Chapter 5 Notes Flashcards
x-ray production requires what
a source of electrons, an appropriate target material, a high voltage, and a vacuum
the x-ray tube consists of
a cathode and anode enclosed within an envelope and then encased in a protective housing
the cathode is the _____ side of the x-ray tube
negative
function of the cathode
produce a thermionic cloud, conduct the high voltage to the gap between cathode and anode, and focus the electron stream as it heads for the anode
cathode assembly consists of
the filament or filaments, focusing cup, and associated wiring
what is the filament
a small coil of thin, thoriated tungsten wire
the focusing cup serves to
narrow the thermionic cloud as it is driven to the anode
the anode is the _____ side of the x-ray tube
positive
anodes are divided into what two types
stationary and rotating
anodes are made using what
tungsten
tungsten is used to make anodes because
high atomic number, high melting point, and good heat-conducting ability
anode assembly consists of
anode, stator, and rotor
the portion of the anode where the high-voltage electron stream impacts is called
target, focus, focal point, focal spot, or focal track
effective focal spot describes
the area of the focal spot that is projected out of the tube toward the object being radiographed
line-focus principle is used to
reduce the effective area of the focal spot (permits the best detail in as large of an area as possible)
effective focal-spot is controlled by
the filament size and the anode target angle
when the target angle is less than _____ degrees, the effective focal spot is smaller than the actual focal spot
45 degrees
anode heel effects states that
radiation intensity is greater on the cathode side than on the anode side
what can cause serious degradation of radiographic image quality?
off-focus or extrafocal radiation
off-focus radiation comprises
photons that were not produced at the focal spot
three types of heating charts
radiographic tube rating charts, anode cooling charts, and housing cooling charts
heating charts are available to help radiographers
avoid thermal damage to x-ray tubes
which heating chart is most valuble
radiographic rating charts because they provide a guide regarding the maximum technical factor combinations that can be used without overloading the tube
what is the melting point of tungsten
3,370 degrees C
what are also good choices for filament material
rhenium and molybdenum
most modern x-ray tubes will last between how many exposures
10,000-20,000 exposures
focusing cup is composed of
nickel
grid-biased tubes changes charge of focusing cup from
negative to positive
grid-biased tubes permit
very short, rapid sequencing of exposures
rotating anode made of
tungsten-rhenium alloy
anode layering assists with
heat loading
when should tube warm-up procedure be performed
if tube has been idle for an extended period of time (>2 hours)
rotating anode can increase heat capacity by
300x
the actual focal spot is
the physical area of the focal track being hit by electrons
the effective focal spot is
the projected area of x-ray beam
with the anode heel effect, beam intensity can vary as much as
45%
what is the only part of the x-ray tube located outside the envelope
stator
rotor is made of
copper cylinder connected to anode disk by molybdenum stem
what is the Gyroscopic effect
when high speed anode rotations add stress to x-ray tube design
envelope is made of
heat tolerant Pyrex glass or metal
two functions of envelope
supports anode/cathode assemblies and maintains a vacuum
off-focus radiation contributes up to what percent of the total primary beam
25%
ways to extend x-ray tube life
- follow recommended tube warm-up procedures
- avoid frequent and successive “boosting” of tube
- use low mA settings when possible
- use low speed rotor rotation when possible
- follow rating charts
- avoid rough handling of x-ray tube head
- listen for unusual sounds
