A radiographic image is the result of:
differential absorption of the primary x rays that interact with varying tissue composition of the anatomic area of interest
Beam attenuation occurs:
when the primary x ray beam loses energy as it interacts with anatomic tissue
X rays have the ability to do what with atoms within anatomic tissue
eject electrons (ionization) from atoms
What are the three primary processes that occur during x ray interaction with anatomic tissue
absorption, transmission, scattering
Total absorption of the incoming x ray photon is the result of:
photoelectric effect
Scattering of the incoming x ray photon is a result of:
Compton effect
Scatter radiation reaching the image receptor provides:
no useful information and creates unwanted density or fog
A radiographic image is composed of:
varying amounts of densities that structurally represent the anatomic area of interest
The process whereby a radiographic image is created by passing an x ray beam through anatomic tissue:
differential absorption
what processes occur during beam attenuation
absorption, photon transmission, scattering
The ability of an x ray photon to remove an atom's electron is a characteristic known as
ionization
The x ray interaction responsible for absorption:
photoelectric
The x ray interaction responsible for scattering
Compton
Exit radiation is composed of
transmitted radiation, scattered radiation
What interaction creates unwanted fog
Compton
The low density areas on an x ray image are created by
absorbed radiation
An anatomic part that transmits the incoming x ray photon will create an are of what on the x ray image
high density
How do x rays interact with an object
transmit, absorb, scattered
X rays interact with object in 3 ways
pass through unaffected, are absorbed by structures in object, are scattered by the object
X ray probability or the chance of an event happening depends on
energy of photon, type of matter x rays are passing through and where on the atomic chart that matter is
Is there absorption with coherent, compton, or photoelectric
coherent no, Compton partial, photoelectric total
What photon KEV range results in coherent scatter
low energy, less than 10 KEV, long wavelength, interacts with whole atom
What photon KEV range results in photoelectric and Compton
intermediate energy, 10 to 10000 KEV, middle wavelength, photon interacts with electron cloud
What happens when high energy photons interact
greater than 1 MEV the photon interacts with the nucleus, photodisintegration
What are 5 types of photon interactions
Transmit through matter unchanged.
Change direction with no energy loss (coherent scatter).
Change direction and lose energy (Compton).
Deposit all energy into matter (photoelectric, pair production, photodisintegration).
What are the 2 primary forms of x ray interaction in diagnostic x ray
Compton scattering and photoelectric absorption
Coherent scattering AKA
classical, Thompson
What is the only x ray interaction that does not cause ionization
coherent scattering
How does coherent scattering occur
low energy photon (less than 10 KEV) enters atom.
atom excited by photon.
releases photon of same KEV and wavelength.
new photon travels in different direction but usually forward (small scatter angle).
no ionization occurs.
What is the result of coherent scattering
photon change in direction with no energy loss.
does not ionize matter.
may cause slight film fogging (forward direction).
What increases probability of coherent scattering
Low atomic Z number (fat more likely than bone).
Lower photon energies (5 KEV more likely than 10 KEV)
What occurs with Compton scattering
photon ejects outer shell electron.
photon loses some energy.
atom is ionized.
How is fog created
Compton scattering
In Compton scattering, the angle of scatter can be directed at any angle from incident beam: true or false
true
What creates backscatter
Compton scatter
What is the source of film fog (decreased contrast)
Compton scattering
What is the source of personnel and patient exposure
Compton scatter
What are the results of Compton scatter
photon changes direction with loss of energy.
atom is ionized.
this is source of patient and personnel exposure and film fog/decreased contrast
Compton scatter probability
increased probability with increased density of matter.
increases in matter with abundant hydrogen.
increase with decreased photon energy.
What is density of matter
how tight electrons are
What happens with photoelectric effect
intermediate photon energy.
total absorption of incident x ray photon.
photoelectron emitted with some KE.
photon loses all energy.
characteristic radiation is emitted when hole filled, cascade of photons.
photoelectric effect results
photon disappears.
atom is ionized.
characteristic photon emitted.
secondary radiation.
cascade effect
Probability of photoelectric effect
decreases sharply as photon energy/KVP increases.
increases with increased atomic/Z #.
Increases atomic # = increased of inner shell electrons.
Pair production
not diagnostic used in PET and nuc med.
occurs with high photon energy only.
photon interacts with nuclear force field.
uses 1.022 MEV to produce positron and negatron.
photon ceases to exist.
Pair production AKA
annihilation radiation.
negatron cannot exist withouth KE.
positron and negatron combine and are destroyed.
matter converts back to energy.
2 photons of .511 MEV emitted.
What happens in pair production
photon disappears.
negatron e+ and positron e- created.
annihilation radiation.
probability of pair production
energy must be at least 1.022 MEV.
increases with increased atomic #, larger nuclear field.
photodisintegration AKA total absorption occurs with
super high photon energy, greater than 10 MEV.
photon absorbed by nucleus.
nucleus is excited and ejects particles and returns to ground state.
result of photodisintegration
photon disappears.
nucleus changes form and becomes a different nuclide.
attenuation versus transmission
attenuation happens with 99.5% of x ray beam.
attenuation is removal of photon.
attenuation is caused by absorption and scattering.
transmission happens with .5% of x ray beam.
transmission are photons that do not interact with subject.
transmission contr
what is differential absorption
attenuation and transmission in tissue that results in image formation.
what creates contrast
differential absorption
What effect does transmitted photoelectric, and compton have on contrast
transmitted black.
photoelectric are clear.
Compton are gray.
What is differential absorption dependent on
KVP or exposure.
atomic #/Z # of absorber.
Mass density of absorber.
KVP versus differential absorption
inverse relationship.
Decreased KVP = increased DA due to higher probability of photoelectric effect and low energy and this increases patient dose.
Increased KVP = decreased DA due to more transmitted photons, decreases patient dose.
Atomic # versus differential absorption
Direct relationship.
DA increases as Z # increases because probability of photoelectric effect increases with increased Z#. Z# has no effect on Compton.
DA decreases as Z # decreases due to probability of more transmitted photons with decreased Z#.
Mass density versus differential absorption
Direct relationship.
DA increases as density increases as quantity of matter per unit per volume increases. Increased matter per volume = increased interaction of both photoelectric and Compton.
Tissue densities.
Contrast agents have what effect:
increase DA, increase photoelectric absorption and Compton scatter.
Barium and iodine used because of high atomic #.
Air is a negative contrast agent.