Although the clinical use of ultrasound (US) now extends over almost 50 years, the arrival of the twenty-first century promises an exciting array of new US technology poised to impact patient care. New challenges and directions in both research and technology development will provide radiologists, referring physicians, and patients with opportunities in enhanced features and expanded uses of US in diagnosis and treatment.

Following its discovery by Pierre Curie in the 1880s, little use was made of the piezoelectric effect until the development of sonar before World War II. Beginning with the translation of wartime sonar technology to peaceful purposes in the 1950s, a series of technologic advances have led to the modern US scanners of today. The A-mode systems of the 1950s were followed by the development of the simple B-mode display, compound scanning instruments used by clinical pioneers in the 1960s. A major technologic advance came in the early 1970s with the development of gray-scale B-mode imaging using analog scan converters. The improvements in diagnostic image quality resulting from this development led to the establishment of US as a robust clinical tool, despite competition from other imaging methods including CT and later MR imaging. With the introduction of gray-scale imaging, duplex Doppler, and real-time imaging in the 1970s, clinical applications expanded rapidly and ultrasound's role was firmly established. The impact of US on the radiographic imaging of the 1970s and early 1980s is reflected in the procedures that US helped to render obsolete: the oral cholecystogram and cholangiogram, percutaneous cholangiography, X-ray pelvimetry, abdominal radiography to determine fetal position and number, radionuclide placentography, contrast venography, screening carotid angiography, and aspiration for simple cyst diagnosis.

The next major advance in US technology occurred as a result of the incorporation of digital technology into US instrumentation in the early 1980s. Substantial improvements in image quality and reliability of scanners were the primary benefits of this fundamental change in the architecture of scanners. In addition to enhanced beam-forming and signal-processing capabilities, new transducer technology expanded the range of clinical use to include endovaginal, endorectal, and endoscopic applications. Further influencing the growth of US during the mid-1980s was the development of color Doppler, a technology that facilitated the use of Doppler by radiologists on a large scale. The impact of rapid increases in computing power continued to have a major impact on the development of US technology throughout the remainder of the 1980s and 1990s. A steady series of innovations were introduced into clinical practice during this time including power Doppler, broad bandwidth transducers, improved signal processing, and three-dimensional acquisition and display. Paralleling these technologic developments has been the introduction of microbubble contrast agents and investigation into novel approaches to imaging.

Despite the fact that US can now be regarded as a mature imaging modality, with a history of clinical use extending over nearly half a century, there is reason to believe that the steady progress in technology that has produced the improvements of the past 30 years will continue well into the future. New technology currently under development or in the early stages of clinical testing promises to have a substantial impact on the practice of US. In this update on new and evolving US technologies only a few representatives of the many candidate technologies likely to impact clinical practice are discussed. These include approaches to improve image contrast and spatial resolution, three-dimensional US, and novel methods including hybrid detector arrays and elastography. High-frequency transducers providing submillimeter spatial resolution, multidimensional arrays to improve contrast resolution and reduce artifacts, improved flow detection, perfusion imaging, and three-dimensional imaging are already having a major impact on imaging. Advanced image processing though tissue harmonic imaging and real-time spatial compounding promise reduction in noise and speckle and are now commercially available. Novel approaches to display differences in tissue elastic properties and new approaches to treatment will further expand the role of US in the management of disease. Finally, contrast agents offer new opportunities for assessment of tumor angiogenesis and response to new therapeutic regimes. These developments in technology are accompanied by the opportunity to combine improved instrumentation, signal processing, and microbubble contrast agents into a formidable tool for the evaluation of antiangiogenesis therapies. New approaches using US are likely to substantially improve the value of US in diagnosis and screening.