The x ray imaging system

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The x ray imaging system

  1. 1. Chapter 8 The X-ray Imaging System <ul><li>The Console is the part of the machine that the operator controls the operation of the x-ray machine. </li></ul><ul><li>All machine console are a little different but there are always similarities. The console is where we control x-ray tube current and voltage. </li></ul>
  2. 2. The Console Controls <ul><li>The console will have controls for: </li></ul><ul><li>mA and time or mAs </li></ul><ul><li>kVp </li></ul><ul><li>Focal Spot </li></ul><ul><li>Line Voltage Compensation </li></ul><ul><li>Automatic Exposure Control </li></ul>
  3. 3. Symbols Used to Draw Circuits <ul><li>We will be using the symbols to define the circuits in the x-ray machine </li></ul>
  4. 4. Console Circuits
  5. 5. Line Compensation <ul><li>At the bottom left is the controls for line voltage compensation. </li></ul><ul><li>Most machine are designed to operate at 220 volts while some will work with 110 volts or 440 volts </li></ul>
  6. 6. Line Compensation <ul><li>The power company often cannot provide exactly 220 volts at all times. </li></ul><ul><li>Elevators and Air Conditioners may reduce the voltage available for the x-ray unit. </li></ul>
  7. 7. Line Compensation <ul><li>Older machine have a meter to monitor the line voltage attached to the autotransformer. </li></ul><ul><li>The operator can adjust the taps on the transformer to account for low or high incoming voltage. </li></ul>
  8. 8. Line Compensation <ul><li>More modern units automatically adjusts for the incoming power so a meter is not provided. </li></ul><ul><li>Often over looked by the operator. </li></ul><ul><li>Results in improper exposure. </li></ul>
  9. 9. Autotransformer <ul><li>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. </li></ul>
  10. 10. Autotransformer <ul><li>The autotransformer has only one winding and one core. </li></ul><ul><li>The single winding has a number of connection or electric taps. </li></ul>
  11. 11. kVp Adjustment <ul><li>Most consoles will have one or two knobs that change the taps on the autotransformer for major and minor kVp. </li></ul><ul><li>Modern units have a LED readout of kVp. </li></ul>
  12. 12. kVp Adjustment <ul><li>Setting the desired kVp will determine the voltage applied to the step-up transformer in the high voltage section of the machine. </li></ul>
  13. 13. kVp Adjustment <ul><li>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. </li></ul>
  14. 14. mA Control <ul><li>The tube current, the number of electrons crossing from the cathode to anode per second is measured in milliapmeres (mA). </li></ul><ul><li>The quantity of electrons is determined by filament temperature. </li></ul>
  15. 15. mA Control <ul><li>The filament normally operates at currents between 3 and 6 A. </li></ul><ul><li>The Tube Current is controlled through a separate circuit called the filament circuit </li></ul>
  16. 16. mA Control <ul><li>Voltage is provided by taps of the autotransformer. This voltage is reduced with precise resisters to a value corresponding to the mA stations available. </li></ul>
  17. 17. mA Control <ul><li>Tube current is usually not continuously variable, usually only currents of 50, 100, 150, 200 & 300 mA and higher are provided. </li></ul><ul><li>Newer units are continuously variable. </li></ul>
  18. 18. mA Control <ul><li>The voltage is then delivered to the filament transformer. The filament transformer lowers the voltage so it is called a step down transformer. </li></ul>
  19. 19. mA Control <ul><li>The selection of the small or large filament is connected to the mA selection or as a separate control. </li></ul>
  20. 20. Exposure Timers <ul><li>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. </li></ul><ul><li>The timer circuit is separate from the other main circuits. </li></ul>
  21. 21. Exposure Timers <ul><li>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. </li></ul>
  22. 22. Types of Timers <ul><li>There are five types of timers: </li></ul><ul><li>Mechanical Timers </li></ul><ul><li>Synchronous Timers </li></ul><ul><li>Electronic Timers </li></ul><ul><li>mAs Timers </li></ul><ul><li>Phototimers </li></ul>
  23. 23. Mechanical Timers <ul><li>Very simple device that has a clock mechanism. </li></ul><ul><li>Operator turns the dial to the desired time. As it unwinds, the exposure is made. </li></ul><ul><li>Can be used for exposure time longer than 250 milliseconds. </li></ul><ul><li>Very old machine and dental units. </li></ul>
  24. 24. Electronic Timers <ul><li>Most sophisticated, complicated and most accurate timer. </li></ul><ul><li>Consists of complex circuit based upon the time required to charge a capacitor through a variable resister. </li></ul><ul><li>Depending upon the incoming power accurate to 1 ms. Most units have this type timer. </li></ul>
  25. 25. mAs Timers <ul><li>Most modern machine are designed to accurately control the tube current and exposure time. </li></ul><ul><li>The product of mA and time (mAs) determines the number of x-ray photons emitted and the density on the film. </li></ul>
  26. 26. mAs Timer <ul><li>A special type of timer monitors the product of mA and terminates the exposure when the desired mAs has been attained. </li></ul><ul><li>This is a mAs timer. </li></ul>
  27. 27. mAs Timer <ul><li>Designed to provide the shortest exposure and the highest safe tube current for the given filament. </li></ul><ul><li>Some have the ability to change mA manually. </li></ul>
  28. 28. mAs Timer <ul><li>Since it monitors the actual tube current, it is on the secondary side of the H.V. Circuit </li></ul><ul><li>Units here have mAs timers. </li></ul>
  29. 29. mAs Timer <ul><li>APR or Anatomically Programs Timers have computers that store the technical factors in the machine. </li></ul><ul><li>Select the view and enter the patient size and the machine is ready!!!! </li></ul>
  30. 30. Phototimers <ul><li>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. </li></ul><ul><li>Offered in addition to a manual timer. </li></ul>
  31. 31. Phototimers <ul><li>There are two types of phototimers: </li></ul><ul><li>1. Photomultiplier tube that reads a fluorescent screen behind the film. </li></ul><ul><li>2. Ion chamber between the grid and film. </li></ul>
  32. 32. Phototimers <ul><li>Ion Chambers is used on most modern x-ray units. </li></ul><ul><li>It is flat and radiolucent so it will not interfere with the image. Multiple chambers can be used to optimize the image. </li></ul>
  33. 33. Phototimers <ul><li>Commonly referred to as Automatic Exposure Control or AEC. </li></ul><ul><li>Widely used in Medical Radiography. </li></ul><ul><li>Used at our Benton Clinic. </li></ul>
  34. 34. AEC Console <ul><li>With AEC, the operator can select: </li></ul><ul><li>Where to read the radiation. </li></ul><ul><li>The desired film density </li></ul><ul><li>kVp and backup mAs </li></ul>
  35. 35. AEC Console <ul><li>Many operators do not measure the patient and set a arbitrary back up mAs or time. </li></ul><ul><li>Ideally, the patient is measured and the back up mAs is set at 2X the normal mAs. </li></ul>
  36. 36. AEC Console <ul><li>This allows the AEC to adjust exposure for the patient’s habitus and area density. </li></ul><ul><li>Radiation is measured at the center of the film or off to the sides of the film. </li></ul>
  37. 37. AEC Console <ul><li>The center is read for most radiography and especially for the spine. </li></ul><ul><li>The sides are read for PA chest, abdomen and rib radiography. </li></ul>
  38. 38. Other functions on the Control Console. <ul><li>The console will also have the exposure button or buttons. </li></ul><ul><li>The prep button is depressed to prepare the tube for exposure. </li></ul><ul><li>The rotor will spin up to 3400 RPM. </li></ul>
  39. 39. Exposure Button <ul><li>A green light will let you know that the machine is ready to make the exposure. </li></ul><ul><li>The exposure button is then depressed and the exposure is initiated. </li></ul>
  40. 40. Exposure Button <ul><li>The button must be held down until the exposure is complete. </li></ul><ul><li>If your finger slips off the button, the exposure is terminated. </li></ul>
  41. 41. Exposure Button <ul><li>The exposure control buttons are referred to as a”Dead man Switch” </li></ul><ul><li>After the buttons are released, the rotor motor reverses and the rotor reduces speed. </li></ul>
  42. 42. Exposure Button <ul><li>During the exposure you will hear an audible tone so you will know that the exposure is in progress. </li></ul>
  43. 43. Chapter 8 High Voltage Section <ul><li>The high voltage section converts low voltage from incoming power to kilo-voltage of the correct wave form. </li></ul><ul><li>It is usually enclosed in a large metal container in the x-ray room. </li></ul>
  44. 44. High Voltage Section <ul><li>It consists of three primary sections: </li></ul><ul><ul><li>High voltage step up transformer </li></ul></ul><ul><ul><li>Filament Transformer </li></ul></ul><ul><ul><li>Rectifiers ( Diodes) </li></ul></ul><ul><li>All components immersed in oil. </li></ul>
  45. 45. High Voltage Transformer <ul><li>The high voltage transformer is a step-up transformer. </li></ul><ul><li>There will be more winding on the secondary side compared to the primary side. </li></ul><ul><li>The ratio of windings is referred to as the turns ratio. </li></ul>
  46. 46. High Voltage Transformer <ul><li>The only difference between the primary and secondary waveforms is the amplitude. </li></ul><ul><li>The turn ratio for most x-ray high voltage transformers is between 500 and 1000. </li></ul><ul><li>Incoming Volts converted to output: Kilovolts. </li></ul>
  47. 47. Voltage Rectification <ul><li>Transformers operate with alternating current. </li></ul><ul><li>Remember that x-ray tubes operate on direct voltage ( electron moving in one direction). </li></ul><ul><li>To convert AC to DC we use rectifiers. </li></ul>
  48. 48. Half-Wave Rectification <ul><li>Sometimes the x-ray tube alone will work as the diode this is called self-rectification . </li></ul><ul><li>When one or two diodes are placed in the circuit that stops the negative flow of electrons it is called Half Wave Rectification. </li></ul><ul><li>60 pulses per second. </li></ul>
  49. 49. Full-Wave Rectification <ul><li>Full wave rectified x-ray machines contain at least four diodes. </li></ul><ul><li>It changes the polarity of the negative half of the wave. </li></ul><ul><li>This allows 120 pulses of x-ray per second. </li></ul><ul><li>The exposure time can be cut in half compared to half-wave systems. </li></ul>
  50. 50. Three-Phase Power <ul><li>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. </li></ul><ul><li>Technical factor cut in half due to more efficient power. </li></ul><ul><li>Too expensive got office use. </li></ul>
  51. 51. High Frequency Generator <ul><li>By changing the frequency from 60 Hz to a higher frequency of 500 to 1000, the ripple is reduced to less than 1%. </li></ul><ul><li>Single phase machine operating on 220 volts and even 110 volts are more efficient that machine operating on three-phase power. </li></ul>
  52. 52. Types of X-ray Generators <ul><li>The type of generator will determine the efficiency of the machine. </li></ul>
  53. 53. Wave Forms of Different Generator Types <ul><li>As the ripple effect decreases, the efficiency increases. </li></ul><ul><li>There is one more type of generator. It uses is called stored energy. </li></ul>
  54. 54. Stored Energy Generators <ul><li>If 220 volt power is not available, the operator may choose a stored energy machine. </li></ul><ul><li>A battery charger is powered by typical house hold current. </li></ul><ul><li>If produces direct current. </li></ul>
  55. 55. Stored energy or Capacitor Discharge Generators <ul><li>There is a short charging time before the exposure can be made. </li></ul><ul><li>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 . </li></ul>
  56. 56. Generator Types Pros & Cons <ul><li>Single phase half or self rectified: Cheap but not efficient. Full wave rectified better. </li></ul><ul><li>Three phase: Expensive to install but cheaper to maintain. Too costly for most offices. 6 pulse less costly than 12 pulse </li></ul><ul><li>High Frequency: very efficient and works with single or three phase power. </li></ul><ul><li>Stored energy: works on conventional 110 volt power but batteries must be replaced. </li></ul>
  57. 57. The Basic X-ray Circuits <ul><li>Circuits that make up the basic x-ray machine. </li></ul>
  58. 58. Other Parts of the X-ray Room <ul><li>The tube is suspended on the tube stand . </li></ul><ul><li>The tube stand may be wall and floor mounted or ceiling suspended.Locks are provided for horizontal and vertical movement. </li></ul>
  59. 59. Other Parts of the X-ray Room <ul><li>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). </li></ul>
  60. 60. Other Parts of the X-ray Room <ul><li>When the tube is aimed at the table, the vertical lock allows us to set the SID. </li></ul><ul><li>Hanging on the wall grid cabinet is the non-Bucky film holder. </li></ul><ul><li>It allows erect non-grid films. </li></ul>
  61. 61. Other Parts of the X-ray Room <ul><li>X-ray tables may be bolted to the floor or mobile. The table will also have a grid cabinet for grid radiography. </li></ul><ul><li>We will discuss grids in greater detail next week. </li></ul>
  62. 62. Collimator and Angle Indicator <ul><li>The tube stand also has an angle indicator attached parallel to the tube. </li></ul><ul><li>There are views that will require tube angles. </li></ul>
  63. 63. Collimator and Angle Indicator <ul><li>The Collimator is attached to the x-ray tube below the glass window where the useful beam is emitted. </li></ul><ul><li>Lead shutters are used to restrict the beam. </li></ul>
  64. 64. Collimator and Angle Indicator <ul><li>A mirror and light source allows us to restrict the beam to the area of interest. </li></ul><ul><li>Collimation is our greatest tool in keeping patient exposure as low as possible. </li></ul>
  65. 65. Other items that may be in a x-ray room. <ul><li>Fluoroscopy Equipment: Allows dynamic imaging of the body. </li></ul><ul><li>Consists of: </li></ul><ul><li>Image intensifier with television camera and monitor. </li></ul><ul><li>Spot-film device for making radiographs or </li></ul><ul><li>Motion picture camera or digital imaging. </li></ul>
  66. 66. Image Intensifier & Fluoroscopy <ul><li>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. </li></ul><ul><li>This resulted in the first x-ray death. </li></ul><ul><li>Dose is still relatively high compared to plain film radiography. </li></ul>
  67. 67. Image Intensifier & Fluoroscopy <ul><li>Plain film radiography uses up to several hundred mA and fractions of seconds. </li></ul><ul><li>Fluoroscopy tubes operate at less than 5 mA but for minutes. 2 to 4 mA is normal. </li></ul><ul><li>In California Fluoroscopy is beyond the chiropractic scope of practice. </li></ul>
  68. 68. Image Intensifier & Fluoroscopy <ul><li>Shortly after WW2, Bell Laboratories invented the photomultiplier tube. This was developed into the modern image intensifier. </li></ul><ul><li>The multiplication of the light emitted by a input fluorescent screen is picked up by a cesium photocathode and converted into electrons. </li></ul>
  69. 69. Image Intensifier & Fluoroscopy <ul><li>A potential of about 25,000 volts is maintained between the photocathode and the anode. </li></ul><ul><li>There are electronic optics and electrostatic focusing lenses between the photocathode and output phosphor. </li></ul>
  70. 70. Image Intensifier & Fluoroscopy <ul><li>The output phosphor can be viewed via mirror optics or a video monitoring system. </li></ul><ul><li>A Videotape recorded can be placed into the video chain. </li></ul><ul><li>Fluoroscopy allows the evaluation of the internal structures in motion. Normal uses include: </li></ul>
  71. 71. Uses of Fluoroscopy <ul><li>Dynamic spinal imaging of range of motion and with contrast called myelograms. </li></ul><ul><li>Dynamic studies of joints with or without contrast media. </li></ul><ul><li>Studies of the digestive system. </li></ul><ul><li>Studies of arteries and blood flow called angiography. </li></ul>
  72. 72. Uses of Fluoroscopy <ul><li>When connected to a computer, for digital fluoroscopy and spot films. </li></ul><ul><li>With digital fluoroscopy, digital angiography is possible. </li></ul><ul><li>By over-lapping an image without contrast, digital subtraction angiography is performed where the bone is removed. </li></ul>
  73. 73. End of Lecture Return to Lecture Index Return to Physics Homepage

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