The numerical value for axial resolution is _______ the value for lateral resolution.

• A. Greater Than
• B. Not Defined
• C. Equal To
• D. Less Than

Answer: (D)

Axial resolution is about distinguishing two reflectors along the direction of the ultrasound beam and is governed by the spatial pulse length. Shorter pulses, achieved with higher bandwidth, produce a shorter pulse length in tissue, so two points closer together along the beam can be resolved, giving a smaller numerical value for axial resolution. Lateral resolution, on the other hand, depends on the width of the ultrasound beam across the beam's path. This beam width is determined by the transducer aperture and diffraction and tends to be larger than the axial dimension, so the smallest separable distance side-to-side is typically greater. Because axial resolution benefits from shorter pulse lengths (smaller numbers) and lateral resolution is limited by beam width (larger numbers), the numerical value for axial resolution is less than the value for lateral resolution.