Reduction in the number of x-ray photons in the beam and subsequent loss of energy as the beam passes through matter; result of x-ray photons interacting with matter and losing energy through these interactions
3 things the x-ray can interact with depending on the energy of the photon
Directly with the nucleus
What are low energy photons most likely to interact with?
What do intermediate energy photons generally interact with?
What are very high energy photons capable of interacting with?
Nucleus (radiation therapy)
What are diagnostic interactions most common with?
Energy required to remove an electron from a shell
What is the approximate K-shell binding energy of the average atom in the soft tissue of the body?
5 basic interactions between x-rays and matter
With each of these interactions the x-ray photons either interact and change direction = scattering or are absorbed by the atom
When a photon is absorbed, all of the energy of the photon is transferred to the matter and the photon no longer exists
If a photon interacts and scatters, the photon still exists but usually possesses less energy than before
Partial energy from the photon is transferred to the matter during the interaction and the lower-energy photon then continues along its new path until again it either interacts and scatters or is absorbed
One photon may scatter several times before it’s finally absorbed completely by the matter
Photoelectric absorption Coherent scattering Compton scattering Pair production Photodisintegration
Involves low-energy photons: below 10 keV
Electrons excited and vibrate at photon frequency; the excited/vibrating atom immediately releases this excess energy by producing a secondary photon that has the same energy and wavelength as the incident photon but travels in a dif direction from the initial photon
No electrons ejected and no ionization takes place
2 types of coherent scattering that have the same basic interaction results
Involves a single electron in the interaction
Involves all the electrons of the atom in the interaction
Incident photon energy completely absorbed by inner-shell electrons
Most likely to occur when x-ray photon has just slightly more energy than binding energy (Eb) of a K or L-shell e-Incident photon ejects the electron from its inner shell and is totally absorbed in the interaction
Dominate areas where there’s higher Z# materials (bone, contrast, etc.)
Incident photon needs an energy that’s slightly greater than the binding energies of the K-shell electrons are very low
X-ray photon absorbed in body
Most photoelectric interactions in the body result in the kinetic energy of the photoelectron; photoelectron is matter (not just energy) & therefore won’t travel far & is usually absorbed w/in 1-2 mm in soft tissue
When is an ion pair formed?
Electron ejected from atom
Remaining atom has vacancy in inner electron shell
Electron ejected from atom
Photoelectron energy can be determined with what equation?
Ei = Eb + Eke Ei = energy of the incident photon Eb = binding energy of the electron Eke = kinetic energy of the photoelectron
Inner shell electron vacancy makes atom electrically unstable
Vacancy instantly filled by an electron from the L-shell (or less commonly M-shell or free electron); in the vast majority of cases the electron transfers from an outer shell to inner shell and as it does it releases energy in the form of this
Characteristic photon (characteristic cascade)
Radiation that originates from irradiated material outside x-ray tube
Production similar to characteristic x-rays production within target (electron transfers from one shell to another)
Created at the x-ray target
Characteristic photons emitted from atoms of patient after PE absorption interaction
Low Z# in tissue = low energy secondary radiation, higher Z# with contrast agents = higher energy
3 rules that govern the possibility of a photoelectric interaction
Incident x-ray photon energy must be greater than or equal to binding energy of inner-shell electron
A photoelectric interaction is more likely to occur when the x-ray photon energy and electron binding energy are nearer to one another
A photoelectric interaction is more likely to occur with an electron that is more tightly bound in its orbit
Occurs when an incident x-ray photon interacts with a loosely bound outer-shell electron, removes the electron from its shell and the proceeds in a different direction as a scattered photon
Dislodged electron when part of the energy of the incident photon is used to remove the outer-shell electron and impart kinetic energy to it
Photon that exits the atom in a different direction, possesses less energy than the incident photon and therefore has a lower frequency and longer wavelength
Compton scattered photon
What equation expresses the energy transfer in the Compton effect?
Ei = Es + Eb + Eke Ei = incident photon energy Es = Compton scattered photon energy Eb = electron binding energy of the Compton electron Eke= kinetic energy given to the Compton electron
When a scattered photon is deflected back toward the source, it’s traveling in the direction opposite to the incident photon; most photons will scatter in a more forward direction, especially when incident photon energy increases
2 interactions between x-ray and matter that have an impact on an x-ray image
2 circumstances where photoelectric interactions predominate
In the lower-energy ranges (25-45 keV) produced by 40-70 kVp
When high atomic number elements are introduced
When just comparing body tissues, in soft tissue (water), interactions are about 50/50 photoelectric absorption versus Compton at about what keV
When just comparing body tissues, in bone, interactions are about 50/50 photoelectric absorption versus Compton at about what keV?
Steps up voltage so amperage can go down
Steps down voltage so amperage can go up
Can be step-up or down, works on self-induction