Visualization of Thin Cylindrical Scatterers in Pulse Acoustic Microscopy

One of the areas of application of long-focus high-frequency ultrasonic beams is visualization of the volume microstructure of materials. Visualization of the microstructure in acoustic microscopy in the reflection mode is provided by recording signals during reflection or scattering of the ultrasonic beam on the elements of the internal structure. Some of the elements can be acoustically rigid thin cylindrical scatterers. The radius of thin cylindrical elements is significantly smaller than the size of the focal spot of the probing beam, therefore, all thin elements of small diameter are displayed in acoustic images with the same size equal to the diameter of the focal spot. To estimate the sizes of thin cylindrical elements visible in the images, a theoretical approach is proposed that describes the formation of the amplitudes of output signals during the interaction of a focused ultrasonic beam with thin cylindrical elements of the structure. The analytical description of the interaction takes into account the radius of the element and the sensitivity of the receiving acoustic system. Taking into account the parameters of the emitter/receiver and the geometry of the cylinder, the inverse problem of estimating the scatterer size depending on the received signal is solved. The theoretical approach is experimentally confirmed by visualization of thin polymer fibers using a scanning pulse acoustic microscope and acoustic lenses at frequencies of 50 and 200 MHz, forming focused beams of different geometries. Based on the results of comparison of experimental data and theoretical calculations, a conclusion is made about the applicability of the described analytical approach within the framework of assumptions and approximations for long-focus beams with a small-angle aperture when estimating the minimum dimensions of cylindrical scatterers visible in acoustic microscopy.