Axial resolution is typically expressed in which unit?

• A. Seconds
• B. Millimeters
• C. Centimeters
• D. Micrometers

Answer: (B)

Axial resolution is a measure of how close two structures along the direction of the beam can be while still appearing as separate objects. It’s a distance, not a time or a rate. In ultrasound, this resolution is determined by the spatial pulse length, which depends on how many cycles are in the emitted pulse and the wavelength of the sound in tissue. Since tissue sound speed is about 1540 m/s and typical transducer frequencies place the wavelength in the fraction-of-a-millimeter to millimeter range, the smallest separations you can resolve along the beam are on the order of tenths to a few millimeters. That’s why axial resolution is commonly expressed in millimeters. The other options describe time or much smaller scales that aren’t the standard way this distance is reported in ultrasound practice.