Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

Ultrasound is the term that describes sound waves of frequencies exceeding therange of human hearing and their propagation in a medium. Medical diagnosticultrasound is a modality that uses ultrasound energy and the acoustic propertiesof the body to produce an image from stationary and moving tissues. Ultrasoundimaging uses a "pulse echo" technique to synthesize a gray-scale tomographicimage of tissues based on the mechanical interaction of short pulses of high-frequencysound waves and their returning echoes. Generation of the sound pulsesand detection of the echoes is accomplished with a transducer, which also directsthe ultrasound pulse along a linear path through the patient. Along a given beampath, the depth of an echo-producing structure is determined from the timebetween the pulse emission and the echo return, and the amplitude of the echo isencoded as a gray-scale value (Fig. 16- 1). In addition to two-dimensional (20)tomographic imaging, ultrasound provides anatomic distance and volume measurements,motion studies, blood velocity measurements, and three-dimensional(3D) imaging.Historically, medical uses of ultrasound came about shortly after the close ofWorld War II, derived from undetwater sonar research. Initial clinical applicationsmonitored changes in the propagation of pulses through the brain to detect intracerebralhematoma and brain tumors based on the displacement of the midline.Ultrasound rapidly progressed through the 1960s from simple "A-mode" scans to"B-mode" applications and compound "B-scan" images using analog electronics.Advances in equipment design, data acquisition techniques, and data processingcapabilities have led to electronic transducer arrays, digital electronics, and real-timeimage display. This progress is changing the scope of ultrasound and its applicationsin diagnostic radiology and other areas of medicine. High-resolution, real-timeimaging, harmonic imaging, 3D data acquisition, and power Doppler are a few ofthe innovations introduced into clinical practice. Contrast agents for better delineationof the anatomy, measurement of tissue perfusion, precise drug deliverymechanisms, and determination of elastic properties of the tissues are topics ofcurrent research.This chapter describes the characteristics, properties, and production of ultrasound;the modes of ultrasound interaction, instrumentation, and image acquisition;signal processing; image display; Doppler flow measurement; quality control;and ultrasound bioeffects.