Attenuation and Attenuation Coefficients Flashcards
What is attenuation?
reduction of the intensity of the primary beam following penetration through a given thickness of a material
in x-rays; photoelectric absorption and Compton scatter
Photoelectric Absorption
X-rays in the diagnostic range may undergo ionizing interactions with the inner shell electrons
Inner-shell electron absorbs all the energy carried by the x-ray photon
has kinetic energy equla to the difference between the energy of the incident x-ray and the binding energy of the electron
what happens to the ejected photoelectrons?
they interact with other atoms within the tissue and never penetrate out of the patient’s body
- don’t impact image
what energy must have the incident x-ray have in PE?
must have energy equal to or greater than the electron binding energy
PE probability
inversely proportional to the 3rd power of the photon energy
directly proportional to the third power of the atomic number of the absorbing material
directly proportional to density
PE and secondary characteristic x-rays
ejection of a k-shell electron results in an orbital vacancy
outer shell electron immediately drops into vacancy
emission of an x-ray with energy equal to the difference between the binding energies of the shells involved
- secondary characteristic radiation
no effect on radiographic image
Compton scattter
x-ray photon hits a lossely-bound outer shell electron
electron absorbs some of the photon’s energy and is ejected
photon, having lost some energy it is deflected and scattered
can lose upto 1/3 its original energy
compton scatter mathematically
Ep=Es+Ebe+Eke
Ep is primary photon energy
Es is energy of scattered photon
Ebe is binding energy of orbital electron
Eke is kenetic energy of recoil electron
compton scatter energy
most energy from incident photon is retained
scattered x-rays with higher energy are deflected less; are more likely to strike the image receptor
- 84% retained has a 90 angle
primary contributor of radiation dose to personnel
compton scatter probability
inversely proportional to xray energy
directly proportional to density
differential attenuation
x-rays that undergo PE interactions are completely absorbed and represent structures with higher Z and density - radiopaque
transmitted x-rays produce the dark areas of a radiograph - radiolucent
inherent subject contrast is generated by the difference in attenuation between structures
differential attenuation and kVp
for the most part, we try to maximize differential attenuation
differential attenuation increases as kVp is decreased
reducing kVp does however also increases patient dose
Differential absorption and Z; kVp
much more absorption in bone than soft tissue
- probability decreases with increasing energy
- contrast decreases as energy decreases
the probability of compton scatter for bone and soft tissue is more equal
- probability decreases with increasing energy
- more predominant than PE at higher kVp’s
Differential attenuation and mass density
mass density is the quantity of matter per unit volume; how tightly atoms of a substance are packed together
the interaction of x-rays with tissue is proportional to the mass density of the tissue regardless of the type of interaction
affects imagin at high and low kVp’s
Contrast agents
enhance differential absorption in homogenous tissue
Barium (Z=56) and Iodine (Z=53) compounds are used as an aid for imagin internal organs with x-ray
much higher atomic number and mass densities than body tissues
As x-ray energy increases
fewer compton interactions
even fewer PE interactions
more transmission through tissue
as tissue Z increases
no change in compton
many more PE interactions
Less x-ray transmission
As tissue mass density increases
proportional increase in compton
proportional increase in PE
proportional reduction in X-ray transmission
attenuation coefficients
tells us the effectivenes of different material as attenuators of radiation
absoption is t
scatter is sigma
total linear attenuation coefficient
value of total linear coefficient depends on probability of interactions between x-ray photons and atoms of the attenuator
number of interactions per unit distance travelled through a medium deoends on:
- beam energy
- Z number
- the spacing between the atoms in the medium - interactions are more likely to occur when atoms are densely packed
total mass attenuation coefficient
common to divide the total linear attenuation coefficient by its density
comprised of T/p and sigma/p
PEMAC
probability that a photon will under go a PE interaction is proprtional to 1/E3
t/p = 1/E3
t/p=Z3
t/p = p
CEMAC
Probability of CS decreases as energy increases
sigma/p=1/E
probability of CS increases as density increases
sigma/p=p
Factors affecting attenuation coefficients
Better attenuators have higher u value
higher beam energy = lower u
therefore u is dependent on tissue type and energy of the incident beam
Within the patient’s body, a 30 keV x-ray undergoes a
photoelectric interaction with a K-shell electron in a calcium
atom. If the electron binding energy of a K-shell electron in
a calcium atom is 4 keV, what will be the kinetic energy of
the photoelectron speeding away from the atom?
26 keV
