Diagnostic Radiology I

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Diagnostic Radiology I

• X-ray production
• X-ray tubes

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X-ray production

• X-rays produced when highly energetic electrons
  interact with matter and convert their kinetic
  energy into electromagnetic radiation
• A device that accomplishes this task consists of
• An electron source
• An evacuated path (vacuum) for electron
  acceleration
• An external energy source to accelerate the
  electrons

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Schematic of x-ray production
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Bremsstrahlung spectrum

• As electrons from the cathode travel to the
  anode, they are accelerated by the electrical
  potential difference between these electrodes
• The kinetic energy gained by an electron is
  proportional to the potential difference between
  the cathode and the anode
• The energies of electrons accelerated by
  potential differences of 20 and 100 kilovolt peak
  (kVp) are 20 and 100 keV, respectively

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Bremsstrahlung (cont.)

• On impact with the target, the kinetic energy of
  the electrons is converted to other forms of
  energy
• Majority of interactions produce heat
• Heating limits the number of x-rays that may be
  produced in a given time without destroying the
  target
• About 0.5 of the time, an electron comes close
  to a nucleus in the target electrode
• Coulombic forces attract and decelerate the
  electron
• Kinetic energy lost by the electron produces an
  x-ray photon with energy equal to that lost by
  the electron

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Bremsstrahlung radiation
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Bremsstrahlung energy distribution
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Unfiltered and filtered spectra

• Unfiltered bremsstrahlung spectrum shows a
  ramp-shaped relationship between the number and
  the energy of the x-rays produced
• Highest x-ray energy determined by peak voltage
  (kVp) applied across the x-ray tube
• Filtered spectrum shows distribution with no
  x-rays below about 10 keV
• Lower energy x-rays preferentially absorbed
• Average x-ray energy typically about one third to
  one half of the highest x-ray energy in spectrum

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X-ray production efficiency

• Major factors that affect x-ray production
  efficiency include the atomic number of the
  target material and the kinetic energy of the
  electrons

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Efficiency (cont.)

• For 100-keV electrons impinging on tungsten
  (Z74), the approximate ratio of radiative to
  collisional losses is 0.9
• More than 99 of the incident energy creates heat
• For 6-MeV electrons, the ratio of radiative to
  collisional losses is approximately 54
• Excessive heat becomes less of a problem at
  higher energies

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Characteristic radiation

• Binding energies are unique to a given element,
  and so are their differences
• Emitted x-rays have discrete energies that are
  characteristic of that element
• The most prevalent characteristic x-rays in the
  diagnostic energy range result from K-shell
  vacancies, which are filled by electrons from the
  L, M, and N shells

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Generation of a characteristic x-ray
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Characteristic radiation (cont.)

• The shell capturing the electron designates the
  characteristic x-ray transition
• A subscript of ? or ? indicates whether the
  transition is from an adjacent shell (?) or
  nonadjacent shell (?)
• Within each shell (other than the K shell), there
  are discrete energy subshells, which result in
  the fine energy splitting of the characteristic
  x-rays

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Filtered spectrum of bremsstrahlung and
characteristic radiation from a tungsten target
at 90 kVp
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Characteristic radiation (cont.)

• Characteristic x-rays other than those generated
  by K-shell transitions are unimportant in
  diagnostic imaging because they are almost
  entirely attenuated by the x-ray tube window or
  added filtration
• Characteristic K-shell x-rays are emitted only
  when the electrons impinging on the target have a
  kinetic energy which exceeds the binding energy
  of a K-shell electron
• Bremsstrahlung x-rayelectron interactions via
  the photoelectric effect also contribute to
  characteristic x-ray production

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K-shell characteristic x-ray energies (keV) of
common x-ray tube materials
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X-ray tubes

• Major components
• Cathode
• Anode
• Rotor/stator
• Glass (or metal) envelope
• Tube housing

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Rotating-anode x-ray tube
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X-ray tube and housing components
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Typical operating conditions

• For diagnostic imaging, peak voltages range from
  20,000 to 150,000 V (20 to 150 kVp)
• Tube currents (the rate of electron flow from the
  cathode to the anode) are in the range of
• 1 to 5 mA for continuous fluoroscopy
• 100 to 1,000 mA for projection radiography (with
  short exposure times, often less than 100 msec)
• The kVp, mA and exposure time are the three major
  selectable parameters on the x-ray generator
  control panel

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Cathode

• The cathode consists of a helical filament of
  tungsten wire surrounded by a focusing cup
• The filament circuit provides a voltage up to
  about 10 V to the filament, producing a current
  of up to about 7 A
• Electrical resistance heats the filaments and
  releases electrons
• Electrons liberated from the filament flow
  through the vacuum of the tube to the anode when
  a positive voltage is applied to the anode
  relative to the cathode

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Filaments and focusing cup
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X-ray tube cathode structure
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Focusing cup

• Surrounds the filament and shapes the electron
  beam width
• An insulated focusing cup may be biased with a
  more negative voltage (about 100 V less) than the
  filament
• Creates a tighter electric field around the
  filament
• Reduces the spread of the beam
• Results in small focal spot width

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Effect of focusing cup
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Focusing cup

• Width of focusing cup slot determines the focal
  spot width 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 filament determines
  area of electron distribution (large or small
  focal spot) on the target

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Filament current

• Filament current determines filament temperature
  and thus the rate of thermionic electron emission
• When no voltage is applied between the cathode
  and the anode, an electron cloud (space charge
  cloud) builds around the filament
• Application of high positive 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 tube current

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Relationship of tube current to filament current
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Space charge

• Existence of the space charge cloud shields the
  electric field for tube voltages of 40 kVp or
  lower
• Only a portion of the free electrons are
  instantaneously accelerated to the anode
• Operation of the x-ray tube is space charge
  limited (places an upper limit on the tube
  current, regardless of the filament current)

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Emission-limited operation

• Above 40 kVp, the space charge cloud effect is
  overcome by the applied potential difference
• Tube current is limited only by the emission of
  electrons from the filament
• Filament current controls the tube current in a
  predictable way
• Higher kVp produces slightly higher tube current
  for the same filament current
• Beyond a certain kVp, saturation occurs whereby
  all of the emitted electrons are accelerated
  toward the anode

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Anode

• The anode is a metal target electrode that is
  maintained at a positive potential difference
  relative to the cathode
• Tungsten is the most widely used anode material
  because of its high melting point (3422C) and
  high atomic number (Z74)
• Tungsten anode can handle substantial heat
  deposition without cracking or pitting of its
  surface

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Anode configurations

• Simplest type of x-ray tube has a stationary
  (fixed) anode
• Consists of tungsten insert imbedded in a copper
  block
• Copper supports the tungsten target, and it
  removes heat efficiently from the target
• Small target area limits heat dissipation rate,
  limiting the maximum tube current and thus the
  x-ray flux
• Used in dental x-ray units, portable x-ray
  machines, portable fluoroscopy systems

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Fixed-anode x-ray tube
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Rotating anode

• Rotating anodes used for most diagnostic x-ray
  applications
• Greater heat loading and consequent higher x-ray
  output capabilities
• Electrons impart energy to a continuously
  rotating target, spreading thermal energy over a
  large area and mass

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Rotating-anode x-ray tube (showing heat
dissipation)
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Anode and rotor assembly of a rotating-anode
x-ray tube
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Rotor and stator

• Rotor consists of copper bars arranged around a
  cylindrical iron core
• Electromagnets surrounding the rotor outside the
  x-ray tube make up the stator
• Alternating current passes through the stator
  windings, causing rotor to spin
• Rotation speeds are 3,000 to 3,600 (low speed) or
  9,000 to 10,000 (high speed) revolutions per
  minute (rpm)

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Cross-sectional end view of stator/rotor assembly
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Rotor assembly

• Rotor bearings are heat sensitive
• Often the cause of x-ray tube failure
• Bearings are in the high-vacuum environment of
  the tube and require special heat-insensitive,
  nonvolatile lubricants
• Molybdenum stem attaches anode to rotor/bearing
  assembly
• Very poor heat conductor and reduces heat
  transfer from the anode to the bearings

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Cooling of rotating anodes

• Because it is thermally isolated, the anode must
  be cooled by radiative emission
• Heat energy emitted from hot anode as infrared
  radiation
• Transfers heat to the x-ray tube insert and
  ultimately to the surrounding oil bath
• Rotating anode with a 5-cm focal track radius and
  1-mm track width provides a focal track with an
  annular area 314 times greater than that of a
  fixed anode with a focal spot area of 1 mm ? 1 mm