Axial resolution is approximately SPL/2. Which factor primarily determines axial resolution?

• A. Wavelength
• B. Spatial pulse length (SPL)
• C. SPL/2
• D. Pulse duration

Answer: (B)

Axial resolution is governed by how long the ultrasound pulse lasts in space along the beam. The relevant distance is the spatial pulse length (SPL), which is the physical length of one pulse: SPL equals the wavelength times the number of cycles in the pulse. Because two closely spaced reflectors along the beam are distinguished by roughly half of that pulse length, axial resolution is about SPL/2. Therefore, the shorter the SPL, the better the axial resolution.

Wavelength and pulse duration influence SPL, but they are not the direct measure of axial resolution themselves. Shorter SPL can be achieved with higher frequency (shorter wavelength) or with fewer cycles in the pulse. In short, SPL is the primary determinant of axial resolution.