Chapter 8 The X-ray Imaging System

1
Chapter 8 The X-ray Imaging System

• 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.

2
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.

9
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.

10
Autotransformer

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

11
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.

13
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

16
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.

18
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.

19
mA Control

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

20
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.

21
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.

22
Types of Timers

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

23
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.

24
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.

25
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.

26
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.

27
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.

28
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.

29
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!!!!

30
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.

31
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.

32
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.

33
Phototimers

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

34
AEC Console

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

35
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.

36
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.

37
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.

38
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.

39
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.

40
Exposure Button

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

41
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.

42
Exposure Button

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

43
Chapter 8 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.

44
High Voltage Section

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

45
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.

46
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.

47
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.

48
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.

49
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.

50
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.

51
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.

52
Types of X-ray Generators

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

53
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.

54
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.

55
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.

56
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.

57
The Basic X-ray Circuits

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

58
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.

59
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).

60
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.

61
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.

62
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.

63
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.

64
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.

65
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.

66
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.

67
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 is beyond the
  chiropractic scope of practice.

68
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.

69
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.

70
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

71
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.

72
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.

73
End of Lecture

• Return to Lecture Index
• Return to Physics Homepage