X-ray Imaging System (Midterms)

• Three Principal Parts of X-ray Imaging System

  1. X-ray tube
  2. Operating console
  3. High-voltage generator
• Operating Console

  • allows RT to control the x-ray tube current and voltage so useful x-ray beam is of proper quantity and quality
  • provides for control of line compensation, kVp, mA, and exposure time
  • located in an adjoining room with a protective barrier separating the two (most systems)
• X-ray tube

  • located in the examination room
• High-voltage generator

  • housed in an equipment cabinet positioned against a wall
  • always close to the x-ray tube, usually in the examination room
  • responsible for increasing the output voltage from the autotransformer to the kVp necessary for x-ray production
• Radiation quantity

  • number of x-rays or the intensity of the x-ray beam
  • expressed in milligray (mGya) or milligray/milliampere-second (mGya/mAs)
• Radiation quality

  • penetrability of the x-ray beam
  • expressed in kilovolt peak (kVp) or half-value layer (HVL)
• Meters

  • provided for monitoring kVp, mA, and exposure time
• 220 V power

  • V power that most x-ray imaging systems are designed to operate
  • some are 110 V or 440 V
• 5%

  • voltage provided to an x-ray unit may vary by as much as this percent
• Line Compensator

  • measures the voltage provided to the x-ray imaging system and adjusts that voltage to precisely 220 V
• Autotransformer

  • has a single winding and is designed to supply a precise voltage to the filament circuit and to the high-voltage circuit of the x-ray imaging system
  • where power supplied to the x-ray imaging system is first delivered
  • works on the principle of electromagnetic induction but is very different from the conventional transformer
  • can be designed to step up voltage to approximately twice the input voltage value
• Primary connections

  • A and A' conduct the input power to the autotransformer

  Secondary connections

  • C allows autotransformer to increase voltage
  • D and E allow a decrease in voltage

  
• Primary voltage

  • voltage it receives
  • operated by autotransformer
• Secondary voltage

  • voltage it provides
  • operated by induction device
• Autotransformer Law

  • same as transformer law
• Minor kilovolt peak Selection

  • "fine tunes"
  • represent two separate series of connections on the autotransformer
• Major kilovolt peak Selection

  • represent two separate series of connections on the autotransformer
• kVp

  • determines the quality of the x-ray beam
• 220 V Primary voltage

  • controllable from about 100 to 400
• kVp Meter

  • placed across the output terminals of the autotransformer and therefore actually reads voltage, not kVp
• Scale of the kVp meter

  • registers kilovolts because of the known multiplication factor of the turns ratio
• Pre-reading kVp Meter

  • kVp meter registers, even though no exposure is being made and the circuit has no current
  • allows the voltage to be monitored before an exposure
• Milliamperes (mA)

  • measure of x-ray tube current, crossing from cathode to anode
• Temperature of the filament

  • determines the number of electrons emitted by the filament
• Filament temperature

  • controlled by the filament current
  • measured in amperes (A)
• 3 to 6 A

  • currents of normally operated filaments
• Thermionic emission

  • release of electrons from a heated filament
• Filament circuit

  • separate circuit that controls x-ray tube current
• Connections

  • provide voltage for the filament circuit
• Precision resistors

  • used to reduce this voltage to a value that corresponds to the selected milliamperage
  • result in fixed stations that provide tube currents of 100, 200, or 300 mA, and higher
• mAs

  • product of x-ray tube current (mA) and exposure time(s)
  • also "electrostatic charge (C)"
• mA meter

  • connected at the center of the secondary winding of the high-voltage step-up transformer
  • how x-ray tube current is monitored
• Secondary voltage

  • alternating at 60 Hz such that the center of this winding is always at zero volts
• Filament Transformer

  • "Filament heating isolation step-down transformer"
  • steps down the voltage to approximately 12 V and provides the current to heat the filament
• • Primary windings
  • thin copper and carry a current of 0.5 to 1 A and approximately 150 V
• Secondary windings

  • thick and approximately 12 V electric potential and carry a current of 5 to 8 A