Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

+ + : ::: ::::::: ••FIGURE 3-3. A: Electron scattering results in the path length of the electron being greater than itsrange. B: Heavily charged particles, like alpha particles, produce a dense nearly linear ionizationtrack, resulting in the path and range being essentially equal.Linear Energy TransferThe amount of energy deposited per unit path length is called the linear energytransfer (LET) and is usually expressed in units of eV/cm. The LET of a chargedparticle is proportional to the square of the charge and inversely proportional to theparticle's kinetic energy (i.e., LET proportional to Q2/Ek). LET is the product ofspecific ionization (IP/cm) and the average energy deposited per ion pair (eV/IP).The LET of a particular type of radiation describes the energy deposition density,which largely determines the biologic consequence of radiation exposure. In general,"high LET" radiations (alpha particles, protons, etc.) are much more damagingto tissue than "low LET" radiations, which include electrons (e-, ~-, and W)and ionizing electromagnetic radiation (gamma and x-rays, whose interactions setelectrons into motion).Scattering refers to an interaction resulting in the deflection of a particle or photonfrom its original trajectory. A scattering event in which the total kinetic energy ofthe colliding particles is unchanged is called elastic. Billiard ball collisions, forexample, are elastic (disregarding frictional losses). When scattering occurs with aloss of kinetic energy (i.e., the total kinetic energy of the scattered particles is lessthan that of the particles before the interaction), the interaction is said to be inelastic.For example, the process of ionization can be considered an elastic interactionif the binding energy of the electron is negligible compared to the kinetic energy ofthe incident electron (i.e., the kinetic energy of the ejected electron is equal to thekinetic energy lost by the incident electron). If the binding energy that must beovercome to ionize the atom is significant (i.e., the kinetic energy of the ejectedelectron is less than the kinetic energy lost by the incident electron), the process issaid to be inelastic.