X-Ray Interaction with Matter PDF

X-RAY INTERACTION WITH MATTER Module 8 Prepared by: SYM Five Forms of X-ray Interactions Classical or Coherent Scattering Compton Effect Photoelectric Effect Pair production Photodisintegration Two Forms of X-ray Interactions Important to Diagnostic X-ray Compton Effec...

X-RAY INTERACTION WITH MATTER Module 8 Prepared by: SYM Five Forms of X-ray Interactions Classical or Coherent Scattering Compton Effect Photoelectric Effect Pair production Photodisintegration Two Forms of X-ray Interactions Important to Diagnostic X-ray Compton Effect Photoelectric Effect Classical or Coherent Scattering ▪ Also called Classical Scattering/Rayleigh/Thomson Scattering/Unmodified. ▪ Is an interaction between low energy x-rays and atoms. The x-ray looses no energy but changes direction slightly. ▪ The wavelength of the incident x-ray is equal to the wavelength of the scattered x-ray. ▪ It is of little importance to diagnostic radiology. Classical or Thompson Scattering There is no loss of energy and no ionization. Because these are low energy x- rays, they are of little importance. At 70 kVp only a few percent of the x-rays undergo this form of scattering. Classic Scatter may contribute to the graying of the image called film fog. Compton Effect ▪ It occurs between moderate-energy x-rays and outer shells electrons. ▪ It results in ionization of the target atom, change in x-ray direction and reduction of x-ray energy. ▪ The wavelength of the scattered x- ray is greater than that of the incident x-ray. ▪ It reduces contrast in an x-ray image. Compton Effect reduced its energy and ionizes the atom as well. The outer shell electron is ejected. This is called Compton Effect or Compton Scattering. The scattered photon and secondary electron will retain most of its energy so it can interact many times before it lose all of it’s energy. The scattered photon will ultimately be absorbed photoelectrically. Compton Effect The probability of Compton Effect is about the same for soft tissue or bone. This decreases with increasing photon energies. Compton scatter decreases with increased kVp. Features of Compton Scattering Most likely to occur With outer-shell electrons With loosely bound electrons. As x-ray energy Increased penetration through increases tissue w/o interaction. Increased Compton relative to photoelectric scatter. Reduced total Compton scattering. Photoelectric Effect ▪ Occurs when an incident x-ray is totally absorbed during the ionization of an inner shell electron. ▪ The incident photon disappears and the K-shell electron, now called a photoelectron, is ejected from the atom. ▪ This is a total x-ray absorption interaction. Photoelectric Effect Therefore the kinetic energy of the photoelectron will be proportionally lower. Characteristic x-rays are produced following a photoelectric interaction to those produced in the x-ray tube. These characteristic x-rays are also secondary radiation and acts like scatter. The probability of a photoelectric interaction is a function of the photon energy and the atomic number of the target atom.  Features of the Photoelectric Effect Most likely to occur With inner-shell electrons With tightly bound electrons. When the x-ray energy is greater than the electron- binding energy. Pair Production If the incident x-ray has sufficient energy, it may escape the electron cloud and come close enough to the nucleus to come under the influence of the strong electrostatic field of the nucleus. Pair Production The interaction with the nucleus strong electrostatic field causes the photon to disappear and in its place appear two electrons. One is positively charged and called a positron while the other remains negatively charged. This is called Pair Production. Pair Production It takes a photon with 1.02 MeV to undergo Pair Production. Therefore it is not important to diagnostic x-ray. Photodisintegration High energy x-ray photons with energies above 10 MeV can escape interaction with both the electrons and nucleus electrostatic fields. It is absorbed into the nucleus that excites the nucleus resulting in the release of a nucleon or other nuclear material. This is referred to as Photodisintegration. Like pair production, the high energy needed to cause this makes it unimportant to diagnostic radiography. Differential Absorption Only Compton and Photoelectric Effects are important interactions that the x-ray may have with matter in the diagnostic spectrum. It should be clear that Compton Scatter X-rays contribute no useful information. These scattered x-rays result in film fog, a generalized dulling of the image on the radiograph by film densities not representing diagnostic information. To reduce this type of fog, we use techniques and apparatus to reduce the amount of scatter reaching the film. Differential Absorption X-rays that undergo photoelectric interaction provide diagnostic information to the image receptor. Since they do not reach the film, these x-rays are representative of anatomic structures with high x-ray absorption characteristics. These structures are said to be Radiopaque (appears bright/white on the radiograph). The other x-rays that penetrate the body and are transmitted without interaction are said to be Radiolucent. Radiolucent (appears dark/black on the radiograph) matter appears as high density or dark areas on the radiograph. Differential Absorption The radiographic image is the result of the difference between those x-rays absorbed photoelectrically and those not absorbed at all. This characteristic is called differential absorption. Differential Absorption increases as the kVp is lowered but lowered kVp results in a higher patient radiation exposure. A compromise is needed for each examination. Differential Absorption Notice how much of the x-rays are absorbed photoelectrically in bone compared to the soft tissue.