Diagnostic ultrasound has experienced tremendous technological advances. Over the past 50 years, ultrasound has evolved from a single specialty tool with large bulky machines to a technology that is highly compact and portable. The development of smaller, less expensive ultrasound systems has increased the number of medical specialties utilizing ultrasound. Many are discovering the benefits of “point-of-care” diagnostic ultrasound. Medical students, nurses, mid-level providers, and physicians have embraced ultrasound as a tool to facilitate patient evaluation and improve outcomes of invasive procedures. The operator must have a basic understanding of the physical principles of ultrasound. It is these principles upon which ultrasound bases its ability to be an effective tool in medical imaging.
The simplest way to describe ultrasound is in the pulse-echo principle. Sonar can be used as an example of the forerunner of diagnostic ultrasound. A submarine that possesses sonar capability can precisely control when an acoustic pulse is generated. It assumes a relative propagation speed as it travels through a specific medium (water). The amount of elapsed time required for the “echo” to return subsequent to striking an object allows the relative distance to be calculated to the target of interest.
Diagnostic ultrasound uses the same concept of the pulse-echo principle. Electric current is passed through crystals in the transducer and generates a sound wave. This piezoelectric effect generates a constant pulse of high-frequency, longitudinal, mechanical sound waves that can be measured and used in calculations. This pulse travels at a relatively constant speed until it encounters a reflective surface, which causes a fraction of the sound to reflect back toward the transducer crystal. When the returning sound wave strikes the crystal, it generates an electrical impulse that is processed into a diagnostic image. Based on the assumption that sound travels at the same speed through all tissues (1540 m/s), a computer measures the round-trip time and intensity of each returning “echo.” The amount of time required for the returning echo determines its relative distance from the transducer while the returning intensity is proportional to the grayscale assignment of the pixel. Each returning echo is presented as a pixel (dot) of information on the display device.
Sound waves are actually a series of repeating mechanical pressure waves that propagate through a medium (Figure 3-1). These pressure waves are measured in hertz (cycles/second). Typically, audible sound ranges between 16,000 and 20,000 Hz. Ultrasound is technically defined as a “sound” having a frequency in excess of 20,000 Hz. In medicine, ultrasound used for diagnostic purposes incorporates frequencies that generally range between 2 and 15 million cycles/second, or 2 and 15 MHz, well above the range of human hearing.
Time versus pressure graph of a sound wave. Amplitude: peak pressure of a wave. Period: time required to complete a single cycle. (Courtesy of SonoSite)