Elevational resolution (ultrasound)

Elevational (azimuthal) resolution represents the extent to which an ultrasound system is able to resolve objects within an axis perpendicular to the plane formed by the axial and lateral dimensions. As one component of overall spatial resolution, the elevational axis represents the height or “thickness” of the beam itself ¹. 

Explanation

Elevational resolution is governed by beam height in the same way that lateral resolution is governed by beam width; two discrete objects returning echoes from a single beamline will falsely appear conjoined on visualisation ². Beam height itself depends on the height of the individual piezoelectric transducer elements, which is fixed for a given transducer ¹. Overall, reduced transducer element height equates to reduced beam height and thus, improved elevational resolution (via reduced requirement for volume averaging during image processing). 

Use of a fixed focal point acoustic lens across an entire linear array surface allows ultrasound beamlines to be focused in the elevational axis ^2,3. As such, beamline height is minimised (and elevational resolution maximised) at the lens' focal point. Objects visualised in the near or far field, relative to this focal point, will be of significantly poorer elevational resolution, due to being captured in a comparatively taller beam. It is this phenomenon which contributes to section-thickness (partial-volume) artifact, and the clinical implications thereof ¹.

1.5-D transducers utilise multiple (commonly 5-7) stacked rows of linear transducer arrays, forming a matrix with a different focal point in the elevational axis for each individual element. This allows for dynamic focusing of beamlines in the elevation dimension, with the goal of minimising beamline height (and thus maximising elevational resolution) across a wide range of distances ².