SU-E-I-37: A Renewed Look at CT Numbers as Encountered in the Diagnostic and Therapeutic Range

Medical Physics
J FredrikssonH Olafsdottir

Abstract

With the development of RT treatment planning systems that use the relationship between CT number and Electron Density (ED), rather than the Linear Attenuation Coefficient (LAC) of a given material, a number of authors have pointed out the divergence between the linearity of CT numbers vs. LAC in the diagnostic range compared to the relationship between the CT number and ED. This paper will review the differences and similarities, and describe a new set of phantom test objects and automated software that can be used to automatically assess both scales. In particular the relative importance of atomic number (Z) and the relative impact of Coherent vs. Incoherent effects at high Z levels will be evaluated. A newly developed phantom (Catphan® 700) has an expanded set of sensitometric samples of known density, electron density, and chemical composition (Z effective). CT scans of the phantom were obtained at various energies (kVp) and the measured CT numbers were compared to the known physical characteristics mentioned above. Regressions between measured CT numbers, linear attenuation coefficients (with and without coherent scattering effects), and electron density for the materials were performed. Effects of different materials and ...Continue Reading

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