Acoustical Imaging by Means of Multi-Frequency Hologram Matrix

Abstract

A multi-frequency holographic imaging method has been developed, which is an advanced application of the hologram-matrix imaging method suggested and developed by the authors. This new method is based on the idea that axial resolution as well as transverse one can be obtained by properly synthesizing transversely focused beams of different frequencies. This idea has been included into the hologram-matrix imaging method. An image is reconstructed by a linear transformation from hologram matrices of different frequencies. Grating-lobe artifacts usually encountered in the multi-frequency imaging methods are eliminated using nonuniformly spaced frequencies. Thus an imaging of objects placed in the near field by a holographic synthetic aperture method has become practical with a small transducer array and simple electronics.

A design way of transducer arrays and frequency series for this imaging method is introduced in this paper. Three examples of the design are evaluated in their point-spread functions and imaging abilities from simulated hologram matrices of a multi-point object. The transmitter is of one element, the receiver is composed of 31 or 37 elements, and the number of frequencies is 40.

Experimental results also shows that this method is promising. Measurement of the multi-frequency hologram matrices can be done within 30 ms. It may take only few minutes to reconstruct an image of 80×80 pixels by a high-speed mini-computer.