Charge-coupled devices for use in electronically-focused ultrasonic imaging systems

Pulse-echo ultrasonic imaging techniques make possible the dynamic visualisation and characterisation of many soft-tissue structures with no known patient risk; however, the extraction of this information requires an ultrasonic imaging system capable of producing a high-resolution image over a sizeable field of view in real time. This paper describes the development of the cascade charge-coupled device (C3D) designed specifically for electronically-focused ultrasonic imaging systems. The C3D lens described here further enhances the desirability of such systems because the delay-line hardware needed to focus and beam steer a linear array of piezoelectric transducers is reduced to a single l.s.i. device, and because clock-frequency control does not require individual control signals for each transducer channel and therefore greatly limits the amount of external circuitry necessary to support the imaging system. The analysis of the fundamental limits of the C3D approach to ultrasonic imaging includes the effects of the asynchronous transfer of charge between sections operating at different clock frequencies, with emphasis on the impact of this process on the time-delay accuracy of the C3D lens and on the maximum level of sidelobe suppression. The first-generation C3D lens has been fabricated and tested in a static water tank. These measurements indicate that the lens produces near-theoretical lateral resolution as predicted by the aperture theory of linear arrays. Based on preliminary results, a second-generation device is proposed whose additional capabilities would include full interpolation of the field of view for a scan-line-free image presentation and on-chip apodisation to obtain higher degrees of sidelobe suppression.