Tissue Substitutes for the Livermore Torso Phantom Suitable for a Continuous Spectrum of Low-energy Photons

Substitute materials that accurately reproduce the radiological properties of human tissues are required for direct in vivo measurement of internally deposited radioactive materials to estimate associated health risk, especially for the respiratory tract. The Livermore torso phantom, the de facto standard for calibrating detector systems that measure radioactive materials deposited in the lungs, liver, and thoracic lymph nodes, was designed with tissue substitute materials that match the density and attenuation coefficient exhibited by natural human tissue when exposed to single low-energy x rays associated with the decay of plutonium. In this study, we evaluated the radiometric tissue equivalence of new tissue substitutes for muscle, rib, sternum, lung, and cartilage that are suitable for a continuous low photon energy spectrum from approximately 30 to 120 keV. The formulation for each of the tissue substitutes was developed using a novel method that determines the optimized quantity of base material and additives to produce a material that best matches the density and photon transmission exhibited by the natural human tissue present in the thoracic cavity. Measurements of the mass attenuation coefficient (i.e., ) from approximately 30 keV up to 120 keV for each substitute tissue were within 8% or better to expected values calculated using the photon cross section database XCOM from the National Institute for Standards and Technology.