Extending the concept of weighted CT dose index to elliptical phantoms of various aspect ratios.

The purpose of this study was to extend the concept of weighted CT dose index (CTDIw) to the elliptical phantoms. Based on the published body dimension data, eight body aspect ratios were chosen between 1 (perfectly circular) and 1.72 (extremely elliptical). For each aspect ratio, two elliptical cylinders were created digitally to represent adult and pediatric bodies. Their cross-sectional areas were identical to the standard 32- and 16-cm CTDI phantoms. For each phantom, CTDI100 at center and periphery were simulated for tube voltages between 70 and 140 kVp using a validated Monte Carlo program. The simulations also provided the average dose over the cross-sectional area, CTDIxsec. Values of CTDIxsec and CTDI100 allowed linear systems of equations to be established, from which central and peripheral weighting coefficients were solved. Regardless of phantom shape, only two weighting coefficients were needed: w(1) for the central CTDI100 and w(2) for the average of the four peripheral CTDI100's. Over the full range of aspect ratios, w(1) increased linearly from 0.37 to 0.46, whereas w(2) decreased linearly from 0.63 to 0.54, allowing the concept of CTDIw to be readily extended to the elliptical phantoms. When cross-sectional area (hence volume) was kept constant, all phantoms had the same CTDIxsec regardless of shape. (C) 2017 Society of Photo-Optical Instrumentation Engineers (SPIE)