Axial resolution is determined chiefly by:

• A. Beam width
• B. Transducer diameter
• C. Pulse duration
• D. Line density

Answer: (C)

Axial resolution is about distinguishing two reflectors that lie along the beam path, so what sets it most is how long the ultrasound pulse is in space. This length, called the spatial pulse length, is determined by how many cycles are in the pulse and by the wavelength. For a given frequency (and thus a fixed wavelength), shortening the pulse by using fewer cycles reduces the spatial extent of the pulse. A shorter spatial pulse length means echoes from two closely spaced interfaces won’t blur into one reflection, so you can separate them more clearly along the axis. That’s why pulse duration is the key factor for axial resolution.

The other options affect other image aspects: beam width governs lateral resolution (side-to-side along the beam), transducer diameter influences beam divergence and focusing, and line density affects sampling on display rather than the intrinsic axial detail.