A Phantom Study of Optimal Examination Conditions for Ankle CT: Focusing on Noise and Sharpness

CT scans utilize relatively higher levels of radiation compared to conventional X-ray examinations. To address this issue and optimize imaging outcomes, various technologies have been developed, among which Automatic Exposure Control (AEC) represents a significant advancement. AEC is particularly effective in areas with considerable thickness variations, such as the thorax or abdomen; however, it may result in unnecessary radiation exposure in smaller anatomical structures, such as the extremities, which can compromise image quality. The objective of this study was to quantitatively evaluate the contrast resolution and spatial resolution under fixed tube current conditions in ankle CT scans, with the aim of identifying the optimal tube current settings. The methodology involved using a phantom to conduct scans across a range of tube currents, starting at 50 mAs and increasing in increments of 50 mAs up to 250 mAs, followed by image acquisition for subsequent analysis. Contrast resolution was assessed using Noise and Contrastto- Noise Ratio (CNR), while spatial resolution was evaluated by obtaining line profiles at the bone edges of the ankle. Measurements of angle and distance were recorded at the 25% and 75% density value points on the y-axis to facilitate comparisons of spatial resolution. The results revealed significant differences in contrast resolution between the 50 mAs and 250 mAs settings, with smaller differences noted between the 100 mAs to 150 mAs and 200 mAs to 250 mAs comparisons. Regarding spatial resolution, the most pronounced difference was observed between the 50 mAs and 100 mAs settings, with diminishing differences noted beyond the 150 mAs threshold. Consequently, image quality improved with each 50 mAs increment up to 200 mAs, with negligible differences observed between images acquired at 150 mAs and 200 mAs. These findings suggest that a tube current setting of 150 mAs may be optimal for achieving high-quality imaging while minimizing radiation exposure.