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imaging equipment 1-3
Terms in this set (38)
give 4 recommendations for extending tube life
-warm up anode
-do not hold rotor switch for extended time
-use low mA when possible
-use low speed rotor when possible
-do not make multiple exposures near tube limits
-do not rotate tube assembly roughly
-do not use tube when you hear loud rotor bearings
what does the anode cooling chart determine?
the limited heat capacity that the anode can handle
How do you determine if an exposure is safe or unsafe in regards to a tube rating chart?
1) find intersection of kV and time
2) determine corresponding mA
3)under the curve is safe and above the curve is unsafe
what is the purpose of x-ray exposure rating charts?
-helps determine operational limits of the tube for multiple and single exposures and determines allowable heat load for anode and tube housing
What are 4 ways that tube cooling is achieved?
-dielectric oil filling the spaces between the envelvope absorb some of the heat created within the tube
-at the end of the tube there is an expandable gasket
-high heat tubes have a recirculation system
-sometimes includes a fan
what is "coast time"?
-the time it takes for the rotor to come to rest after use
what is the rotor made out of? what is the shaft made out of that attaches the rotor to the anode?
-rotor is a hollow copper cylinder for the purpose that it can be affected by changing magnetics
how does the anode spin?
the stator and rotor together form an electromagnetic induction(production) motor
**The alternating current supplied to the stator produces a magnetic field that causes the rotor to spin.
4 things that affect the anode heal
1) the angle of the anode- smaller the angle the larger the anode heel effect
2)the side of the image receptor- a larger IR will > AHE
3) source to image distance - as it increases, AHE decreases (the further away from the central beam we are, the more dramatic the change is)
4)focal spot size- results in the smaller EFS with less radiation intensity on the anode side of the x-ray beam
Anode Heel Effect
The heel effect results in reduced x-ray intensity on the anode side of the useful beam caused by absorption in the "heel"
line focus principle
The line focus principle allows for smaller effective focal spot with increased heat capacity.
effective focal spot
refers to the area of the focal spot that is projected out of the tube. the EFS is as seen from the IR's perspective
actual focal spot
refers to the area on the anode exposed to the electrons. it is the physical part of the focal track that is struck by electrons. the size of the AFS is determinded by the length of the filament and the width of the focusing cup slot
why do stress relieved anodes have slits in them?
to allow for great expansion under excessive heat
-used in CT and angio procedures
what is the anode target area made of and what is its purpose?
-made of tungsten
-it is where x-ray photons are created and the portion of anode where electrons impact
why are there two focal spots?
small- used when better spatial resolution is required
large- when large body parts are imaged or with high heat production
-filament wires break, thin or vaporize
-tungsten deposits inside the envelope
-free gas molecules inside the vacuum
what is the focusing cup made of and what is its purpose?
-made of nickel
-its negative charge "focuses" the electrons and limits the thermionic cloud
the "cloud" of negative electrons boiled off from the heated filament and repelling each other before accelerated to the anode
explain the filament process
1. a current is supplied to the filament
2. the resistance of the filament cause it to heat up and release electrons
3. a change in filament current will change its temperature
4. controls rate of thermionic emission
what are the 3 components of the cathode?
what is the purpose of the focusing cup?
it is negatively charged so that it electrostatically confines the electron beam to a small area of the anode
-holds the filament
what is the most common cause of tube failure?
tungsten vaporization with deposition on inside of the glass enclosure
2 reasons filaments are usually made of tungsten
-high melting point= not likely to burn out like the filament of a light bulb
-doesnt vaporize quickly therefore doesn't become gassy quickly and slowly coats internal parts with tungsten
** 1-2% thorium addition helps prolong tube life
filament(made of tungsten) becomes heated-> emits electrons
*outer shel electrons of the filament atoms are "boiled off" and ejected from the filament
which is the positive and which is the negative component of the x-ray tube?
what happens when a little gas gets in the enclosure of the x-ray tube?
the electron flow from cathode to anode is reduced, fewer x-rays are produced and more heat is generated
**metal enclosure tubes maintain constant electric potential between electrons of tube current and enclosure
what does Isotropic mean?
equal intensity distributed in all directions
what does kVp stand for?
kilovolt peak- max amount of energy any photon can have as it exits the x-ray tube
mA (milliamperage) is the measurement of what?
In what part of the equipment are the x-rays produced?
what kind of equipment is "CT equipment" used for?
for detailed images, sometimes 3D
what kind of imaging is "mammography equipment" used for?
to image the breast
what kind of imaging is "fluoroscopic equipment" used for? and how are they identified?
-for dynamic studies
-identified by their degree of tilt
what kind of imaging is "mobile equipment" used for?
takes x-rays to the patient
what kind of imaging is "general duty" equipment used for?
skeletal and other images
what kind of couches contribute to reduced patient dosage?
3 main components for x-ray production
-high voltage generator
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