Focal spot size is one of the crucial factors that affect the image quality of any x-ray imaging system. It is, therefore,
important to measure the focal spot size accurately. In the past, pinhole and slit measurements of x-ray focal spots were
obtained using direct exposure film. At present, digital detectors are replacing film in medical imaging so that, although
focal spot measurements can be made quickly with such detectors, one must be careful to account for the generally
poorer spatial resolution of the detector and the limited usable magnification. For this study, the focal spots of a
diagnostic x-ray tube were measured with a 10-μm pinhole using a 194-μm pixel flat panel detector (FPD). The twodimensional
MTF, measured with the Noise Response (NR) Method was used for the correction for the detector blurring.
The resulting focal spot sizes based on the FWTM (Full Width at Tenth Maxima) were compared with those obtained
with a very high resolution detector with 8-μm pixels. This study demonstrates the possible effect of detector blurring on
the focal spot size measurements with digital detectors with poor resolution and the improvement obtained by
deconvolution. Additionally, using the NR method for measuring the two-dimensional MTF, any non-isotropies in
detector resolution can be accurately corrected for, enabling routine measurement of non-isotropic x-ray focal spots. This
work presents a simple, accurate and quick quality assurance procedure for measurements of both digital detector
properties and x-ray focal spot size and distribution in modern x-ray imaging systems.