What is measured with a test object containing closely spaced, highly reflective targets along the...

Comprehensive and Detailed Explanation From Exact Extract:

Axial resolution is the system's ability to distinguish two structures located along the direction of the ultrasound beam (parallel to the beam axis). Test objects or phantoms contain closely spaced pins or reflectors along this axis to evaluate axial resolution.

Principles and Instrumentation state:

"Axial resolution is determined by the spatial pulse length and is tested using targets positioned along the beam axis."

Sensitivity (A) relates to detection of weak echoes.

Specificity (B) refers to diagnostic accuracy, not phantom testing.

Lateral resolution (C) is evaluated using side-by-side (perpendicular) targets.

Therefore, the correct answer is D: Axial resolution.

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