tissueequivalent

Tissue-equivalent refers to materials whose radiological interaction with ionizing radiation resembles that of human tissue. Such materials are used to design phantoms for dosimetry and imaging and to calibrate equipment. Equivalence is energy dependent: a material may be tissue-equivalent for diagnostic X rays but not for high-energy therapy beams, and no single substance perfectly mimics all tissue types across all modalities.

Key properties include electron density, effective atomic number, and mass density, which govern photon attenuation, scatter, and charged-particle stopping power. Designers select materials so that their attenuation coefficients and scattering match those of the target tissue within the energy range of interest. In practice, water is the simplest reference tissue, and a variety of polymers and plastics—such as A-150 tissue-equivalent phantom material, PMMA, and polystyrene—are used to approximate soft tissue. Bone-equivalent substitutes use higher density materials and higher effective Z.

Applications include construction of dosimetric and imaging phantoms for radiotherapy QA, CT and MRI calibration, and experimental studies of radiation interactions. Phantoms may be homogeneous or anthropomorphic to reproduce organ geometries and tissue composition. Standards and recommendations come from professional bodies such as ICRU, IEC, and ASTM, which define acceptable material properties, reference phantoms, and calibration procedures. The choice of tissue-equivalent material depends on the modality, energy range, and required accuracy, balancing manufacturability, durability, and compatibility with detection systems.