Physical area of the focal track that’s impacted (what is on the anode, actual focal spot)
Actual focal spot
Positive side of the x-ray tube; serves as a target surface for high-voltage electrons from the filament, conducts the high-voltage from the cathode back into the x-ray generator circuitry, and serves as the primary thermal conductor
Gap between filament and target
3 parts of the anode assembly
Permit the calculation of the time necessary for the anode to cool enough for additional exposures to be made
Anode cooling chart
Because of the geometry of an angled anode target, the radiation intensity is greater on the cathode side
This can cause as many as 20% more photons at the cathode end of the tube and 25% fewer photons at the anode end
A total variation of approximately 45% exists parallel to the anode-cathode axis; the 45% variation is significant enough to cause a visible dif in exposure during radiographic exams when large film sizes are used at short distances
Can be seen when using short SID’s
Because the cathode end of the x-ray tube has a more intense beam, it should be positioned toward the denser/thicker part of the body
Certain parts of body should go under cathode instead of anode (humerus, femur, lower leg & thoracic spine)
Anode heel effect
The negative of the x-ray tube; produces a thermionic cloud, conducts the high voltage to the gap between the cathode and anode and focuses the electron stream as it heads for the anode
3 parts of the cathode assembly
Two filament wires on all diagnostic machines
Doesn’t require separate ground conductors, common ground used
Left = longer big around, right = shorter not as big around
Area of the focal spot that is projected out of the tube toward the object being radiographed; because the anode sets at an angle, what patient/IR sees of that focal spot is different
Effective focal spot
The entire cathode and anode assembly is housed within Pyrex glass or metal (more common now because glass can vaporize = cloudy and can’t see inside so vaporization becomes another filter and absorbs x-rays)
10” long, 6” central diameter, 2” peripheral/ends diameter
Radiation produced in the x-ray tube someplace other than the anode/composed of photons that were not produced at the focal spot; occurs when the high-voltage electrons striking the focal spot produce scattered electrons or photons
Upper shutters help get rid of it
Contributes up to 25 percent of total primary beam
Can change histogram during post processing in digital radiography
A small coil of thin thoriated tungsten tungsten wire
The portion of the anode where the high-voltage electron stream will impact
The circular path that will be impacted by the electron beam; precise point at which x-ray photons are created
Composed of nickel
Low negative potential applied
Helps bring electrons back together and keeps them focused to the anode in a convergence pattern (because electrons all possess negative charges and their tendency is to diverge rather than travel in straight lines so it’s provided with a low negative potential)
Compresses thermionic cloud as it’s driven toward the anode
Goes around filament
Very small focal spot that’s a fraction of a millimeter in size
Fractional focal spot
Permit the calculation of the time necessary for the housing to cool enough for additional exposures to be made
Housing cooling chart
Any photons that escape from the housing except at the port, radiation coming out from where we don’t want it to
Reduce the effective focal spot to permit the best resolution of detail while permitting as large an area as possible (to increase thermal conductivity)
As actual focal-spot size increases, effective focal-spot size increases
When the target angle is less than 45 degrees, the effective focal spot is smaller than the actual focal spot
Effective focal spot’s vertical dimension is one that is stated as the focal-spot size
Focal-spot size increases/blooms as mA is increased (doesn’t have resounding effect on recorded detail
Composed of cast steel and is capable of absorbing most of the unwanted photons; normally only lead lined on cathode side (because more x-rays on this side due to anode heel effect)
Part you see
Most valuable because they provide a guide regarding the max technical factor combinations that can be used without overloading the tube
Radiographic tube rating chart
Anode assembly that turns during exposure
Functions as source of x-ray photons and is the primary thermal conducting device is enhanced by the use of rhenium alloyed tungsten as the target focal material
Range from 5-13 cm in diameter and are composed of molybdenum
Turn during the exposure thus presenting a much larger target area, faster it rotates = better heat dissipation
Have a dynamic target area and are designed to greatly increase the target area
Can increase the target area up to 300 times depending on the anode disk diameter
Have much greater heat loading capacities than stationary, high-speed greater than regular speed
3 purposes of the protective housing
Controls leakage and scatter radiation
Isolates voltage through dielectric oil
Provides means to cool tube
4 conditions that must exist for x-rays to be produced
Source of electrons (filament)
High-voltage = main circuits (filament circuit = current)
Electrons at the filament wire get together, all negative and want to get away from each other so they begin to oppose the emission of additional electrons
Why you won’t see mA’s above 1000 in diagnostic because there’s too many electrons that can get away
Space charge effect
3 primary reasons tungsten metal is the choice for the source of x-ray photons
High atomic number (more electrons for x-rays to interact with)
High melting point
What is the advantage of a high-speed rotor?
Have much greater heat loading capacities than regular speed
Suck out all air so electrons flow from cathode to anode without encountering the gas atoms of air
Electrons can freely go from cathode to anode
An incident electron interacts with the force field of the nucleus, causing the incident electron to slow down, thus diverting the electron’s course
The electron loses energy and changes direction
The energy lost is a bremsstrahlung proton, and the photon energy is half the difference between the entering and exiting kinetic energy of the electron
The reaction of electrons dropping into the holes created during a characteristic interaction until there is only a hole in the outer shell
Hole in inner shell and must be filled by electron from outer shell; must move fast to get inside and get rid of energy to do this (in the form of x-rays)
Electron energy difference: closer to the inner shell have more energy bc it had to move faster and gave off more energy
Secondary photons produced
Occurs when K-shell emissions reach their effective energy range of 69 keV (increased output due to tube potential change to 69 or 70)
The electrons form the thermionic cloud that arrive at the anode target
Actions that take place when the electrons strike their target or focal spot
What is the approximate percentage of electron energy that is converted to x-ray photon energy in the x-ray tube?
Only 1% x-rays
The majority of the electron energy in the x-ray tube is converted to what form of energy?
99.8% of incident electrons’ kinetic energy is converted to heat
Only happen when incident electron interacts with K-/inner shell electron
Incident electron must have enough energy to knock an inner-shell electron from orbit thereby ionizing the atom
Incident electron continues in slightly different direction
Kinetic energy must overcome binding energy (occurs in techniques using 70 kVp or higher)
Energy is exactly the difference between the binding energy of the outer and inner shells between which the electron dropped
Characteristic cascade: hole in inner shell and must be filled by electron from outer shell; must move fast to get inside and get rid of energy to do this (in the form of x-rays)
Electron energy difference: closer to the inner shell have more energy bc it had to move faster and gave off more energy
Secondary photons produced
Only electron that drops into K-shell will contribute to beam and be of diagnostic value
Characteristic target interaction
What is the average keV of the primary beam as compared to the kVp?
Average primary beam photon has a keV energy of only approximately 30-40% of selected kVp
What effect does increasing mAs have on the total x-ray emission spectrum?
Changes beam amplitude
Higher mA = higher amplitude (looks exactly the same except taller)
Height is the only thing that changes
What effect does increasing kVp have on the total x-ray emission spectrum?
Changes beam amplitude and average energy due to increase in kinetic energy provided to incident electrons
No increase in electrons striking target
Increase in amplitude represents more emitted photons due to higher energy of each incident electron striking the target
As you increase kV, electrons have more energy and more x-rays are produced
Any filtration that occurs outside the tube and housing before the IR
Used to even out unequal densities
The use of two or more materials to complement one another in their absorbing abilities
Any material designed to effectively absorb photons from the x-ray beam
The amount of absorbing material that will reduce the intensity of the primary beam to one-half its original value
Indirect measure of the total filtration in the path of the x-ray beam
Expressed as Al/Eq
Half-value layer (HVL)
Results from the composition of the tube and housing
Tube aging increases this because vaporized tungsten coats tube window which makes HVL testing important
A type of compound filter consisting of tin, copper and aluminum, in that order, typically used in radiation therapy
Sum of inherent and added filtration
Does not take into account any compound or compensating filtration
Percentage of photons attenuated decreases as photon energy increases, even when filtration is increased
Type of compensating filter used to even out densities such as in the mediastinum
Type of compensating filter used to even out densities such as in the foot or lower extremities
Process of eliminating undesirable low-energy x-ray photons by insertion of absorbing materials into primary beam
Allows radiographer to shape emission spectrum
“Hardening” of beam removes low energy “soft” photons and increases average beam energy
Low energy photons cannot penetrate the part and only contribute to patient dose
What is the standard filter material used in diagnostic radiography?
When more than one filtering material is used, as in compound filter, how are the materials arranged in relationship to the x-ray source?
Place the highest atomic number material closest to the tube and lowest atomic number material closest to the patient, final layer usually aluminum
2 most common compensating filters
How does filtration affect patient dose and beam intensity?
Ideally, filtration would only remove low-energy photons
Some high energy photons are removed
Results in decrease in radiographic density that must be compensated for with increase in technique
Coil of wire that acts as a conductor located on the anode side between the stator electromagnets
Part of the rotating anode that turns during the exposure thus presenting a much larger target area, located on the anode side between the bearings
Induction-motor electromagnets that turn anode, bank of electromagnets that work on mutual/electromagnetic induction
Located outside the vacuum of the envelope
The electromagnetic effect that causes the rotor to turn can function through the envelope, permitting electrical isolation of the stator coils from the high voltage of the exposure
If this fails the rotor ceases to turn and immediate melting of a spot on the target because rotating anode targets aren’t designed to absorb the heat of a high-voltage exposure while stationary
Copper cylinder connected to anode disk by molybdenum shaft, located inside the stator and envelope
Cuff is the true this that’s affected by the electromagnetic field of the stator, causing it to turn
Inside of rotor contains silver plated steel ball _______ around a shaft that’s anchored in the envelope; noise heard when rotor switch is depressed prior to an exposure
Supplies the filament of the x-ray tube with properly modified power located inside the envelope on the cathode side
2 reasons tungsten is selected as the filament material
High melting point
Difficult to vaporize
The boiling off of electrons from the surface of the wire due to increased heat, causing an electron cloud
Filament heated by filament circuit
Causes electrons to be released from filament
Function of filament is to provide sufficient resistance to the flow of electrons so that the heat produced will cause this to occur
Causes electrons to leave the surface of the filament wire and form a thermionic cloud; when the high voltage is released at exposure, the entire cloud is available to be driven toward the anode target where x-ray photons will be produced
Thermionic emission/space charge cloud
Reduces particles that deposit on other surfaces and reduce the vacuum within the tube
Cloudy and can’t see inside so this becomes another filter and absorbs x-rays
Vaporized tungsten collection on envelope
Causes old tubes to have mirrored appearance and can eventually cause high-voltage this when sufficient current is attracted to a deposit during an exposure
Immediately destroys the tube
Evaporation deposits on glass envelope cause increased filtration of the primary beam which decreases tube filtration (metal envelopes can be grounded to significantly reduce this problem)
What is the average tube filament life?
2 types of tube failure
In high kVs there can be so many electrons being sent across the tube that it is saturated with electrons
Achieved when there are no further thermionic elections to be driven toward the anode (kVp won’t increase the tube mAs)
3 functions of the anode assembly
Target surface for high voltage electrons from the filament thus becoming the source of the x-ray photons
Conducts high voltage from cathode back into the x-ray generator circuitry
Serves as primary thermal conductor because it rotates
Where the high-speed electrons from the filament are suddenly stopped, resulting in the production of x-ray photons
Serves as path for the high-voltage flow during exposure
Positive side of the tube
2 types of anode
The circular path that will be impacted by the electron beam
The area of the focal track that is impacted by the electron beam at one time
Assists with heat loading
Backed with molybdenum and/or graphite (graphite-backed anodes can double heat-loading capabilities without increasing bearing wear)
2 reasons molybdenum is used as a target material in mammographic equipment
Creates needed lower energy photons which permits a better soft tissue image
High melting point
Why is there a specialized glass window made of beryllium in mammographic equipment?
Because the glass used in most x-ray tubes absorbs too much of the low-energy beam
Normal use of rotating anode will eventually vaporize sufficient target focal track material to roughen the target area, reduces the efficiency of the tube
Gradually warms anode and prevents cracking
Helps maintain vacuum (“vacuum pump”)
Stress relieved anode dissipates heat much more efficiently and doesn’t require an elaborate tube this
Point source of x-ray photons where all tube-to object and IR distances are measured
2 things the effective focal spot is controlled by
Actual focal spot (controlled by length of filament)
Anode target angle
What degree anode angle in most diagnostic x-ray tubes?
At how many revolutions per minute (rpm) do common rotating anodes revolve?
Common rotating anodes revolve at 3200-3600 rpm
At how many revolutions per minute (rpm) do high-speed rotating anodes revolve?
High-speed anodes operate at 10000-12000 rpm
Wobbling or melting anode disk can crack from the heat of exposure which can divert the electron and/or photon stream toward the envelope and crack the envelope permitting implosion of the vacuum which can suck the insulating oil into contact w/the superheated anode assembly; this can cause vaporization of the oil in a violent explosion (move patient)
What is a particular problem high speed anodes have that’s caused by the tone produced by their rotating?
At between 5000-7000 rpm, the harmonics produced by the rotating cuff are at a frequency capable of shattering the glass envelope
What happens if gyroscopic effect produced by the centrifugal force of the rotation of a high-speed anode is great enough?
If a high-speed anode tube housing is quickly rotated from one position to another, the gyroscopic effect can cause trauma to the anode disk and bearing causing the destruction of the tube
Ball bearings eventually become imperfectly round leading to wobbling of the rotor that throws the focal track off center and tube efficiency drops dramatically
Bad bearings caused by long use at high temperatures
What happens if the electron stream overheats the target area of the anode focal track?
It melts the tungsten which drips into the envelope and destroys the tube
Why are metal envelopes more common?
More common now because glass can vaporize (cloudy and can’t see inside so vaporization becomes another filter and absorbs x-rays)
Structure where the primary x-ray beams exits the envelope, which allows less absorption or scatter of photons
Area where glass/metal is thinner so x-rays we want to come out of tube do
What is the leakage radiation limit?
100 milliRoentgens per hour (mR/hr) at 1 meter (m)
3 functions of dielectric oil in tube
Sometimes circulated through heat exchanger
How are anode cooling curves/charts calculated?
kVP x mA x time x rectification constant
3 rectification constants
Single-phase = 1
High frequency = 1.4
Three-phase = 1.35
Manmade in tube when the high speed electrons from the cathode strike an anode target
Not manmade, products of nuclear radioactive decay
2 things that happen when you increase kinetic energy of incident electrons
Increase quality and quantity
Increase number of target interactions
Electrons undergo how many interactions to dissipate excess energy?
Target interactions all occur within how many millimeters of target surface?
0.25 to 0.5 mm
3 target interactions (happen at anode)
2 types of target interactions that can produce diagnostic-range x-ray photons
Incident electrons transfer kinetic energy to excite outer shell electrons of target atoms and causes them to emit infrared radiation
Electrons then return to their normal state– where they will be re-excited again and again, each time emitting infrared radiation as heat
2 target materials
3 reasons why tungsten and rhenium are used as target materials
High atomic numbers
High melting points
Similar electron binding energies
What is the target material in mammography?
How much do brems interactions account for the percent of the beam?
85-100 percent of beam
2 things that occur as incident electrons get closer to nucleus in Brems interactions
Photon energy increases
Larger deflection of incident electron
What happens with direct interaction between nucleus and incident electron in Brems interactions?
Maximum energy photon
Possible, but not probable
What is the minimum you have to set your kV to to create characteristic interactions?
What is the binding energy of tungsten k-shell?
At between 80-100 kVp what percentage of the beam is produced by brems and characteristic interactions?
Between 80-100 kVp, about 80-90% of the primary beam is produced by brems interactions and 10-20% by characteristic
Brems and characteristic emissions combined
Selected kVp will determine maximum keV possible for any photon
What happens to the emission spectrum with a change in generator?
Single phase to three-phase inncreases amplitude and energy
Soft tissue penetration requires approximately how many kiloelectronvolt (keV) photons?
What is aluminum filtration expressed as?
Aluminum equivalency (Al/Eq)
5 types of filtration
Inherent filtration Added filtration Compound filtration Compensating filtration Total filtration
Typical tube might have total inherent filter of what Al/Eq?
0.5-1.0 mm Al/Eq
3 structures that contribute to the inherent filtration
Dielectric oil bath
Glass window of housing (most comes from this)