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70 Cards in this Set

  • Front
  • Back
the ability of xray photons to penetrate matter is dependent on
their energy
the energy level of x-ray photons determines
the amount of penetration that you get rather than absorption
how are x-ray photons released
- a tungsten filament is heated until it releases electrons in a cloud
- electrons are acclerated through an electromagnetic field
- they collide with a tungsten target, transferring their energy and releasing it as an x-ray photon
x-rays are generated out of
k, l, m orbitals
components of a x-ray head
- tube
- power source
- collimator
- aiming cylinder
- oil
- aluminum filter
why is the head filled with oil
to dissipate heat
standard for aluminum filtration
1.5 mm to 70 kVp (2.5 above 70 kVp)
what does the collimator do
narrows the x-ray beam
aiming cylinder also known as the
cone
what does aiming cylinder do
also narrows the beam
why do we use tungsten
- large atom with a lot of electrons (high atomic number)
- high thermal conductivity
- high melting point
- low vapor pressure
what does the focusing cup do
localizes electrons
electrons are sped up by the
kVp setting
kVp setting determines the
energy level of electrons
quality setting
kV
quantity setting
milliampage and exposure time
focal spot
the focal spot is the area on the tungsten target to which the focusing cup directs the electrons from the filament
the sharpness of the radiographic image __________ as the size of the focal spot __________
increases; decreases
effective focal spot
the projection of the actual focal spot, perpendicular to the electron beam
actual focal spot size
1 mm x 3 mm
effective focal spot size
1 mm x 1 mm
why is angulation of the target importent
it decreases the size of the effective focal spot, which increases sharpness
typical angulation of target
20 degrees
as the angulation of the target increases
the focal spot increases
how does emulsion effect sharpness
emulsion on both sides decreases sharpness
bremsstrahlung radiation is produced by
- direct hit of electron on nucleus in the target
- or by the passage electrons near the nucleus, which results in the electrons being deflected and decelerated
maximum energy of a photon is generated when
a high energy electron directly hits the nucleus and transfers energy
when you double exposure time ______________________________
you double the relative number of photons generated
what happens to the mean energy level of the electron beam as you double exposure time
it remains the same
characteristic radiation
occurs when an electron from the filament displaces an electron from a shell of a tungsten target atom
what happens if you double the mA setting
you double the amount of photons generated
what happens when you increase kVp
1- increase number of photons
2- increase mean energy level
3- increase maximal energy
high energy photons have a higher probability of __________________
penetrating matter
low energy photons have a higher probability of ____________________
being absorbed
how do you double darkness? what if its too dark?
change the kVp setting by 15; decrease the time
how collimation of an x-ray beam acheived
by restricting its useful size
name different types of collimators
round collimators
rectangular collimator
target-to-film distance in short cones
8 inches
target-to-film distance in long cones
16 inches
which has a narrower electron beam: long or short cone?
long cone
compton absorption
occurs when an incident photon interacts with an outer electron, producing a scattered photon of lower energy than the incident photon and a recoil electron ejected from the target atom
compton absorption makes up what percentage of the interactions of photons
49%
coherent scattering
low-energy incident photon interacts with an outer electron, causing electron to vibrate momentarily. a scattered photon of the same energy level is emitted at a different angle
coherent scattering makes up what percentage of photon interactions
7%
photo-electric absorption
an incident photon gives up all its energy to an inner electron, which is ejected from the atom (photo-electron)
photo-electric absorption makes up what percentage of photon interactions
23%
exposure = ?
the measure of radiation quantity to ionize air
what is the SI unit for exposure
KERMA (kinetic energy released in matter)
KERMA =
joules/kg
absorbed dose (Dt)
measure of the energy absorbed by unit mass by any type of matter
unit of absorbed dose (Dt)
grey (used to be rads)
1 grey = ?
100 rads
equivalent dose (Ht)
compares the biological effects of different types of radiation on an organ or tissue
unit of measure of equivalent dose
sievert
Wr for photons =
1
Wr for 5 keV neutrons =
5
Wr for alpha particles =
20
how do you find Ht
the sum of all Wr multiplied by Dt
effective dose (E)
estimates risk of radiation in humans or whole body
Wt for breast, colon, lung, stomach, red bone marrow
.12
Wt for gonads
.2
Wt for bladder, esophagus, liver, thyroid
.04
Wt for bone surface, skin, salivary glands, brain
.01
how is E calculated
sum of Wt multiplied by equivalent dose
radioactivity
describes the decay rate of radioactive materials
SI unit of radioactivity
bacquerel (Bq)
key interproximal space for PA's
between 1st molar and 2nd premolar - it should be open
attenuation
the reduction of intensity of a photon beam as it passes through matter, resulting through interactions of individual photons with atoms of the absorber
main means of attenuation of a photon beam
- coherent scattering
- photoelectric absorption
- compton absorption
- 9% of photons pass through the head without interaction
which form of attenuation is most important for high quality radiographs
photoelectric absorption