X-Ray Tube Focal Spot Assessment Using Pinhole Digital Radiography with a Compact Detector: Signal Extraction and Mechanical Design
Abstract
Purpose
The x-ray tube focal spot is a key determinant of imaging system resolution. Digital pinhole radiography yields the source point spread function (PSFsource) in addition to the nominal size. Using a compact digital detector of a type used in dentistry and noting the IEC Standard 60336:2020 pinhole radiography requirements, we investigate extraction of faint image patterns in the presence of x-ray statistics and detector noise, and design a robust and versatile mechanical system for source-pinhole-detector alignment.
Methods
A CsI(Tl)/CMOS 20-µm 1000x1500 element detector (Hamamatsu S11684-12) and pinholes of 75 and 30 µm (RaySafe) were used to evaluate a Picker Dunlee PX412A/DU304 x-ray tube, focal spot nominal values 0.6, 1.2, by summing multiple exposures acquired in detector manual mode. Fixed pattern noise from the row-sequential readout was minimized via normalization (MATLAB). Several mechanical designs were considered for alignment plus measurement of enlargement factor (image size/source size) via a scale object when direct distance measurement is obstructed.
Results
The detector software’s standard flat-field normalization was insufficient to remove background degradation of faint focal spot patterns. Normalizing the focal spot pattern, using regions external to the focal spot in the same image, provided better data. Most IEC requirements were met, but SNR > 100 was not. A camera structure has been designed but not yet fabricated. The design comprises stackable aluminum box frames of heights 10, 20, 40 cm, the combinations of which will give pinhole-to-detector distances 10, 20, 30, ..., 70 cm.
Conclusion
Digital focal spot pinhole radiography, employing multiple exposures and fixed pattern normalization using part of same image, gives directly PSFsource. Due to low focal spot image fluence the IEC requirement on SNR is difficult to meet, but in any case, measurement uncertainty can be propagated to the final nominal size. A versatile camera mechanical system has been designed.