Which parameter measures the ability to distinguish two structures that are close along the beam direction?

• A. Axial resolution
• B. Lateral resolution
• C. Elevational resolution
• D. Penetration

Answer: (A)

The ability to distinguish two reflectors that are close along the beam direction is governed by axial resolution. This metric depends on how short the transmitted pulse is in space, which is tied to spatial pulse length (the number of cycles in the pulse and the wavelength). Shorter pulses (higher frequency or fewer cycles) yield shorter spatial pulse lengths, allowing two closely spaced interfaces along the depth of the beam to be resolved as separate echoes. If the separation is smaller than the axial resolution, their echoes overlap and they appear as a single reflection; if it’s larger, you’ll see two distinct interfaces. Lateral resolution, by contrast, concerns structures separated perpendicular to the beam and is limited by beam width. Elevational resolution deals with thickness in the third dimension (slice thickness), and penetration relates to how deep the ultrasound can image—not to resolving structures along the beam.