Theoretical study of harmonic generation sound beams in case of uniform, exponential and cosinusoidal apertures

摘要

The non-linear propagation of ultrasound in medical imaging has recently been exploited to improve image resolution and remove near field artifacts generated by overlying tissue structures. The images are formed using the second harmonic energy generated by nonlinear propagation. Second harmonic beams have narrower beam width and lower side lobes than the fundamental. The second harmonic draws energy from the fundamental continuously along the propagation path. These characteristics contribute to improve the quality of medical ultrasound images. In this paper, our objective is to show that the choice source aperture has significant consequences on the quality of the field transmitted in term of directivity. We also studied the field generated by three apertures sources, in a nonlinear medium. Theoretical approach, based on the Khokhlov-Zabolotskaya-Kuznetsov (KZK) parabolic approximation is used in order to consider the diffraction effects related to the use of a focusing real source. The fundamental ultrasonic fields and the second harmonic are compared at three distances from the source. Width and side lobes were used when recording three distributions of the beam form.