The image produced by an ultrasound machine begins with the transducer. Transducer comes from the Latin transducere, which means to convert. In this case, electrical impulses are converted to mechanical sound waves via the piezoelectric effect .

In ultrasound imaging the transducer has a dual function as a sender and a receiver. Sound waves are transmitted into the body where they are at least partially reflected. The piezoelectric effect occurs when alternating current is applied to a crystal containing dipoles [1]. Areas of charge within a piezoelectric element are distributed in patterns which yield a “net” positive and negative orientation. When alternating charge is applied to the two element faces, a relative contraction or elongation of the charged areas occurs resulting in a mechanical expansion and then a contraction of the element. This results in a mechanical wave which is transmitted to the patient (Fig. 2.4).

Reflected mechanical sound waves are received by the transducer and converted back into electrical energy via the piezoelectric effect. The electrical energy is interpreted within the ultrasound instrument to generate an image which is displayed upon the screen.

For most modes of ultrasound, the transducer emits a limited number of wave cycles (usually two to four) called a pulse. The frequency of the two to four waves within each cycle is usually in the 2.5–14 MHz range. The transducer is then “silent” as it awaits the return of the reflected waves from within the body (Fig. 2.5). The transducer serves as a receiver more than 99 % of the time.

Pulses are sent out at regular intervals usually between 1 and 10 kHz which is known as the pulse repetition frequency ( PRF ) . By timing the pulse from transmission to reception, it is possible to calculate the distance from the transducer to the object reflecting the wave. This is known as ultrasound ranging (Fig. 2.6). This sequence is known as pulsed – wave ultrasound .

The amplitude of the returning waves determines the brightness of the pixel assigned to the reflector in an ultrasound image. The greater the amplitude of the returning wave, the brighter the pixel assigned. Thus, an ultrasound unit produces an “image” by first causing a transducer to emit a series of ultrasound waves at specific frequencies and intervals and then interpreting the returning echoes for duration of transit and amplitude. This “image” is rapidly refreshed on a monitor to give the impression of continuous motion. Frame refresh rates are typically 12–30 per second. The sequence of events depicted in Fig. 2.7 is the basis for all “scanned” modes of ultrasound including the familiar gray-scale ultrasound.

As ultrasound waves are transmitted through human tissue, they are altered in a variety of ways including loss of energy, change of direction, and change of frequency. An understanding of these interactions is necessary to maximize image quality and correctly interpret the resultant images.

Attenuation refers to a loss of kinetic energy as a sound wave interacts with tissues and fluids within the body [2]. Specific tissues have different potentials for attenuation. For example, water has an attenuation of 0.0 whereas kidney has an attenuation of 1.0 and muscle an attenuation of 3.3. Therefore, sound waves are much more rapidly attenuated as they pass through muscle than as they pass through water (Fig. 2.8). (Attenuation is measured in dB/cm/MHz.)

The three most important mechanisms of attenuation are absorption, reflection, and scattering. Absorption occurs when the mechanical kinetic energy of a sound wave is converted to heat within the tissue. Absorption is dependent on the frequency of the sound wave and the characteristics of the attenuating tissue. Higher frequency waves are more rapidly attenuated by absorption than lower frequency waves.

Since sound waves are progressively attenuated with distance traveled, deep structures in the body (e.g., kidney) are more difficult to image. Compensation for loss of acoustic energy by attenuation can be accomplished by the appropriate use of gain settings (increasing the sensitivity of the transducer to returning sound waves) and selection of a lower frequency.

Refraction occurs when a sound wave encounters an interface between two tissues at any angle other than 90°. When the wave strikes the interface at an angle, a portion of the wave is reflected and a portion transmitted into the adjacent media. The direction of the transmitted wave is altered (refracted). This results in a loss of some information because the wave is not completely reflected back to the transducer, but also causes potential errors in registration of object location because of the refraction (change in direction) of the wave. The optimum angle of insonation to minimize attenuation by refraction is 90° (Fig. 2.9).

Reflection occurs when sound waves strike an object or an interface between unlike tissues or structures. If the object has a relatively large flat surface, it is called a specular reflector, and sound waves are reflected in a predictable way based on the angle of insonation. If a reflector is small or irregular, it is called a diffuse reflector. Diffuse reflectors produce scattering in a pattern which produces interference with waves from adjacent diffuse reflectors. The resulting pattern is called “speckle” and is characteristic of solid organs such as the testes and liver (Fig. 2.10).

When a sound wave travels from one tissue to another, a certain amount of energy is reflected at the interface between the tissues. The percentage of energy reflected is a function of the difference in the impedance of the tissues. Impedance is a property of tissue related to its “stiffness” and the speed at which sound travels through the tissue [3]. If two adjacent tissues have a small difference in tissue impedance, very little energy will be reflected. The impedance difference between kidney (1.63) and liver (1.64) is very small so that if these tissues are immediately adjacent, it may be difficult to distinguish the interface between the two by ultrasound (Table 2.1).

Fat has a sufficient impedance difference from both kidney and liver that the borders of the two organs can be distinguished from the intervening fat (Fig. 2.11).

If the impedance differences between tissues are very high, complete reflection of sound waves may occur, resulting in acoustic shadowing (Fig. 2.12).

Sound waves are emitted from the transducer with a known amplitude, direction, and frequency. Interactions with tissues in the body result in alterations of these parameters. Returning sound waves are presumed to have undergone alterations according to the expected physical principles such as attenuation with distance and frequency shift based on the velocity and direction of objects they encountered. The timing of the returning echoes is based on the expected velocity of sound in human tissue. When these expectations are not met, it may lead to image representations and measurements which do not reflect actual physical conditions. These misrepresentations are known as “artifacts.” Artifacts, if correctly identified, can be used to aid in diagnosis.