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RTE 141 ch. 7
What are the 2 internal primary parts of the xray tube?
Cathode -- negative end, consists of filaments, focusing cup, and the source of electrons needed for x-ray production
Anode -- positive end
Both cathode and anode are an?
electrode; 2 or more equals diode
3 parts of the external structures of the xray tube
support structure, protective housing, glass or metal enclosure
3 types of support structures
Floor to ceiling
Most frequently used support structure
Has 2 perpendicular sets of ceiling mounted rails (allows for long. and transverse travel of xray tube)
Ceiling support structures
Support structure with single column with rollers at each end
1 attached to a ceiling mounted rail and the other attached to a floor mounted rail
Floor to ceiling support structures
Support structure used in angiointerventional radiology suites
Ceiling mounted, flexible
Variations called L-arm or U-arm
C-arm support structures
Position when the x-ray tube is centered above the exam table at SID
Properly designed protective housing reduces the level of radiation to less than
100 mR/hr at 1m, when operated at maximum conditions
X-rays that escape through protective housing and contribute to nothing
When x-rays are produced, they are emitted _____, which means equal intensity in all directions
X-rays used only thru this special section
X-rays emitted thru the window are called
Protective housing guards against these 2 things
Protective housing also provides
Mechanical support for the xray tube; protects from damage caused by rough handling
The protective housing around some x-ray tubes contains oil that serves as both an ______ and _______ to dissipate heat.
Some protective housings have a ____ ____ to air-cool the tube or the oil in which the xray tube is immersed.
An x-ray tube is an electronic vacuum tube with components contained with in a _____ or ____ enclosure
Glass or metal
What is a glass enclosure made of?
Made of Pyrex to enable it to withstand the heat generated
What does a metal enclosure do?
Maintains constant electric potential between the electron of the tube current and the enclosure
Why is a metal enclosure preferred?
Less likely to fail
As glass tubes age, what happens?
Some tungsten vaporizes and coats the inside of the glass enclosure
This alters the electrical properties of the tube
Results to arcing and tube failure
2 types of internal components
2 filaments supply separate electron beams to produce ____ _____ ____
2 focal spots
What is a filament?
Coil of wire made of tungsten w/ 1%-2% thorium added
2 mm diameter 1-2 cm long
Emits electrons when heated
What is thermionic emission?
The filament is heated and electrons are boiled off (electrons are emitted)
What are filaments made of ?
Why use thoriated tungsten?
Provides for a higher thermionic emission
Melting point 3410 degrees Celsius;
Doesn't vaporize easily
Enhances the efficiency of thermionic emission Prolongs tube life
2 disadvantages of Tungsten
Upsets some of the electric characteristics of tube which can cause arcing and lead to tube failure
What if the focusing cup?
Metal cup, made of nickel
Holds the filament
Has a negative charge so that it electrostatically confines the electron beam to a small area of the anode
Effectiveness of the focusing cup is determined by these factors
Filament size and shape
Position of the filament in the focusing cup
What is designed to be turned off and on very rapidly?
Grid controlled x-ray tube
What is a grid controlled x-ray tube used in?
Portable capacitor discharge imaging systems, digital subtraction angiography
What does the term "grid" refer to ?
Refers to an element in the tube that acts as the switch
The term "grid" refers to an element in the tube that acts as the switch.
In a grid controlled x-ray tube the ___ ___ is the ____ , and therefore, the exposure switch.
Focusing cup; grid
What is the filament current?
The current used to the heat the cathode to the operating temperature
At low filament current, there is no _____ ____ because the filament does not get hot enough for the thermionic emission.
The x-ray tube current is adjusted by controlling what?
What is the space charge effect?
The excess of electrons or ions in a given volume
Define saturation current
Increasing the kVp does not result in a higher mA because all of the available electrons have been used. In general, it means the maximum current which can be obtained under certain conditions.
________ is not reached at lower kVp because of space charge limitation
Name 2 focal spot sizes
* usually made with the mA station selector on the operating console.
Large focal spot size is used for?
Used when large body parts are imaged
Small focal spot size used for?
Used when better spatial resolution is required
What is an anode?
Positive side of the x-ray tube
It conducts electricity, radiates heat and contains the target
Name 2 types of anodes
What type of anode is used in dental x-rays (some portable)?
What type of anode is capable of producing high intensity x-ray beams in short time?
List 3 functions of the anode
Electrical conductor - receives electrons emitted by the cathode
Mechanical support for target
Thermal dissipator (dissipates heat)
3 most common anode materials
What is the target?
The area of the anode struck by the electrons from the cathode
Stationary anode target consists of what materials?
Tungsten alloy embedded in the copper anode
What is the target in a rotating anode tube?
The entire rotating disc is the target
What materials does high capacity x-ray tubes have layered under the Tungsten target?
Why is molybdenum and graphite used in high-capacity x-ray tubes?
Because they have lower mass density than tungsten, making the anode lighter and easier to rotate
Name 3 reasons why tungsten is the material of choice for the target
- Atomic number (high, 74)
- Thermal conductivity (nearly equal to copper, efficient for heat dissipation)
- High melting point (3400 C)
What allows the electron beams to interact with a much larger target area as opposed to stationary; and is not confined to one small spot?
Heat capacity can be further improved by increasing the speed of
Most rotating anode revolve at 3,600 rpm; or up to 10,000 rpm for high-capacity tubes
What is an induction motor?
Electromagnetic induction motor used to turn the anode.
What is the outside enclosure of the induction motor?
Consists of a series of electromagnets, equally spaced around the neck of the tube
What is the inside enclosure of the induction motor?
Shaft made of bars of copper and soft iron fabricated into one mass
What is coast time?
"coasting to rest" after use.
In a new x-ray tube, the cost time is approximately 60s
With age, what happens to the coast time?
The coast time is reduced because of wear of the rotor bearings
What is the line-focus principle?
Results in an effective focal spot size smaller than actual focal spot by angling the target
What is the actual x-ray source?
Diagnostic x-ray tubes have target angles that vary from approx. _____ degrees.
Define the heel effect
Intensity of x-rays that are emitted through the "heel" of the target is reduced because they have longer path through the target, and therefore increased absorption
The smaller the anode ____, the larger is the heel effect
What is the source of electrons?
In the useful beam, what is the imaginary line generated by the centermost x-ray in the beam?
Central ray = 100%
Intensity on cathode side = 120%
Intensity on anode side = 75%
Define off-focus radiation
X-ray created in areas outside the actual focal spot
The use of a _____ doesn't reduce off focus radiation
How is the x-ray tube life extended?
By using minimum radiographic factors of mA, kVp, and exposure time appropriate for each exam
Heat is generated in the ____ during exposure
Must be dissipated for it to continue
List 3 ways heat is dissipated
What is the transfer of heat by emission on infrared radiation?
* most of the heat is dissipated by radiation during exposure
What is the transfer of energy from one area of an object to another?
What is the transfer of heat by the movement of a heated substance from one place to another?
Define open filament
With excessive heat of the filament, the filament wire becomes thinner and eventually breaks
List the 3 types of x-ray tube rating charts
Radiologic radiation chart
Anode cooling chart
Housing cooling chart
Which chart is most important because it conveys which technique is safe to use and which is not?
Radiologic radiation chart
* combination of kVp and time that lie below the mA curve is safe
The thermal capacity of an anode and its heat dissipation characteristics are contained in which rating chart?
Anode cooling chart
* also used to determine length of time required for complete cooling after high-heat input
What does a housing cooling chart tell you?
Complete cooling required for the x-ray tube housing
* after maximum heat capacity, requires 1-2 hours, and twice that amount of time without fan-powered air circulation.
What is the measure for thermal energy?
Heat units (HU)
Size of the x-ray tube window
Higher tube currents and shorter exposure times are possible with what?
What is the formula for heat units?
1 kVp x 1 mA x 1 s
What is the HU formula for a single phase?
kVp x mA x s
What is the HU formula for a 3-phase/high-frequency?
1.4 x kVp x mA x s
What is the area being bombarded by the filament electrons?
The actual focal spot
What is the effective focal spot?
The origin of the x-ray beam and is the area as seen from the patient's perspective