ARDMS Sonography Principles and Instrumentation (SPI) Practice Exam 2025 - Free Sonography Exam Questions and Study Guide

Axial resolution in ultrasound refers to the ability to distinguish two points that are positioned along the axis of the beam (or directly in line with the ultrasound beam). It is primarily determined by the spatial pulse length, which is defined as the length of the ultrasound pulse in the direction of propagation.

To calculate the axial resolution, one typically considers the formula that states the axial resolution is approximately half of the pulse length. This is because the smallest distance that can be resolved is related to how long the ultrasound pulse is; two structures must be separated by at least half of the pulse length to be identified as distinct entities.

In this case, if the pulse length is 4.5 mm, the axial resolution would be calculated as follows:

Axial resolution = Pulse length / 2

Axial resolution = 4.5 mm / 2 = 2.25 mm

Thus, the correct answer of 2.25 mm represents the ability to resolve two points that are 2.25 mm apart along the axis of the ultrasound beam in soft tissue. This reflects the practical limit of resolution due to the characteristics of the ultrasound pulse being used.