Author: Matthew R. Hoerner, Maryam Naseri, Mena Shenouda 👨🔬
Affiliation: Yale University School of Medicine, Yale University 🌍
Purpose:
Historically film has been the gold standard receptor for measurements of x-ray tube focal spot size. Flat Panel Digital Radiography Detectors (FPDRDs) have lower spatial resolution compared to film. This limitation compromises the accuracy of focal spot size measurements.
Methods:
This study compares three methods for focal spot size measurement—NEMA, TG 150, and Nickoloff ‘s empirical method—and proposes an alternative approach to correct for detector blur using a radial line pair phantom (star pattern). While NEMA and TG 150 are widely used standards, they do not account for detector blurring and use two different formulas to determine the optimal magnification. Nickoloff’s method includes an empirical correction factor for detector blur. In our method, an additional image of the star pattern is acquired directly on the detector to determine the blur profile. This correction is incorporated into the calculation of focal spot size to improve measurement accuracy. Measurements were performed using typical small (0.6 mm) and large (1.2 mm) nominal focal spots, two different spoke angle star patterns (1 and 2 degree), and FPDRDs with varying pixel sizes (139-150 µm).
Results:
Our results demonstrate that our proposed method aligns closely with Nickoloff’s empirical approach. However, both TG 150 and NEMA methods yield larger focal spot size measurements, especially for small focal spots, as neither adequately considers the effect of detector blur. The study also highlights discrepancies in TG 150's claim that the suggested magnification ensures the blur diameter is 50% of the spoke diameter. As the nominal focal spot size increases the blur diameter to spoke diameter ratio decreases.
Conclusion:
These findings highlight the limitations of current standards and emphasize the need to integrate detector blur corrections into focal spot size measurement protocols, particularly when using FPDRDs.