Compensating transducer diffraction effects in synthetic aperture imaging for immersed solids

One of the fundamental requirements for the successful application of the classical synthetic aperture focusing technique (SAFT) is the use of a transducer that emits spherical (cylindrical) waves. For a planar transducer, the performance of the SAFT algorithm will deteriorate if its active area becomes too large comparing to the wavelength. This is due to the spatial impulse responses (SIRs) associated with the transducer that no longer resemble Dirac functions since the emitted waves is not spherical. Therefore, to achieve a high resolution or finite-sized transducers, the SIRs must be taken into consideration. Here, we propose a method that is based on a discrete linear model of the imaging system. The method uses a spatio-temporal deconvolution technique designed to minimize the mean squared reconstruction error of the imaging system. To demonstrate the performance of the proposed method we present experiments using a phased array for the inspection of a copper specimen. The results obtained using the deconvolution method for finite apertures are compared to those obtained with a time-domain SAFT algorithm and a focused phased array.