1. X-Ray Tube Function

    The x-ray tube is an electronic device that converts ____ energy into ____ energy (x-radiation)
    Conditions needed:
    • electrical,
    • electromagnetic

    • A source of electrons
    • A high voltage
    • An appropriate target material
    • A vacuum
  2. The Three Main Parts of the X-ray Tube are?
    • - Cathode (-)
    • - Anode (+)
    • - Envelope

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  3. Cathode

    Negative (-) electrode of the x-ray tube


    Cathode assembly:
    • 1. Produce a thermionic cloud of electrons
    • 2. Conduct high voltage to the gap between the cathode & anode
    • 3. Focus the electron stream as it heads for the anode

    • - Filament(s)
    • - Focusing cup
    • - Wiring
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  5. Filaments

    Coils of thoriated tungsten wire (__-__ __ long)
    ____ used because of its high melting point of 3,370° C (approximately 6,100° F) & its resistance to _____
    _____ enhances the efficiency of thermionic emission & prolongs tube life

    _____ & _____ are also used for x-ray tube filaments
    Set inside the focusing cup
    Most diagnostic tubes have 2 filaments (1 large & 1 small). These are called dual focus tubes.
    The filaments of the cathode (large & small) correspond to the focal spots (large & small) located on the anode
    • 7-15 mm,
    • Tungsten, vaporization
    • Thorium

    Rhenium, molybdenum
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  7. Thermionic Emission

    ____ are produced at the ____ by passing an electric current through it as determined by the __ set
    - Electrical current produces _____, which in turn heats up the filament wire. As mA is increased, _____ _____ increases, & electrons are literally boiled off from the ____ ____ of the thorium & tungsten atoms. The higher the mA set, the more electrons are produced.

    The number of electrons produced at the filament therefore affects the _____ of radiation produced at the ____
    Electrons, filament, mA

    • resistance
    • filament temperature
    • outer shells

    quantity, anode
  8. Focusing Cup

    A metal shroud made of ___ that has 2 shallow depressions to hold the filaments. Its main purpose is to ___ the electron cloud as it travels toward the ___.
    Electrons at the filament repel each other due to being like charges
    Would result in divergence of the electrons & poor x-ray production at anode
    Focusing cup charged with a ___ ___ ___, which electrostatically confines the electron beam (“focuses” the electrons towards one another as they head to the anode)
    • nickel
    • narrow, anode
    • low negative voltage
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    Focusing Cup
  10. At a certain point during thermionic emission, the negative charges built up in the area of the filament opposes the emission of additional electrons – called the ___ ___ ___

    Phenomenon limits most x-ray tubes to a maximum mA range of 1,000-1,200 mA
    space charge effect
  11. As kV is increased, a greater % of electrons are driven toward the anode. X-ray tube current (mA) rises with increasing voltage to a maximum value. Increasing kV further will not result in higher mA because all of the available electrons have been used – called the ___ ___

    Phenomenon requires that filament amperage be adjusted for kV changes; x-ray circuitry does this automatically when kV & mAs is changed
    saturation current
  12. Anode

    Positive (+) electrode of the x-ray tube


    Anode assembly:
    1. Serves as a target surface for electrons to strike & the source of x-ray photons

    2. Acts as an electrical conductor; high-voltage electrons from the cathode are conducted through it & back into the x-ray generator circuitry

    3.Serves as a thermal conductor; heat generated from the production of x-rays is dissipated through it

    • - Anode
    • - Rotor
    • - Stator
  13. Rotating Anode Disks

    A 5-13 cm disk:
    ___ (___-___) - portion of the disk where electrons strike & x-rays are emitted. Actually a circular path (or area) on the face of the disk. Also called the focal track.
    Disk is designed to rotate at high speed (___-___ rpm)
    Advantage over stationary anodes – allows the electron beam to interact with a larger target area so heating of the anode is not confined to one spot. Dissipates heat over a 360° arc which results in more efficient x-ray production (increased quantity & quality of radiation)

    Target bombarded with electrons for only 7-50 microseconds; the faster the anode rotates, the better the heat dissipation
    • Target (tungsten-rhenium)
    • 3,200-3,600
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    Anode types
  15. _____ is the metal of choice for the target because:
    1.It has a high atomic number (Z #) of __ - enhances the production of diagnostic-range photons
    2.High melting point (3,370° C or 6,100° F)
    3.Good thermal conductivity – allows heat to dissipate
    • Tungsten
    • 74

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  16. ___ added because:
    It provides added mechanical strength to withstand the stress of high-speed rotation & the effects of thermal expansion & contraction

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    Large Focal Spot
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    Small Focal Spot
  19. Using the large filament & focal spot provides higher heat capacity & can produce larger quantities of radiation (higher mA)
    - ___ ___ produced at target because electrons strike a larger area

    Using the small filament & focal spot provides lower heat capacity & is limited in the quantities of radiation that can be produced (lower mA)
    - ___ ___ produced at target because electrons strike a smaller area
    Less heat

    More heat
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    Effective Focal Spot
  21. ___ ___ ___ – physical area on the target/focal track that is impacted by the electrons
    Sizes: .1 mm to 3.0 mm
        Small (.6 mm)
        Large (1.2 mm)

    ___ ___ ___ – area of the focal spot that is projected out of the tube toward the object being imaged

    The effective focal-spot size is controlled by the actual focal spot size
    Actual focal spot

    Effective focal spot
  22. ___-___ ___ - Technique used to reduce the area of the effective focal spot

    Angling the target of the anode disk provides as large an actual focal spot as possible (for larger heat capacity), while keeping the effective focal spot smaller (for better image resolution/detail)

    Angles vary from 7°-17° with __° most common (12° needed to cover a 14” x 17” field size)
    Line-Focus Principle

  23. The geometric shape of the anode & the line-focus principle causes the radiation intensity to be uneven when it exits the tube, known as the ___-___ ___
    anode-heel effect
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    Heel Effect & Intensity
  25. ___ - Cylinder made of bars of copper & soft iron fabricated into one mass
    Attached to the anode disk by a molybdenum shaft
    Contains silver-plated steel ball bearings inside
    Sound that is heard when exposure control/button is depressed
    Turns at high rpm (3,200-3,600 rpm) when stator energized, which in turn rotates the shaft & anode disk
  26. ___ - Series of electromagnets that surround the rotor separated by glass or metal
    Only part of the anode assembly outside of envelope
    Along with rotor makes up an electromagnetic induction motor
    Current to stator is reversed to stop the rotor from turning at the end of exposure
  27. Tube Failure Related to Rotor & Stator

    ___ ___ ___ – rotor ceases to turn & electrons will melt a spot on the rotating anode target
    Electrical stator failure
  28. Tube Failure Related to Rotor & Stator

    ___ ___ ___ __ __ ___ – results in wobble of the rotor throwing the target/focal track off center
    Ball bearing wear in the rotor
  29. Tube Failure Related to Rotor & Stator

    _____________________ – using high mAs & kV settings & taking repeated exposures without interruption could exceed the heat limits of the anode & cause melting of the target (even if it is rotating correctly)
    Overheating the target area of the anode
  30. Tube Failure Related to Rotor & Stator

    ___________________ – using high mAs & kV on a “cold” anode & not performing proper warm-up procedures can cause thermal expansion & cracking of the anode
    Not warming up the tube
Card Set
RTE 1418 Imaging I - Unit 6, The X-Ray Tube