What type of transducer has the same measurement for elevational and lateral resolution?

• A. Convex linear array
• B. Phased linear array
• C. Annular array
• D. Sequential linear array

Answer: (C)

Resolution in ultrasound imaging has two in-plane directions: lateral and elevational. Lateral resolution depends on how tightly the beam can be focused side-to-side as it travels, while elevational resolution depends on how thin the imaging slice is, which is tied to the transducer’s height and how well the beam is focused in the out-of-plane direction. An annular array uses concentric rings, and by applying the right delays to those rings, it can focus energy in both the lateral and elevational directions with similar beam widths. This symmetric focusing leads to approximately equal, or isotropic, resolution in those two directions at a given depth. In contrast, other transducer geometries—linear, phased, or sequential linear—tend to produce good lateral resolution but have a slice thickness determined largely by the crystal height, which makes elevational resolution poorer and not matched to lateral. So the annular array is the type that gives the same measurement for elevational and lateral resolution.