Ch 6, 7 & 10 Flashcards by Courtney W

Physical area of the focal track that’s impacted (what is on the anode, actual focal spot)

Actual focal spot

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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

Anode

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3 parts of the anode assembly

Anode
Stator
Rotor

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Permit the calculation of the time necessary for the anode to cool enough for additional exposures to be made

Anode cooling chart

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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

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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

Cathode

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3 parts of the cathode assembly

Filament
Focusing cup
Associated wiring

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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

Dual focus

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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

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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

Envelope

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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

Off-focus/extrafocal radiation

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A small coil of thin thoriated tungsten tungsten wire

Filament

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The portion of the anode where the high-voltage electron stream will impact

Focal point/spot/focus

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The circular path that will be impacted by the electron beam; precise point at which x-ray photons are created

Focal track

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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

Focusing cup

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Very small focal spot that’s a fraction of a millimeter in size

Fractional focal spot

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Permit the calculation of the time necessary for the housing to cool enough for additional exposures to be made

Housing cooling chart

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Any photons that escape from the housing except at the port, radiation coming out from where we don’t want it to

Leakage radiation

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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

Line-focus principle

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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

Protective housing

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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

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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

Rotating anode

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3 purposes of the protective housing

Controls leakage and scatter radiation
Isolates voltage through dielectric oil
Provides means to cool tube

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4 conditions that must exist for x-rays to be produced

Source of electrons (filament)
Target (anode)
High-voltage = main circuits (filament circuit = current)
Vacuum

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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 Heat-conducting ability

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

Vacuum

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

Bremsstrahlung interaction

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

Characteristic cascade

Occurs when K-shell emissions reach their effective energy range of 69 keV (increased output due to tube potential change to 69 or 70)

Characteristic peak

The electrons form the thermionic cloud that arrive at the anode target

Incident electron

Actions that take place when the electrons strike their target or focal spot

Target interaction

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

Added filtration

Used to even out unequal densities

Compensating filter

The use of two or more materials to complement one another in their absorbing abilities

Compound/K-edge filter

Any material designed to effectively absorb photons from the x-ray beam

Filter

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

Inherent filtration

A type of compound filter consisting of tin, copper and aluminum, in that order, typically used in radiation therapy

Thoreaus filter

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

Total filtration

Type of compensating filter used to even out densities such as in the mediastinum

Trough filter

Type of compensating filter used to even out densities such as in the foot or lower extremities

Wedge filter

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

Filtration

What is the standard filter material used in diagnostic radiography?

Aluminum

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

Wedge | Trough

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

Armature

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

Rotating portion

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

Stator

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

Rotor

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

Bearings

Supplies the filament of the x-ray tube with properly modified power located inside the envelope on the cathode side

Filament circuit

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

Vaporization

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)

Tube arcing

What is the average tube filament life?

6-9 hours

2 types of tube failure

Tube arcing | Filament breakage

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)

Saturation current

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

Anode assembly

2 types of anode

Stationary | Rotating

The circular path that will be impacted by the electron beam

Focal track

The area of the focal track that is impacted by the electron beam at one time

Target Focus Focal points Focal spot

Assists with heat loading Backed with molybdenum and/or graphite (graphite-backed anodes can double heat-loading capabilities without increasing bearing wear)

Anode layering

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

Pitting

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

Warm-up procedure

Point source of x-ray photons where all tube-to object and IR distances are measured

Target

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)

Tube impaling

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

Window

What is the leakage radiation limit?

100 milliRoentgens per hour (mR/hr) at 1 meter (m)

3 functions of dielectric oil in tube

Insulates Promotes cooling 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

X-rays

Not manmade, products of nuclear radioactive decay

Gamma rays

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?

1000+

Target interactions all occur within how many millimeters of target surface?

0.25 to 0.5 mm

3 target interactions (happen at anode)

Heat production Bremsstrahlung interactions Characteristic interactions

2 types of target interactions that can produce diagnostic-range x-ray photons

Bremsstrahlung interactions | Characteristic interactions

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

Heat production

2 target materials

Tungsten | Rhenium

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?

Molybdenum

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?

70 kV

What is the binding energy of tungsten k-shell?

69.5 keV

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

Emission spectrum

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?

30-40 keV

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

Glass envelope Dielectric oil bath Glass window of housing (most comes from this)