FIELD LOCATIONS BY COMPUTER SIMULATION

Carlos (SP) -BrasilABSTRACTThis paper proposes a method of evaluating X-ray tube focal spots and the corresponding image sharpness bycomputer simulation based on the transfer functions theory. This theory was chosen due to its quantitative as well asqualitative response for the radiographic systems performance, which provides less subjective evaluations and betterpredictions about the characteristics of the imaging process. The present method uses as input data the effective focal spotdimensions in the field center and the value of the target angulation. An ideal pinhole which scans the entire radiation field issimulated. It allows to obtain the Point Spread Functions (PSFs) for any region of interest. The Modulation TransferFunctions (MTF5) are then determined from two-dimensional Fourier transformation from the PSFs. This provides toevaluate the focal spot projection in all field locations and therefore to predict the sharpness of the associated image.Furthermore the computer simulation reduces greatly the number of practical procedures required for obtaining the datawhich provides the MTF evaluation of radiographic systems.Keywords: Modulation Transfer Function; Point Spread Function; radiographic systems quality assurance.1. INFRODUCTIONQuality assurance of. radiologic images has been considered essential to radiologic departments since the quality ofthe information provided to the radiologist is directly related to the operational conditions of the system. Hence, sincedecades ago, investigators have been trying to improve the techniques dedicated to this aim.The most important causes for the poor resolution of a radiographic system are the size and the shape of the focalspot. They are responsible for a region known as "penumbra" in the image. Penumbra is the blurring in the borders of theimage from any object, and it is caused by the finitedimensions of the focal spot. This blurring is avoidable only if the objectis in contact with the image plane, but it hardly occurs in the practice conditions of an actual exam, where the displacementof anatomical structures to be radiographed is common. In addition, as the radiographic systems are not isoplanatic, theimage from an object displaced along the radiation field will be different for each location"2.This well known effect of the focal spot size and shape on the radiographic image quality has let many researchersto propose different methods in order to determine the focal spot size of radiographic systems. They have also investigatedthe relationship between this size with the image quality since the 50's as by experimental procedures as by computersimulations. From these techniques, those which make use of the transfer functions basis are considered the most accuratebecause they provide not only qualitative but also quantitative data about the frequency response of the s1stems. Hence, theyallow less subjective analysis, as well as predictions relative to the imaging process of particular objects3'.The transfer function theory was applied firstly to the analysis of communication systems in electronics. Latter, itwas extended and fitted to several areas, as radiography systems. The transfer functions allow to investigate the systems