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RadTherapy
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The x-ray tube and housing assembly are quite heavy; what is required so the radiology tech can position them?
support mechanism
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What support system is probably the most frequently used?
ceiling support
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What is telescoping column attaches the x-ray tube housing to the rails?
source-to-image receptor distance (SID)
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When the x-ray tube is centered above the examination table at the standard SID, the x-ray tube is in what position?
preferred detent position
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Some ceiling-supported x-ray tubes have a single control that removes all locks, allowing the tube to ____.
"float"
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Which support system have a single column with rollers at each end, one attached to a ceiling mounted rail and the other attached to the floor-mounted rail?
floor-to-ceiling support system
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On which support system the x-ray tube slides up and down the column as the column rotates?
floor-to-ceiling support system
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Which support system has the colum positioned with one or two floor-mounted rails?
Single floor support system
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What system is ceiling mounted and provide for very flexible x-ray tube positioning and shaped like a C?
C-arm
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When x-rays are produced with equal intensity in all directions, how are they emitted?
isotropically
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We use only x-rays emitted through the special section of the x-ray tube called
window
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The x-rays emitted through the window are called
useful beam
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X-rays that escape through the protective housing are called
leakage radiation
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What contribute nothing in the way of diagnostic information and result in unnecessary exposure of the patient and the rad tech?
leakage radiation
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Properly designed protective housing reduces the level of leakage radiation to less than ____ at 1 m when operated at max conditions
1 mGya/hr
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What does protective housing guard against (2)
- excessive radiation exposure
- electric shock
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What incorporates specially designed high-voltage receptacles to protect against accidental electric shock?
protective housing
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What protective housing support provide for the x-ray tube and protects the tube from damage caused by rough handling
mechanical support
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The protective housing around some x-ray tubes contains oil that serves as both ____ and ____.
- insulator (against electric shock)
- thermal cushion (to dissipate heat)
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An x-ray tube is an electronic vacuum tube with components contained within a ____ or ____
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What is a special type of vacuum tube that contains two electrodes?
x-ray tube
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X-ray tubes contain two electrodes called
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X-ray tubes are relatively large, perhaps ___ to ___ cm long and ____ cm in diameter.
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Which enclosure is made of Pyrex glass to enable it to withstand the tremendous heat generated?
glass enclosure
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Which enclosure maintains a vacuum inside the tube and allow for more efficient x-ray production and a longer tube life?
glass enclosure
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Which tube were not vacuum tubes but rather contained controlled quantities of gas within the enclosure?
Crookes tube (early x-ray tubes)
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Which tube is a vacuum tube, if it becomes gassy, x-ray production falls, and the tube can fail?
Coolidge tube (modern x-ray tube)
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As a glass enclosure tube ages, what vaporizes and coats the inside of the glass enclosure?
tungsten
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What is the result of the electrical properties of the tube allowing tube current to stray and interact with the glass enclosure? (2)
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What tube maintain a constant electric potential between the electrons of the tube current and the enclosure. Therefore, they have a longer life and are less likely to fail.
Metal enclosure tubes
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What are designed with a glass or a metal enclosure?
x-ray tubes
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What is the negative side of the x-ray tube; it has two primary parts, a filament and a focusing cup?
cathode
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What are the cathode 2 primary parts?
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What is a coil of wire similar to that in a kitchen toaster, but it is much smaller?
filament
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An x-ray tube filament emits electrons when it is ____.
heated
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When the current through the filament is sufficiently high, the outer-shell electrons of the filament atoms are ____ ____ and ____ from the filament.
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When the current through the filament is sufficiently high, the outer-shell electrons of the filament atoms are "boiled off and ejected from the filamemts. This phenomenon is known as what?
thermionic emission
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What are filaments usually made of?
thoriated tungsten
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What provides for higher thermionic emmission than other metals?
tungsten
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What vaporization with deposition on the inside of the glass enclosure is the most common cause of tube failure.
Tungsten
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The filament is embedded in a metal shroud called
focusing cup
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All of the electrons accelerated from cathode to anode are electrically (positive or negative)
negative
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Because all of the electrons accelerated from cathode to anode are electrically negative, the electron beam tends to spread out owing to what?
electrostatic repulsion
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Some electrons can miss the ____ completely.
anode
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What is negatively charged so that it electrostatically confines the electron beam to a small area of the anode?
focusing cup
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Effectiveness is determined by its size and shape, its charge, the filament size and shape, and the position of the filament in what?
focusing cup
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Most rotating anode x-ray tubes have two filaments mounted in the cathode assemble "side by side" creating ____ and ____ focal spot sizes.
large and small
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The x-ray tube current is adjusted by controlling what?
filament current
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Because electrons carry (positive/negative) charges, they repel one another and tend to form a cloud around what?
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What is the cloud of electrons called
space charge
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What makes it difficult for subsequent electrons to be emitted by the filament because of electrostatic repulsion.
space charge
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Cloud of electrons called a space charge, makes it difficult for subsequent electrons to be emitted by the filament because of electronstatic repulsion. This phenomenon is called
space charge effect
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What emission at low kVp and high mA can be space charge limited?
Thermionic emission
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What rises with increasing voltage to a maximum value?
x-ray tube current
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The x-ray tube current rises with increasing voltage to a maximum value is ____ current
saturation current
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Most diagnostic x-ray tubes have two focal spots:
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Which focal spot that is used when better spatial resolution is required?
small focal spot
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Which focal spot is used when large body parts are imaged and when other techniques that produce high heat are required?
large focal spot
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*NOTE*
Normally, either fialment can be used with the lower mA station - approx 3-- mA or less. At approx 400 mA and up, only the larger focal spot is allowed because the heat capacity of the anode could be exceeded if the small focal spot were used.
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Which focal spot range from 0.1 to 1 mm?
small focal spot
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Which focal spot range from 0.3 to 2 mm?
large focal spot
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What are the two types of anodes?
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Which anode x-ray tubes are used in dental x-ray imaging systems, some portable imaging systems and other special-purpose units in which high tube current and power are not required?
stationary anode
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Which anode are general-purpose x-ray tubes are capable of producing high-intensity x-ray beams in short time?
rotating anode
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What is the positive side of the x-ray tube; it conducts electricity and radiates heat and contains the target?
anode
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The anode serves 3 functions in an x-ray tubes:
- electrical conductor
- mechanical support
- thermal dissipater
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What recieves electrons emitted by the cathode and conducts them through thr tube to the connecting cables and back to the high voltage generator?
electrical conductor
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The anode that provides support for the target?
mechanical support
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When the projectile electrons from the cathode interact with the anode, more than 99% of their kinetic energy is converted into heat is called
thermal dissipater
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What is the area of the anode struck by the electrons from the cathode?
target
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In stationary anode tubes, what consists of a tungsten alloy embedded in the copper anode?
target
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In rotating anode tubes, the entire rotating disc is the ____.
target
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What is the material of choice for the target for general radiography?
tungsten
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Tungsten is the material of choice for the target for general radiography for three main reason:
- atomic number
- thermal conductivity
- high melting point
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Tungsten results in high-efficiency x-ray production and in high-energy x-rays
Atomic number, 74
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Tungsten nearly equal to that of copper. It is therfore an efficient metal for dissipating the heat produced.
Thermal conductivity
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Any material, if heated sufficiently, will melt and become liquid
High melting point (3400 degrees C compared with 1100 degrees C)
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X-ray tube allows the electron beam to interact with a much larger target area: therefore heating of the anode is not confined to one small spot, as in a stationary anode tube.
rotating anode
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Higher tube currents and shorter exposure times are possible with which anode
rotating anode
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What electromagnetic motor is used to turn the anode?
induction motor
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What is powered by an electromagnetic induction motor?
rotating anode
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What is the area of the target from which x-rays are emitted
the focal spot
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What spot is required in the Line-Focus Principle for the better spatial resolution of the image
small focal spots (because the smaller the focal spot, the better)
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Unfortunately, as the size of the focal spot decreases, the heating of the target is concentrated onto a smaller area; This is the limiting factor to ____ ____ ____.
focal spot size
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What is the actual x-ray source
focal spot
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Before the rotating anode was developed, another design was incorporated into x-ray tube targets to allow a large area for heating while maintaining a small focal spot; this design is known as
line-focus principle
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Diagnostic x-ray tubes have target angles that vary from approx ___ to ___ degrees.
5 to 20 degrees
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What results in an effective focal spot size much less than the actual focal spot size?
line-focus principle
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What is one unfortunate consequence of the line-focus principle is that the radiation intensity on the cathode side of the x-ray field is greater than that on the anode side?
Heel Effect
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What interact with target atoms at various depths into the target?
electrons
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The ____ the anode angle, the ____ the heel effect. (larger or smaller)
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What effect results in reduced x-ray intensity on the anode side of the useful beam caused by absorption in the "heel" of the target?
heel effect
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The difference in radiation intensity across the useful beam of an x-ray field can vary by as much as ___ %
45%
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What is the useful beam is the imaginary line generated by the centermost x-ray in the beam?
central ray
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If the radiation intensity along the central ray is designated as ____ %, than the intensity on the cathode side may be as high as ___ %, and that on the anode side may be as low as ___ %
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What effect is important when one is imaging anatomical structures that differ greatly in thickness or mass density?
heel effect
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*NOTE*
In general, positioning the cathode side of the x-ray tube over the thicker part of the anatomy provides more uniform radiation exposure of the image receptor
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What effect results in smaller effective focal spot and less radiation intensity on the anode side of the x-ray beam?
heel effect
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X-ray tubes are designed so that projectile electrons from the cathode interact with the target only at the focal spot.
Off-Focus Radiation
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Some of the electrons bounce off the focal spot and then land on other areas of the target, causing x-rays to be proced frome outside of the ____ ____.
focal spot (called off-focus radiation)
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What are the 3 ways heat can be dissipated?
- radiation
- conduction
- convection
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What is the transfer of heat by the emission of infrared radiation. Heat lamps emit not only visible light but also infrared radiation?
radiation
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What is the transfer of energy from one area of an object to another. The handle of a heated iron skillet becomes hot because of this?
Conduction
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What is the transfer of heat by the movement of a heated substance form one place to another. Many homes and offices are heated by this?
convection
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What results in reduced x-ray tube life?
Excessive heat
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*NOTE*
- Maximum radiographic techniques should never be applied to a cold anode
- Example: just like your car has to warm up
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*NOTE*
A second type of x-ray tube failure results from maintaining the anode at elevated temperatures for prolonged periods.
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A final cause of tube failure involved the ____.
filament
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The most frequent cause of abrupt tube failure is electron arcing from the filament to the enclosure beacuse of the ____ ____.
vaporized tungsten
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What is the primary support structure for the x-ray tube, which allows the greatest ease of movement and range of position?
ceiling support system
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What covers the x-ray tube and provides the following 3 functions?
protective housing
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What are the 3 funcitons of protective housing?
- reduces leakage radiation to less than 1 mGya/hr at 1 m
- provides mechanical support, thereby protecting the tube from damage
- serves as a way to conduct heat away from the x-ray tube target
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What surrounds the cathode (-) and the anode (+), which are the electrodes of the vacuum tube?
glass or metal enclosure
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What contains the tungsten filament, which is the source electrons?
cathode
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What anode is the tungsten-rhenium disc, which serves as a target for electrons accelerated from the cathode?
rotating anode
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What results from angled targets?
line-focus principle
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What is the variation in x-ray intensity across the x-ray beam that results from absorption of x-rays in the heel of the target?
heel effect
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Safe operation of the x-ray tube is the responsibilty of who?
radiographers
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Tube failure can be prevented. There are 3 caused of tube failure:
- A single excessive exposure causes pitting or cracking of the anode.
- Long exposure time causes excessive heating of the anode, resulting in damge to the bearing in the rotor assembly. bearing damage causes warping & rotational friction of the anode
- Even with normal use, vaporization of the filament causes tungsten to coat the glass or metal enclosures; this eventually causes arcing
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