Two-dimensional synthetic focusing using large apertures—Deconvolution of the spatial impulse response

Abstract

The synthetic aperture focusing technique (SAFT) has been used successfully in both medical and nondestructive evaluation (NDE) applications of ultrasound. Transducer size is an important issue when implementing SAFT algorithms. The classical SAFT method is based on the assumption that a point source is used for emitting ultrasonic waves, which means in practice that the transducer aperture is so small compared to the wavelength that it emits spherical waves. If the transducer aperture is large its spatial impulse response (SIR) significantly differs from a Dirac pulse and nonspherical waves will be generated. Therefore, to successfully perform synthetic focusing using large apertures, the transducer’s SIR must be taken into account. A new method for compensating transducer SIR in synthetic focusing is presented in the paper. The method employs a stochastic, time-domain Wiener filtering technique for two-dimensional synthetic focusing. Results of the experiments performed in water using a linear phased array are presented to demonstrate the performance of the proposed method. The use of an array enabled altering the aperture without changing the electrical characteristics of the whole system. The results obtained using the proposed technique for finite apertures are compared to those obtained with a classical SAFT algorithm.