|NDT.net May 2004 Vol. 9 No.05|
| CT-IP 2003 Proceedings
Unsharpness characteristics of digital detectors for industrial radiographic imagingU. Zscherpel, K. Osterloh, U. Ewert, BAM Berlin
SummaryThe 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.
The experimental set-up is shown in Fig.1. A X-ray tube Seifert Isovolt 450/10 (1.8 mm nominal focal spot) was taken as a radiation source. The focus detector distance was kept always at 3m to obtain a geometrical unsharpness, which is sufficiently small for object detector distances of 200 mm. All investigations have been done at two different radiation qualities :
An AGFA D4 Vacupac (digitised with 50 ľm pixel size), a CR system FUJI XG-1 (100 ľm pixel size) and a flat panel detector AGFA DirectRay (139 ľm pixel size, direct conversion based on amorphous Selenium, am-Se) were used as radiation detectors.
Object scatteringEdge profiles of an aluminium step-wedge in dependence on the object detector distance are shown in Fig. 2. An explanation for the distorted shape is given below in Fig. 3.
The detector (regardless if film, imaging plate or flat panel) receives any radiation from all directions in space, without distinguishing between primary and scattered radiation. The latter one is most intensive at the object surface and decreases from there on in all directions with the square of the distance. Thus scattered radiation is most disturbing if objects are close to the detector. This effect can be measured preferably at edges. Outside the shadow of the object X-ray radiation hits directly the detector and scattered radiation, generated within the object, adds to the dose received. Inside the shadow close to the rim scattered radiation is lacking from absent mass outside the object body. This results in an edge enhancement as shown in the experimental results (fig. 2a c). By increasing the distance between object and detector (200 mm in fig. 2 d - f) it is possible to reduce the scattered radiation from the object according to the 1/R2 distance law so far, that it has no noticeable influence on the detector.
The following dependencies were observed regarding this effect :
As a consequence of the effect of object scattering, it is necessary to observe a minimum distance between object and detector or a suitable intermediate filtering. This will reduce the distortion of edge responses by scattered radiation (long-range influence in the range of some cm!) from the object.
Detector scatteringIn addition to the scattered radiation originated from the object further scattered radiation may be generated directly in the vicinity of the detecting layer inside the detector casing that also influences the edge response. In Fig. 5 the altering edge response of a Pt or Cu edge is shown depending on the radiation quality. A film AGFA D4 Vacupac was used as detector. Caused by the small thickness of this detector (0.6 mm overall thickness: plastic bag, 2 lead screens and the film itself), the effect of scattering inside this detector is low and no substantial difference can be seen in fig. 5 between 100 kV and 220 kV + 8 mm Cu.
In contrast, Fig. 6 shows edge profiles visibly modified by the radiation quality (from 100 kV to 220 kV + 8 mm Cu) in case of the flat panel detector AGFA DirectRay. Symmetrical spurs (wings), spreading over larger distances in the cm-range, are generated increasingly with raising radiation energies. The total thickness of this detector is about 45 mm (4 mm carbon fibre composites (CFC) protection plate, 0.5 mm Se detection layer, several glass layers with detector electronics, as well as electronic boards and a mm-thick Pb-layer to protect the electronics from X-ray radiation).
A similar effect was observed for imaging plates when using with CFC cassettes without Pb screens (see Fig. 7). Symmetrical spurs on the edge within cm-range are generated by the adjacent cassette, which is provided for easy handling and automated read-out by the CR reader.
For comparison, the Pt edge response was studied with a D4 Vacupac film, having the film directly behind the CFC protection plate that has been dismounted from the flat panel detector (see Fig.8). An increasing effect on the spurs is evident like in CR cassettes. The flat panel with and without the CFC protection plate did not show any difference. No further influence could be seen by the protection plate. This gave reason to assume that the scattering effects were generated from scattering layers within the detector.
Fig. 8 shows the influence of cassettes and objects near the detector on the edge response profiles. This is a simple test for the determination of additional scatter unsharpness from the environment surrounding the detector.
While the image sharpness of the edge (or slope of the edge function) is determined by scatter effects within the detector layer itself (inner detector unsharpness, Fig. 9a), an additional scatter contribution is arising from detector surrounding material layers leading to a secondary, strongly smeared image of the radiation profile in the detector plane (Fig. 9b). The superposition of both scattering effects results in an edge profile as measured (Fig. 9c), i.e. a steep slope at the edge rounded with extended spurs. Such an superposition has not yet been reported in literature nor any theoretical description, e.g. by Monte-Carlo simulations of the scatter contributions (in analogy to /2/). The described effect can be also measured on flat panel detectors from other manufacturers and with different working principles than described here (e.g. indirect converting flat panels by Perkin-Elmer or Varian with fluorescence screens as radiation detector).
Effect on MTF measurementWhile the effect of long-range spurs is evident in the edge response function (see Fig. 10a), it can be neglected for the corresponding line spread function (see Fig. 10b, by differentiation of the edge spread function). This is the result of the high-pass property of the derivative.
The normalized magnitude spectrum of the Fourier transform of the line spread function (the modulation transfer function, MTF, see Fig. 11) reflects the long-range spurs of the edge profile as steep decay (like a peak) at low spatial frequencies. Usually the value of the 20% MTF decay is taken for determination of the spatial resolution of the detector. This characterises (without disturbing influences) the inner detector unsharpness (i.e. the steep slope of the edge). The 20% value of the MTF is shifted considerably to lower spatial resolutions by this additionally scattering contribution, i.e. the spatial resolution of the detector is apparently reduced.
Consequences for CT reconstructionThe long-range spurs of the edges caused by scattering parts within the detector device raise additionally artefacts in the reconstructed images from 3D-CT equipment, whenever flat panel detectors are employed together with higher X-ray tube voltages (> 200 kV, as usual in industrial applications). Artefacts, pretending apparent material closely outside the real object, are generated in the CT reconstruction result (see Fig.12), or vice versa, in thin walled regions more transparent material or even a cavity may appear visually. Solutions for this problem are discussed in /3,4/. Compensation of the long-range constituents of the unsharpness caused by scattering within the detector device, e.g. by deconvolution, can be achieved successfully if the radiation quality (radiation spectrum) depends only marginally on the position in the detector plane, if hardening effects from dense material layers can be neglected.
ConclusionsTwo separate scattering processes were identified as superimposed in radiographs. They can be related to different sites of origin (inside object or radiation detector) and appear in images of 2D detectors in different ways, as follows:
AcknowledgementThe authors thank C. Bellon for helpful discussions and MCNP calculations.