Introduction to the special issue on coded waveforms

Although coded waveforms were first explored in the field of Radar in the years following the Second World War, it was not until the very mid-seventies and early eighties that their application was discussed in the context of ultrasound imaging. Among the earliest discussions of coded waveforms in the field of nondestructive testing are those by Newhouse and Furgason in the mid 1970´s using a random noise generator as a signal source and performing correlation on the received echo signals and a reference copy of the transmitted noise signal. A few years later, in 1980, Lee and Furgason described the use of Psuedo Random Binary Sequences (PRBS), including Golay codes, in the context of phased array-based flaw detection. About the same time, Takeuchi described the use of spread spectrum techniques for medical imaging. It is interesting to reflect on the fact that these early works were presented practically simultaneously with the introduction of phased array medical ultrasound imaging. However, many significant challenges impeded the introduction of coded waveforms in commercial ultrasound scanners. For example, short time-bandwidth products are necessary to avoid distortion of received echos due to dynamic received focusing. This distortion, and indeed any other form of distortion, typically results in elevated sidelobe levels following the pulse compression stage in the receiver. Additionally, at least in the case of medical imaging, one has to contend with phase aberrating effects due to sound velocity inhomegeneity, nonlinear propagation, differential rates of frequency dependent attenuation, limited available transducer bandwidth and target motion between successive pulses. During the 1980´s, O´Donnell´s group at General Electric Research further investigated coded imaging resulting in publications and patents describing in detail the parameters and design tradeoffs that must be taken into account when designing a coded waveform phased array digital scanner- However, it was not until the late 1990´s that General Electric successfully introduced coded waveforms to diagnostic ultrasound scanners. This was largely a result of the efforts of a team led by Chiao and Thomas whose contributions are recorded in a series of related patents. This proof that coded imaging can make a significant contribution has proven to be a catalyst to renewed interest in the field of coded waveforms. Among more recent contributions are analyses of FM chirp-based waveforms (Misaridis and Jensen) and investigations of various other binary and continuous waveforms. This recent growth in activity, as indicated by the increasing number of coded waveforms papers submitted to the IEEE Transactions and to the IEEE Ultrasonics Symposium, were the motivation for organizing this Special Issue.