Chapter 2: Radiation Safety and Protection

INTRODUCTION

The Health Physics Society defines radiation as "energy that comes from a source and travels through space."¹ The source could be atomic particles such as alpha and beta emissions as well as electromagnetic energy associated with AM/FM radio, radar, visible light, ultraviolet light, x-rays, and gamma rays. Radiation with enough energy to remove an electron from an atom is termed ionizing radiation.² A characteristic of x-rays, gamma rays, alpha and beta particles, radiation having this ability can lead to biological damage when absorbed in human tissue.

This chapter will introduce the common units to describe radiation, sources of radiation exposure, radiation dose limits, and an introduction to radiation biology. The chapter will further focus on radiation safety and protection regulations pertinent to the practice of Nuclear Cardiology. These regulations are governed by the Nuclear Regulatory Commission (NRC) and are found in Title 10, Parts 19, 20, and 35 of the Code of Federal Regulations (CFR). NRC NUREG 1556 Volume 9, Revision 2 provides guidance specific to radioactive materials licensing and offers suggested policies and procedures for radiation safety compliance.

RADIOLOGICAL UNITS

There are several conventional terms used when describing radiation. These include exposure, absorbed dose, and dose equivalent. Named after Wilhelm Roentgen, the scientist who discovered x-rays in 1895, the Roentgen (R) is the unit of radiation exposure in air.³ In comparison to the International System of Units (SI), one Roentgen corresponds to the amount of radiation required to liberate 2.58 × 10^-4 Coulombs per kilogram of air.

The Rad, or Radiation Absorbed Dose, is the measure of the amount of energy absorbed by an object as radiation passes through.⁴ The amount of energy absorbed is dependent on the energy of the incident photon and the composition of the material. The f-factor is a tissue weighting factor used to convert exposure in air (R) to absorbed dose (rad) in tissue taking into account the x-ray or gamma ray energy and effective atomic number of the tissue exposed. For example, a 100-keV gamma photon incident on fat will transfer 91% of its energy, whereas the same photon will deliver 96% of its energy to muscle tissue. Therefore, a source of radiation exposing a point in air to 100 R will deliver a dose of 91 rad to fat tissue and 96 rad to muscle tissue at the same reference point.

Dose equivalent is a term used to quantify the amount of energy deposited in tissue along with the associated biological risk from the type of radiation.⁵ The conventional unit for dose equivalent is rem which is calculated by multiplying the radiation absorbed dose (rad) by a radiation quality factor or QF.⁶ Table 2-1 illustrates that the QF for x-rays, gamma rays, and beta particles is equal to 1 whereas the QF for alpha particles is 20.⁷ This ...