What is ultrasound? It may be defined as waves of sound beyond the ordinary limits of hearing. Therefore, simplified to high-frequency sound waves, sound waves are a vibratory phenomenon and they require matter for their transmission. Sound also travels best in substances composed of many molecules therefore, it will travel better through solids composed of multiple molecules than it will in gases such as air, in which there are fewer molecules. For our purposes, we will limit our discussion of sound waves to high-frequency waves since these are the type used in medical diagnosis.
Unlike audible sound, high-frequency sound can be directed in a beam (much like light) and in so doing follows the laws of reflection and refraction, making it possible to reflect off objects of very small size. The main disadvantage of high-frequency sound is that it travels poorly through air, requiring an airless contact with the body during examination. This explains the need to use mineral oil or jelly as a coupling agent to body areas being examined.
Sound is actually a series of compressions and rarefactions. The combination of one compression and one rarefaction represents one cycle. The distance between one cycle and the onset of next cycle is the wave length (λ). The speed at which sound travels through a given medium is its velocity. Now, we can define frequency as the number of cycles over a given period of time. Therefore, the velocity is equal to the frequency multiplied by the wavelength (V = F × λ). By this equation, you can see that frequency and wavelength are inversely related. The higher is the frequency, and the smaller is the wavelength.
How sound travels through a medium is often referred to as the acoustic impedance of that medium. For example, as sound travels through the bladder it is going in essentially a straight line. When it reaches an interface (or border) between the bladder and the uterus, a portion of the beam is reflected or refracted back to the transducer while the majority of the beam continues in a straight line through the body.2
Ultrasound with a higher frequency (or smaller wavelength) can reflect sound from smaller objects, since a higher frequency ultrasound beam has greater resolution. Resolution is the ability to visualize objects of interfaces close to one another. For example, the transducer used most often in abdominal and obstetrical sonography is the 2.25 or 3.5 MHz transducer, because the average depth of the area of interest in this case is 20–40 cm. However, in scanning the thyroid, (which is usually less than 7 cm away from the transducer) a 10 MHz transducer is better since it sacrifices depth for better resolution or detail nearer the surface.
The area closest to the transducer is termed the near field and that farthest away, the far field. The ultrasound beam encounters numerous interfaces on its path through the body. At each of these interfaces, part of the sound beam is reflected back and a smaller portion passes through the interface. This diminishes the sonic beam and leaves progressively less and less available for deep penetration. In addition to the reflection of the sound beam there is also sonic absorption and scatter, causing even more weakening of the sonic beam. This phenomenon is referred to as attenuation of the sound beam. A term used to express the amount of absorption and attenuation of the sound beam is half value layer. This term may be defined as the distance sound will travel in a given medium before its energy is attenuated one half its original value. The fluid and homogenous tissue are excellent conductors of sound while bone has a very low half value layer, presenting a veritable barrier to the sonic beam.
The velocity (speed) of sound depends on the density and elasticity of the medium through which it travels. Velocity also depends on temperature of the medium. However, since the human body temperature is relatively constant, temperature changes are not usually important in medical diagnostic work. The velocity of sound through living human soft tissue is 1540 m/s. The only significant change of velocity encountered by the ultrasound beam would be as it traveled through gas or bone. The greater the acoustic difference, the more sound will be reflected back to the transducer. Another important factor governing the amount of sound reflected is the angle at which the beam strikes the interface (angle of incidence). The closer the angle of incidence is to 90° to the interface, the more the sound is reflected.