High spatial resolution measurement of the X-ray intensity pattern in a radiograph of the thorax

The X-ray intensity pattern in a radiograph of the thorax was measured. The measurements were carried out in the detector plane with high spatial resolution. A special phantom prepared from the lung of a dead body in which all fine details were existing was used for these measurements. This phantom was exposed to X-rays applying the standard examination conditions for thoracic imaging according to the "European Guidelines on Quality Criteria for Diagnostic Radiographic Images", and the image was stored on industrial X-ray film normally used for non-destructive testing. These images were digitized with a high-resolution drum film scanner. It is shown that the image degradation by the industrial X-ray film and the scanner was very small in the range of the estimated spectrum of the intensity pattern and that no problems;with aliasing due to the scanning process would arise. The characteristic curves of the film and the scanner were evaluated. The X-ray intensity pattern of the thorax was calculated on the basis of these curves. Different methods to evaluate the information content of the image in the frequency domain are presented. The results obtained by these methods have been used to propose requirements for digital detector systems.