Biomedical Imaging

$\begin{array}{l}{\text{H}}_2\mathrm{O}\stackrel{\text{ion}\text{.rad}\text{.}}{\to }\text{H}\cdot \text{+OH}\cdot \\ {\text{H}}_2\mathrm{O}\stackrel{\text{ion}\text{.rad}\text{.}}{\to }{\text{H}}_2{\mathrm{O}}^{+}{\text{+e}}_{\text{aq}}^{-}\left(5.78\right)\end{array}$

This is immediately followed by further reactions, like

$\begin{array}{cc}\begin{array}{c}{\text{H}}_2{\mathrm{O}}^{+}\to {\text{H}}^{+}+\mathrm{O}\text{H}\cdot \\ \text{OH}\cdot \text{+OH}\cdot \to {\text{H}}_2{\text{O}}_2\\ {\text{O}}_2+{\text{e}}_{\text{aq}}^{-}\to {\text{O}}_2^{-}\\ {\text{O}}_2^{-}+{\text{H}}_2\text{O}\to {\text{H}}_2{\mathrm{O}}_2+2{\text{OH}}^{-}+{\text{O}}_2\end{array}& \left(5.79\right)\end{array}$

which produce so called reactive oxygen species (ROS). The hydroxyl radical OH· with an unpaired electron, hydrogen peroxide H2O2 and the superoxide anion ${\mathrm{O}}_2^{-}$are chemically highly reactive and can cause numerous modifications of biomolecules, including several types of DNA damage discussed below.45

In irradiation with photons and electrons, the indirect action of radiation via radicals and ROS created from water and molecular oxygen accounts for about 70% of radiation induced cell death, whereas with direct action of radiation, the immediate ...