The fallacy of sharpness in determining digital radiographic image quality

B S White 

Sharpness is an important factor in digital radiography, contributing towards the detection of discontinuities. However, the overall image quality is dependent upon the brightness, sharpness and overall noise level in the image. Radiography can be used to detect internal discontinuities such as holes, inclusions, shrinkage and cracks. Typically, discontinuities are missing material in a part with a specific material type and thickness. When an image is displayed for interpretation, what ultimately matters is whether the relevant features are sufficiently visualised. Image quality metrics provide a description of the detection capability. Medical radiology and industrial radiography use different terms to describe image quality, however both terminologies illustrate the same phenomenon. A discussion of detective quantum efficiency (DQE), modulation transfer function (MTF), noise power spectrum (NPS), basic spatial resolution (SRb), signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR) and equivalent penetrameter sensitivity (EPS) will be presented as part of this paper.