CHARACTERIZATION OF ULTRASONIC-IMAGING SYSTEMS USING TRANSFER-FUNCTIONS

The transfer functions for focussed and defocussed, coherent and confo cal optical imaging systems have been applied to the equivalent ultras onic imaging systems. The transfer functions with varying degrees of d efocus were calculated to show the defocus effects for ultrasonic imag ing systems. Assuming that the acoustic waves are reflected perfectly on the surface of the step edge, the theoretical line-scans for small amplitude signals across a step edge, with various degrees of defocus, were generated. The first derivative of the line-scan for a step edge is shown theoretically to yield the same impulse response as that cal culated using the inverse Fourier transform applied to the original tr ansfer function. These results show how the real and imaginary parts o f the transfer functions contribute to ultrasonic image formation. A m ethod for the experimental determination of the impulse response, and the transfer functions for the characterization of an ultrasonic imagi ng system, such as an acoustic microscope, is provided.