P.M. Hurculak, Positron emission tomography, Canadian Journal of Medical Radiation Technology, vol. 18, G E. Knoll,Single-Photon emission computed tomography, Proceedings of the IEEE, vol. 71, no 3, P. 320, A Macovski, Medical Imaging Systems, Englewood Cliffs, N]: Prentice-Hall, 1983. H.R. Schelbert, Future perspectives: Diagnostic possibilities with positron emission tomography, " Roentgen Blaetter, vol. 43, no 9, PP. 384-390, Sept. 1990 G L. Wolf and C. Popp, NMR, A Primer for Medical Imaging, Thorofare, N J. Slack, Inc., 1984 Further information The journal IEEE Transactions on Medical Imaging describes advances in imaging techniques and image pro- cessing. Investigative Radiology published by the Association of University Radiologists, emphasizes research carried out by hospital-based physicists and engineers. Radiology, published by the North American Society of Radiologists, contains articles which emphasize clinical applications of imaging technology. Diagnostic imaging publishing by Miller Freeman, Inc, is a good source of review articles and information on the imaging 116.2 Ultrasound Leon a. frizzell Ultrasound, acoustic waves at frequencies higher than those audible by humans, has developed over th 35 years into an indispensable clinical diagnostic tool. Currently, ultrasound is used to image most parts body. More than half of all pregnant women in the United States are examined with ultrasound. This widespread utilization has resulted from ultrasounds proven clinical utility for imaging soft tissues compared to more expensive imaging techniques. The development of ultrasound, particularly for fetal examinations, has also been fostered by its safety record; no case of an adverse biological effect induced by diagnostic ultrasound has ver been reported in humans [AIUM, 1988]. Diagnostic ultrasound systems are used primarily for soft tissue imaging, motion detection, and flow mea- surement. Except for some Doppler instruments, these systems operate in a pulse-echo mode. a brief summary of some of the fundamentals of acoustic wave propagation and the principles of ultrasound imaging follows. als of a Unlike electromagnetic waves, acoustic waves require a medium for propagation. The acoustic wave phenom enon causes displacement of particles(consisting of many molecules), which results in pressure and density hanges within the medium. For a traveling sinusoidal wave, the variation in acoustic pressure( the difference between the total and ambient pressure), excess density, particle displacement, particle velocity, and particle acceleration can all be represented by the form p=Pe-aux cos(ot-kx) (116.1) for a wave propagating in the positive x direction, where p is the pressure(or one of the other parameters listed above), P is its amplitude, o is the angular frequency, and @= 2nf where f is the frequency in hertz, k is the propagation constant and k=o/c where c is the propagation speed, a is the attenuation coefficient, and t is the time. The wave can experience significant attenuation, as represented by the exponential decay of amplitude with distance, during propagation in tissues. The attenuation coefficient varies greatly among tissues [ Goss et al., 1978, 1980; Haney and OBrien, 1986] but is low for most body fluids, much higher for solid tissues, and very high for bone and lung(see Table 116.1). The skin depth is the distance that the wave can propagate before being attenuated to e of its original amplitude and is thus simply the inverse of the attenuation coefficient. e 2000 by CRC Press LLC© 2000 by CRC Press LLC P.M. Hurculak, “Positron emission tomography,” Canadian Journal of Medical Radiation Technology, vol. 18, no. 1, March 1987. G.F. Knoll, “Single-photon emission computed tomography,” Proceedings of the IEEE, vol. 71, no. 3, p. 320, March 1983. A. Macovski, Medical Imaging Systems, Englewood Cliffs, N.J.: Prentice-Hall, 1983. H.R. Schelbert, “Future perspectives: Diagnostic possibilities with positron emission tomography,” Roentgen Blaetter, vol. 43, no. 9, pp. 384–390, Sept. 1990. G.L. Wolf and C. Popp, NMR, A Primer for Medical Imaging, Thorofare, N.J.: Slack, Inc., 1984. Further Information The journal IEEE Transactions on Medical Imaging describes advances in imaging techniques and image pro￾cessing. Investigative Radiology, published by the Association of University Radiologists, emphasizes research carried out by hospital-based physicists and engineers. Radiology, published by the North American Society of Radiologists, contains articles which emphasize clinical applications of imaging technology. Diagnostic Imaging, publishing by Miller Freeman, Inc., is a good source of review articles and information on the imaging marketplace. 116.2 Ultrasound Leon A. Frizzell Ultrasound, acoustic waves at frequencies higher than those audible by humans, has developed over the past 35 years into an indispensable clinical diagnostic tool. Currently, ultrasound is used to image most parts of the body. More than half of all pregnant women in the United States are examined with ultrasound. This widespread utilization has resulted from ultrasound’s proven clinical utility for imaging soft tissues compared to more expensive imaging techniques. The development of ultrasound, particularly for fetal examinations, has also been fostered by its safety record; no case of an adverse biological effect induced by diagnostic ultrasound has ever been reported in humans [AIUM, 1988]. Diagnostic ultrasound systems are used primarily for soft tissue imaging, motion detection, and flow mea￾surement. Except for some Doppler instruments, these systems operate in a pulse-echo mode. A brief summary of some of the fundamentals of acoustic wave propagation and the principles of ultrasound imaging follows. Fundamentals of Acoustics Unlike electromagnetic waves, acoustic waves require a medium for propagation. The acoustic wave phenom￾enon causes displacement of particles (consisting of many molecules), which results in pressure and density changes within the medium. For a traveling sinusoidal wave, the variation in acoustic pressure (the difference between the total and ambient pressure), excess density, particle displacement, particle velocity, and particle acceleration can all be represented by the form p = P e –ax cos(wt – kx) (116.1) for a wave propagating in the positive x direction, where p is the pressure (or one of the other parameters listed above), P is its amplitude, w is the angular frequency, and w = 2pf where f is the frequency in hertz, k is the propagation constant and k = w/c where c is the propagation speed, a is the attenuation coefficient, and t is the time. The wave can experience significant attenuation, as represented by the exponential decay of amplitude with distance, during propagation in tissues. The attenuation coefficient varies greatly among tissues [Goss et al., 1978, 1980; Haney and O’Brien, 1986] but is low for most body fluids, much higher for solid tissues, and very high for bone and lung (see Table 116.1). The skin depth is the distance that the wave can propagate before being attenuated to e–1 of its original amplitude and is thus simply the inverse of the attenuation coefficient