An alternative approach to acquire transmission travel time data is proposed, exploiting the geometry of devices commonly used in ultrasound computed tomography for medical imaging or non-destructive testing with ultrasonic waves. The intent is to (i) shorten acquisition time for devices with a large number of emitters, (ii) to eliminate the calibration step, and (iii) to suppress instrument noise. Inspired by seismic ambient field interferometry, the method rests on the active excitation of diffuse ultrasonic wavefields and the extraction of deterministic travel time information by inter-station correlation. To reduce stochastic errors and accelerate convergence, ensemble interferograms are obtained by phase-weighted stacking of observed and computed correlograms, generated with identical realizations of random sources. Mimicking an imaging setup, the accuracy of the travel time measurements as a function of the number of emitters and random realizations can be assessed both analytically and with spectral-element simulations for phantoms mimicking the model parameter distribution. The results warrant tomographic reconstructions with straight- or bent-ray approaches, where the effect of inherent stochastic fluctuations can be made significantly smaller than the effect of subjective choices on regularisation. This work constitutes a first conceptual study and a necessary prelude to future implementations.
Ultrasound computed tomography (USCT) is a promising imaging modality for breast cancer screening. Three challenges commonly arising in time-of-flight USCT are (1) to choose a physical forward model that describes acoustic wave propagation in an inhomogeneous medium appropriately (2) to effectively mitigate the ill-posedness for an adequate reconstruction of the model and (3) to efficiently deal with large data sets. In this contribution, we investigate methods that address these three challenges by developing an optimization strategy based on a stochastic descent method that adaptively subsamples the data and analyze its performance in combination with different sparsity-enforcing regularization techniques.
We present a novel approach to obtain time-of-flight measurements between transducer pairs in an Ultrasound computed tomography (USCT) scanner by applying the interferometry principle, which has been used successfully in seismic imaging to recover the subsurface velocity structure from ambient noise recordings. To apply this approach to a USCT aperture, random wavefields are generated by activating the emitting transducers in a random sequence. By correlating the random signals recorded by the receiving transducers, we obtain an approximation of the Green's functions between all receiver pairs, where one is acting as a virtual source. This eliminates specific source imprints, and thus avoids the need for reference measurements and calibration. The retrieved Green's functions between any two measurement locations can then be used as new data to invert the sound speed map. On the basis of the cross-correlation travel times a ray-based time-of-flight tomography is developed and solved with an iterative least-squares method. As a proof of concept, the algorithm is tested on numerical breast phantoms in a synthetic 2D study.
<div> <div> <div> <p>We propose a translation of widely-used seismic ambient noise tomography to active noise tomography in medical ultrasound. This is intended to eliminate time-consuming transducer calibration and to improve illumination of the target.</p> <p>Ultrasound computed tomography (USCT) is an emerging visualization modality in medical imaging and is especially apt to screen soft human tissue such as the breast. Currently, USCT applications are developed for breast cancer detection using a collection of ultrasound scans that measure the pressure wavefield emitted by individual transducers. To obtain good coverage, a large number of emitter-receiver pairs is required, as well as careful calibration of transducers using reference measurements in water at constant temperature. Standard acquisition and calibration are time consuming processes, placing major constraints on the integration of USCT for breast cancer detection in medical practice.</p> <p>We present a novel approach to obtain traveltime measurements between transducer pairs in USCT by applying random field interferometry, as developed in seismic imaging. Since ambient noise sources are absent in the medical application, we generate random wavefields actively by firing sources in a random sequence. Cross-correlation of the recordings provides an approximation of Green&#8217;s functions between receivers, from which traveltime measurements can be extracted.</p> <p>The proposed method has two major benefits: (1) Since cross-correlation eliminates time shifts caused by the a priori unknown source wavelet, the tedious calibration step can be avoided. (2) Coverage improves because the implicit use of reflections off the device boundary overcomes limited illumination caused by the small opening angle of typical ultrasound transducers.</p> <p>The traveltimes extracted from the Green&#8217;s function approximations can be used as new data in a ray-based traveltime tomography. As a proof of concept, we test the algorithm on numerical breast phantoms, and we show that the latter can be reconstructed successfully from the cross-correlation traveltimes. In summary, random field interferometry opens new perspectives to shorten and facilitate the acquisition and tomographic inversion of USCT datasets.</p> </div> </div> </div>
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