An Investigation on the Non-uniform Distribution of Radiation Intensity Output of Diagnostic X-ray Tubes

Objective: This study aimed at investigating the air kerma distribution of the radiation output in two and three dimensions of three general radiographic units by physical measurements. Methods: A 6-cm(3) ionization chamber was placed at 5-cm intervals of 225 locations in a 70 x 70 cm(2) x-ray beam field. The chamber was placed at a level of 30 cm above the x-ray table to measure the air kerma intensity of the x-ray output across thewhole area. Three different brands of radiographic units with different anode target angles and at different tube aging stages were studied. The same exposure parameters were used for each location, and measurements were made for three different tube potentials at 60, 85, and 110 kilovoltage peaks (kV[p]). Results: At the tube potential of 85 kV(p), the relative maximum percentage differences on the average air kerma measured along the central anode-cathode axis (x-axis) of the x-ray tube were found to be 31.5%, 15.5%, and 43.1% more at the cathode ends than those at the anode ends of the three radiographic units. Results of a paired samples t test (confidence intervals at 95%, two-tailed) on three of the four fixed pair-wise sampling points along the x-axis were found to be statistically significant in their relative percentage differences in air kerma intensity with respect to the central point (labeled as 35 cm) with a P value <=.03. However, those measured at four fixed pair-wise points at 90 degrees across the anode-cathode direction were all found to be statistically nonsignificant (P >=.29). Similar trends were observed for the other two tube potentials tested. From the three-dimensional displays of radiation output intensity, a similar unique pyramidal shape pattern with a skewed peak toward the cathode end was observed for all three diagnostic x-ray tubes tested. Conclusions: The anode heel effect was found to be enhanced in the xray tube with a smaller anode target angle, more aging, and higher tube potentials. Three-dimensional air kerma distribution maps of the three x-ray units tested were found to be similar in pattern, and this was displayed in detail regarding the extent of the anode heel effect.