The Spatial Frequency Spectrum: A Means for Studying the Quality of Radiographic Imaging Systems

Abstract

introduction: general considerations regarding radiographic image quality The basic purpose of radiographic imaging systems is to produce, with a minimum of patient exposure, images of such quality that diagnostically important detail can be seen readily by the radiologist. Systems designers and radiologists are confronted daily by the question: Does the system which I am designing or using fulfill its basic purpose better than any other system I might be designing or using? Usually, the question takes this short form: Is the image quality of this system better than that of other systems? In this short form the question is deceptively simple and potentially misleading since closer examination of the foregoing remarks reveals that in reality it is immensely complex, involving not only the imaging system itself but, by implication, what we shall call the entire radiological process. This is indicated by three key phrases contained in the foregoing description of the basic purpose of radiographic imaging systems: “to produce images,” “diagnostically important detail,” and “seen by the radiologist.” In a general manner, the radiological process can be looked upon as a process involving exposing, recording, visual detection, and recognition operations as outlined in Figure 1. The exposing operation involves only the x-ray tube and the object. It is affected by the geometry of the exposure setup, the energy of the x rays, and the absorption and scattering properties of the object being radiographed. The result of the exposing operation is the input to the recording system. The recording operation is performed by the imaging and recording system and, thus, is affected by the system's transfer characteristics, which are a set of parameters determining the system output (the radiographic image) for a given input (the x-ray pattern). The visual detection operation involves the viewing system, which includes the eye of the observer and the illuminator or other optical systems, and is affected also by the ambient illumination. The recognition operation takes place in the brain of the radiologist and involves such intangibles as comparison of a given radiographic image with a remembered “normal” or standard image established through education and experience, and the pattern-recognition ability of the radiologist. The sharp division between visual detection and recognition postulated here may appear to be rather artificial since these operations are closely interwoven in practice. It is certainly true, however, that the significance of radiographic images cannot be recognized by the radiologist unless they are first recorded detectably.