The evolution of medical ultrasonic imaging systems

Abstract

A limit to effective soft tissue imaging is set by the acoustic properties of tissue. The attenuation is roughly proportional, and the size of a resolution element inversely proportional to frequency. Thus the number of resolution elements per image is fixed. Accordingly, acoustic frequencies from about 1 to 50 MHz are required. Effective imaging has required the development and application of innovative high-frequency hardware; particularly the transducers. These have progressed from single element types to 512 element arrays in production, with two-dimensional arrays having 1282 elements in development. The electronics now require many more channels with a dynamic range exceeding 100 dB and, with the introduction of digital methods, even multiple very fast A/D converters in the beamformer. Display devices have gone from WWII CRT’s with 200 spots per radius to digital image stores on CRT’s with thousands of spots available. These can now do image processing with software. The earlier systems survive in some applications that require them. The single element types are still used in high-frequency catheters, for example. The wideband transducers have opened up new methods, enhanced Doppler, contrast and tissue imaging that use a wider range of frequencies than previously possible. The single element types are still used in high-frequency catheters, for example.