An in-house step-wedge phantom for the calibration of pixel values in CT localizer radiographs for water-equivalent diameter measurement

Introduction: To develop an in-house acrylic-based step-wedge phantom with several thickness configurations for calibrating computed tomography (CT) localizer radiographs in order to measure the water-equivalent diameter (D_w) and the size-specific dose estimate (SSDE).

Method: We developed an in-house step-wedge phantom using 3 mm thick acrylic, filled with water. The phantom had five steps with thicknesses of 6, 12, 18, 24, and 30 cm. The phantom was scanned using a 64-slice Siemens Definition AS CT scanner with tube currents of 50, 100, 150, 200, and 250 mA. The relationship between pixel value (PV) and water-equivalent thickness (t_w) was obtained for the different step thicknesses. This was used to calibrate the CT localizer radiographs in order to measure D_w and SSDE. The results of D_w and SSDE from the radiographs were compared with those calculated from axial CT images.

Results: The relationship between PV and t_w from CT localizer radiographs of the phantom step-wedge produced a linear relationship with R² > 0.990. The linear relationships of the D_w and SSDE values obtained from CT localizer radiographs and axial CT images had R² values > 0.94 with a statistical test of p-value > 0.05. The D_w difference between those from CT localizer radiographs and axial CT images was 3.7% and the SSDE difference between both was 4.3%.

Conclusion: We have successfully developed a step-wedge phantom to calibrate the relationship between PV and t_w. Our phantom can be easily used to calibrate CT localizer radiographs in order to measure D_w and SSDE.