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In the previous article we introduced a set of test objects for quality control of digital subtraction fluorography (DSF) systems. These test objects have been evaluated using several commercial DSF systems in their clinical environment. In this second article we present test images and results obtained during these trials and explain how the test objects can be used to analyse the imaging performance of DSF systems.

A set of X-ray test objects has been developed for quality control in digital subtraction fluorography (DSF). Factors which can be assessed include dynamic range, highlight overload and signal-to-noise ratio, low contrast sensitivity, unsharpness, and systematic misregistration. These test objects complement, both in design and calibration, those previously developed to evaluate the imaging performance of television fluoroscopy and small-format fluorography systems. In this article, the first of two, we describe the Leeds DSF test objects, paying particular attention to the reasoning behind their design.

In this paper we show that the presence of structure noise, shading and vignetting limits the low-contrast performance of radiological imaging systems which incorporate an X-ray image intensifier. As these imperfections are reproduced exactly at each exposure they are eliminated, along with the patient's background anatomy, during digital subtraction angiography. We have developed an imaging algorithm, based on the digital subtraction technique, which eliminates systematic imperfections from grey-scale (radiographic) images. Preliminary studies indicate that this algorithm, which we call “digital greyscale fluorography” (DGF), offers new opportunities to apply digital imaging techniques in classical radiography.A radiologist's ability to discriminate low-contrast lesions is limited by, among other factors, the power of the noise in the diagnostic image. If a single static image is recorded via a digital X-ray image intensifier television system, four sources of noise can be identified: X-ray quantum mottle, electronic noise arising within the television channel, noise arising from the digitisation process and fixed-pattern or structure noise (Hay & Cowen, 1981). Structure noise arises from localised and generalised inhomogeneities in the sensitivity of the various image transfer processes within the X-ray image intensifier and in the photoconductive target layer of the television camera tube. In addition, vignetting and irregular shading due to the radiation field and the coupling optics introduce a powerful contribution of low spatial frequency noise that limits the degree to which it is possible to take advantage of display windowing techniques.