What is the typical frequency range for diagnostic ultrasound?

• A. 2–10 MHz
• B. 0.5–1 MHz
• C. 20–40 kHz
• D. 50–100 MHz

Answer: (A)

Diagnostic ultrasound relies on frequencies in the megahertz range because you need a balance between image sharpness and how deep the sound can travel through tissue. The speed of sound in soft tissue is about 1540 m/s, so the wavelength gets shorter as frequency increases. At around 2 MHz the wavelength is roughly 0.8 mm, and at about 10 MHz it’s about 0.15 mm. Shorter wavelengths give better spatial resolution, especially along the direction the sound travels, which helps you distinguish small structures. But higher frequency also attenuates more quickly, so deep structures become harder to image. That’s why clinical probes are designed for the 2–10 MHz range: enough penetration to see organs like the liver or abdomen while still providing good resolution.

Frequencies much lower, like 0.5–1 MHz, would penetrate deeper but with poorer resolution; much higher frequencies, like 50–100 MHz, would offer excellent resolution but extremely limited depth, making them unsuitable for standard diagnostic exams. Frequencies in the range of 20–40 kHz are far below medical imaging needs and not used for diagnostic ultrasound. Hence, 2–10 MHz is the typical range.