Which statement best describes the units used in the wavelength formula for ultrasound?

Which statement best describes the units used in the wavelength formula for ultrasound?

• A. Wavelength in millimeters and frequency in megahertz.
• B. Wavelength in meters and frequency in hertz.
• C. Wavelength in kilometers and frequency in kilohertz.
• D. Wavelength in micrometers and frequency in gigahertz.

Answer: (A)

Wavelength comes from dividing the speed of sound by the frequency: lambda = c / f. In ultrasound practice, the speed in soft tissue is about 1540 m/s. If the frequency is expressed in megahertz (MHz), the resulting wavelength falls in millimeters because 1540 m/s divided by 1,000,000 Hz gives 0.00154 meters, or about 1.54 mm per MHz. This makes mm for wavelength and MHz for frequency the most convenient, commonly used units. Using meters with hertz would work mathematically, but the mm/MHz pairing keeps numbers simple in typical ultrasound contexts.

Wavelength comes from dividing the speed of sound by the frequency: lambda = c / f. In ultrasound practice, the speed in soft tissue is about 1540 m/s. If the frequency is expressed in megahertz (MHz), the resulting wavelength falls in millimeters because 1540 m/s divided by 1,000,000 Hz gives 0.00154 meters, or about 1.54 mm per MHz. This makes mm for wavelength and MHz for frequency the most convenient, commonly used units. Using meters with hertz would work mathematically, but the mm/MHz pairing keeps numbers simple in typical ultrasound contexts.