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, and physicians have embraced ultrasound as a tool to facilitate patient evaluation and improve outcomes of invasive procedures. However, these increased applications introduce a level of potential risk if the operator is not appropriately trained. The operator must have a basic understanding of the physical principles of ultrasound. It is these principles upon which ultrasound rests 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 though crystals in the transducer creating 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, at which time a fraction of the sound is reflected back toward the transducer crystal. When the returning echo strikes the crystal, it generates an electrical impulse that is eventually converted into information that is processed into diagnostic images. Sound is calculated to travel through human tissue at body temperature at approximately 1,540 m/s. The ultrasound system measures the round-trip time and intensity of the 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. This information may then be represented 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. 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 20 MHz (megahertz), or 2 and 20 million cycles/second, well above the range of human hearing. While diagnostic ultrasound frequencies extending beyond 15 MHz are available for medical imaging, they are often used in catheter-based technology.