X-ray Physics

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X-ray Physics

  1. 1. X-ray Physics In a nutshell By Dr. Jill Davis
  2. 2. What you need to know:  The tube – how x-rays are produced  The body – how x-rays interact with the body  The image – how x-rays interact with film  Film processing
  3. 3. What are X-rays?  Made of photons  Travel at speed of light  Travels in a straight line  Has no mass nor charge (cannot be focused by magnets)  X-ray beam has a mix of energies  Maximum energy in a beam = kVp  Diagnostic X-ray range 20-150 kVp
  4. 4. What are X-rays?
  5. 5. The X-ray tube
  6. 6. The X-ray tube parts:  Cathode (-)  Filament made of tungsten  Anode (+) target  Tungsten disc that turns on a rotor  Stator  motor that turns the rotor  Port  Exit for the x-rays
  7. 7. X-ray production  Push the “rotor” or “prep” button  Charges the filament – causes thermionic emission (e- cloud)  Begins rotating the anode.  Push the “exposure” or “x-ray” button  e-’s move toward anode target to produce x-rays
  8. 8. Hitting the target  e-’s hitting the target creates x-rays two different ways:  Characteristic x-rays – are due to the material the e-’s hit (tungsten). Only occurs above 70 kVp  Bremsstrahlung (braking) x-rays – due to slowing down of e- beam.  < 70 kVp – 100% of X-rays are of this type  > 70 kVp – 85% of X-rays are of this type
  9. 9. Characteristic
  10. 10. Bremsstralung
  11. 11. Anode Heel Effect
  12. 12. Exposure Factors:  kVp – kilovoltage peak  mA – miliamps (current)  s – seconds (duration of exposure)  mAs – product of mA and s  Exposure factors are set by radiographer
  13. 13. X-ray Quality vs. Quantity  Quality = penetrating power / energy  Quantity = # of X-rays in beam  ↑kVp = ↑ speed of e- = ↑ quality  ↑ kVp = efficiency of x-ray production = ↑ quantity  ↑ mA = more e- hit target = ↑ quantity  ↑s = longer exposure time = ↑ quantity
  14. 14. What you need to know:  The tube – how x-rays are produced  The body – how x-rays interact with the body  The image – how x-rays interact with film  Film processing
  15. 15. Interactions in the Body:  Three things can happen to x-rays as they hit the body:  Absorption (photoelectric effect) – x-ray is absorbed by tissues – does not contribute to image.  Scatter (Compton effect) – contributes to “fog”  Transmission – penetrates through body to hit radiographic film.
  16. 16. Interactions in the Body
  17. 17. Problem:  Only x-rays of sufficient energy (quality) can transmit through body to create an image.  Low energy x-rays don’t contribute to the image, but add to patient radiation dose.  Also, different thicknesses, and composition of body parts will determine amount of x-ray penetration.  Therefore we need to reduce low energy (low quality) x-rays, but at the same time have the right quantity of x-rays hitting the body part.
  18. 18. Filtration  How we fix the problem is with filtration  Three kinds of filtration:  Inherent – due to tube housing, insulation, etc.  Added – aluminum shielding that blocks low energy x-rays.  Special – used to image body parts that have varying thickness or density.  Filtration is measured in terms of “half-value layer”
  19. 19. “Special Filtration”
  20. 20. What you need to know:  The tube – how x-rays are produced  The body – how x-rays interact with the body  The image – how x-rays interact with film  Film processing
  21. 21. Image Quality
  22. 22. Density  Controlling Factors:  mA and s  ↑mAs = ↑quantity of photons reaching film = ↑density
  23. 23. Density  Influencing factors:  ↑kVp = ↑quality (penetration) = ↑density  ↑SID (source-image distance) = ↓density  Due to inverse square law – intensity of x-ray is inversely proportional to the square of the distance from source.  ↑OID (object-image distance) = ↓density  Grids (discussed later) = grids ↓density  ↑Film/screen speed = ↑density  ↑body part thickness = ↓density  ↑filtration = ↓density
  24. 24. Density and kVp
  25. 25. Density and SID
  26. 26. Image Quality
  27. 27. Contrast  ↑contrast = short scale = more black and white (less detail)  ↓contrast = long scale = mores shades of grey (more detail)
  28. 28. Contrast  Controlling factor  kVp  ↑kVp = ↓ contrast (more shades of grey)
  29. 29. Contrast  Influencing factors:  Grid –↓fog (scatter) = ↑contrast  Collimation – narrow collimation = ↓ scatter = ↑contrast  Anatomic part – variation in tissue density visible on film  What are the 5 tissue densities?  Air, Fat, Water/Tissue, Bone, Metal
  30. 30. Image Quality
  31. 31. Recorded Detail  The “sharpness” of structural lines in the image  Geometric unsharpess  Image receptor unsharpness  Motion unsharpnesss
  32. 32. Geometric Unsharpness  ↑SID = ↓divergence of rays = ↓ unsharpness  ↑OID = ↑divergence of rays = ↑ unsharpness  Penumbra = geometric unsharpness along the edges of the film.
  33. 33. Image receptor unsharpness  ↑film/screen speed = ↓detail = ↑unsharpness
  34. 34. Motion Unsharpness  ↑ motion of patient, image receptor, or tube = ↑unsharpness  Prevention of motion unsharpness:  ↓ exposure time  Patient instruction (i.e. hold breath)  immobilization
  35. 35. Image Quality
  36. 36. Distortion  Size Distortion  ↑OID = ↑size distortion (magnification)  ↑SID = ↓size distortion  Shape distortion  Occurs when anatomical part is not parallel to the image receptor (elongation or foreshortening)  Reduced by proper patient positioning and/or tube tilt.
  37. 37. Collimation  Is located under the port of the X-ray tube.  Has a light in it for radiographer to see where x-rays would hit the patient  Purpose- restricts beam  ↓ patient dose  ↓scatter (↑contrast)  Collimation should be visible on a minimum of three sides of the film
  38. 38. Grids  Part of the “bucky” that hold the film cassette  Reduces scatter radiation that hits film  Grid is made of lead strips  Grid ratio – height/width of interspace  Hitting prep button causes grid to vibrate to blur out grid lines (doesn’t show up on film)
  39. 39. What you need to know:  The tube – how x-rays are produced  The body – how x-rays interact with the body  The image – how x-rays interact with film  Film processing
  40. 40. Film  Photographic film has several layers:  Supercoat – protective covering  Emulsion – is radiation and light sensitive  Made of silver halide and gelatin  Base – plastic; for stability  Film is available in different “speeds” just like 35 mm camera film: the faster the speed, the less radiation is needed to produce an image.
  41. 41. Image formation  Latent image – invisible image caused by light or radiation exposure  Manifest image – shows up after film is developed
  42. 42. Intensifying screen  Is located in the cassette that film is placed inside of.  Screen contains “phosphors” – that fluoresces when exposed to x-rays.  Purpose – screens amplify x-rays that hit the film so you need a lot less mAs to produce an image .  Drawback – lose some recorded detail  Screens also come in different “speeds” – i.e. the degree to which it fluoresces upon exposure.
  43. 43. Film Processing  May become obsolete as the industry moves to digital  Steps of processing (automatic)  Developer – converts latent image to manifest image (22 sec)  Fixer – acetic acid  Wash- water removes residual chemicals  Dry – blow dryer in the processor
  44. 44. Radiation Dosimetry - definitions  Roentgen – unit of radiation that will liberate a charge of 2.58 x10(-4) coulombs per kilogram of air.  Coulomb – unit of electrical charge  RAD = radiation absorbed dose – 1 rad is equal to the radiation necessary to deposit 100 ergs (unit of energy) in 1 gram of irradiated material  SI unit: 1 gray = 100 RAD
  45. 45. Radiation Dosimetry - definitions  REM – rad equivalent man – is the unit of absorbed dose equivalent; is a measure of the biological effect of radiation.  SI unit: 1 sievert = 100 REM
  46. 46. Radiosensitivity  Radiation damages DNA  Tissues that are sensitive to radiation are:  High – lymphocytes, spermatogonia, erythroblasts, intestinal crypt cells  Intermediate- endothelial cells, osteoblasts, spermatids, fibroblasts  Low – muscle cells, nerve cells, chondrocytes.  Rule of thumb – the cells that proliferate more are more sensitive
  47. 47. Positioning Tips  SID (aka FFD or TFD)  Is either 40” or 72”  Think 40” for all except FS, lat or oblique C-sp (air gap), P-A chest  Tube Tilt  For every 5° tube tilt, move tube 1” closer  When to use tube tilt – to reduce shape distortion  Example – A-P lower cervical 15° cephalad = 37”
  48. 48. Positioning Tips  Central Ray  Generally aim at middle of anatomy you want to see.  Film Size – small film for small part  8 x 10, 10 x 12, 14 x 17, 14 x 36  Collimation – how much do you restrict beam?  Collimation – visible on film at least 3 sides  Include anatomy you want to see
  49. 49. Positioning Tips  Ten day rule  For females of childbearing age  X-rays not taken after 10 days of start of menstrual period.  Shielding / filters  Gonadal shield any A-P view that includes the pelvis  Lead apron over body parts not to be visualized (extremity views)  Filters – wedge filters (example, for A-P FS, wedge over the superior half of spine.
  50. 50. You’re done!

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