The Generation of X-ray:

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Chapter 2

• The Generation of X-ray
• X-ray Tubes

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The Generation of X-ray

• X-rays are produced whenever electrons, traveling
  at high speeds, collide with matter in any form.

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The Generation of X-ray

• There are three essentials that must be fulfilled
  before x-rays can be produced. They are
• 1. A source of electrons
• 2. A means of accelerating controlling their
  movement (via a difference in potential)
• 3. A place to stop them with great suddenness (a
  point of impact)

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The Generation of X-ray

• The Law of Conservation of Energy - energy can
  neither be created or destroyed.
• Binding Energies - The forces that hold electrons
  in orbit around the nucleus. The nucleus ()
  attracts the electron (-), but the spin of the
  electron keeps them from collapsing toward the
  nucleus.

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The Generation of X-ray

• Ground State - Inner shells have more energy
  available due to their proximity to the nucleus
  (stronger interaction). Outer shells have less
  energy available because they have weaker
  interaction with the nucleus.

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The Coolidge Hot-Cathode Tube

• In modern x-ray tube the glass bulb is exhausted
  to as complete a vacuum as is possible to attain.
  The cathode (-) is composed of a small spiral
  filament of tungsten wire, about 1 cm long 0.2
  to 0.3 cm in diameter, which is housed in a
  focusing cup. The anode () is a solid rod of
  copper molybdenum in the opposite end of the
  tube.

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The Coolidge Hot-Cathode Tube

• The surface of the anode () facing the cathode
  (-) filament is beveled at between7 and 17
  degrees has a block of tungsten set into it it
  is only a few centimeters from the filament.

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How The Three Essentials are Fulfilled in the
Coolidge Hot-Cathode Tube

• The source of electrons in modern x-ray tubes is
  the tungsten filament. This is connected to a
  step-down transformer heated to incandescence
  by current from it. The heating of the wire
  alters its atomic stability in that the electrons
  are less firmly combined with the nucleus of the
  atom.

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How The Three Essentials are Fulfilled in the
Coolidge Hot-Cathode Tube

• These loosely bound electrons hover about the
  cathode like a cloud. The greater the heat to the
  filament, the more electrons available at the
  face of the cathode.
• This is called Thermionic Emission or Boiling Off
  of Electrons (the source of electrons)

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How The Three Essentials are Fulfilled in the
Coolidge Hot-Cathode Tube

• If a high voltage current is applied to the tube
  (negative to the cathode) these electrons will be
  repelled from the cathode (like charges
  repelling) towards the anode (unlike charges
  attracting) with one-third to one-half the speed
  of light (a means of acceleration).

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How The Three Essentials are Fulfilled in the
Coolidge Hot-Cathode Tube

• These electrons will strike the anode with great
  force (a point of impact) be converted into
  x-rays heat.
• The energy of the speeding electrons is converted
  into two type of energy
• Greater than 99 Heat
• Less than 1 X-Ray
• (exothermic reaction - heat gt energy)

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The Focal Spot of the X-Ray Tube

• Most of the electrons bombard the target over a
  small area near its center. This is the actual
  focal spot of the tube. The actual focal spot has
  an area nearly equal in size to the overall
  dimensions of the filament.

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The Focal Spot of the X-Ray Tube

• The smaller the focal spot, the greater is the
  detail produced on the radiograph, but the
  smaller is the capacity of the tube to produce
  x-rays. This is explained by the fact that all
  the energy is expended at the focal spot .The
  smaller the focal spot, the more intense will be
  the heat developed. Longer exposure time is
  necessary with such tubes.

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The Focal Spot of the X-Ray Tube

• To control the size of the focal spot a sleeve of
  molybdenum can be placed around the filament
  given a negative charge which repels the
  electrons from all directions, this will form a
  more narrow stream.

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The Focal Spot of the X-Ray Tube

• The principle of line focus is a method used to
  give a smaller focal spot with a larger target
  area. The actual focal spot is a rectangle
  approximately 3 times as long as it is wide.

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The Focal Spot of the X-Ray Tube

• When the beveled anode () face is viewed from
  the patients point of view, the focal spot
  appears to be nearly square. This is the
  effective projected focal spot. It serves to
  bring the x-ray source closer to being one point
  while still maintaining the larger area for
  impact.

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Double Focus Tubes

• Are those tubes having 2 focal spots - one fine
  (0.3 mm) focus for maximum detail one large
  (2.0 mm) for heavier exposures. A mechanical
  switch includes the focus of choice in the
  circuit. Most x-ray machines have this built in.

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Double Focus Tubes

• Penumbra - blurring of the edge of an organ or
  bone due to the size of the focal spot.
• Today the large focal spots are 0.8 mm - 1.0 mm,
  and are actually smaller than the old small focal
  spots. This is accomplished via new ways to cool
  the tubes.

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Double Focus Tubes Summary

• Large Filament larger effective focal spot -
  used for larger body parts - looses some detail.
• Small Filament smaller effective focal spot -
  used for small body parts such as extremity
  fractures (hair line) and fine detail.(this
  causes incredible heat build up)

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Methods of Cooling the Anode

• Sufficient heat is generated in the operation of
  an x-ray tube to melt the tungsten target (3370
  C) that methods had to be developed to dissipate
  the heat protect the tube.
• Construction of the anode with two metals - one
  with a very high melting point (tungsten) the
  other with a high conductivity for heat (copper)
  - was an important first step.

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Methods of Cooling the Anode

• The main cooling methods that are used or have
  been used are as follows
• Natural Radiation - In this method heat is lost
  via the glass tube into the air. Low capacity
  tubes may be used for short periods without any
  means of cooling.

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Methods of Cooling the Anode

• Air Cooling by Radiation - A radiator (series of
  metal discs) may be attached to the extreme end
  of the anode to increase the surface area which
  can give off heat into the air.
• Water Cooling - Obsolete today - water was
  carried away through a hollowed out anode stem.

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Methods of Cooling the Anode

• Oil Cooling - Almost all x-ray tubes in use today
  are surrounded by oil. The oil insulates as well
  as cools. oil air cooling may be combined.
• The Rotating Anode Tube - As the name implies the
  anode target rotates during the exposure. This
  allows us to increase the exposure because of the
  tremendous ability to dissipate heat.

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The Rotating Anode

• The anode in the tube is a beveled tungsten disc
  attached to a rotor that revolves when the tube
  is on. The cathode filament is offset to one side
  so that the electron stream hits near the edge of
  the revolving disc.
• The rotating anode continually presents a
  different area on the target to the electron
  stream.

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The Rotating Anode

• The focal spot remains fixed in space while the
  circular anode rotates during the exposure to
  provide a cooler surface for the electron stream
  to strike.
• The heat is distributed over a broad band, thus
  maintaining the temperature rise well within safe
  limits. As the capacity of the tube to withstand
  heat is increased, the capacity of the tube to
  produce x-rays is increased.

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The Rotating Anode

• It also permits manufacturers to produce tubes
  with smaller effective focal spots.
• The disadvantages are
• the tube is very delicate
• special lubricants are necessary for the motor
  which will not produce volatile gases

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Tube Capacity

• The tube capacity or their ability to produce
  x-rays is affected by the rotating anode.
• Tubes are rated in terms of
• Kilovoltage (kV)
• Miliamperage (MA)
• Time of exposure (S)

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Tube Capacity

• These factors are dependent on the rectification
  system, cooling method and focal spot size.
• kV - capacity is determined by the distance from
  the filament to the target.
• MA - capacity is determined by size of the focal
  spot the rectification system used.
• S - capacity is determined by the anode tube
  cooling rates.

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The Heel Effect

• Heel effect is the term applied to the fact that
  x-ray radiation does not exit the long axis of
  the tube in uniform intensities.
• The intensity of the beam is equal to the number
  of rays diminishes fairly rapidly from the
  central ray to the anode side of the patient,
  while increasing slightly toward the cathode side
  of the patient.

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The Heel Effect

• We can use this to our advantage in radiography
  if we remember to position the tube with the
  anode end of the tube towards the more easily
  penetrated body part.
• Originally it was thought that the heel effect
  was due to the angulation of the anode was
  based upon the theory of refraction.

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The Heel Effect

• Today we believe that the electrons which are
  traveling at high speeds, bombard the target and
  xray is produced in 360 degrees of direction. 180
  degrees of this x ray produced is absorbed by
  the anode itself.
• Therefore, only 180 degrees of x-rays leave the
  anode.
• Of those, some are absorbed by the lead shutters
  of the collimator, allowing only those travelling
  in the desired direction to exit the tube.

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The Heel Effect

• X-ray is not uniform along the film surface.
  Therefore the effects of the heel effect are seen
  toward the edges of the film.
• To decrease the heel effect, increase the
  collimation decrease the film size.
• With increased collimation you get less heel
  effect.

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The Heel Effect

• For example, in taking a radiograph of the
  cervical-thoracic area, the neck area should
  receive the rays from the anode portion of the
  beam as the neck is thinner than the thoracic
  region.