Evaluation Method to Determine the Focal Spot Size of a CT System with a new Designed Resolution Test-chart

State-of-the-art micro-focus X-ray sources are extensively used in industrial computed tomography systems to achieve micrometric resolution, which is fundamental when measuring for example micro-components or parts characterized by micro-features and structures. Resolution limitations are mainly connected to the size of the interaction cross-section between the electron beam and the target material (i.e. focal spot). Therefore, much effort has been put into research reducing the size of the focal spot of X-ray sources. However, the accurate measurement of the effective focal spot size is still challenging. Within this scope, an evaluation method to determine the spot size and shape of the source of a computed tomography system is explained in detail. The concepts of modulation transfer function in the Fourier regime and the full width at half maximum in the real space are used for the evaluation. Different voltages and powers are investigated, as the spot size strongly depends on these parameters. These measurements are compared to computer simulated reference values obtained with a simulated resolution chart.