Ultrasound physics: principles of ultrasonography

Abstract

Sound has wave characteristics and can thus be described in terms of wave length, frequency, and velocity. The frequency of audible sound is usually in the range of 15 Hz to 20 KHz, whereas diagnostic ultrasound is usually in the frequency range of 1 to 30 MHz. 1 Most general diagnostic ultrasound transducers function at frequencies of 2.5 to 7.0 MHz; endoscopic ultrasound frequencies are usually in the 5 to 25 MHz range. The frequency of the ultrasound utilized will affect both the depth of tissue that can be examined and the diagnostic quality of the image produced. The velocity of sound in human soft tissue tends to be fairly constant, at approximately 1540 meters per second, with slight variation from tissue to tissue. 1-5 This constant velocity in soft tissue is important for ultrasound image production. As with light waves, sound can be reflected and refracted. Reflection is critical for image formation. Reflection of sound occurs when sound strikes an interface between two tissues or media that differ in acoustic impedance.l4, 6 Acoustic impedance is an inherent property in all tissues that signifies each tissue's ability to propagate sound waves. If the difference in acoustic impedance between two tissues at an interface is high, a large amount of the sound striking this interface will be reflected. If the difference in acoustic impedance at the interface is low, only a small amount of sound will be reflected and, therefore, a large amount of sound will be transmitted. 5 Acoustic coupling agents, such as ultrasound gel or a water bath, allow for transmittance of ultrasound emitted from a transducer. Without such agents, the ultrasound would be entirely reflected at either the