Two basic principles need to be understood regarding how ultrasound is generated and an image is formed. The first is the piezoelectric effect, which explains how ultrasound is generated from ceramic crystals in the transducer.4 An electric current passes through a cable to the transducer and is applied to the crystals, causing them to deform and vibrate. This vibration produces the ultrasound beam. The frequency of the ultrasound waves produced is predetermined by the crystals in the transducer.

The second key principle is the pulse-echo principle, which explains how the image is generated. Ultrasound waves are produced in pulses, not continuously, because the same crystals are used to generate and receive sound waves, and they cannot do both at the same time. In the time between the pulses, the ultrasound beam enters the patient and is bounced or reflected back to the transducer. These reflected sound waves, or echoes, cause the crystals in the transducer to deform again and produce an electrical signal that is then converted into an image displayed on the monitor. The transducer generally emits ultrasound only 1% of the time; the rest of the time is spent receiving the returning echoes.

Reflection takes place when ultrasound waves are bounced back to the transducer for image generation. The portion of the ultrasound beam that is reflected is determined by the difference in acoustic impedance between adjacent structures.5 Acoustic impedance is the product of a tissue's density and the velocity of the sound waves passing through it; therefore, the denser the tissue, the greater the acoustic impedance. The large differences in density and sound velocity between air, bone, and soft tissue create a correspondingly large difference in acoustic impedance, causing almost all of the sound waves to be reflected at soft tissue-bone and soft tissue-air interfaces. On the other hand, because there is little difference in acoustic impedance between soft tissue structures, relatively few echoes are reflected to the transducer from these areas.

Scattering refers  to the redirection of ultrasound waves as they interact with small, rough, or uneven structures. This tissue interaction occurs in the parenchyma of organs, where there is little difference in acoustic impedance, and is responsible for producing the texture of the organ seen on the monitor. Scattering increases with higher-frequency transducers, thus providing better detail or resolution.

Absorption occurs when the energy of the ultrasound beam is converted to heat. This occurs at the molecular level as the beam passes through the tissues.

Refraction occurs when the ultrasound beam hits a structure at an oblique angle. The change in tissue density produces a change in velocity, and this change in velocity causes the beam to bend, or refract. This type of tissue interaction can also cause artifacts that need to be recognized by the sonographer.

Image Optimization

To obtain good-quality images, the sonographer must know what type and size of transducer to use and how to use the available ultrasound controls. There are many transducers or probes from which to choose, and selection of the appropriate one depends on the location of structures to be imaged and the size of the patient.

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