Evaluation of CT calibration curves from stoichiometric and tissue substitute methods according to tissue characteristics

Abstract

In order to optimize and increase the precision of dose determination, it is important to obtain the relationship between CT Hounsfield units and electron densities, as the basic inputs for radiotherapy planning systems. Therefore, the aim of this study was to plot and compare the CT calibration curves resulted from both tissue substitute calibration (based on the tissue equivalent materials) and stoichiometric calibration (based on biological tissues) methods for a specific CT scanner at tube voltage of 120 kVp by applying a precise homemade physical phantom. Hence, the Hounsfield units of all phantom materials were measured experimentally and also calculated by related formulas. Considering the CT numbers and the electron densities of various tissue equivalent materials and real tissues, the calibration curves were plotted. According to the results, the higher accuracy of the stoichiometric method relative to tissue substitute method was confirmed in this study. On the other hand, since estimating Hounsfield unit of each human tissue strongly depends on the physical properties of tissue substitutes of the physical phantom, the high precision of this phantom led to increase the accuracy of the calibration curves obtained based on the stoichiometric method.