TH‐EF‐BRA‐08: Extending the Concept of Weighted CT Dose Index to Elliptical Phantoms of Various Aspect Ratios

Abstract

Purpose: Standard CT dosimetry phantoms are currently perfectly circular cylinders. Lately elliptical cylinders have been proposed as an alternative because they better represent the shapes of the human body and allow the evaluation of tube current modulation systems. The purpose of this study was to extend the concept of CTDIw to elliptical phantoms of various aspect ratios. Methods: Based on published adult and pediatric data, eight body aspect ratios were chosen from the full range between 1 (perfectly circular) and 1.72 (extremely elliptical). For each value, two elliptical cylinders were created digitally to represent adult and pediatric bodies. They had the same cross‐sectional areas as the standard 32‐cm and 16‐cm CTDI phantoms. For each digital phantom, CTDI₁₀₀ at central and peripheral locations were simulated for tube voltages between 70–140 kVp using a Monte Carlo program previously validated for a clinical CT system (SOMATOM Definition Flash, Siemens Healthcare). The simulation also output the average dose over the cross‐sectional area, Dxsec, the quantity CTDIw is intended to represent. Values of Dxsec and CTDI₁₀₀ allowed linear systems of equations to be established, from which central and peripheral weighting coefficients were solved. Results: All elliptical phantoms had the same Dxsec as the standard circular phantoms. Regardless of phantom shape, only two weighting coefficients (w1 and w2) were needed: w1 for central CTDI₁ ₀₀ and w2 for the average of the four peripheral CTDI₁₀₀'s. For perfectly circular phantoms, w1 and w2 were 0.37 and 0.63, respectively, agreeing well with the conventional weighting coefficients of 1/3 and 2/3. Over the full range of aspect ratios, w1 increased linearly from 0.37 to 0.46, whereas w2 decreased linearly from 0.63 to 0.54. Conclusion: CTDI₁₀₀ weighting coefficients are linear functions of phantom aspect ratio, allowing the concept of CTDIw to be easily extended to elliptical phantoms of various shapes. This research is supported in part by a Faculty Startup Fund from Cleveland State University.