Chapter 3 Interaction of X-radiation with Matter

fundamental physics concepts that relate to_____ and ____ are reviewed.
radiation absorption, scatter
the highest energy level of photons in the x-ray beam (kVp)
peak kilovoltage
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the product of electron tube current and the amount of time in seconds that the x-ray tube is activated (mAs)
milliampere-seconds
controls the quality, or penetrating power, of the photons in the x-ray beam and to some degree also affects the quantity, or number of photons, in the beam
peak kilovoltage
x-rays are carriers of
man-made, electromagnetic energy
If x-rays enter a material such as human tissue, they may:
-interact with the atoms of the biologic material in the patient
-pass through without interaction
If an interaction occurs, electromagnetic energy is transferred from the x-rays to the atoms of the patient’s biologic material
absorption
the amount of energy absorbed per unit mass is (D)
absorbed dose
diagnostic x-ray beam is produced when a stream of high-speed electrons bombards a positively charged target in a highly evacuated glass tube. the target is made of metal tungsten or a metal alloy, tungsten rhenium.
anode
These materials have: (anode)
-high melting points
-high atomic numbers (tungsten 74 and rhenium 75)
the combination of the x-ray tube glass wall and the added aluminum placed within the collimator may be
permanent inherent filtration
emerging x-ray photon beam is collectively referred to as
primary radiation
Although all photons in a diagnostic x-ray beam do not have the same energy, the most energetic photons in the beam can have______ than the electrons that bombared the target.
No more energy
The energy of the electrons inside the x-ray tube is expressed in terms of the_______.
electrical voltage applied across the tube
In diagnostic radiology, this is expressed in thousands of______(kV).
volts, or kilovolts
Because the voltage across the tube fluctuates, it is usually expressed in ____(kV).
kilovolt peak
Therefore, a 100-kVp beam contains photons having energies of 100 keV or less, with an average energy of approximately ____.
33 keV
when an x-ray beam passes through a patient, it goes through a process and rather broad, may be used to refer to any process decreasing the intensity of the primary photon beam that was directed toward a destination. Refers to both absorption and scatter processes that prevent photons from reaching a predefined destination.
attenuation
some primary x-ray photons also traverse the patient with interacting
direct transmission
These noninteracting x-ray photons reach the IR which may be phosphor plate, digital radiography receptor, and radiographic fim
Radiographic Image Receptor
Several methods have been devised to limit the effects of indirectly transmitted x-ray photons. These methods are_________.
-air gap techniques
-radiographic grids
bending of photon’s path that is part of the exit or image-formation radiation and it degrades the appearance of a completed radiographic image by blurring the sharp outlines of dense structures
small-angle scatter
Greater overall exposure of the IR occurs, thus interfering with the radiologist’s ability to distinguish different structures in the image and this undesirable, additional exposure is
radiographic fog
1-50 kVp, an atom site, less probable than photoelectric absorption, energy: unchanged; direction after interaction: slight change, no interaction
Coherent scattering
1-50 kVp, inner-shell electron site,energy: absorbed; direction after interaction: not applicable, most responsible for radiation dose in this energy range, photoelectron interaction
Photoelectric absorption
60-90 kVp and also ranges 200 kVp-2 MeV, outer-shell electron, energy: reduced; directionafter interaction: changed, both it and photoelectric interactions occur in this energy range(60-90 kVp), predominantly responsible for radiation dose in this energy range and produce many more it and photoelectric interaction on their own (200 kVp-2 MeV), compton-scattered electron interaction and compton-scattered photon interaction
Compton scattering
begins at about 1.022 MeV; important at 10 MeV, dominants at 50 MeV and greater; nucleus of atom site, energy: disappers after interaction with nucleus; transformed into 2 new particles that annihilate each other; direction after interaction: energy reappears in form of 2 .511-MeV photons moving in opposite directions, positive electron and ordinary electron interactions with 2 .511 MeV photons
Pair production
greater than 10 MeV, nucleus of atom site, energy: absorbed by nucleus after collision with high-energy photon, excess energy in nucleus creates instability that is usually alleviated by emission of a neutron, other emissions possible, neutron interaction, photon or proton-neutron combination interaction, alpha particle interaction
Photodisintegration
5 types of interactions between radiation and matter:
coherent scattering, photoelectric absorption, compton scattering, pair production, photodisintegration
Coherent scattering is sometimes called the following terms:
Classical, elastic, unmodified scattering
not important in any energy range
coherent scattering
diagnostic radiology
photoelectric absorption
diagnostic radiology and therapeutic radiology
compton scattering
therapeutic radiology
pair production
therapeutic radiology
photodisintegration
in honor of the scientist who 1st explained it, before the concept of the photon was known by using wave analysis alone
Rayleigh scattering
in which the low-energy photon interacts with one or more free electrons
Thompson scattering
no importance in any energy range when the low energy x-ray photon interacts with an atom of human tissue, it does not lose kinetic energy and the photon merely changes direction by 20 degrees or less, no ionization of the biologic atom occurs
coherent scattering
Formula for electron shells
2n^2
coherent scattering
making excited
coherent scattering
photoelectric absorption
contrast and inner shell interaction
important to x-ray
photoelectric absorption
compton scatter
weak energy and important to x-ray
compton scatter
pair production
way above x-ray
pair production
photodisintegration
way above x-ray
photodisintegration
the “released” energy is carried off in the form of a photon that is
characteristic photon or x-ray
from photoelectric interactions within human tissue are low enough in energy that they are predominantly absorbed within the body
fluorescent radiation
“______occurs when an inner electron is removed from an atom in a photoelectric interaction, causing an inner-shell vacancy, the energy liberated when this vacancy is filled, but instead of emerging from the atom as fluorescent radiation, can be transferred to another electron of the atom, thereby ejecting the electron. “
Auger effect
The number of x-rays emitted per inner-shell vacancy.
fluorescent yield
The by- products of photoelectric absorption include the following:
– Photoelectrons
-Charactreristic x-ray photons
The probability of the occurrence of photoelectric absorption depends on the ___ of the incident xray photons and the ____ of the atoms comprising the irradiated objects.
energy, atomic number
(POPA) It increases markedly as the engery of the incident photon ____ and the atomic number of the irradiated atoms_____.
decreases, increases
is the most important mode in the interaction between x-radiation and the atoms of the patient’s body in the energy range used in diagnostic radiology because this interaction is responsible for both the patient’s dose and contrast in the image.
Photoelectric absorption
effective atomic number is composite Z value for when multiple chemical elements comprise a material
Zeff
measured in grams per cubic centimeter
mass density
_____ of body parts also plays a role.
Thickness
degree of overall blackening on a radiographic film
radiographic density
(IR) is used because radiographic film is no longer used as the primary IR
image receptor exposure
is the intensity of the display monitor’s light emission and a monitor function that can change the lightness of darkness of the image on a display monitor
Brightness
_____ is density and _____ is the brightness
Darkness, contrast
difference between black and white
contrast
sets the midpoint of the range of densities visible on the image
window level
Adjusting the window level, also known as ______, refers to changing the brightness , either to be increased or decreased throughout the entire range of densities.
windowing
The greater difference is in the amount of photoelectric absorption, the _____ the contrast in the radiographic image will be between adjacent structures of differing atomic numbers.
greater
Substances (solids, liquids, or gases) used in diagnostic imaging procedures that permit the radiologist to distinguish between various body soft tissue structures; may be injected, swallowed, inhaled, or introduced by rectum like barium
contrast media
compton scattering is also known by the following terms:
incoherent, inelastic, modified scattering
posses excess kinetic energy and is capable of ionizing other atoms and freed electron( secondary, recoil electron)
compton scatttered electron
In diagnostic radiology, the probability of occurrence of compton scattering relative to that of the photoelectric interaction____ as the energy of the x-ray photon ______.
increases
Compton scattering and Photoelectric absorption in tissue are equally probable at approximately ___ keV.
35
Therefore, in a 100-kVp x-ray beam when the photons have an average energy in the range of ___ to ___ keV, compton events occur.
30, 40
does not occur unless the energy of the incident x-ray photon is at least 1.022 mega electron volts
pair production
radiation is used in positron emission tomography (PET)
annihilation
is an interaction that occurs at more than 10 MeV in high-energy radiation therapy treatment machines
photodisintegration
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