Chapter 8 The X-ray Machine - PowerPoint PPT Presentation

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Chapter 8 The X-ray Machine

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Chapter 8 The X-ray Machine
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Chapter 8 The X-ray Machine

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  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.

  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 patient’s 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 a”Dead 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. 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.