P1A-12 Using the Phase Modulation Imposed by Tissue Inhomogeneity to Determine the Full Acoustic Near Field

Abstract

Ultrasound is used in many different clinical contexts, and often in tissue with inclusions such as cavities, vessels or lesions. If the acoustic impedance of the medium within the inclusion is different to that of the surroundings, the propagating ultrasonic field will be altered, especially near the object's boundaries and in the distal shadow zone. The diffractive field magnitude modulations are complicated. The corresponding phase modulations are, however, quite regular, and this observation is the basis for the method presented in this paper. The tissue objects we model are small to mid-sized compared to the ultrasound wavelength, with scaled wave number ka ~ 1-60, and the sound speed variations are lsim 10%. The acoustic field at any point can be expressed, in integral form, using the Green's function as the kernel. The integrand also involves the unknown field inside the tissue inhomogeneity, which, in the widely used Born approximation, is replaced by the incident wave. Our results for biomedical parameters show that the field magnitude inside the object is greatly overestimated, with errors much greater than 100% for sound speed differences of more than a few percent. In this study we present a different treatment of the integrand that produces far more accurate results. We show that seeding the integrand with a better estimate of the phase modulation imposed by the tissue gives much more accurate and reliable results for the full field, both within and near the object. Only one evaluation of the scattering integral is required. We call this approach the Phase Corrected Scattering Integral (PCSI). Results are surprisingly accurate even when the tissue phase is estimated using a simple ray model. The range and accuracy of the PCSI approach is demonstrated in two dimensions using cylindrical geometry, and in three dimensions using spherical geometry. The method can be readily adapted to more complex scattering objects, such as non-circular cross sections, shells, nonuniform sound speed distributions and even multiple scattering objects.