Which option correctly defines ultrasound?

• A. Frequency greater than 20,000 cycles per second
• B. Light-based imaging modality
• C. Ionizing radiation technology
• D. Electrical current therapy

Answer: (A)

Ultrasound is defined by its frequency: sound waves with frequencies above the range of human hearing. Because these high-frequency waves are not audible, they can be used to probe tissues and create images from the echoes they produce. This distinguishes ultrasound from light-based imaging, ionizing radiation technologies, and electrical current therapies. The key defining trait is a frequency greater than about 20,000 cycles per second.