X-Ray Interaction with Matter PDF

X-Ray Interaction with matter Dr. Nahla Atallah X-RAYS INTERACT with matter in the following five ways: (1) coherent scattering, (2) Compton scattering, (3) photoelectric effect, (4) pair production, and (5) photodisintegration.  Only Compton scattering and photoelectric effect are impor...

X-Ray Interaction with matter Dr. Nahla Atallah X-RAYS INTERACT with matter in the following five ways: (1) coherent scattering, (2) Compton scattering, (3) photoelectric effect, (4) pair production, and (5) photodisintegration.  Only Compton scattering and photoelectric effect are important in making an x-ray image. The conditions that govern these two interactions control differential absorption, which determines the degree of contrast of an x-ray image.  Coherent Scattering X-rays with energies below approximately 10 keV interact with matter by coherent scattering, sometimes called classical scattering or Thompson scattering (Figure 9-1). J.J. Thompson was the physicist to first describe coherent scattering.  In coherent scattering the incident x-ray interacts with a target atom, causing it to become excited. The target atom immediately releases this excess energy as a scattered x-ray with wavelength equal to that of the incident x-ray (λ = λ′) and therefore of equal energy. However, the direction of the scattered x-ray is different from that of the incident x-ray. The result of coherent scattering is a change in direction of the x-ray without a change in its energy. There is no energy transfer and therefore no ionization.  Most coherently scattered x-rays are scattered in the forward direction. Coherent scattering primarily involves low-energy x-rays, which contribute little to the medical image. Compton Scattering  X-rays throughout the diagnostic range can undergo an interaction with outer-shell electrons that not only scatters the x-ray but reduces its energy and ionizes the atom as well.  This interaction is called Compton scattering (Figure 9-2).  In Compton scattering 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. The x-ray continues in a different direction with less energy. The energy of the Compton-scattered x-ray is equal to the difference between the energy of the incident x-ray and the energy of the ejected electron.  During Compton scattering, most of the energy is divided between the scattered x-ray and the Compton electron. Both the scattered x-ray and the Compton electron may have sufficient energy to undergo additional ionizing interactions before they lose all their energy. Scattered x-rays provide no useful information on the radiograph. In general, the probability of Compton scattering decreases as x-ray energy increases. The probability of Compton scattering does not depend on the atomic number of the atom involved.  Any given x-ray is just as likely to undergo Compton scattering with an atom of soft tissue as with an atom of bone (Figure 9-3).  The scattered x-rays from Compton scatterings can create a serious radiation exposure hazard in radiography and particularly in fluoroscopy. A large amount of radiation can be scattered from the patient during fluoroscopy. Photoelectric Effect  X-rays in the diagnostic range also undergo ionizing interactions with inner-shell electrons. The x-ray is not scattered, but it is totally absorbed. The electron removed from the atom is called photoelectron and escapes with kinetic energy equal to the difference between the energy of the incident x-ray and the binding energy o the electron. Mathematically, this is shown as follows:  Characteristic x-rays are produced after a photoelectric interaction in a manner similar to that described in Chapter 7. Ejection of a K-shell photoelectron by the incident x-ray results in a vacancy in the K shell. This unnatural state is immediately corrected when an outershell electron, usually from the L shell, drops into the vacancy.  This electron transition is accompanied by the emission of an x- ray whose energy is equal to the difference between binding energies of the shells involved. These characteristic x-rays consist of secondary radiation and behave in the same manner as scattered radiation. They contribute nothing of diagnostic value and fortunately have sufficiently low energy that they do not penetrate to the image receptor. The probability that a given x-ray will undergo a photoelectric interaction is a function of both the x-ray energy and the atomic number of the atom with which it interacts.  A photoelectric interaction cannot occur unless the incident x-ray has energy equal to or greater than the electron binding energy. A barium K-shell electron bound to the nucleus by 37 keV cannot be removed by a 36-keV x-ray. If the incident x-ray has sufficient energy, the probability that it will undergo a photoelectric effect decreases with the third power of the photon energy (1/E)3. The probability that a given x-ray will undergo a photoelectric interaction is a function of both the x-ray energy and the atomic number of the atom with which it interacts.  This relationship is shown graphically in Figure 9-5 for soft tissue and bone.  As the relative vertical displacement between the graphs of soft tissue and bone demonstrates, a photoelectric interaction is much more likely to occur with high-Z atoms than with low-Z atoms (see Figure 9-5).  Pair Production If an incident x-ray has sufficient energy, it may escape interaction with electrons and come close enough to the nucleus of the atom to be influenced by the strong 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. This process is called pair production (Figure 9-8).  Because two electrons are formed in pair production interaction, the incident x-ray photon must have at least 1.02 MeV of energy. An x-ray with less than 1.02 MeV cannot undergo pair production. Any of the x-ray’s energy in excess of 1.02 MeV is distributed equally between the two electrons as kinetic energy. The electron that results from pair production loses energy through excitation and ionization and eventually fills a vacancy in an atomic orbital shell.  The positron unites with a free electron, and the mass of both particles is converted to energy in a process called annihilation radiation. Because pair production involves only x-rays with energies greater than 1.02 MeV, it is unimportant in x-ray imaging, but it is very important for positron emission tomography (PET) imaging in nuclear medicine. Photodisintegration  X-rays with energy above approximately 10 MeV can escape interaction with electrons and the nuclear field and be absorbed directly by the nucleus. When this happens, the nucleus is raised to an excited state and instantly emits a nucleon or other nuclear fragment. This process is called photodisintegration (Figure 9-9).