A virtual source method for evaluating ultrasound propagation through tissues mapped from medical images

Abstract

A numerical model which uses digitized layer interfaces was developed for calculating multilayer ultrasound propagation, reflection, and refraction. This model was used to project the acoustic field from a spherical phased array (diameter 10 cm, F number 1, frequency 1 MHz) into multilayer tissues, and was verified by performing phantom measurements. A section on each layer interface in the propagation path was used as a virtual source. Digitized profiles of the tissue layer interfaces of arbitrary shapes can be obtained from MRI, CT, or ultrasound. The phase delay for each element of the phased array was first calculated, and then applied to each element for phase correction. With no phase correction, the focus of the array was shifted and defocused. By using the phased array, the shifted focal point was corrected and the side lobes were reduced. The effects of the array element size (number) were investigated for a deep sonication where the layer thickness information was obtained from a series of MRI scans of a volunteer. For this particular array, it is shown that the element size of 0.39 cm2 is small enough to produce a near-optimum focus. [Work supported by NCI Grant No. CA 46627.]