LIFESCI 1D03 : Medical Imaging Physics [email protected] Page 1 Effects of Radiation: Topic summary 1. Introduction In radiobiology we are concerned with looking at the effects of energy absorption in cells. We need to look at what happens when a charged particle moves through the cells in tissue releasing energy along a track. The energy released by these ionising particles can lead to the formation of highly reactive chemical species, which in turn can damage DNA and lead to cell death. Some terms and definitions: Exposure: Radiation output from an x-ray source. Radiation exposure is measured using an air ionisation chamber. Units - Roentgen 1R = 2.58X10-4 C/kg Dose: Dose D is the amount of energy deposited per unit mass (Gy) 1 gray = 1 J/kg Equivalent Dose H = D x W r (Sievert Sv) Photons (all energies) W r = 1 Electron (all energies) W r = 1 Neutrons W r = 5-20 depending on energy Protons W r = 5 Alpha W r = 20 Effective dose. Takes into account different organs in the body and how they react to dose. (Sievert Sv) 2. Charged particle tracks If a charged particle passes through a material, it will leave a track of ionised and excited atoms. The track characteristics will depend on the type of charged particle i.e. an electron and an alpha particle having the same initial energy will produce very different looking tracks. For the electron, the events along the track are far more widely spaced than from the alpha particle track. The alpha particle is said to be more densely ionising.
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