Comparison between two methods for estimation of Equilibrium Dose in MSCT procedures

Abstract

The CTDI100 underestimates the dose indicator due the scattering beyond the scanning length of 100mm of the measurement. A proposed method to obtain the dose (including the “tail” of the dose distribution) is a measurement on a large phantom to estimate the dose indicator on an ideal infinitum large phantom (equilibrium dose). As the scanning length increases, the dose indicator grows asymptotically. Measuremente previously obteined with three tied phantoms of PMMA were adopted in the present work. It includes a set of length measured for different combinations of protocols of voltage, pitch, diameter and axis of measurement. Using an empirically characterized function of the experimental data, two methods to estimate the equilibrium dose were evaluated. One method is manipulating algebraically of the function, and another is the use of Levenberg-Marquardt least-squares. The comparative evaluation of both methods demonstrate that in higher values of the pitch results in a higher difference of both methods. The results demonstrate a good compatibility of the methods for the estimation of the quantity Deq. However, the results for alfa and Leq are significantly different when both methods are considered.