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Sub-megahertz transmission is not usually adopted in pre-clinical small animal experiments for focused ultrasound (FUS) brain therapy due to the large focal size. However, low frequency FUS is vital for preclinical evaluations due to the frequency-dependence of cavitation behavior. To maximize clinical relevance, a dual-aperture FUS system was designed for low-frequency (274.3 kHz) cavitation-mediated FUS therapy. Combining two spherically curved transducers provides significantly improved focusing in the axial direction while yielding an interference pattern with strong side lobes, leading to inhomogeneously distributed cavitation activities. By operating the two transducers at slightly offset frequencies to modulate this interference pattern over the period of sonication, the acoustic energy was redistributed and resulted in a spatially homogenous treatment profile. Simulation and pressure field measurements in water were performed to assess the beam profiles. In addition, the system performance was demonstrated in vivo in rats via drug delivery through microbubble-mediated blood-brain barrier disruption. This design resulted in a homogenous treatment profile that was fully contained within the rat brain at a clinically relevant acoustic frequency. [ABSTRACT FROM AUTHOR]

A shaped-based ultrasound tomography method is proposed to reconstruct ellipsoidal objects using a linearized scattering model. The method is motivated by the desire to detect the presence of lesions created by high intensity focused ultrasound (HIFU) in applications of cancer therapy. The computational size and limited view nature of the relevant three-dimensional inverse problem renders impractical the use of traditional pixel-based reconstruction methods. However, by employing a shape-based parametrization it is only necessary to estimate a small number of unknowns describing the geometry of the lesion, in this paper assumed to be ellipsoidal. The details of the shape-based nonlinear inversion method are provided. Results obtained from a commercial ultrasound scanner and a tissue phantom containing a HIFU-like lesion demonstrate the feasibility of the approach where a 20 mm×5 mm×6 mm ellipsoidal inclusion was detected with an accuracy of around 5%. [ABSTRACT FROM AUTHOR]

Conventional ultrasound transducers used for medical diagnosis generally consist of linearly aligned rectangular apertures with elements that are focused in one plane. While traditional beamforming is easily accomplished with such transducers, the development of quantitative, physics-based imaging methods, such as tomography, requires an accurate, and computationally efficient, model of the field radiated by the transducer. The field can be expressed in terms of the Helmholtz-Kirchhoff integral; however, its direct numerical evaluation is a computationally intensive task. Here, a fast semi- analytical method based on Stepanishen’s spatial impulse response formulation [J. Acoust. Soc. Am. 49, 1627–1638 (1971)] is developed to compute the acoustic field of a rectangular element of cylindrically concave transducers in a homogeneous medium. The pressure field, for lossless and attenuating media, is expressed as a superposition of Bessel functions, which can be evaluated rapidly. In particular, the coefficients of the Bessel series are frequency independent and need only be evaluated once for a given transducer. A speed up of two orders of magnitude is obtained compared to an optimized direct numerical integration. The numerical results are compared with Field II and the Fresnel approximation. [ABSTRACT FROM AUTHOR]

A common technique to determine the electromechanical response of a spherically focusing transducer is to use a reference pulse echo from a flat plate in the focal plane of the transducer. We show that when the pressure focusing gain of the transducer is much greater than unity, the focal plane reflection is a valid approximation of the desired electromechanical response. An alternative calibration target is a point scatterer and we show theoretically and experimentally that this waveform is the double time differential of the flat-plate response. The use of calibration to describe general scatterers through a Born approximation (Jensen, J. A. (1991), J. Acoust. Soc. Am. 89, 182-190) is discussed. [ABSTRACT FROM AUTHOR]

We consider geometric inversion methods designed to directly determine information concerning the size, shape, location, and perhaps number of anomalies in a region of interest [E. L. Miller, M. E. Kilmer, and C. M. Rappaport, IEEE Trans. Geosci. Remote Sens. 38, 1682 (2000); O. Dorn, E. L. Miller, and C. Rappaport, Inverse Probl. 16, 1119 (2000)] for diffusive inverse problems arising in medical imaging and environmental remediation. First, a parametric approach to the problem is derived and validated using a diffuse optical tomography sensing example. A second technique to identify boundaries of an unknown number of objects is based on the idea of curve evolution. This approach mathematically "shrink wraps" a deformable surface in 3D or curve in 2D to the boundary of one or more objects. We demonstrate the utility of this method using an electrical resistance tomography sensing example in three dimensions. [ABSTRACT FROM AUTHOR]

A shaped-based ultrasound tomography method is proposed to reconstruct ellipsoidal objects using a linearized scattering model. The method is motivated by the desire to detect the presence of lesions created by high intensity focused ultrasound (HIFU) in applications of cancer therapy. The computational size and limited view nature of the relevant three-dimensional inverse problem renders impractical the use of traditional pixel-based reconstruction methods. However, by employing a shape-based parametrization it is only necessary to estimate a small number of unknowns describing the geometry of the lesion, in this paper assumed to be ellipsoidal. The details of the shape-based nonlinear inversion method are provided. Results obtained from a commercial ultrasound scanner and a tissue phantom containing a HIFU-like lesion demonstrate the feasibility of the approach where a 20 mm x 5 mm x 6 mm ellipsoidal inclusion was detected with an accuracy of around 5%.