Which measurement is used to assess axial resolution in a phantom?

• A. Lateral width
• B. Depth range
• C. Closely spaced targets along the beam
• D. Elevational width

Answer: (C)

Axial resolution is about distinguishing two reflectors that sit along the ultrasound beam’s path, in depth. In a phantom, you test this by placing two tiny targets one behind the other along the beam and varying how close they are in the depth direction. When the separation is large enough, you see two distinct echoes; when the targets are too close, their echoes merge into one. The smallest separation that still yields two separate echoes defines the axial resolution. The other measurements don’t test this axis: lateral width covers side-to-side resolution, elevational width relates to slice thickness in the out-of-plane direction, and depth range is about how deep you can image, not how closely two echoes along the beam can be resolved.