Towards a resolution metric for medical ultrasonic imaging

The Rayleigh resolution criterion, or its near analogue, the full-width, half-maximum (FWHM) beam width, is not appropriate for comparing the imaging performance of scanned-beam, medical ultrasound imaging systems: these criteria ignore the low-level outlying parts of the beam that have a significant impact on the image of such wide dynamic range systems. We propose a resolution metric for such systems that is the radius of a spherical void (in a uniformly backscattering continuum) that produces a given (averaged) dip in backscattered signal power when scanned. Which beam pattern produces the best 'cystic resolution' (as we call this measure) is dependent upon the dynamic range of the imaging system. The imaging performance of several beam patterns are compared using the old resolution criteria and this new metric.