Ch 6 The X-ray Tube Flashcards
the anode is the _____ end, the cathode is the ____ end
positive, negative
x-rays travel from _____ to ______
cathode to anode
where is heat the biggest problem?
at the anode
the focal spot is the _____ source, they get there through______
electron, boiling off of the filament
there are ___ filaments
2, a small focal spot and a big focal spot
filaments are usually made out of:
thoriated tungsten
what are the rmp rates for rotating anodes?
3,400 rmp & 10,000 rpm
the _______ is on the anode side
induction motor
the induction motor is made up of 2 parts:
rotor and stators
the rotor:
rotates the anode, it is inside the glass tube proper
the stator:
is made of electromagnets, provides electric and magnetic fields and electron flow, creates the force that creates the rotation, is outside of the glass tube proper
the target is found on the _____ and is made of _____
anode, tungsten
the rotor is on the ______ side and is made of _____
anode, tungsten
the actual focal spot is:
where the electrons actually hit, where the heat is, is bigger to dissipate heat better
the effective focal spot is:
what hits the IR, is smaller, provides detail
cathode end has __% of the beam and it used on ____ parts, generally placed at the ____
> 100% beam, thicker parts, placed at the head end
anode end has __% of the beam and is used on _____ parts, generally places at the ____
<100% beam, thin parts, placed at the feet end
the window is:
made of pyrex glass, is thin, what the photons go through
extra-focal x-rays are:
radiation produced anywhere else besides the anode target area of focal spot
the primary parts of the x-ray tube are:
cathode (-) and anode (+)
any electric tube with 2 electrodes is a:
diode
the external structure of the x-ray includes:
the support structure, protective housing, and a glass or metal enclosure
when x-rays are produced they are emitted:
isotropically (with equal density in all directions)
x-rays emitted through the window are:
the useful beam
leakage radiation:
x-rays that escape through the protective housing, they contribute nothing to diagnostics
protective housing reduces:
the level of leakage radiation to less than 1 mGya/hr at 1m OR 100mR/hr at 1m from the source
*protective housing guards against:
excessive radiation exposure and electric shock
protective housing around some x-ray tubes contains oil that:
serves as an insulator against electric shock and as a thermal cushion to dissipate heat
glass enclosures are made of ____ and are able to ____
pyrex glass, withstand tremendous heat
the enclosure maintains:
a vacuum inside the tube
the modern x-ray tube is called ____
Coolidge tube or vacuum tube
metal enclosures maintain:
a constant electric potential b/w the electrons of the tube current and the enclosure, they have a longer life and are less likely to fail
*x-ray tubes are designed with ___ or ___ enclosures
glass or metal
the x-ray tube window is where ______, it allows____
the useful beam of x-rays is emitted, it allows maximum emission of x-rays with minimum absorption
*the primary parts of the cathode are:
the filament and the focusing cup
a filament is a:
coil of wire
the filament emits:
electrons when it is heated
when the current through the filament is high enough the:
outer shell electrons get boiled off and ejected from the filament, aka thermionic emission
filaments are usually made out of:
thoriated tungsten
*tungsten vaporization causes:
deposits on the inside of the glass enclosure, it is the most common cause of tube failure and can cause arcing
the filament is embedded in:
a metal shroud called the focusing cup
the focusing cup is ___ charged so that is:
negatively, so that is electrostatically confines that electron beam to a small area of the anode
most rotating anodes have __ filaments
2, a large and a small focal spot
grid-controlled tubes are designed to:
turn on and off rapidly
*x-ray tube current is adjusted by:
controlling the filament current
because electrons carry negative charges:
they repel one another and tend to form a cloud around the filament
this cloud of electrons is called _____ and it:
space charge, it makes it difficult for other electrons to be emitted by the filament b/c of electrostatic repulsion (electrons are stuck at the cathode end b/c they are attracted to the focusing cup)
*thermionic emission at ___ kVp and ___ mA can be space charge limited
low kVp and high mA
saturation current:
at any given filament current, the x-ray current rises with increasing voltage to a maximum value, a further increase in kVp doesn’t result in a higher mA because all available electrons have been used
filament current is measured in ____
tube current is measured n ____
A
mA (technique factor)
small FS is used when, measurement:
better spatial resolution is required, 0.1-1mm
large FS is used when, measurement:
large body parts or when techniques that produce high heat are used, 0.3-2mm
stationary anodes are used in:
dental, portables, and special units where high tube current and power are NOT required
*the anode does 3 things:
conducts electricity, radiates heat, and contains the target
the anode receives electrons emitted by the cathode and conducts them through the tube and back to the _____
high-voltage generator
the anode is a ______, and also provides ______, and must be a good ______
electrical conductor, mechanical support for the target, thermal dissipator
more than __% of _____ turns into heat
99% of electron kinetic energy
most common anode materials:
copper, molybdenum, and graphite
in STATIONARY ANODES the target is:
tungsten alloy embedded in a copper anode
in ROTATING ANODES the target is:
a rotating tungsten disc
tungsten is used for the target for 3 reasons:
- high atomic # (74), high energy x-rays
- thermal conductivity, efficient metal for dissipating heat
- high melting point, 3,400C can withstand high tube current
the rotating anode provides ____ times more _____
500x more area to interact with electron beam
*____ tube current and _____ exposure times are possible with the rotating anode
high tube current and shorter exposure times
*the focal spot size is the:
actual x-ray source
*line-focusing principle:
by angling the target, the effective area is much smaller than the actual area
target angles can vary from ______
5-20 degrees
the intensity of the x-rays emitted on the heel end of the target are:
reduced because they have a longer path and increased absorption
*the smaller the angle, the ___ the heel effect
larger
at the CR the beam intensity is __%
100%
at the cathode side the beam intensity:
can be up to 120%
at the anode side the beam intensity:
can be as low as 75%
positioning the cathode end over the _____ anatomy provides _____
thickest anatomy provides more uniform exposure (density)
*heel effect results in ___ effective FS and ___ radiation intensity on the anode side of the beam
smaller effective FS and less radiation intensity
radiation:
transfer of heat by emission of infrared radiation
conduction:
transfer of energy from one area of an object to another
convection:
transfer of heat by the movement of a heated substance from one place to another
*excessive heat results in:
reduced tube life
*maximum techniques should:
never be applied to a cold anode
*most frequent cause of abrupt tube failure is:
electron arcing from the filament to the enclosure because of tungsten vaporization
radiographic rating chart tells you:
which techniques are safe to use and which are not safe to use for x-ray tube operation
in the rating chart the x & y axis are:
kVp & time
the curve represents:
tube currents in mA
any combo of kVp & time that is below the mA curve is:
safe to use
anode cooling chart tells you:
the thermal capacity of the anode (maximum heat capacity) and determines the length of time required for complete cooling
single phase HU=
kVp x mA x s = 0.7J
3 phase/high frequency HU=
kVp x mA x s = 1J
complete cooling of the housing requires ___ hours
1-2 hours
