Cardiac Imaging Using a Phased Array Ultrasound System

Underlying the conception and design of this new imaging system were severai design considerations specifically directed toward its application in clinical cardiology. These include image rates commensurate with cardiac dynamics, a field of view sufficiently large to allow image formation of most or all of the left ventricle, and the utilization of a relatively small, hand-held transducer which would allow some freedom in imaging various planes of the cardiac anatomy through the restricted acoustic windows of the thorax. In addition, efforts were directed toward improving the final image quality by maintaining high azimuthal and range resolution throughout the field of view and by logarithmically compressing the received echo signals from each element of the array. This non-linear processing, aside from improving image resolution and depth of field and reducing sidelobe response, compresses the dynamic range of echo information so that regions in the tissues characterized by diffuse or specular reflectors can be displayed simultaneously on a typical CRT monitor. The high data acquisition rates necessary to tomographically visualize the dynamic cardiac structures are achieved by electronically phasing a linear array of ultrasound transducer elements. This method permits the reorientation of the transmitted ultrasound beam from one transmit operation to the next as well as allowing the effective orientation of the array during reception to coincide with that during transmission. The prototype imaging system uses a linear array of 16 ultrasound transducers during the transmit and receive operations. The fundamental frequency of the heavily damped array is 1.8 MHz and measures 24 mm by 14 mm at the site of contact. The latter