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					  Chapter 2
The Generation
   of X-ray:
 X-ray Tubes
The Generation of X-ray

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

				
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posted:8/25/2011
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