Ultrasonic Nondiffracting Transducer for Medical Imaging

Historically, transducers having Gaussian shading and Fresnel shaped voltage drives on an array of annular elements have heen used for medical imaging of the highest quality. These transducers have good focus within the depth of field and smooth near field patterns. Outside of the depth of held the focus degrades with diffraction effects, Nondiffracting solutions to the wave equation governing propagation in tissues (the scalar Helmholtz equation) have recently been discovered and extensively tested on electromagnetic waves. Very little work has heen done with nondiffracting beams in acoustics and none in medical imaging. A study of these methods for medical imaging is reported. Computer simulatitms and experimental results for a ten-ring annular Bessel shaded transducer are described. Both CW and pulse wave results are shown and compared to conventional Gaussian beams. The nondiffracting beam has about 1.27-mm radius main lobe with a 20-cm depth of field compared to the Gaussian transducer of the same size with a 1.2 7-mm radius main lobe at a focus of 12 cm and 2 ∙ 4-cm depth of field. The side lobes of the nondiffracting beam are the same as the Jv Bessel function. The new beam forming method may have promise in novel imaging and tissue characterization modes whereby the effects of diffraction are eliminated.