In an ultrasound phantom, axial resolution is typically measured using which target configuration?

• A. Multiple round fluid-filled structures
• B. Multiple equidistant rods
• C. Long fluid-filled tubes
• D. Closely spaced targets of varying distances

Answer: (D)

Axial resolution is the ability to distinguish two reflectors that lie along the direction the ultrasound beam travels. It hinges on the spatial pulse length—the shorter the pulse, the better the axial detail you can resolve along the beam path. To measure this in a phantom, you place targets directly in line with the beam so that two closely spaced reflectors appear one after the other along depth. By using closely spaced targets at varying separations, you can determine the minimum axial distance at which the echoes remain distinct, which defines the axial resolution.

If you used long fluid-filled tubes, you’d get a single extended reflector rather than two separate echoes, making it hard to gauge axial discrimination. Equidistant rods arranged across the field test lateral (side-by-side) resolution rather than along the beam axis, and multiple round structures not aligned with the beam don’t specifically reveal axial separability. So, closely spaced targets of varying distances along the beam direction directly probe axial resolution.