TABLE 116.1 Approximate Ultrasonic Attenuation Coefficient, Speed, and Characteristic ic Impedance for Water and Selected Tissues at 3.5 MHz Attenuation Coefficient Speed Characteristic Impedance (m-) (10°Pas/m Water Amniotic fluid Muscle 172 5.70 Lun Ultrasound is typically used to image soft body tissues such as liver, but the sound beam often travels through fluids, for example, through amniotic fluid when imaging the fetus. Generally, bone and lung are not imaged with ultrasound. The attenuation processes include absorption, which is the conversion of acoustic energy to heat, and scattering, which will be addressed later. The attenuation increases roughly linearly with frequency in the 2-to 10-MHz range typically used for medical imaging. This range represents a compromise between increased penetration at lower frequencies(because of decreased attenuation) and improved resolution asso ciated with higher frequencies as discussed below. Thus, the lower frequencies are used when greater penetration is required, such as for fetal imaging in the obese patient, and higher frequencies for lesser penetration, such as the examination of peripheral vascular flow. When an acoustic wave impinges on an interface between two media of different specific acoustic impedance, a portion of the incident energy is reflected. For normal incidence on an infinite plane interface, the pressure reflection coefficient is given by [Kinsler et al., 1982 R=22-2 (116.2) 22+z1 where z, and z, are the specific acoustic impedance of the incident and transmitting media, respectively. For a plane wave the specific acoustic impedance is equal to the characteristic impedance which is the product of the density and acoustic speed in the medium(see Table 116.1). The speed is dependent upon the density and the elastic properties of the medium. Thus, at an interface between media exhibiting different densities or elastic properties, i.e., compressibility, some acoustic energy will be reflected. Although the reflection coefficient at an interface between muscle and bone is large(approximately 0.54)the reflection coefficient between two soft tissues such as liver and muscle is quite small (approximately 0.006). Reflection at oblique incidence obeys Snell's law in the same way it applies to electromagnetic waves In addition to the specular reflection that occurs at an interface between two media of different specific acoustic impedance as described above (where any curvature along the interface is negligible over distances comparable to a wavelength), energy may also be scattered in all directions by inhomogeneities in the medium. An acoustic image is formed by using this scattered energy as well as specular reflections. The fraction of the incident energy reflected or scattered is very small for soft tissues. Although it is convenient to consider plane waves of infinite lateral extent, as was done above, real source generate finite beams of ultrasound. These sources may be unfocused, but for the typical diagnostic system they are focused. Figure 116.4 shows the acoustic field from a typical focused source. The source consists of piezoelectric transducer which converts electrical to acoustic energy and vice versa. Most transducers for medical applications are made from ceramic materials such as a lead zirconate titanate(PZT) mixture. For a circular perture these may be circular disks with a plano-concave lens mounted in front to produce spherical focusing Alternatively, the transducer itself may be a spherical segment that produces a focused field without a lens Some probes utilize electronic focusing methods. Such a phased array probe consists of many individual elements which can be excited with signals having a controlled delay with respect to one another such that the c2000 by CRC Press LLC© 2000 by CRC Press LLC Ultrasound is typically used to image soft body tissues such as liver, but the sound beam often travels through fluids, for example, through amniotic fluid when imaging the fetus. Generally, bone and lung are not imaged with ultrasound. The attenuation processes include absorption, which is the conversion of acoustic energy to heat, and scattering, which will be addressed later. The attenuation increases roughly linearly with frequency in the 2- to 10-MHz range typically used for medical imaging. This range represents a compromise between increased penetration at lower frequencies (because of decreased attenuation) and improved resolution asso￾ciated with higher frequencies as discussed below. Thus, the lower frequencies are used when greater penetration is required, such as for fetal imaging in the obese patient, and higher frequencies for lesser penetration, such as the examination of peripheral vascular flow. When an acoustic wave impinges on an interface between two media of different specific acoustic impedance, a portion of the incident energy is reflected. For normal incidence on an infinite plane interface, the pressure reflection coefficient is given by [Kinsler et al., 1982] (116.2) where z1 and z2 are the specific acoustic impedance of the incident and transmitting media, respectively. For a plane wave the specific acoustic impedance is equal to the characteristic impedance which is the product of the density and acoustic speed in the medium (see Table 116.1). The speed is dependent upon the density and the elastic properties of the medium. Thus, at an interface between media exhibiting different densities or elastic properties, i.e., compressibility, some acoustic energy will be reflected. Although the reflection coefficient at an interface between muscle and bone is large (approximately 0.54) the reflection coefficient between two soft tissues such as liver and muscle is quite small (approximately 0.006). Reflection at oblique incidence obeys Snell’s law in the same way it applies to electromagnetic waves. In addition to the specular reflection that occurs at an interface between two media of different specific acoustic impedance as described above (where any curvature along the interface is negligible over distances comparable to a wavelength), energy may also be scattered in all directions by inhomogeneities in the medium. An acoustic image is formed by using this scattered energy as well as specular reflections. The fraction of the incident energy reflected or scattered is very small for soft tissues. Although it is convenient to consider plane waves of infinite lateral extent, as was done above, real sources generate finite beams of ultrasound. These sources may be unfocused, but for the typical diagnostic system they are focused. Figure 116.4 shows the acoustic field from a typical focused source. The source consists of a piezoelectric transducer which converts electrical to acoustic energy and vice versa. Most transducers for medical applications are made from ceramic materials such as a lead zirconate titanate (PZT) mixture. For a circular aperture these may be circular disks with a plano-concave lens mounted in front to produce spherical focusing. Alternatively, the transducer itself may be a spherical segment that produces a focused field without a lens. Some probes utilize electronic focusing methods. Such a phased array probe consists of many individual elements which can be excited with signals having a controlled delay with respect to one another such that the TABLE 116.1 Approximate Ultrasonic Attenuation Coefficient, Speed, and Characteristic Impedance for Water and Selected Tissues at 3.5 MHz Attenuation Coefficient Speed Characteristic Impedance Tissue (m–1) (m/s) (106 Pa s/m) Water 0.2 1520 1.50 Amniotic fluid 0.7 1510 1.51 Blood 7 1550 1.60 Liver 35 1580 1.74 Muscle 50 1560 1.72 Bone 800 3360 5.70 Lung 1000 340 0.25 R z z z z = - + 2 1 2 1