Investigations of various gamma radiation interaction parameters of human tissues and their tissue substitute materials for dosimetric applications

Highlights

• Values of energy fluence rate are evaluated.

• CT number for various tissues and their tissue equivalent materials are determined.

• Kerma (Jkg^-1) values for tissue equivalent materials (TEMs) of muscle, cortical bone and lungs.

• Effective atomic numbers (Z_eff) to tissue equivalent materials (TEMs) are assigned.

Abstract

The aim of present work is to examine the radiation interaction properties of muscle, cortical bone, lungs and their tissue equivalent materials in variety of radiation based dosimetric applications. The radiation interaction parameters like photon fluence rate, energy fluence rate, kerma, relative kerma, effective atomic number (Z_eff) and computed tomography (CT) number for tissues and their various equivalent materials for energies 60-, 81-, 122-, 184-, 356-,511-, 662-, 834- and 1120-keV respectively are investigated. Effective atomic numbers (Z_eff) of tissue equivalent materials have been assigned using anon-destructive experimental technique of multiple back-scattering of 662 keV photons. The gamma radiation flux of 662 keV photons is detected by 3″x 3″ NaI(Tl) scintillation detector. The numbers of multiply backscattered events, for all the tissue equivalent materials (TEM), are found to be increasing with thickness of TEM and finally saturates. The saturation thickness helps in assigning a precise value of effective atomic number Z_eff for tissue equivalent materials. Monte Carlo simulations support the results of present experiments. Kerma and relative kerma for tissue equivalent materials are evaluated for various gamma photon energies available from standard radioisotope sources. CT number of these tissues and their equivalent materials are evaluated for the first time for which no data are available for comparison with present results.

Introduction

The qualitative measurements of ionizing radiation interaction parameters are undertaken either to establish or to use numerical relationship between them and the biological, physical or chemical effects these radiations produce in them. These effects can only occur with transfer of energy from ionizing radiation to some irradiation material or human tissue. To study the dose received by tissue of any part of human body, when exposed to ionization radiation, several materials have been used to simulate the human tissues and organs. In radio-biological and medical applications, the study of interaction of gamma ray photons is of great importance for exact calculation of radiation dose. The criteria for proper selection of tissue equivalent materials (TEM) are always a challenge to researchers. For their selection, tissue substitute should have the same radiation interaction properties as human tissue and should have precise value of photon interaction parameters. The ICRU report 44, gives the results of various studies on the selection and utilization of tissue equivalent material (TEM) used in dosimetry for diagnostic radiology and radiotherapy applications (International Commission on Radiation Units and Measurements, 1989).

Kienbock (Kienbock (1906) started tissue simulation work on aluminium and concluded that absorption power of 1 mm aluminium foil is equivalent to 10 mm thick water slab and muscle layer. Westman (1924) moulded the wax in the shape of human body and called it as body phantom. Rossi and Failla (1956) prepared many liquids and gels by the mixture of Water (H₂O), Glycerol (C₃H₈O₃), Urea (NH₂.CONH₂) and Sucrose (C₁₂H₂₂O₁₁) in specific ratio, which is equivalent to tissue (Cortical bone). Spiers, 1943, 1946, 1949 had used Plaster of Paris (CaSO₄.2H₂O) and glass as TEM of cortical bone. Cameron (1965) introduces the Cameron wax which is made up of paraffin wax (C₂₅H₅₂) and Calcium carbonate which is used as bone substitute. Other materials such as Bakelite (C₄₃H₃₈O₇)_n, Nylon (C₆H₁₁NO)_n, Lucite (C₅H₈O₂)_n are used as muscle tissue; Magnesium (Mg) and Facey liquid as cortical bone tissue, and gelatin as a lung tissue.

Radiation characteristics of tissue and their substitute (White, 1978) are compared by basic convenient parameters such as total mass attenuation coefficient (μ/ρ), mass energy absorption coefficient (μ_en/ρ), Effective atomic number (Z_eff), kerma, relative kerma and Computed tomography (CT) number. These parameters are useful parameters for choosing a substitute of muscle, cortical bone and lungs. Manjunatha et al. (Manjunatha and Rudraswamy, 2013) studied the effective atomic number (Z_eff) and electron density (N_el) of human organ, and their values have been computed in the energy range of 1 keV–100 GeV using WinXcom (Berger and Hubbell, 1987). Al-Bahri et al. (Al-Bahri and Spyrou, 1998) determined the electron density (N_el) of normal and pathological human breast tissue by using 59.5 keV gamma rays from ²⁴¹Am radioactive source using Compton scattering technique. Sellakumar (Sellakumar et al., 2007) evaluated the water equivalence and radiation transport properties of polymer gel dosimetry over a wide range of energy of gamma photons. Singh et al. (2010) assigned the effective atomic number (Z_eff) to sample of scientific interest and also measured the iodine content in thyroid using an HPGe semiconductor detector for 145 keV incident gamma photons. Singh et al. (2017), using a non-destructive multiple backscattering technique, assigned the effective atomic number (Z_eff) for low Z materials and concluded that the multiple backscattering techniques give the precise result for low atomic number materials.

Kerma is the dosimetric parameter which has been introduced to emphasize two stage processes to take place. Firstly, uncharged particle transfers the energy to the kinetic energy of charged particle. Secondly, those charge particles impart energy to the matter. Kerma is related to energy fluence rate by mass-energy transfer coefficient. Nilsson et al. (Nilsson and Brahme, 1983) determined the relation between kerma and absorbed dose, and also provided the variation of energy fluence rate and kerma with thickness of the tissue equivalent materials.

CT number is one of the useful parameter in radiation diagnosis during CT scanning which provide the detailed images and 3D view of various body tissues such as bone, muscle, lungs and blood vessels at the time of scanning. CT number is directly related to X-ray linear-attenuation coefficient of various tissue and tissue substitute materials. Brayant et al. (Bryant et al., 2012) have given the definition of CT number and dependency of CT number with energy over a wide range and mass-attenuation coefficient of materials with respect to water mass-attenuation coefficient of water. The significant variation in CT number was theoretically calculated with respect to photon energy for blood, lungs, brain, muscle, adipose tissue, breast and cortical bone.

The primary objective of present work is to examine the radiation interaction properties of cortical bone, muscle tissue and lungs tissue in dosimetry for medical, biological and nuclear applications and also measurement of photon fluence rate, energy fluence rate, kerma, effective atomic number Z_eff (theoretically as well as experimentally), relative kerma and CT number for tissues and their substitute materials.