IEM

• Exposure timers


  For any given radiographic examination, the number of x-rays that reach the image receptor is directly related to both the x-ray tube current and the time that the x-ray tube is energized
• Exposure timers


  1. X-ray operating consoles provide a wide selection of x-ray beam-on times

  2. Used in conjunction with the appropriate mA station to provide an even wider selection of values for mAs
• Guard timer


  A timing circuit that will terminate an exposure after a prescribed time, usually approximately 6 s
• Timer circuit


  1. Makes and breaks the high voltage across the x-ray tube

  2. Nearly always done on the primary side of the high-voltage transformer, where the voltage is lower
• Types of timing circuits


  • Synchronous timers
  • Electronic timers
  • mAs timers
  • Automatic exposure control
• Synchronous timers


  • Driven by a synchronous motor to provide precise 60 rps timing
  • Minimum exposure time is 1/60 s (17 ms)
  • Cannot be used for serial exposures as they must be reset after each exposure
• Electronic timers


  • Most sophisticated, complicated, and accurate
  • Based on the time required to charge a capacitor through a variable resistance
  • Allow a wide range of time intervals to be selected, accurate to 1 ms
  • Suitable for rapid serial exposures
• mAs timers


  • Monitor the product of mA and exposure time and terminate exposure when the desired mAs value is attained
  • Provide the highest safe tube current for the shortest exposure for any mAs selected
  • Located on the secondary side of the high-voltage transformer
• Automatic exposure control (AEC)


  • Measures the quantity of radiation that reaches the image receptor
  • Automatically terminates the exposure when the image receptor has received the required radiation intensity
  • Uses an ionization chamber or photodiode detector assembly
• AEC calibration


  1. Making exposures of a test object and adjusting the AEC for the required x-ray intensities

  2. Usually done by the service engineer
• AEC mode


  • Requires particular care, especially in low kVp examinations like mammography
  • The AEC may not respond properly due to varying tissue thickness and composition
• When using AEC


  The electronic timer should be set to 1.5 times the expected exposure time as a backup in case the AEC fails to terminate
• High-voltage generator


  Responsible for increasing the output voltage from the autotransformer to the kVp necessary for x-ray production
• High-voltage transformer


  • A step-up transformer with a turns ratio usually between 500:1 and 1000:1
  • Operates only on alternating current, producing a sinusoidal voltage waveform
• Turns ratio of high-voltage transformer

  Voltage increase is proportional to the turns ratio, according to the transformer law
• Voltage rectification


  Converting alternating current (AC) to direct current (DC) to provide the x-ray tube with electron flow in only one direction
• Rectifier


  An electronic device that allows current flow in only one direction
• 1. rays are produced by the acceleration of electrons from the cathode to the anode and cannot be produced by electrons flowing in the reverse direction</b>
• Semiconductor


  A material that lies between the range of insulators and conductors in the ability to conduct electricity
• Semiconductors are classed into two types: n-type and p-type
• Voltage rectification


  Rectifiers in most modern x-ray generators are the silicon, semiconductor type
• Semiconductors


  • Lie between the range of insulators and conductors in the ability to conduct electricity
  • Tiny crystals have some useful electrical properties and allow semiconductors to serve as the basis for today's solid-state microprocessor marvels
• Types of semiconductors

  • N-type
  • P-type
• 1. type semiconductors


  Have loosely bound electrons that are relatively free to move
• 1. type semiconductors


  Have spaces, called holes, where there are no electrons. Holes are as mobile as electrons.
• 1. n junction


  1. Electrons and holes migrate toward the junction and wander across it

  2. Flow of electrons and holes constitutes an electric current
• Positive potential on n side of p-n junction

  Electrons and holes swept away from junction, no current passes through
• 1. n junction


  Conducts electricity in only one direction, called a solid-state diode
• Rectifiers


  Conduct electric current in only one direction
• Rectification is essential for the safe and efficient operation of the x-ray tube
• Unrectified voltage


  Voltage waveform on secondary side of high-voltage transformer
• Current through x-ray tube only exists during positive half of cycle when anode is positive and cathode is negative
• Half-wave rectification


  Voltage not allowed to swing negatively during negative half of cycle
• Half-wave rectified circuit


  1. Contains one or more diodes

  2. Allows current to pass through x-ray tube during positive portion of AC waveform

  3. Does not conduct during negative portion, resulting in a series of positive pulses
• Full-wave rectification


  Negative half-cycle reversed so anode is always positive
• Full-wave rectified circuit

  1. Contains at least four diodes

  2. Current passed through tube at 120 pulses per second
• Main advantage of full-wave rectification is that exposure time for any given technique is cut in half
• Full-wave rectification


  1. Electrons flow through diode D and x-ray tube during positive half-cycle

  2. Electrons flow through diode A during negative half-cycle
• Half-wave rectified x-ray tube emits x-rays only half of the time, full-wave rectified emits 120 times per second
• Single-phase power


  Results in a pulsating x-ray beam due to alternate swing in voltage from zero to maximum 120 times per second