Effect of the Diameter of the Central Opening on Nonlinear Acoustic Field Characteristics of High-Intensity Focused Ultrasound Transducers

A number of novel noninvasive surgical technologies utilizing high-intensity focused ultrasound (HIFU) are based on using nonlinear acoustic effects that lead to distortion of the wave when it propagates from an ultrasound transducer and formation of shock fronts at the focus. Typically, such transducers that generate a high-power ultrasound beam, have near axially symmetric shape with a central circular opening to accommodate a diagnostic probe for visualization purposes. To predict the focal field parameters of such transducer geometries, an equivalent source model of a spherical segment is convenient, as nonlinear effects in its field are well studied. The equivalent source parameters (diameter, focal length, and amplitude) are optimized to closely approximate the focal region of the original transducer along the axial coordinate. This paper investigates the effect of the central opening size on the nonlinear field characteristics and applicability of the equivalent source model for a typical therapeutic ultrasound transducer with a frequency of 1 MHz and F_# = 0.9. It is demonstrated that the size of the central opening significantly affects the degree of nonlinear waveform distortion in the focal region, and the equivalent source model can be applied only when the diameter of the central opening is less than 20% of the transducer diameter.