread functions (PSFs) obtained using a multi-angle plane wave acquisition sequence and four parallel receive apodizations. We seek to characterize point spread functions over a broad FOV. Based on these data, we propose models to predict PSF angulation within well-defined support regions as a function of position, plane-wave transmit angle, and aperture size. With these models, we aim to extend the support for a localization algorithm valid over the full FOV.

    

Computational approach to investigating acoustic response of aerogels to ultrasound waves in aqueous and non-aqueous environments

Aerogels are a highly porous and lightweight materials with a unique combination of physical and chemical properties that can be customized for specific applications [1]. Tunability of key parameters such as pore diameter, Young’s modulus, bulk density, and thermal and electrical conductivity have been shown [2]. The range of aerogel’s applications have significantly grown in the last few years. The low density which characterizes aerogels is correlated to their high acoustic impedance, allowing the noninvasive tracking and monitoring of in vivo aerogel implants using routine diagnostic ultrasound techniques.. Most of the work conducted so far has been experimental in nature [ S Ghimire et al, 2021] and as such has been able to only explore a limited parameter space. Experimental work has been limited by the complexity of the experimental setup and detection mechanism needed to accurately map the wave-aerogel interaction and wave propagation behavior.

Here, the authors present a computational approach to studying the acoustic response of aerogels to ultrasound waves in two different environments, aqueous and non-aqueous. Using K-wave, MATLAB tool acoustic wave propagation via aerogel structure were simulated to obtain absorption, transmission and attenuation characteristics. Aerogels with densities in the range of 50 – 200 kg/m3 and porosities in the range of 0 - 70 % were investigated. The Attenuation and transmission loss behavior was studied for sound waves in the frequency range of 0.5 – 1 MHz. Extremely low-density aerogels (50 kg/m3) suspended in air were found to have very low transmission loss with opposite results for the same aerogel when submerged in water. Overall, a computational approach of determining acoustic properties of aerogels has been developed and investigated thoroughly.