Chapter 8 The Xray Machine

1
Chapter 8 The X-ray Machine

• The Console is the part of the machine that the
  operator controls the operation of the x-ray
  machine.
• All machine console are a little different but
  there are always similarities. The console is
  where we control x-ray tube current and voltage.

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The Console Controls

• The console will have controls for
• mA and time or mAs
• kVp
• Focal Spot
• Line Voltage Compensation
• Automatic Exposure Control

3
Symbols Used to Draw Circuits

• We will be using the symbols to define the
  circuits in the x-ray machine

4
Console Circuits
5
Line Compensation

• At the bottom left is the controls for line
  voltage compensation.
• Most machine are designed to operate at 220 volts
  while some will work with 110 volts or 440 volts

6
Line Compensation

• The power company often cannot provide exactly
  220 volts at all times.
• Elevators and Air Conditioners may reduce the
  voltage available for the x-ray unit.

7
Line Compensation

• Older machine have a meter to monitor the line
  voltage attached to the autotransformer.
• The operator can adjust the taps on the
  transformer to account for low or high incoming
  voltage.

8
Line Compensation

• More modern units automatically adjusts for the
  incoming power so a meter is not provided.
• Often over looked by the operator.
• Results in improper exposure.

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Autotransformer

• The autotransformer is designed to supply voltage
  of varying magnitude to several different
  circuits of the x-ray machine including both the
  filament circuit and high voltage circuits.

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Autotransformer

• The autotransformer has only one winding and one
  core.
• The single winding has a number of connection or
  electric taps.

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kVp Adjustment

• Most consoles will have one or two knobs that
  change the taps on the autotransformer for major
  and minor kVp.
• Modern units have a LED readout of kVp.

12
kVp Adjustment

• Setting the desired kVp will determine the
  voltage applied to the step-up transformer in the
  high voltage section of the machine.

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kVp Adjustment

• If a meter is provided, it is placed across the
  output terminals of the autotransformer and
  therefore it reads voltage and not kVp. The scale
  will read in kVp.

14
mA Control

• The tube current, the number of electrons
  crossing from the cathode to anode per second is
  measured in milliapmeres (mA).
• The quantity of electrons is determined by
  filament temperature.

15
mA Control

• The filament normally operates at currents
  between 3 and 6 A.
• The Tube Current is controlled through a separate
  circuit called the filament circuit

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mA Control

• Voltage is provided by taps of the
  autotransformer. This voltage is reduced with
  precise resisters to a value corresponding to
  the mA stations available.

17
mA Control

• Tube current is usually not continuously
  variable, usually only currents of 50, 100, 150,
  200 300 mA and higher are provided.
• Newer units are continuously variable.

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mA Control

• The voltage is then delivered to the filament
  transformer. The filament transformer lowers the
  voltage so it is called a step down transformer.

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mA Control

• The selection of the small or large filament is
  connected to the mA selection or as a separate
  control.

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Exposure Timers

• For any given radiographic examination, the
  number of x-rays reaching the image receptor is
  directly related to the tube current and the time
  that the tube in energized.
• The timer circuit is separate from the other main
  circuits.

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Exposure Timers

• It consists of a mechanical or electronic device
  whose action is to make and break the high
  voltage across the tube on the primary side of
  the high voltage section.

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Types of Timers

• There are five types of timers
• Mechanical Timers
• Synchronous Timers
• Electronic Timers
• mAs Timers
• Phototimers

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Mechanical Timers

• Very simple device that has a clock mechanism.
• Operator turns the dial to the desired time. As
  it unwinds, the exposure is made.
• Can be used for exposure time longer than 250
  milliseconds.
• Very old machine and dental units.

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Electronic Timers

• Most sophisticated, complicated and most
  accurate timer.
• Consists of complex circuit based upon the time
  required to charge a capacitor through a variable
  resister.
• Depending upon the incoming power accurate to 1
  ms. Most units have this type timer.

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mAs Timers

• Most modern machine are designed to accurately
  control the tube current and exposure time.
• The product of mA and time (mAs) determines the
  number of x-ray photons emitted and the density
  on the film.

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mAs Timer

• A special type of timer monitors the product of
  mA and terminates the exposure when the desired
  mAs has been attained.
• This is a mAs timer.

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mAs Timer

• Designed to provide the shortest exposure and the
  highest safe tube current for the given filament.
• Some have the ability to change mA manually.

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mAs Timer

• Since it monitors the actual tube current, it is
  on the secondary side of the H.V. Circuit
• Units here have mAs timers.

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mAs Timer

• APR or Anatomically Programs Timers have
  computers that store the technical factors in the
  machine.
• Select the view and enter the patient size and
  the machine is ready!!!!

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Phototimers

• A phototimer that measures the quantity of
  radiation reaching the receptor and terminates
  the exposure when sufficient radiation needed to
  produce the correct density on the film.
• Offered in addition to a manual timer.

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Phototimers

• There are two types of phototimers
• 1. Photomultiplier tube that reads a fluorescent
  screen behind the film.
• 2. Ion chamber between the grid and film.

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Phototimers

• Ion Chambers is used on most modern x-ray units.
• It is flat and radiolucent so it will not
  interfere with the image. Multiple chambers can
  be used to optimize the image.

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Phototimers

• Commonly referred to as Automatic Exposure
  Control or AEC.
• Widely used in Medical Radiography.
• Used at our Benton Clinic.

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AEC Console

• With AEC, the operator can select
• Where to read the radiation.
• The desired film density
• kVp and backup mAs

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AEC Console

• Many operators do not measure the patient and set
  a arbitrary back up mAs or time.
• Ideally, the patient is measured and the back up
  mAs is set at 2X the normal mAs.

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AEC Console

• This allows the AEC to adjust exposure for the
  patients habitus and area density.
• Radiation is measured at the center of the film
  or off to the sides of the film.

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AEC Console

• The center is read for most radiography and
  especially for the spine.
• The sides are read for PA chest, abdomen and rib
  radiography.

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Other functions on the Control Console.

• The console will also have the exposure button or
  buttons.
• The prep button is depressed to prepare the tube
  for exposure.
• The rotor will spin up to 3400 RPM.

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Exposure Button

• A green light will let you know that the machine
  is ready to make the exposure.
• The exposure button is then depressed and the
  exposure is initiated.

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Exposure Button

• The button must be held down until the exposure
  is complete.
• If your finger slips off the button, the exposure
  is terminated.

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Exposure Button

• The exposure control buttons are referred to as
  aDead man Switch
• After the buttons are released, the rotor motor
  reverses and the rotor reduces speed.

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Exposure Button

• During the exposure you will hear an audible tone
  so you will know that the exposure is in progress.

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High Voltage Section

• The high voltage section converts low voltage
  from incoming power to kilo-voltage of the
  correct wave form.
• It is usually enclosed in a large metal container
  in the x-ray room.

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High Voltage Section

• It consists of three primary sections
• High voltage step up transformer
• Filament Transformer
• Rectifiers ( Diodes)
• All components immersed in oil.

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High Voltage Transformer

• The high voltage transformer is a step-up
  transformer.
• There will be more winding on the secondary side
  compared to the primary side.
• The ratio of windings is referred to as the turns
  ratio.

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High Voltage Transformer

• The only difference between the primary and
  secondary waveforms is the amplitude.
• The turn ratio for most x-ray high voltage
  transformers is between 500 and 1000.
• Incoming Volts converted to output Kilovolts.

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Voltage Rectification

• Transformers operate with alternating current.
• Remember that x-ray tubes operate on direct
  voltage ( electron moving in one direction).
• To convert AC to DC we use rectifiers.

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Half-Wave Rectification

• Sometimes the x-ray tube alone will work as the
  diode this is called self-rectification.
• When one or two diodes are placed in the circuit
  that stops the negative flow of electrons it is
  called Half Wave Rectification.
• 60 pulses per second.

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Full-Wave Rectification

• Full wave rectified x-ray machines contain at
  least four diodes.
• It changes the polarity of the negative half of
  the wave.
• This allows 120 pulses of x-ray per second.
• The exposure time can be cut in half compared to
  half-wave systems.

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Three-Phase Power

• If three phases of power are combines with the
  phase off by one step, the normal reduction of
  voltage back to zero is removed. Commonly called
  the Ripple.
• Technical factor cut in half due to more
  efficient power.
• Too expensive got office use.

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High Frequency Generator

• By changing the frequency from 60 Hz to a higher
  frequency of 500 to 1000, the ripple is reduced
  to less than 1.
• Single phase machine operating on 220 volts and
  even 110 volts are more efficient that machine
  operating on three-phase power.

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Types of X-ray Generators

• The type of generator will determine the
  efficiency of the machine.

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Wave Forms of Different Generator Types

• As the ripple effect decreases, the efficiency
  increases.
• There is one more type of generator. It uses is
  called stored energy.

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Stored Energy Generators

• If 220 volt power is not available, the operator
  may choose a stored energy machine.
• A battery charger is powered by typical house
  hold current.
• If produces direct current.

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Stored energy or Capacitor Discharge Generators

• There is a short charging time before the
  exposure can be made.
• The disadvantage to the design is a drop in power
  at the end of the exposure of about 1 kV/mAs.
  This is called a falling load generator.

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Generator Types Pros Cons

• Single phase half or self rectified Cheap but
  not efficient. Full wave rectified better.
• Three phase Expensive to install but cheaper to
  maintain. Too costly for most offices. 6 pulse
  less costly than 12 pulse
• High Frequency very efficient and works with
  single or three phase power.
• Stored energy works on conventional 110 volt
  power but batteries must be replaced.

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The Basic X-ray Circuits

• Circuits that make up the basic x-ray machine.

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Other Parts of the X-ray Room

• The tube is suspended on the tube stand.
• The tube stand may be wall and floor mounted or
  ceiling suspended.Locks are provided for
  horizontal and vertical movement.

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Other Parts of the X-ray Room

• When the tube is angled toward the wall grid
  holder, the horizontal lock allow us to set the
  distance between the tube and the film (SID).

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Other Parts of the X-ray Room

• When the tube is aimed at the table, the vertical
  lock allows us to set the SID.
• Hanging on the wall grid cabinet is the non-Bucky
  film holder.
• It allows erect non-grid films.

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Other Parts of the X-ray Room

• X-ray tables may be bolted to the floor or
  mobile. The table will also have a grid cabinet
  for grid radiography.
• We will discuss grids in greater detail next week.

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Collimator and Angle Indicator

• The tube stand also has an angle indicator
  attached parallel to the tube.
• There are views that will require tube angles.

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Collimator and Angle Indicator

• The Collimator is attached to the x-ray tube
  below the glass window where the useful beam is
  emitted.
• Lead shutters are used to restrict the beam.

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Collimator and Angle Indicator

• A mirror and light source allows us to restrict
  the beam to the area of interest.
• Collimation is our greatest tool in keeping
  patient exposure as low as possible.

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Other items that may be in a x-ray room.

• Fluoroscopy Equipment Allows dynamic imaging of
  the body.
• Consists of
• Image intensifier with television camera and
  monitor.
• Spot-film device for making radiographs or
• Motion picture camera or digital imaging.

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Image Intensifier Fluoroscopy

• Thomas Edison invented the fluoroscope in 1896.
  Early units consisted of a fluorescent hand held
  viewer that the doctor held in from of the
  patient during continuous exposure.
• This resulted in the first x-ray death.
• Dose is still relatively high compared to plain
  film radiography.

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Image Intensifier Fluoroscopy

• Plain film radiography uses up to several hundred
  mA and fractions of seconds.
• Fluoroscopy tubes operate at less than 5 mA but
  for minutes. 2 to 4 mA is normal.
• In California Fluoroscopy requires additional
  training and license or permits for technologist
  and doctors.

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Image Intensifier Fluoroscopy

• Shortly after WW2, Bell Laboratories invented the
  photomultiplier tube. This was developed into the
  modern image intensifier.
• The multiplication of the light emitted by a
  input fluorescent screen is picked up by a cesium
  photocathode and converted into electrons.

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Image Intensifier Fluoroscopy

• A potential of about 25,000 volts is maintained
  between the photocathode and the anode.
• There are electronic optics and electrostatic
  focusing lenses between the photocathode and
  output phosphor.

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Image Intensifier Fluoroscopy

• The output phosphor can be viewed via mirror
  optics or a video monitoring system.
• A Videotape recorded can be placed into the video
  chain.
• Fluoroscopy allows the evaluation of the internal
  structures in motion. Normal uses include

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Uses of Fluoroscopy

• Dynamic spinal imaging of range of motion and
  with contrast called myelograms.
• Dynamic studies of joints with or without
  contrast media.
• Studies of the digestive system.
• Studies of arteries and blood flow called
  angiography.

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Uses of Fluoroscopy

• When connected to a computer, for digital
  fluoroscopy and spot films.
• With digital fluoroscopy, digital angiography is
  possible.
• By over-lapping an image without contrast,
  digital subtraction angiography is performed
  where the bone is removed.

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