Radiation Quantities and Units

Introduction

Accurate measurement of radiation is crucial in all medical uses of radiation, including diagnosis and treatment. In diagnostic imaging procedures, image quality must be optimized to obtain the best possible image with the lowest radiation dose, minimizing the risk of morbidity. In radiotherapy, the prescribed dose must be delivered precisely to maximize the tumor control probability (TCP) and minimize the normal tissue complication probability (NTCP).

Key Radiation Quantities and Their Definitions

Below are some of the most important radiation quantities used in medical applications of radiation:

Radiation Quantities
Quantity	Definition	SI Unit	Old Unit	Conversion Factor
Exposure (X)	Ability of radiation to ionize air	Coulomb per kilogram (C/kg)	Roentgen (R)	1 R = 2.58 × 10⁻⁴ C/kg
Kerma (K)	Kinetic Energy Released in Matter	Gray (Gy)	—	1 Gy = 1 J/kg
Dose (D)	Energy absorbed per unit mass of medium	Gray (Gy)	Rad	1 Gy = 100 rad
Equivalent Dose (Hₜ)	Absorbed dose weighted by radiation type	Sievert (Sv)	Rem	1 Sv = 100 rem
Effective Dose (E)	Sum of weighted equivalent doses to tissues	Sievert (Sv)	Rem	1 Sv = 100 rem
Activity (A)	Number of nuclear decays per unit time	Becquerel (Bq)	Curie (Ci)	1 Ci = 3.7 × 10¹⁰ Bq

Mathematical Formulas and Constants

Here are some key mathematical formulas and constants used in radiation science:

Energy of a photon: $E = h \cdot ν$ , where $h = 6.626 x 10 - 34 J·s$ is Planck’s constant, and $ν$ is the frequency of the radiation.
Relationship between Energy and Wavelength: $E = h \cdot c / λ$ , where $c = 2.998 x 10 ⁸ m/s$ is the speed of light, and $λ$ is the wavelength of the radiation.
Energy equivalence: $E = m \cdot c ²$ , where $m$ is mass, $c$ is the speed of light, and $E$ is the energy equivalence of mass.

Radiation Shielding and Dosimetry

The effectiveness of shielding against radiation is quantified using the following formula:

Half-Value Layer (HVL) - The thickness of a material required to reduce the radiation intensity by half. It is given by:

I = I₀ e - μ x

, where

I

is the radiation intensity,

I₀

is the initial intensity,

μ

is the attenuation coefficient, and

x

is the thickness of the material.