attenuation Flashcards
primary components of x ray machine
- tube and power supply in tube head; head supported by arm
- control panel to adjust duration, energy, and exposure rate
kvp controls
how strong beam is
mA controls
filament heating
when do you change kvp and ma
for larger or pedo patients
high mA and kV means
high density
mA is
quantity of x ray
kV is
quality of x ray
alternating current
when flow of electrons move in one direction and then reverse in opposite direction
frequency
of alternating current cycles/second = 60 Hz
voltage
electrical pressure pushing electrons thru a wire
current (ampere)
number of electrons flowing in a wire/second
x ray generator
supplies electric power to tube
low voltage circuit
- heats tungsten filament
- provides electrons by heating the filament to incandescence
high voltage circuit
- create flow of electrons bw anode and cathode
- provides potential difference between anode and cathode
step up transformer
high voltage current (60-120)
creates potential difference between anode and cathode
step down trans
low voltage current
heats tungsten filament
primary functions of autotransformer
- stabilize incoming electricity (voltage)
- adjusts the voltage flowing to the tube head
mA controls
temperature of filament and size of electron cloud
- high–> more e- strike target making more xrays
- low–> fewer e- strike target making fewer x rays
longer time–>
more x rays
shorter time–>
fewer x rays
double exposure time–>
2X more radiation
in old x rays, only —- were usable
60 of alterations
modern dental x rays
-replace 60 cycle with high f power supply resulting in constant potential bw cathode and anode
tube rating
-guide the longest exposure time for range of KV and mA without damage
duty cycle
freq with which successive exposures can be made without overheating the anode
quantity
number of photons
quality
energy of photons
intensity
total energy (quality x quantity) contained in the beam per unit time per unit area
how manufacturer controls intensity
- target material
- filtration
how practitioner controls intensity
kVp
mA
time
source-receptor distance
why use tungsten for target
- high atomic number
- high melting temp
- large # of electrons
filtration
reduces intensity of primary x ray beam by selectively absorbing out useless, low energy x rays
inherent filtration
- glass wall of tube, oil, aperture window
- not sufficient to meet NCRP standards
added filtration
thin sheet of aluminum
total filtration=
added + inherent
influence of filtration
- mean photon energy increases
- total number of photons decrease
- most photons removed are lower energy photons
- helps reduce harmful effects of radiation
50-70 kVp –>
1.5 mm Al
above 70 kVp–>
2.5 mm Al
what happens if you double the mA
twice as many electrons will be released but mean energy doesn’t change
the longer the exposure time, the —-
more x rays are produced
exposure time affects the —, not the —- of the beam
affects the quantity not the quality
increasing kVp–>
increases number of x rays produced (quantity) and also increases energy of beam (quality)
15 kVp increase
increases intensity by 2X
15 kVp decrease
decreases intensity by 1/2 or 50%
half value layer
- expresses quality of beam
- thickness of an absorbing material necessary to reduce the x ray intensity to 1/2 of its original intensity
what are allowed to exit the tube housing
only useful, primary beam xrays
the intensity —- as distance from the focal spot —-
intensity decreases as distance from focal spot increases
inverse square law
intensity of radiation varies inversely as square of the source-film distance
change from 8 to 16 BID – whats the intensity
(distance doubled)
1/(2^2)=1/4 original intensity
three interactions bw patient and x ray
- photons scattered
- photons absorbed
- photons pass completely thru patient
coherent scattering
incoming photon interacts with atom and sets it in motion and then disappears
- low energy photon
- interacts with whole atom & incident photon ceases to exist
- excited atom returns to stable state and generates another photon
- secondary photon has same energy as incident photon
- minimal contribution to film fog
compton scattering
- x ray interacts with outer shell e- in pt’s tissue
- electron ejected
- scattered photons and e continue and interact
- photons travel in all directions and cause image degradation/film fog
photoelectric absorption
- incident x ray has more energy than binding energy of k shell e- in tissue
- incident x ray completely disappears
- photoelectron is generated (ejected from atom)
no interaction =
black/radiolucent
scattering=
hazy (fog)
absorption=
radiopaque (white and gray)
factors influencing absorption
- photon energy (kVp)
- thickness of structure
- density of structure
- atomic number
higher kVp results in
shorter wavelengths
- increased energy –> greater penetration of radiation
- higher energy–> less absorption
the thicker the structre, the greater—
the amount of absorption
density of tissues
compactness of material
- mass/volume
- greater density–> greater absorption
atomic number’s effect on absorption
more effect than density or thickness
- increased probability of interactions
- greater atomic number–> greater absorption
differential absorption
different tissues absorb x rays in varying degrees
