Topics • Radiation intensity • Attenuation • Linear attenuation coefficient • Differential absorption • Half-value layer • Transmission Fraction • X-ray Interaction Mechanism
Radiation Intensity • The intensity of radiation is defined as the rate of emitted energy from unit surface area through unit solid angle.
Attenuation • The total reduction in the number of x-rays remaining in an x-ray beam after penetration through a given thickness of tissue. • Attenuation is the product of absorption and scattering.
Linear Attenuation Coefficient • The attenuation coefficient is a quantity that characterizes how easily a material or medium can be penetrated by a beam of light, sound, particles, or other energy or matter.
Differential Absorption • The difference in x-ray interaction. • Increases as the kVp is reduced.
Differential absorption and attenuation of the x-ray beam depend on the following factors: • The atomic number (Z) of the atoms in tissue. • The mass density of the atoms in tissue. • The x-ray energy.
Half Value Layer (HVL) The Thickness of an absorber needed to reduce the intensity of the x-ray beam into half of its original value.
X-ray Interaction Mechanism • Coherent Scattering • Compton Scattering • Photoelectric Effect • Pair Production • Photodisintegration
Coherent Scattering • Energies below approximately 10 keV. • Sometimes called classical scattering or Thompson scattering, Rayleigh Scattering. • No ionization.
Compton Effect • In the Compton effect, the incident x-ray interacts with an outer shell electron and ejects it from the atom, thereby ionizing the atom. The ejected electron is called a Compton electron or a secondary electron.
Compton Effect • The probability of the Compton effect is inversely proportional to x-ray energy (1/E) and independent of atomic number.
Photoelectric Effect • Interacts with inner shell electrons • X-rays are absorbed • The electron removed from an atom is called photoelectron.
Photoelectric Effect • The probability if the photoelectric effect is inversely proportional to the third power of the x-ray energy (1/E)3. • The probability of photoelectric effect is directly proportional to the third power of the atomic number of the absorbing material (Z3).
Atomic Number and K-shell Electron binding energy of radiologically important elements
Pair Production • Incident electron interacts with the nuclear field. • The interaction between the x-ray and the nuclear field causes the x-ray to disappear, and in its place, two electrons appear, one positively charged (positron) and one negatively charged (electron).
Pair Production • Occurs above 1.02 MeV
Photodisintegration • Occurs with x-ray energies above 10 MeV. • The nucleus is raised to an excited state and instantly emits a nucleon or other nuclear fragment.