Evaluating the focal spot MTF in all field locations by computer simulation

This paper proposes a method of evaluating x-ray tube focal spots and the corresponding image sharpness by computer simulation based on the transfer functions theory. This theory was chosen due to its quantitative as well as qualitative response for the radiographic systems performance, which provides less subjective evaluations and better predictions about the characteristics of the imaging process. The present method uses as input data the effective focal spot dimensions in the field center and the value of the target angulation. An ideal pinhole which scans the entire radiation field is simulated. It allows to obtain the point spread function (PSFs) for any region of interest. The modulation transfer functions (MTFs) are then determined from 2D Fourier transformation from the PSFs. This provides to evaluate 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 data which provides the MTF evaluation of radiographic systems.