|INTRODUCTION AND OVERVIEW|
|Photon Concentration (Fluence)|
|Surface Integral Exposure|
|Dose Area Product (DAP)|
|Computed Tomography Dose Index (CTDI)|
|Mean Glandular Dose (MGD)|
|RADIO FREQUENCY RADIATION|
|Specific Absorption Rate (SAR)|
INTRODUCTION AND OVERVIEW
There are many different quantities and units used to quantify radiation, because there are a number of different aspects of an x-ray beam or gamma radiation that can be used to express the amount of radiation. The selection of the most appropriate quantity depends on the specific application. The primary objective of this chapter is to help the reader develop a conceptual understanding of the various radiation quantities and units and gain sufficient factual knowledge to support their usage.
A complicating factor is that American society is undergoing a slow change in the units used to express a variety of physical quantities. In everyday life we see this as a change from the conventional British unit system (feet, pounds, miles) to the metric system (meters, kilograms, kilometers). In radiology we are experiencing a change not only to the general metric units but also to the proposed adoption of a set of fundamental metric units known as the International System of Units (SI units). The adoption of SI radiation units is progressing rather slowly because there is nothing wrong with our conventional units, and SI units are somewhat awkward for a number of common applications. Throughout this text we use the units believed the most useful to the reader. In this chapter both unit systems are discussed and compared.
The table below is a listing of most of the physical quantities and units encountered in radiology. It is a useful reference especially for the conversion of one system of units to another.
|Exposure||Conventional Unit||SI Unit||Conversions|
|Exposure||roentgen (R)||coulomb/kg of air (C/kg)||1 C/kg = 3876 R|
|1 R = 258 uC/kg|
|Dose||rad (R)||gray (Gy)||1 Gy = 100 rad|
|Dose equivalent||rem||sievert (Sv)||1 Sv = 100 rem|
|Activity||curie (Ci)||becquerel (Bq)||1 mCi = 37 mBq|