A CMOS image sensor method of focal spot size measurement

A phosphor opto-coupled monochrome CMOS image sensor with a slit diaphragm was used to investigate focal spot characteristics. Images were captured during x-ray exposure with a triggered frame grabber and subsequently enhanced. Dimensions of the focal spot width (1.39mm) and length (1.92mm) were determined from the focal spot intensity profiles and their corresponding Full Width at Half Maxima (FWHM) in two orthogonal orientations. The CMOS image sensor measurements demonstrated differences in the measured width and length dimensions when compared to film measurements. The obtained nominal focal spot values however showed that image-sensor determined focal spot dimensions agreed with the direct film and film-screen methods when based on the AS/NZS defined nominal focal spot values. The CMOS image sensor tested appears to lack the measurement accuracy required for the measurement of small focal spot sizes due in part to its limited camera sensitivity.