Physical Principles of Ultrasound

At the completion of this chapter the student should be able to:

• Discuss the Physical Properties of Sound Waves
• Discuss the Sound Wave Interaction with Tissues
• Explain Transducers

Sound waves are mechanical vibrations that induce alternate rarefaction (expansion) and compression of any physical medium through which the sound wave travels . Sound waves have four physical properties: (Figure 1.1.1)

The wavelength is the length of one wave cycle (i.e. from peak to peak) which includes one rarefaction and one compression and is measured in millimeters (mm). The propagation velocity is the velocity of the wave through a medium. The propagation velocity is dependent upon the medium's characteristics (i.e. density, temperature). The propagation velocity of average human tissue is 1540 m/sec. In Table 1.1.2 common propagation velocities are listed for different tissues.

The frequency of a sound wave is the number of cycles of a sound wave per second or Hertz (Hz). The frequency can be calculated by dividing wavelength by time (Figure 1.1.3). A small wavelength will yield a higher frequency, whereas a larger wavelength will yield a smaller frequency. The human hearing range is 20-20,000 Hz. Ultrasound is greater than 20,000 Hz. Medical ultrasound is typically 1-20 MHz, or megahertz. One megahertz(MHz) is equal to 1 million Hertz.

Propagation velocity, c, is the product of wavelength and frequency . If the medium is human soft tissue, then the propagation velocity (c) is 1540 m/sec. The wavelength can be calculated by dividing 1540 m/sec by the frequency. Note that wavelength is inversely proportional to frequency. A higher frequency will have a shorter wavelength.

As the ultrasound beam penetrates a medium, the beam is attenuated or loses energy. As a beam penetrates tissue, some of the beam is reflected, refracted or absorbed as heat generation. The amount of penetration will determine the depth of the scanning area. Penetration is directly related to wavelength. Smaller wavelengths are more easily reflected or refracted in the superficial tissues than longer wavelengths. As wavelength is increased (or frequency decreased) the ultrasound will penetrate deeper. As the wavelength is decreased (or frequency is increased) the ultrasound beam will have a shallower penetration. Low frequency ultrasound has superior penetration.

At the high penetration/low resolution settings the deeper structures are more easily visualized at the loss of resolution of the more superficial structures. At the opposite setting, the high resolution/low penetration setting, the deep structures are not well visualized while the superficial structures are sharp and exhibit enhanced detail.

Amplitude is equivalent to loudness. If the echocardiographic machine cannot 'hear' the returned signal (echo) then, if the machine 'yells louder', it may hear the louder echo.

2D Echocardiography uses a change in amplitude to display images. Returned signals that have a higher amplitude are displayed as brighter than returned signals with a smaller amplitude. Doppler uses a change in frequency to display images. A returned signal that has a much higher frequency will be displayed with a higher velocity color, whereas, a signal with the lower frequency will be displayed with a lower velocity color. A display that has both 2D and Doppler images displayed simultaneously is called a duplex image or a duplex scan.