Dosimetry introduction

KermaThis is a measure of the absorption of radiation in a small mass of a medium.It is no different to absorbed dose except for higher energy radiations, whichare not encountered in diagnostic radiography. Higher energy radiationsproduce energetic electrons which could in turn cause ionisation remote fromthe irradiated area, & thus cannot be part of the original small mass underconsideration.Kerma approximately replaces the quantity “exposure”. It is best thought ofas the “dose to air” & is usually measured with an air filled ionisation chamber.Kerma is derived from kinetic energy released per unit mass of tissue byindirectly ionising radiation, ie anything that is not a charged particle. Use ofthis quantity implies that no secondary radiation is produced within the patient;the energy is transferred to electrons already present in the tissue. Kermacan be quantified with x-rays, gamma rays & uncharged neutrons, but not withcharged particles (ie alpha, beta or protons).In the diagnostic x-ray range, air kerma is equal to absorbed dose in airAir kerma of 1 Gy is equal to an absorbed dose in air of 1 GyReferencesSimone Plaut (1993) Radiation protection in the X-ray department, London,Butterworth HeinemannGraham & Cloke (2003) Principles of Radiological Physics, 4 th Edition,London, Churchill Livingstone6