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X ray Imaging System

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					X-RAY IMAGING SYSTEM
The Discovery of X-ray
   November 8, 1895
   Wilhelm Conrad Roentgen
   Cathode ray tube – Crookes
     Large   partially evacuated glass tube
   Screen of barium platinocyanide would fluoresce
   Investigating various materials that would stop the
    rays he inadvertently x-rayed his own hand
   Roentgen rays
Radiation protection / ALARA
   Time/Distance/Shielding
   Do not allow familiarity to result in false security
   Stay out of the primary beam
   Protective apparel
   Radiation monitor at the collar
   Do not hold patients
   Person holding must use shielding
   Gonadal shielding
   1st trimester pregnancy
   Collimate!
ALARA
   Time
   Distance
   Shielding
Principles of radiation protection
   Radiation protection can be divided into
       occupational radiation protection
           is the protection of workers
       medical radiation protection
           which is the protection of patients
       public radiation protection
            which is protection of individual members of the public, and of
            the population as a whole.
       The types of exposure, as well as government regulations
        and legal exposure limits are different for each of these
        groups, so they must be considered separately.
Cardinal Rules
   Time:
     Reducing the time of an exposure reduces the effective
      dose proportionally.
   Distance:
     Increasingdistance reduces dose due to the inverse
      square law.
   Shielding:
     Adding  shielding can also reduce radiation doses. The
      radiation getting through falls exponentially with the
      thickness of the shield.
General Purpose Room
   Overhead tube
   Fluoroscopic table/tube
   Radiation protection
     Lead curtain
     Bucky slot cover

     lead apron and gloves

     Protected viewing
      window
General Purpose Room
Producing X-rays
   When fast moving electrons slam into a metal
    object, x-rays are produced.
   Kinetic energy from the electrons is transformed into
    electromagnetic energy
   The law of conservation of energy states that the
    total amount of energy in an isolated system remains
    constant.
       A consequence of this law is that energy cannot be
        created nor destroyed. The only thing that can happen
        with energy in an isolated system is that it can change
        form
X-ray Production
1.   X-ray machine turned on, small amount of current
     sent to the filament to warm it and ready it for
     much higher current
2.   Radiographer warms tube with warm up exposures
3.   Tech chooses exposure factors on console (kVp,
     mAs) appropriate for exam
4.   Electricity incoming to department adjusted (line
     voltage compensator) to maintain it at a constant
     level
5.   Tech presses rotor switch and exposure switch in
     one motion
6.   Induction motor spins the anode as filament get
     hotter
7.   Filament heats, boils off electrons (thermionic
     emission) creating space charge (thermionic cloud)
     around the filament
8.   At same time AC sent to step up transformer and
     boosted to kilovoltage levels
9.    Kilovoltage creates a high potential difference
      making anode less negative (relatively positive)
      and the cathode highly negative
10.   High potential difference causes electrons (e-) to
      move at very high speed (half the speed of light)
      from cathode anode
11.   At the target the collision of these e- causes a
      conversion of kinetic energy (100%) to heat
      (99.8%) and x ray (0.2%)
Main Components
   X-ray tube
   Operating console
   High voltage generator

   Table
     Uniformly  radiolucent
     Fixed/tilting

   Tube support
     Overhead
     floor
Image receptor/ holders
   Table bucky
     grid

   Upright bucky
     Grid



    CR cassette
    Film cassette
Console
   Technical factors
     mAs

     kVp

   Body part selection
   AEC / phototiming
X-Ray Tube
The X-ray Tube
   Power
     Incoming line current 60Hz AC 200-240volts
     Single phase power

     Three phase power

     High frequency

   X-ray circuit
     Main circuit
     Filament circuit
Main x-ray circuit
   Modifies incoming line power
     Boosts the voltage
     Permits the radiographer to adjust amperage, voltage
      and length of exposure

    Main switch : with circuit breakers at power box
    Exposure switch: permits current to flow through the circuit
      designed to begin but not end exposure
    Timer: ends the exposure at an accurate, preset time
Generating X-ray Photons
   X-ray photons are produced when high speed
    electrons from the cathode (-)strike the anode (+)
    target.
The exposure switch
   Exposure switch connected to the switch that causes
    the anode to rotate
   Anode must be rotating at high speed to prevent
    melting of the target area by high kilovoltage
    exposure
   Most are pressed halfway to activate anode
    rotation then fully depressed to expose
   Tube manufacturers recommend depression in one
    motion to extend tube life
The exposure switch cont..
   Much be attached to the console to avoid exposing
    operator
   On mobile equipment; must be six feet in length
   All exposure switches must be dead man type
     Release   of switch terminates exposure
The timer circuit
   Electronic Timers
     Most  common
     Capable of accurate exposures as short as 0.001
      second with 1 msec delay
     Charges a Silicon controlled rectifier which triggers the
      exposure
     Milliampere-Second Timers
       Monitor  the product of mA and time (mAs) on the secondary
        side of the high voltage step up transformer
       When desired mAs is reached the timers interrupt the circuit
        to stop the exposure
Filament Circuit
   Modifies the incoming line power to produce the
    thermionic emission from the filament wire of the x-
    ray tube
   The incoming line must be modified to 3-5 amperes
    and 6-12 volts
   Current control device regulates the amperage
    supplied to the filament in the tube
     Adjusted by radiographer at the console
     To equivalent of mA ratings of 50,100,200,300, 400
Filament Circuit cont…
   Filament circuit supply is drawn directly from the
    main circuit supply
   The current is then sent to a step down transformer
    that modifies the amperage that will be sent to the
    filament itself

   All of the radiographer-operated controls are on
    the low voltage side of the circuit to prevent
    operators from high voltage shock hazards
X-ray Production
1.   X-ray machine turned on, small amount of current
     sent to the filament to warm it and ready it for
     much higher current
2.   Radiographer warms tube with warm up exposures
3.   Tech chooses exposure factors on console (kVp,
     mAs) appropriate for exam
4.   Electricity incoming to department adjusted (line
     voltage compensator) to maintain it at a constant
     level
5.   Tech presses rotor switch and exposure switch in
     one motion
6.   Induction motor spins the anode as filament get
     hotter
7.   Filament heats, boils off electrons (thermionic
     emission) creating space charge (thermionic cloud)
     around the filament
8.   At same time AC sent to step up transformer and
     boosted to kilovoltage levels
9.    Kilovoltage creates a high potential difference
      making anode less negative (relatively positive)
      and the cathode highly negative
10.   High potential difference causes electrons (e-) to
      move at very high speed (half the speed of light)
      from cathode anode
11.   At the target the collision of these e- causes a
      conversion of kinetic energy (100%) to heat
      (99.8%) and x ray (0.2%)

				
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posted:3/29/2011
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