By comparing rods labeled A and B in the phantom image, which conclusion is supported?

• A. Lateral resolution has deteriorated with depth
• B. Elevational resolution has deteriorated with depth
• C. Lateral resolution is best with the rods labeled B
• D. Axial resolution is best with the rods labeled B

Answer: (A)

Lateral resolution is what lets you tell two objects apart side by side in the image, and it depends on how wide the ultrasound beam is at a given depth. As sound travels, the beam spreads out (diffracts) and becomes broader unless you use focusing. That broader beam at greater depths means two closely spaced reflectors will appear merged or less distinct sideways. In the phantom image, the rods at deeper depth look less separable than those that are shallower, showing that the ability to distinguish them laterally has worsened with depth. This is exactly what happens to lateral resolution as depth increases. Axial resolution, tied to pulse length, and elevational resolution, tied to slice thickness, aren’t what's being demonstrated by the observed change in lateral separation with depth.