Conference Paper

Optimal focusing through aberrating media: a comparison between time reversal mirror and time delay correction techniques

Lab. Ondes et Acoust., Paris Univ.
DOI: 10.1109/ULTSYM.1991.234304 Conference: Ultrasonics Symposium, 1991. Proceedings., IEEE 1991
Source: IEEE Xplore

ABSTRACT

The problem of focusing an ultrasonic beam through an
inhomogeneous medium was investigated experimentally for conventional
time delay focusing, adaptive time delay focusing, and time reversal
focusing. It is shown that neither time delay based technique gives a
properly focused beam for all aberrator locations. Only time reversal
focusing allows a robust focusing regardless of the aberrator location.
The matched filter theory of signal processing allows an interpretation
of this result: time reversal focusing provides the optimal input to the
linear system consisting of the acousto-electric and diffraction impulse
response

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    • "Propagation path effects in aberration estimation and correction in the absence of a point target using statistical methods have been investigated by Waag and astheimer [64], astheimer et al. [65], and Tillet et al. [66]. The time-reversal mirror technique [67] has demonstrated success at focusing through aberrating layers [68]– [72] by addressing the aberration problem as a complex filtering operation, allowing variation of both amplitude and phase with frequency and more fully addressing the various degradation phenomena described previously. The basic principle is that the matched filter producing optimum snr is the time reversal of the received signal. "
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    ABSTRACT: Having previously presented the ultrasound brain helmet, a system for simultaneous 3-D ultrasound imaging via both temporal bone acoustic windows, the scanning geometry of this system is utilized to allow each matrix array to serve as a correction source for the opposing array. Aberration is estimated using cross-correlation of RF channel signals, followed by least mean squares solution of the resulting overdetermined system. Delay maps are updated and real-time 3-D scanning resumes. A first attempt is made at using multiple arrival time maps to correct multiple unique aberrators within a single transcranial imaging volume, i.e., several isoplanatic patches. This adaptive imaging technique, which uses steered unfocused waves transmitted by the opposing, or beacon, array, updates the transmit and receive delays of 5 isoplanatic patches within a 64° x 64° volume. In phantom experiments, color flow voxels above a common threshold have also increased by an average of 92%, whereas color flow variance decreased by an average of 10%. This approach has been applied to both temporal acoustic windows of two human subjects, yielding increases in echo brightness in 5 isoplanatic patches with a mean value of 24.3 ± 9.1%, suggesting that such a technique may be beneficial in the future for performing noninvasive 3-D color flow imaging of cerebrovascular disease, including stroke.
    Full-text · Article · Mar 2013 · IEEE transactions on ultrasonics, ferroelectrics, and frequency control
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    ABSTRACT: Abstruct- A theoretical model for time-reversal cavities to optimize focusing in homogeneous and inhomogeneous media is described. The concept of the cavity can be understood as the most realistic approximation to an exact three-dimensional (3-D) time-reversal of ultrasonic fields; it is also a generalization of the time-reversal mirrors realized experimentally in our laboratory. The proposed method is based on an approach in the transient regime, that is more general than the monochromatic formalism used in optics to analyze the phase conjugation mirrors efficiency. This method uses impulse diffraction theory to obtain the impulse response of the cavity in any inhomogeneous medium. It is also proposed an original interpretation of the limitations due to diffraction observed in wave field propagation in terms of the different waves generated inside the cavity. The time-reversal focusing process using a closed cavity in a weakly inhomogeneous medium is finally compared with more classical techniques to compensate wavefront distortions, thus illustrating the focusing improvement due to the time-reversal method.
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    ABSTRACT: For pt.II see ibid., vol.39, no.5, p.567-78 (1992). A theoretical model for time-reversal cavities to optimize focusing in homogeneous and inhomogeneous media is described. The concept of the cavity can be understood as the most realistic approximation to an exact three-dimensional (3-D) time-reversal of ultrasonic fields; it is also a generalization of the time-reversal mirrors realized experimentally in the laboratory. The proposed method is based on an approach in the transient regime that is more general than the monochromatic formalism used in optics to analyze the phase conjugation mirrors efficiency. This method uses impulse diffraction theory to obtain the impulse response of the cavity in any inhomogeneous medium. An original interpretation of the limitations due to diffraction observed in wave field propagation in terms of the different waves generated inside the cavity is also proposed. The time-reversal focusing process using a closed cavity in a weakly inhomogeneous medium is compared with more classical techniques to compensate wavefront distortions, thus illustrating the focusing improvement due to the time-reversal method.< >
    Preview · Article · Oct 1992 · IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control
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