Effect of X-Ray Tube Focal Spot Size on Radiovisiograph Resolution

Abstract

IntroductionThe goal of this work was to discuss recent informa tion available from international sources indicating that X ray focal spot size has an effect on the quality of images obtained using radiovisiographs [1,2]. It was reported in [2] that focal spot size reduction from 0.8 × 0.8 to 0.4 × 0.4 mm increased sharpness of dental images.In the domestic medical literature the problem of correlation of X ray focal spot size and image quality had been discussed in detail [3]. The effects on the image sharpness, contrast, exposure dose, and power consump tion were discussed for the case of a dental X ray tube with diameter of 0.1 mm [4]. In collaboration with one of the leading domestic manufacturers of X ray equipment, further research was performed to study the special case of dental radiovisiography. MaterialsStandard contact X ray was used in the study [5]. The X ray focal spot diameter d was 1 mm. The distance f from the object to the X ray source was large, while the distance to the detector was very short to provide contact X ray. The focal spot diameter d and distance f from the object to the X ray source exerted a significant effect on image quality (sharpness). The distance f was selected to meet the requirements for X ray image sharpness taking into account the actual values of focal spot diameter d 1 and object thickness. Even slight changes in the distance Δf resulted in considerable reduction of the image sharpness.The ratio ( f + Δf ) determines magnification of the image of the object m: At focal spot diameter d and detector resolution R n the maximal resolution is calculated from [6]:This equation shows that the maximal resolution of modern dental systems depends on focal spot diameter. The dependence for the typical value of R n = 20 line pairs per mm is given in Table 1.Focal spot diameter d = 0.1 mm and R n = 20 line pairs per mm provide a 10% increase in the X ray system resolution relative to contact X ray with d = 0.7 mm. Focal spot diameter d = 0.05 mm provides a 1.4 fold increase in the X ray system resolution. MethodsContact X ray of a reference test pattern and a dental X ray of a skeletonized mandible with soft tissue phantom