ENTC 4390

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ENTC 4390

• PRODUCTION OF X RAYS

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• INTRODUCTION

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• To produce medical images with x rays, a source
  is required that
• 1. Produces enough x rays in a short time
• 2. Allows the user to vary the x-ray energy
• 3. Provides x rays in a reproducible fashion
• 4. Meets standards of safety and economy of
  operation

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• Currently, the only practical sources of x rays
  are radioactive isotopes, nuclear reactions such
  as fission and fusion, and particle accelerators.

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• Only special-purpose particle accelerators known
  as x-ray tubes meet all the requirements
  mentioned above.
• In x-ray tubes, bremsstrahlung and characteristic
  x rays are produced as high-speed electrons
  interact in a target.

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• A heated filament releases electrons that are
  accelerated across a high voltage onto a target.
• The stream of accelerated electrons is referred
  to as the tube current.
• X rays are produced as the electrons interact in
  the target.
• The x rays emerge from the target in all
  directions but are restricted by collimators to
  form a useful beam of x rays.
• A vacuum is maintained inside the glass envelope
  of the x-ray tube to prevent the electrons from
  interacting with gas molecules.

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ELECTRON SOURCE

• A metal with a high melting point is required for
  the filament of an x-ray tube.
• Tungsten filaments (melting point of tungsten
  3370 C) are used in most x-ray tubes.
• A current of a few amperes heats the filament,
  and electrons are liberated at a rate that
  increases with the filament current
• The filament is mounted within a negatively
  charged focusing cup.
• Collectively, these elements are termed the
  cathode assembly.

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• The focusing cup, also called the cathode block,
  surrounds the filament and shapes the electron
  beam width.
• The voltage applied to the cathode block is
  typically the same as that applied to the
  filament.
• This shapes the lines of electrical potential to
  focus the electron beam to produce a small
  interaction area (focal spot) on the anode.

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• A biased x-ray tube uses an insulated focusing
  cup with a more negative voltage (about 100 V
  less) than the filament.
• This creates a tighter electric field around the
  filament, which reduces spread of the beam and
  results in a smaller focal spot width.

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• Although the width of the focusing cup slot
  determines the focal spot width, the filament
  length determines the focal spot length.
• X-ray tubes for diagnostic imaging typically have
  two filaments of different lengths, each in a
  slot machined into the focusing cup.
• Selection of one or the other filaments
  determines the area of the electron distribution
  (small or large focal spot) on the target.

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• The filament current determines the filament
  temperature and thus the rate of thermionic
  electron emission.
• As the electrical resistance to the filament
  current heats the filament, electrons are emitted
  from its surface.
• When no voltage is applied between the anode and
  the cathode of the x-ray tube, an electron cloud,
  also called a space charge cloud, builds around
  the filament.
• Applying a positive high voltage to the anode
  with respect to the cathode accelerates the
  electrons toward the anode and produces a tube
  current.
• Small changes in the filament current can produce
  relatively large changes in the rube current .

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• The existence of the space charge cloud shields
  the electric field for tube voltages of 40 kVp
  and lower, and only a portion of the free
  electrons are instantaneously accelerated to the
  anode.
• When this happens, the operation of the x-ray
  tube is space charge limited, which places an
  upper limit on the tube current, regardless of
  the filament current.
• Above 40 kVp, the space charge cloud effect is
  overcome by the applied potential difference and
  the tube current is limited only by the emission
  of electrons from the filament.
• Therefore, the filament current controls the tube
  current in a predictable way (emission-limited
  operation).

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• The tube current is five to ten times less than
  the filament current in the emission-limited
  range.
• Higher kVp produces slightly higher tube current
  for the same filament current
• For example, at 5 A filament current, 80 kVp
  produces 800 mA and
• 120 kVp produces 1,100 mA, approximately as
  kVp15.
• Beyond a certain kVp, saturation occurs whereby
  all of the emitted electrons are accelerated
  toward the anode and a further increase in kVp
  does not significantly increase the tube current.

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Anode

• The anode is a metal target electrode that is
  maintained at a positive potential difference
  relative to the cathode.
• Electrons striking the anode deposit most of
  their energy as heat, with a small fraction
  emitted as x-rays.
• Consequently. the production of x-rays, in
  quantities necessary for acceptable image
  quality, generates a large amount of heat in the
  anode.

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• To avoid heat damage to the x-ray tube, the rate
  of x-ray production must be limited.
• Tungsten (W, Z 74) is the most widely used
  anode material because of its high melting point
  and high atomic number.
• A tungsten anode can handle substantial heat
  deposition without cracking or pitting of its
  surface.
• An alloy of 10 rhenium and 90 tungsen provides
  added resistance to surface damage.
• The high atomic number of tungsten provides
  better bremssrahlung production efficiency
  compared with low-Z elements.

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• Molybdenum (Mo, Z 42) and rhodium (Rh, Z 45)
  are used as anode materials in mammographic x-ray
  tubes.
• These materials provide useful characteristic x
  rays for breast imaging.

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Anode Configurations

• X-ray tubes have stationary and rotating anode
  configurations.
• The simplest type of x-ray tube has a stationary
  (i.e., fixed) anode.
• It consists of a tungsten insert embedded in a
  copper block.
• The copper serves a dual role
• it supports the tungsten target, and
• it removes heat efficiently from the tungsten
  target.

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• Unfortunately, the small target area limits the
  heat dissipation race and consequently limits the
  maximum tube current and thus the x-ray flux.
• Many dental x-ray units, portable x-ray machines,
  and portable fluoroscopy systems use fixed anode
  x-ray tubes.

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• Despite their increased complexity in design and
  engineering, rotating anodes are used for most
  diagnostic x-ray applications, mainly because of
  their greater heat loading and consequent higher
  x-ray output capabilities.
• Electrons impart their energy on a continuously
  rotating target, spreading thermal energy over a
  large area and mass of the anode disk.

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• A bearing-mounted rotor assembly supports the
  anode disk within the evacuared x-ray tube
  insert.
• The rotor consists of copper bars arranged around
  a cylindrical iron core.
• A series of electromagnets surrounding the rotor
  outside the x-ray tube envelope makes up the
  stator, and the combination is known as an
  induction motor.

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• Rotation speeds are 3,000 to 3,600 (low speed) or
  9,000 to 10,000 (high speed) revolutions per
  minute (rpm).
• X-ray machines are designed so that the x-ray
  tube will not be energized if the anode is not up
  to full speed
• this is the cause for the short delay (1 to 2
  seconds) when the x-ray tube exposure button is
  pushed.

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• Rotor bearings are heat sensitive and are often
  the cause of x-ray tube failure.
• Bearings are in the high-vacuum environment of
  the insert and require special heat-insensitive,
  nonvolatile lubricants.
• A molybdenum stem attaches the anode to the
  rotor/bearing assembly, because molybdenum is a
  very poor heat conductor and reduces heat
  transfer from the anode to the bearings.

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• Because it is thermally isolated, the anode must
  be cooled by radiative emission.
• Heat energy is emitted from the hot anode as
  infrared radiation, which transfers heat to the
  x-ray tube insert and ultimately to the
  surrounding oil bath.

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• The focal track area of the rotating anode is
  equal to the product of the track length (2pr)
  and the track width (Ar), where r is the radial
  distance from the track to its center.
• A rotating anode with a 5-cm focal track radius
  and a 1-mm track width provides a focal track
  with an annular area 3/4 times greater than that
  of a fixed anode with a focal spot area of 1 mm x
  1 mm.
• The allowable instantaneous heat loading depends
  on the anode rotation speed and the focal spot
  area.
• Faster rotation speeds distribute the heat load
  over a greater portion of the focal track area.

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Anode Angle and Focal Spot Size

• The anode angle is defined as the angle of the
  target surface with respect to the central ray in
  the x-ray field.

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• Anode angles in diagnostic x-ray tubes, other
  than some mammography tubes, range from 7 to 20
  degrees, wirh 12- to 15-degree angles being most
  common.
• Focal spot size is defined in two ways
• The actual focal spot size is the area on the
  anode that is struck by electrons, and
• it is primarily determined by the length of the
  cathode filament and the width of the focusing
  cup slot.

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• The effective focal spot size is the length and
  width of the focal spot as projected down the
  central ray in the x-ray field.
• The effective focal spot width is equal to the
  actual focal spot width and therefore is not
  affected by the anode angle.
• However, the anode angle causes the effective
  focal spot length to be smaller than he actual
  focal spot length.

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• There are three major tradeoffs to consider for
  the choice of anode angle.

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• A smaller anode angle provides a smaller
  effective focal spot for the same actual focal
  area.
• A smaller effective focal spot size provides
  better spatial resolution.

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• However, a small anode angle limits the size of
  the usable x-ray field owing to cutoff of the
  beam.
• Field coverage is less for short
  focus-to-detector distances.

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• The optimal anode angle depends on the clinical
  imaging application.
• A small anode angle (approximately 7 to 9
  degrees) is desirable for small field-of-view
  image receptors, such as cineangiographic and
  neuroangiographic equipment, where field coverage
  is limited by the image intensifier diameter
  (e.g., 23 cm).
• Larger anode angles (approximately 12 to 15
  degrees) are necessary for general radiographic
  work to achieve large field area coverage at
  short focal spot-to-image distances.

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• The nominal focal spot size (width and length) is
  specified at the central ray of the beam.
• The central ray is usually a line from the focal
  spot to the image receptor that is perpendicular
  to the A-C axis of the x-ray rube and
  perpendicular o he plane of a properly positioned
  image receptor.

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• In most radiographic imaging, the central ray
  bisects the detector field.
• X ray mammography is an exception.

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• Tools for measuring focal spot size are
• The pinhole camera,
• The slit camera,
• The star pattern, and
• The resolution bar pattern.

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• The pinhole camera uses a very small circular
  aperture (10 to 30 mm diameter) in a disk made of
  a thin, highly attenuating metal such as lead,
  tungsten, or gold.

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• With the pinhole camera positioned on the central
  axis between the x-ray source and the detector,
  an image of the focal spot is recorded.

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• The slit camera consists of a plate made of a
  highly attenuating metal (usually tungsten) with
  a thin slit, typically 10 mm wide.
• In use, the slit camera is positioned above the
  image receptor, with the center of the slit on
  the central axis and the slit either parallel or
  perpendicular to the A-C axis.
• Measuring the width of the distribution and
  correcting for magnification yields one dimension
  of the focal spot.
• A second radiograph, taken with the slit
  perpendicular to the first, yields the other
  dimension of the focal spot.

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• The star pattern Test tool contains a radial
  pattern of lead spokes of diminishing width and
  spacing on a thin plastic disk.
• Imaging the star pattern at a known magnification
  and measuring the distance between the outermost
  blur patterns (areas of unresolved spokes) on the
  image provides an estimate of the resolving power
  of the focal spot in the directions perpendicular
  and parallel to the A-C axis.
• A large focal spot has a greater blur diameter
  than a small focal spot.
• The effective focal spot size can be estimated
  from the blur pattern diameter and the known
  magnificacion.

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• A resolution bar pattern is a simple tool for
  in-the-field evaluation of focal spot size.
• Bar pattern images demonstrate the effective
  resolution parallel and perpendicular to the A-C
  axis for a given magnification geometry.

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• The focal spot is the volume of target within
  which electrons are absorbed and are produced.
• For radiographs of highest clarity, electrons
  should be absorbed within a small focal spot.
• To achieve a small focal spot, the electrons
  should be emitted from a small or fine
  filament.

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• Radiographic clarity is often reduced by
  voluntary or involuntary motion of the patient.
• This effect can be decreased by using x-ray
  exposures of high intensity and short duration.

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• The smaller, fine filament is used when
  radiographs with high detail are desired and
  short, high-intensity exposures are necessary
• If high-intensity exposures are needed to limit
  the blurring effects of motion, the larger,
  coarse filament is used.

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TUBE VOLTAGE AND VOLTAGE WAVEFORMS

• The intensity and energy distribution of x rays
  emerging from an x-ray tube are influenced by the
  potential difference (voltage) between the
  filament and target of the tube.
• The source of electrical power for radiographic
  equipment is usually alternating (ac).
• This type of electricity is by far the most
  common form available for use, because it can be
  transmitted with little energy loss through power
  lines that span large distances.

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• X-ray tubes are designed to operate at a single
  polarity, with a positive target (anode) and a
  negative filament (cathode).
• X-ray production is most efficient (more x rays
  are produced per unit time) if the potential of
  the target is always positive and if the voltage
  between the filament and target is kept at its
  maximum value
• In most x-ray equipment, ac is converted to
  direct current (dc), and the voltage between
  filament and target is kept at or near its
  maximum value.
• The conversion of ac to dc is called
  rectification.

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• Two electrical currents flow in an x-ray tube.
• The filament current is the flow of electrons
  through the filament to raise its temperature and
  release electrons.
• The tube current flows from the filament to the
  anode across the x-ray tube.

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• The figure illustrates the influence of tube
  voltage and filament current upon tube current.
• One of the factors is space charge.
• At low tube voltages, electrons are released from
  the filament more rapidly than they are
  accelerated toward the target
• The cloud accumulates around the filament.

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• The useful beam of an x-ray tube is composed of
  photons with an energy distribution that depends
  on four factors.
• Bremsstrahlung x rays are produced with a range
  of energies even if electrons of a single energy
  bombard the target.