The rods depicted by the arrows in this phantom are used to determine which feature of the machine?

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

Answer: (D)

The rods test axial resolution, which is the ability to distinguish two interfaces that lie along the path of the ultrasound beam. In this phantom, the rods are oriented in the depth (along the beam) direction, so separating two closely spaced rods depends on how short the transmitted pulse is. A shorter pulse (fewer cycles, higher frequency) gives a shorter spatial pulse length, improving axial resolution. If the rods are closer than this limit, their echoes merge and you can’t tell them apart; if they’re farther apart, you’ll see two distinct echoes. This setup wouldn’t assess lateral resolution (which depends on beam width across the image) or penetration (which relates to how deep the signal can go before it becomes too weak) or contrast resolution (which concerns distinguishing differing echo intensities), so axial resolution is the best fit for what those rods are measuring.