Unsharpness characteristics of digital detectors for industrial radiographic imaging

The investigation of imaging properties of new digital detectors such as imaging plates (used in computed radiography, CR) or flat panels (used as film replacement or in computed tomography, CT) showed energy dependent unsharpness effects. This image unsharpness has essential influences on applications in NDT. Particularly, in CT the precise knowledge of the detector unsharpness characteristics is critical for the evaluation of resulting images in NDT applications. For this reason edge response functions have been determined in dependence on radiation energy as a measure of imaging unsharpness. This analysis reveals a superposition of two distinct unsharpness effects: one effective within a short range (low unsharpness) and another one covering a larger area (high unsharpness). This has considerable consequences to quantitative analysis of unsharpness, as determined either by edge or slit imaging or by MTF calculations. Particularly at lower spatial frequencies the MTF decay is influenced, i. e. the unsharpness is spreading over a larger distance in the X-ray image. At higher spatial frequencies the MTF decay of the detectors does not show any peculiarity. Spatial resolution as measured by line pair or duplex wire IQIs is dominated by the short distance effect. The long distance one can hardly be detected visibly in film images. However, the long range component is clearly measurable for digitised films, for imaging plates and also for flat panel detectors. The long range unsharpness is also one of the major effects, which are responsible for distortions in 3D-CT. MTF and edge response measurements are described for determination of additional unsharpness functions by scattered radiation from material adjacent to the detector layer, e.g. flat panel casings, film and CR cassettes