Resolution and information limitations from transducer arrays.

Abstract

The factors limiting the resolution achievable with array-based imaging methods are considered. The limitations on beamwidth and field of view for beam scanning systems are first examined using plane wave diffraction theory, treating focusing and any array curvature as separable to the plane wave analysis. It is found that in all cases the number of half-beamwidths (maximum to null) across the field of view cannot exceed the number of electrically addressable elements in the array. The minimum beamwidth achievable by strong focusing is shown to be limited to twice the inter-element pitch. Digital reconstruction is then considered as an alternative to beam scanning. Here the number of image points recoverable across the field of view is equal to the number of elements in the array. The mathematical equivalence of the two methods is demonstrated, and the inference is made that the half-beamwidth represents a fundamental measure of lateral resolution. Alternative array configurations, suitable for CT methods, are then compared to probe arrays and it is found that the total number of resolution cells within a two-dimensional image is dependent on the pulse length and number of elements, rather than on the array configuration and imaging method. This again reveals the fundamental similarity of the various methods, essentially in combining position line information from the separate elements, albeit in different orders. Finally an expression for the information capacity of a scanning system is presented which relates the limitations on spatial and contrast resolution to the bandwidth and dynamic range of the system and the number of elements in the array.