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The xray tube and housing assembly are quite heavy; therefore, they require a ____ so the radiologic technologist can position them.
Support Mechanism
-
The ____ support system is probably the most frequently used
Ceiling
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A telescoping column attaches the xray tube housing to the rails, allowing for variable
source to image receptor distance (SID)
-
When the xray tube is centered above the exam table at the standard SID, the xray tube is in a
preferred detent position
-
Some ceiling supported xray tubes have a single control that removes all locks, allowing the tube to
float
-
The ____ has a single column with rollers at each end, 1 attached to a floor mounted rail
floor to ceiling support system
-
On the floor to ceiling support system, what slides up and down the column as the column rotates
xray tube
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A variation of the floor to ceiling support system has the column positioned on a single ___ with 1 or 2 floor mounted rails
floor support system
-
Which systems are ceiling mounted and provide for very flexible xray tube positioning
C arm support system
-
When xrays are produced, they are emitted ____ that is with equal intensity in all directions
isotropically
-
____ is when xrays are produced with equal intensity in all directions
isotropically
-
We only use the xrays that are emitted thru a special section called the
window
-
The xrays emitted thru the window are called the
useful beam
-
xrays that escape thru the protective housing are called
leakage radiation
-
What contributes nothing in the way of diagnostic info and results in unnecessary exposure of patient and radiologic technologist
leakage radiation
-
what guards against excessive radiation exposure and electric shock
protective housing guards
-
Properly designed protective housing reduces the elvel of leakage radiation to less that ____ when operated at maximum conditions
1mGy/hr at 1 m
-
The protective housing protects against
- leakage
- electric shock
- mechanical support
- cooling abilities
-
The protective housing also provides ___ for the xray tube and protects the tube from damage caused by rough handling.
mechanical support
-
The protective housing around some xray tubes contains oil that serves as an ____ against electric shock and as a thermal ____ to dissipate heat
-
The xray tube is a special type of vacuum tube that contains 2
- 2 electrodes
- the cathode and the anode
-
What are 2 electrodes within the xray tube
-
Why is the large 30-50 cm long and 20 cm in diameter glass enclosure made of pyrex glass?
to enable it to with stand the tremendous amount of heat generated
-
The glass/metal enclosure maintains a vacuum inside the tube which allows for ___ and ____
more efficient xray production and a longer tube life
-
What is an example of an early xray tube
Crookes Tube
-
What is an example of a modern xray tube
Coolidge Tube
-
The early xray tubes, modifications of the Crookes tube, were not vacuum tubes but rather contained controlled qtys of ____ within the enclosure
gas
-
The modern xray tubes, Coolidge, is a _____ tube. if it becomes gassy, xray production fails and the tube can fail.
vacuum
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_____maintain a constant electric potential between the electrons of the tube current and the enclosure and therefore have a longer tube life and less liikely to fail
metal enclosure tubes
-
What are some benefits of a metal tube over a glass tube
As the glass tube ages, some tungsten vaporizes and coats the inside of the glass enclosure, which alters the electrical properties allowing tube current to stray and interact with the glass and results in arcing and tube failure, metal tubes do not have this issue and maintain a constant electric potential between the electrons of the tube current and enclosure.
-
The ____ is the negative side of the xray tube and has 2 primary parts ____ and ____
a filament and a focusing cup
-
The ____ is a coil of wire similar to that in a kitchen toaster, but is much smaller
filament
-
What emits electrons when heated
xray tube filament
-
When the current thru the filament is sufficiently high, the outer electrons of the filament atoms are
"boiled off" and ejected from the filament
-
Filaments are usually made of
thoriated tungsten
-
______ provides for higher thermionic emission than other metals
tungsten
-
What is the most common cause of tube failure
Tungsten vaporization with deposition on the inside of the glass enclosure
-
The filament is embedded in a metal shroud called the
focusing cup
-
Because all of the electrons accelerated from the cathode to the anode are electrically ____, the electron beam tends to spread out owing to electrostatic repulsion. Some electrons can even miss the anode
negative
-
The focusing cup is _____ charged so that is electrostatically confines the electron beam to a small area of the anode
negatively charged
-
The focusing cup is negatively charged so that it can electrostatically confines the _____ to a small area of the ____
-
The effectiveness of the focusing cup is determined by
- it shape and size
- position of the filament in the focusing cup
-
Most rotating anode xray tubes have ___ filaments mounted in the cathode assembly side by side creating large and small focal spot sizes
2
-
Most rotating anode xray tubes have 2 filaments mounted in the ____ assemble side by side creating ____ and______
- cathode
- large and small focal spot sizes
-
The xray tube _____ is adjusted by controlling the filament current
current
-
The xray tube current is adjusted by
controlling the filament current
-
When the filament emits the electrons, they hang around before going to the anode causing a cloud of electrons around the filament, this is called a
space charge
-
Space charge makes it difficult for subsequent electrons to be emitted by the filament because of ____. this phenomenon is called
- electrostatic repulsion
- space charge effect
-
What makes it difficult for subsequent electrons to be emitted by the filament because of electrostatic repulsion?
space charge
-
Thermonic emission at low kVp and high mA can be
space charge limited
-
What is filament current that has risen to its max value because all available electrons have been used
saturation current
-
Most diagnostic xray tubes have ____ focal spots
-
Which focal spot is used when better spatial resolution is required
- small focal spot
- like an ankle
-
Whic focal spot is used when imaging a larger body part and when other techniques that produce high heat are required
large focal spot
-
Normally either filament can be used with the lower mA station-approx ____ or less
300 mA
-
At approx ____ and up, only the larger focal spot is allowed because the heat capacity of the anode could be exceeded if you use the small
400 mA
-
At what mA do you use the larger focal spot and why
400 mA because the heat capacity could be exceeded if you use the small
-
Small focal spots range from ____to_____ mm
0.1-1
-
Large focal spots range from _____to______mm
0.3-2mm
-
What are the 2 types of anodes
-
Where are stationary anode xray tubes used
- dental xrays
- portable imaging
- special purpose units in which high tube current and power are not required
-
Where are rotating anode xray tubes used
- general purpose
- because they must be capable of producing high intensity xray beams in a short time
-
What is the positive side of the xray tube and conducts electricity and radiates heat and contains the target
anode
-
What 3 functions do the anodes serve in an xray tube
- electrical conductor
- mechanical support
- thermal dissapater
-
The anode is an/a _______ when it receives electrons emitted by the cathode and conducts them thru the tube to the connecting cables and back to the high voltage generator
electrical conductor
-
The anode also is a thermal dissipater when the projectile electrons from the cathode interact with the anode, more than ___ of their kinetic energy is converted to ____
-
What is the area of the anode struck by the electrons from the cathode
target
-
in stationary anode tubes, the target consists of a ____
tungsten alloy embedded in the copper anode
-
In rotating anodes, the target is the
entire rotating disc
-
Tungsten is the material of choice for the target for general radiography for what 3 reasons
- atomic #
- thermal conductivity
- high melting point
-
Tungsten's ____ results in high efficiency xray production and in high energy xrays
high atomic #, 74
-
Tungsten has a ____ nearly equal to that of copper which is why its an efficient metal fro dissipating the heat produced
thermal conductivity
-
Which anode xray tube allows the electron beam to interact with a much larger target area therefore the heating of the anode is not confined to 1 small spot
rotating anode
-
What are possible with the rotating anode
- higher tube currents
- shorter exposure times
-
Higher tube currents and shorter exposure times are possible with the rotating anode
-
What is used to turn the anode
electromagnetic induction motor
-
Rotating anode is powered by
electromagnetic induction motor
-
Where on the target are xrays emitted
focal spot
-
As the size of the focal spot decreases, the heating of the target is concentrated onto a smaller area. This is the ____ to the focal spot size
limiting factor
-
Radiology requires ___ because the ____ the focal spots, the better the spatial resolution of the image.
-
The ____ is the actual xray source
focal spot
-
Before the rotating anode, the ____ was incorporated into xray tube targets to allow a large area for heating while maintaining a small focal spot
line-focus principle
-
Diagnostic xray tubes have ____ that vary from approx 5-20 degress
target angles
-
What results in an effective focal spot size much less than the actual focal spot size
Line focus principle
-
What is an unfortunate consequence of the line focus principle where the radiation intensity on the cathode side of the xray is greater than that on the anode side
heel effect
-
The heel effect is when the radiation intensity on the ____ side of the xray field is greater than that on the _____ side due to the line focus principle
-
The ____ the anode angle, the ____ the heel effect
-
The difference in radiation intensity across the useful beam of an xray field can vary by as much as
45%
-
The ____ of the useful beam is the imaginary line generated by the centermost xray in the beam
central ray
-
If radiation intensity along the central ray is 100%, then the intensity of the cathode is ____ and that on the anode is ____
- as high as 120%
- as low as 75%
-
What is important when imaging anatomical structures that differ greatly in thickness or mass density
heel effect
-
Positioning the ____ side of the xray tube over the thicker part of the anatomy provides more uniform radiation exposure of the image receptor
cathode side
-
Where are the cathode and anode directions usually indicated
on the protective housing, sometimes near the cable connectors
-
What results in smaller, effective focal spot and less radiation intensity on the anode side of the xray beam
heel effect
-
The heel effect results in smaller effective focal spot and less radiation intensity on the ____ side of the xray beam
anode
-
When electrons bounce off the focal spot and then land on other areas of the target, causing xrays to be produced from outside of the focal spot it is called
off focus radiation
-
Heat from the xray tube can be dissipated in what 3 ways
- radiation
- conduction
- convection
-
What is the transfer of heat by the emission of infared radiation
Radiation
-
What is the transfer of energy from 1 area of an object to another
Conduction
-
What is the transfer of heat by the movement of a heated substance from 1 place to another
convection
-
Excessive heat results in
reduced xray tube life
-
What results in reduced xray tube life?
excessive heat
-
All 3 modes of heat transfer occur in an xray tube. Most of the heat is dissipated by radiation during
exposure
-
What should never be applied to a cold anode
maximum radiographic technique
-
How does a 2nd type of xray tube failure result
from maintaining the anode at elevated temps for prolonged periods
-
What is the most frequent cause of abrupt tube failure
electron arcing from the filament to the enclosure because of vaporized tungsten
-
The filament wire becomes thinner and eventually breaks, producing
an open filament
-
What is the primary support structure for the xray tube which allows the greatest ease of movement and range of position
Ceiling support system
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