Unit 3 (The X-Ray Tube)
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name four elements involved in the production of x-rays:
- a source of electrons
- a target
- a vacuum
give two types of a target within the tube for the electrons to hit:
- stationary anode
- rotating anode
at what rate does the rotating anode move?
what is needed to drive the electrons off the filament and toward the anode?
a high-voltage (low amperage) supply to the cathode side of the x-ray tube.
why is a vacuum needed for the production of x-rays?
it makes x-ray production more efficient by removing molecules that could interfere with x-ray production
where is the stator located in reference to the vacuum?
outside the vacuum, outside the envelope
- A. armature
- B. stator electromagnetics
- C. bearing
- D. tungsten anode
- E. envelope
- F. filament circuit
- G. filament
- H. electron beam
- I. x-ray beam
- J. molybdenum neck and base
- K. rotating portion
what part of the tube is the large magnet and copper wire that spins around the outside of the tube (we often pass it around in class)?
name three elements of the cathode assembly:
- focusing cup
- associated wiring
- A. focusing cup
- B. filament
- C. high voltage circuit
- D. filament circuit
give the measurements and make-up of the filament coil:
- thoriated tungsten
- .1-.2 mm thick
- 1-2 mm wide
- 7-15 mm long
describe the filament coil's relationship to the focal spot and recorded detail:
- smaller filament = smaller focal spot, better recorded detail
- larger filament = larger focal spot, less recorded detail
describe how the color of the tube's envelope darkens with age:
thoriated tungsten of the filament coil heats up, vaporizes/evaporates and floats to the top of the tube where it settles and coats the glass, changing the glass's color
why is tungsten most often used as a primary filament material?
because of its high melting point (3,370°C), it is difficult to vaporize it or turn it into gas
name the primary and two secondary choices as a main filament material, and their melting points:
- tungstun (melting point of 3,370°C)
- rhenium (melting point of 3,170°C)
- molybdenum (melting point of 2620°C)
what is the element of choice for a filament in mammography and why?
- because of its low atomic number, which is needed to produce the lower energy xrays used in imaging the breast
what is the reason for molybdenum being used on the back side of the anode in the x-ray tube?
it helps with expansion, to help the anode to not crack; improves the lifespan of the x-ray tube.
at what point does thermionic emission occur?
the filament has to reach 2,200°C
what is the lifespan of the average filament?
- 6-9 hours total
- = 10,000 - 20,000 exposures
causes electrons to be released from the filament and form a cloud:
- thermionic emission
- (forms a thermionic cloud)
describe what the focusing cup is doing in this diagram:
the focusing is located in the area beneath the filament coil, and it is helping to push the electrons to the upper surface of the filament coil.
What is tube arcing?
when enough vaporized tungsten has collected on the envelope, which eventually depletes the vacuum, electrons may "arc" from the cathode to the tungsten deposits to the anode - causing the tube to eventually become unstable and need replacement.
at what point is the filament considered to be subject to breakage?
when 10% of the filament diameter has been vaporized
what is one thing that can be done to delay the occurance of filament breakage, extending the life of the x-ray tube?
the tech can push the rotor and expose buttons simultaneously, instead of rotoring before exposing.
what is the approximate cost to replace an average diagnostic tube?
what is the focusing cup composed of?
name four effects or occurences that the focusing cup takes a part in:
- low negative potential applied
- compresses the thermionic cloud
- space charge effect
- saturation current
when more and more electrons build up in the area of the filament, and their negative charges begin to oppose the emission of additional electrons:
space charge effect
the space charge effect limits the x-ray unit to a maximum:
a maximum mA of 1000-1,200
when above a specific kVp, small changes are adjusted for automatically by reducing the temperature of the filament slightly to assure a constant current with changing peak kilovoltages:
what is the most common saturation voltage amount for x-ray tubes?
in layman's terms, what is the saturation current doing?
"saturating" the filament coil with as many electrons as possible to maintain a 40kVp (saturation voltage) supplying the filament
what substance has been found to be helpful in maintaining a constant saturation point (higher amperage)?
powdered iron (as opposed to more ferromagnetic elements)
what is the purpose of the biased focusing cup?
- it maintains the cup at a more negative voltage than the filament.
- this results in a narrower electron beam, making the focal spot smaller
what occurs at the biased focusing cup when the tech selects a smaller focal spot?
the biased focusing cup supplies a higher voltage to the focusing cup, which causes the electrons to become a narrower beam without switching to a smaller filament.
higher voltage supplied to the focusing cup causes:
the electron beam to be narrowed
what are the functions of the grid-biased tubes?
- more precise control of the thermionic cloud
- changes the charge of the focusing cup from negative to positive
- after charge is changed, allows the cup to regulate, pulse, and synchronize x-ray production precisely
give two alternate names for the grid-biased tube:
what are the functions of the anode assembly?
- target surface
- conducts high voltage
- primary thermal conductor
give the percentages of the forms of energy produced in the x-ray tube:
- 99% energy in the form of heat
- 1% energy in the form of x-ray photons
give the components of the anode assembly:
- A. bearings
- B. stator
- C. envelope
- D. focal track target
- E. rotor
- F. high voltage circuit
why is a tungsten-rhenium alloy often used in the rotating anode?
- first for their high melting points:
- tungsten (W): 3,370°C
- rhenium (Ru): 3,170°C
- for their heat conducting ability
- tungsten is doped with rhenium because its low atomic number offers more expansion without cracking the tube
- (W) 74 Z# and (Ru) 44 Z#
what is the purpose of anode layering?
- assists with heat loading
- being backed with molybdenum and/or graphite assists with absorption of heat being produced in the tungsten target
give the materials used in mammography equipment and why they are used:
- molybdenum (Mo) target: its low atomic number (42) creates lower energy photons
- beryllium (Be) tube window: its low atomic number (4) makes it less likely to attenuate the beam
give the purposes of warming up the tube:
- gradually warms the anode, which prevents cracking
- helps maintain the vacuum
- stress relieved anode
explain what's happening in this picture:
this is an example of "pitting" of the anode, which is usually caused by a malfunction of the stator or rotor, most common with stator failure
give six names for the portion of the anode that the electron stream comes in contact with:
- target area
- focal point
- focal spot
- focal track
in the rotating anode, turning faster:
dissapates more heat over a larger surface area
in the rotating anode, what is the importance of the diameter of the disk?
the larger the disk, the more heat can be shared over a greater area which helps dissipate heat
states that the effective focal spot is controlled by the actual focal spot and the target angle:
line focus principle
what is the difference in the actual focal spot and the effective focal spot?
- the actual focal spot is the area on the anode where electrons are converted to x-ray photons
- the effective focal spot is the area of the focal spot that is projected out of the tube toward the object being radiographed
Increasing the filament size will ____________ the actual focal spot size and __________ the effective focal spot size and vice versa.
the tube angle (which plays a part in focal spot size) varies from:
7-17° with 12° being typical
according to the line focus principle, if you decrease the tube angle, the actual focal spot size will ___________ and the effective focal spot size will _____________ and vice versa.
describe the anode heel effect:
- the percentage of intensity differs from one end of the anode to the other.
- the cathode end is always hotter
- the anode end is less hot because of less interaction with matter
describe the rotor of the x-ray tube:
- copper cylinder connected to anode disk by molybdenum stem
- turns when stator is energized
- has ball bearings
what is failure of the rotor most often caused by?
- failure of the bearings
- can result in pitting of the anode
describe the envelope of the x-ray tube:
- comprised of pyrex glass or metal
- includes the window (where the x-ray photons go through to reach the patient)
- includes the vacuum
give the dimensions of the envelope:
- pyrex glass or metal:
- 10" long
- 6" central diameter
- 2" peripheral diameter
give the functions of the protective housing of the x-ray tube:
- controls leakage and scatter radiation
- isolates high voltages
- provides a means to cool the tube
what is the protective housing composed of?
lead-lined cast steel
what is the leakage radiation limit when it comes to the protective housing?
100mR/hr at 1 meter
name two components of the x-ray unit that work to manage high voltage isolation and tube cooling:
give the functions of the dielectric oil:
- promotes cooling
- sometimes circulated through heat exchanger
photons that were not produced at the focal spot or extrafocal radiation:
- off-focus radiation
- (can be produced when electrons interact with the tungsten, created another xray)
name three rating charts/cooling curves:
- tube rating charts
- anode cooling curves
- housing cooling curves
how do you calculate heat units?
kVp x mA x time(sec) x rectification constant
give the rectification constants for single phase, three phase, and high frequency:
- single phase: 1.00
- three phase: 1.35
- high frequency: 1.40
when calculating heat units, what are you measuring?
the anode storage capacity of the tube
remember to go over the charts/graphs at the end of the PP and the study packet!!!
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