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This work presents attenuation and sensitivity measurements of radiographic imaging plates (IPs) with quasimonoenergetic X-rays in the 8–60 kiloelectronvolt range. The measurements are used to validate theoretical predictions. A short overview of the theoretical model is given. The model can be used to describe the sensitivity of different detector types to a wide range of X-ray energies.
Concepts for digital image evaluation are presented for Computed Radiography (CR) and Digital Detector Arrays (DDAs) used for weld inspection. The precise DDA calibration yields an extra ordinary increase of contrast sensitivity up to 10 times in relation to film radiography. Restrictions in spatial resolution caused by pixel size of the DDA are compensated by increased contrast sensitivity. First CR standards were published in 2005 to support the application of phosphor imaging plates in lieu of X-ray film, but they need already a revision based on experiences reported by many users. One of the key concepts is the usage of signal-to-noise (SNR) measurements as equivalent to the optical density of film and film system class. The contrast sensitivity, measured by IQI visibility, depends on three essential parameters: The basic spatial resolution (SRb) of the radiographic image, the achieved signal-to-noise ratio (SNR) and the specific contrast (µeff - effective attenuation coefficient). Knowing these 3 parameters for the given exposure condition, inspected material and monitor viewing condition permits the calculation of the just visible IQI element. Furthermore, this enables the optimization of exposure conditions. The new ISO/FDIS 17636-2 describes the practice for digital radiography with CR and DDAs. It considers the first time compensation principles, derived from the three essential parameters. The consequences are described.
The presently active standards on Computed Radiography (CR) need a major revision. It was observed by many users that the image quality for class B of EN 14784-2 is not achievable under the same exposure conditions as used for film exposure. A mathematical model was developed and tested, which allows the calculation of the image quality, proven by image quality indicators (IQI), depending on the fixed pattern noise and the efficiency of the used imaging plate (IP) scanner system. All tested IP scanner systems provide a fixed correlation between the measured signal/noise ratio (SNR) and the grey values in the digital images. The maximum achievable SNRmax depends on the fixed pattern noise of the plate (high dose limit). Depending on the exposure dose an optimum visibility of IQIs can be predicted by calculation and measured considering the attenuation coefficient for calculation of the contrast/noise ratio (CNR). The diameter of the just visible wire is proportional to 1/sqrt(CNR). The optimum tube voltage for best visibility of IQIs and maximum CNR depend on the exposure dose. The optimum tube voltage for best visibility is achieved only at exposures with high dose. A dose dependant optimal tube voltage was not observed for film radiography.
Neutrons are absorbed particularly by hydrogen containing materials so
they can be used as a tool for visualising heterogeneous density distributions of organic materials. Penetration limits are set by the layer thicknesses and the neutron energies applied. In case of specimens with a flat shape the situation may be encountered that the object only could be penetrated in selected directions. In addition, the overall size may exceed the beam diameter and thus the viewing window if a certain region of interest should be studied by tomography without destroying the integrity of the specimen.
This study shows the capabilities and limits of thermal and fast neutrons to investigate flat wooden specimens such as boards and girders by neutron tomography under such circumstances. Taking projections was impaired either by the limits of penetrability or by the total size of the object. As a consequence, projections were included for reconstruction only from a limited angular range of 90°. It could be shown that an approach based on the slice theorem was capable to visualise structural features along the beam directions while simply omitting the perpendicular ones without causing additional artefacts.
Small samples with a thickness of up to 2 cm but several times broader could be studied with the ANTARES facility of the FRM II neutron source in Garching providing thermal neutrons while larger objects required a beam of higher energy as available in the NECTAR facility of the same institution. The fast (fission) neutrons (1.5 – 2 MeV) of this site allowed investigating an area of interest inside a girder with a cross section of 23.5 x 49 cm². Internal features such as inclusions could be detected as well as a heterogeneous density distribution in glue layers.