Rad Science AEC Flashcards
Device that measure the quantity of radiation reaching the IR
Automatic Exposure Control
It automatically terminates the exposure when the IR has received enough radiation to provide the required optical density
AEC
only controls time
AEC
_must be set in case of AEC failure (150% of anticipated exposure)
Back-up timer
-2, -1, +1, +2 increases or decreases densityby _ % for each step
25%
_are used to compensate for pathology, not part thickness
Density controls
2 types of AEC
phototimer
ionization chamber
_ the most common AEC ( ionizes gas inside chamber)
ionization chamber
must have accurate
positioning and photocell selection
The _ (made of tungsten)are located w/in the focusing cup.
Filament
__ has a high melting point and it is hard to vaporize (turn to gas)
Tungsten
most common cause of tube failure is _ w/deposits on the inside of the glass envelope
tungston vaporization
the filament emits electrons when sufficiently heated. The outer shell of electrons are boiled off. This is _ and requires - amps of current
thermionic emission
4-6
Because electrons are negative, they repel each other and sit around the filament waiting to be sent to the anode. This is referred to as the
electron cloud (space charge effect)
2 types of anodes
stationary anode
rotationg anode
Staionary anode is used in
low powered systems (dental)
rotating anode is used in
x-ray tubes
The _ is the area of the anode struck by electrons from the filament
target (focal spot, focal tract)
The filament is made of
tungston and alloyed rhenium
Tungston is used for the target for 3 reasons
atomic #, high 74
thermal conductivity dissipates heat well
high melting point, little pitting
the rotating anode allows for _ over a much larger area
electron interactions
_ is the source of radiation
focal spot
useful radiation; radiation between tube and patient
primary radiation
the _ is located between the anode and rotor (made of molybdenum)
stem
intensity is greater on the _ side of the tube because some intensity is absorbed by the angle of the _ ((heel)
cathode
anode
the smaller the target angle, the larger the
heel effect
place the thicker portion of the subject under the _ side of the tube for more intensity
cathode
electrons should hit the focal spot of the _. Some hit and bounce off and land outside of the focal spot. This can increase _, reduce _ and image tissue collimated off.
anode
pt. dose
contrast
