Unit 1 Flashcards
Parts of a cathode assembly
Filament, Focusing cup, wiring
Filament characteristics
coil of thoriated tungsten; high melting point; difficult to vaporize; smaller coils produce best detail
Thermonic Emission
heating of the filament causes electrons to be released
How to extend tube life
depress exposure switches in one motion, decrease rotor time; excessive rotoring causes tube arching (vaporized tungsten)
Components to rotating anode assembly
anode, stator, rotor
characteristics of anode targets
high atomic number (enhances production of photons); high melting point; heat conducting ability;
Anode layering
backed with molybdenum and graphite to assist with heat loading
Target Area
portion of anode that electron stream contacts; point where x-ray photons created
Actual focal spot
physical area of the focal track that is impacted by electrons
effective focal spot
area of the focal spot that is projected out of the tube
Line-focus principle
effective focal spot is controlled by the size of the actual focal spot and anode target angle; when target angle is less than 45 degrees the effective focal spot is smaller than the actual focal spot
Anode heel effect
radiation intensity is greater on the cathode side because some rays are absorbed by the anode heel
Off focus radiation
composed of photons that were not produced at the focal spot; scatter electrons hit other structures in the tube
Stator
induction motor electromagnets send currents to the rotor to turn the anode; located outside the vacuum
Rotor
hollow copper cylinder attache to the anode disk; currents from the stator cause it to turn
Envelope
constructed around the anode and cathode and sealed tight to maintain vacuum; x-ray beams exit the tube through the window; vacuum allows electrons to flow from cathode to anode without encountering atoms in air
Protective housing
controls leakage and scatter radiation; made of lead; isolates high voltages; provides means to cool the tube
mA
how many electrons are crossing the tube
mAs
how many electrons are crossing the tube per second
Reciprocity law
Density exposure should remain unchanged as long as the intensity and duration of the x-ray (mAs) exposure remains unchanged.
mAs formula
mAs= mA x seconds
kVp
speed of electrons; high kVp = higher x-ray penetrability and quantity
15% rule
And increase of kVp by 15% will cause a doubling in exposure, the same effect as doubling the mA or doubling exposure time
Distance and x-ray emission
As SID increases, beam intensity decreases
Inverse square law
I1/I2 = (D2)^2/(D1)^2
Exposure Maintenance Formula
mAs1/mAs2 = (D1)^2/(D2)^2
Density/ IR exposure
makes detail visible; degree of blackening
Typical optical density range
.25-2.5
mAs controlling density
longer exposures increase density; need at least a 30% change in mAs to make a visible change in density
kVp controlling density
small changes make a large difference; higher kVp increases scatter radiation
Focal spot controlling density
does not effect density
Anode heel effect with density
Density is greater at cathode side of beam
SID effecting density
greater SID = less density
Beam restriction effecting density
more restriction decreases density
anatomical part effecting density
tissue thickness; tissue type; radiopaque (absorbs photons) or radiolucent
Grids affecting density
grid absorbs scatter which decreases density
Contrast
difference between adjacent densities
dynamic range
range of brightness of the display monitor light emission
High contrast
black and white; low kVp
Low contrast
many shades of gray; high kVp
film contrast
range of densities that the film is capable of recording
Subject Contrast
range of differences in intensity of x-ray beam; dependent on kVp, type of irradiated material, amount of irradiated material
kVp with contrast
higher kVp=more penetration, more scatter, lower contrast
amount of irradiated material and contrast
increase in thickness and field size increases scatter = low contrast
type of irradiated material and contrast
materials with high atomic numbers (lead & iodine) absorb x-rays more than low atomic numbers (hydrogen & carbon); large atomic number difference = high contrast
focal spot size with contrast
no effect
anode heel effect with contrast
little to no effect
beam restriction with contrast
restricting the beam reduces amount of photons available and increases contrast
SID with contrast
increase of distance increases contrast
OID with contrast
air gap causes scatter to avoid IR and increases contrast
Filtration with contrast
increases average photon energy which causes more scatter and decreases contrast
Grids with contrast
Grids reduce scatter which improves contrast
