Xray physics & positioning

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Xray physics & positioning
2011-09-04 21:11:52

xray physics
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  1. production of xrays
    • fast moving electrons collide with matter= xrays produced
    • in a tube this is done by directing electrons against a metal target.
    • 1% of their energy is in radiation, the other energy is given up in the form of heat
    • the tube is lined with lead protection
    • 2 electrodes: cathode and anode
  2. cathode consists of
    • large filament
    • small filament
    • focusing cup
  3. filaments
    • source of the electrons
    • made of tungsten
  4. thermionic emission
    • current applied to the filament- coil becomes hot
    • boiling off electrons
  5. temperature of filament controls _
    • QUANTITY of elecrons (mA)
    • number of electrons determines number of xrays created
  6. focusing cup/filament cup
    • part of the cathode, encaes 2 filaments
    • where the thermionic emission occurs
  7. anode
    • positive electrode in the xray tube
    • most xray consist of a rotating anode
    • rotating target receives electrons as they move from cathode to anode
  8. rotating anode target
    • dissipate heat generated
    • exposes are of tungsten called focal spot
  9. smaller filament= ___ detail
  10. beam hardening aka the line focus principle
    • effect of making the actual focal spot smaller
    • smaller target angle, smaller effective spot
  11. milliamperage (mA)
    heats the filament causing boiling off of electrons
  12. milliamps per sec (mAs)
    more current=more electrons produced= more xrays= greater radiograph density (darker)
  13. density
    blackness on film
  14. radiolucent
    structures that produce blackness on film
  15. radio-opaque
    structures that produce whiteness on film
  16. relationship between mAs and density
    directly proportional
  17. changing mAs
    primary method to control blackness

    more mAs= more film exposure
  18. kVp
    • force applied to accelerate electrons
    • greater force= greater photons
  19. relationship between wavelength and frequency
    inversely related
  20. low kvp=low energy=weak penetration
    high kvp= high energy= strong penetration
  21. higher kvp produces
    shorter wavelengths with ability to penetrate tissue
  22. contrast
    difference between structures, makes detail visible
  23. relationship between kvp and contrast
    inversely related
  24. low kVp
    • high contrast= short scale
    • long wavelengths, weak penetration
    • black and white film
    • less scatter, increased exposure
  25. high kVp
    • low contrast= long scale
    • more shades of grey (soft tissue film)
    • more scatter, decreased exposure
  26. 15% rule
    lower contrast- longer scale- more greys: increase kvp by 15% and decrease mAs by 50%

    high contrast- short scale-b&w: decrease kvp by 15% and increase mAs by 50%
  27. kilovoltage and density
    • mAs is the prime method to control density
    • relationship between kvp and density is not proportional
    • 15% increase in kVp by 15% will double the density on the film
  28. differential absorbtion
    • denser tissue absorbs xrays
    • gas/fat= black
    • mm, water, ST= gray
    • bone, metal= white
  29. anode heel effect
    intensity of the xray is greater at the cathode side than the anode side
  30. anode side
    • less xray
    • smaller effective focal spot
    • more detail
    • use on thinner body part
  31. cathode side
    • more xray
    • larger effective focal spot
    • less detail
    • thicker body part
  32. focal film distance/ source image distance
    length of space from focal spot (tube) to the recording medium (film)

    as you increase FFD- image gets smaller and more clear. decreases magnification and distortion

    as you decrease FFD- image gets bigger and blurrier
  33. inverse square law
    explains the intensity of the beam affected by changes in the FFD
  34. Inverse square law
    intensity of radiation varies inversely with the square of the distance from the source

    40 inch FFD: doubled to 80 in FFD

    80 inch FFD the mAs must be 4x amount for blackness maintained
  35. 72" views:
    • lateral cervical
    • flex/ext
    • obliques
    • pa chest/lat chest
  36. Intensifying screens
    • flourescent crystals- the salt changes the xray photons into visible light photons
    • *decrease pt exposure*
  37. definition
    • claritiy and sharpness of an image
    • umbra is the picture made by the beam
    • penumbra is the blury halow parts
  38. object film distance
    • distance b/n patient and film
    • increased ofd: image becomes bigger/blurrier (more penumbra)

    decreased ofd: image becomes smaller/clearer

    *object as close as possible for better detail*
  39. screen film contact
    • pressure between the film holder ont he xray film
    • should be evenly distributed
    • *wire mesh test is best for testing*
  40. poor screen film contact
    reduces the sharpness of the image
  41. distortion
    • size and magnification
    • OFD and FFD
  42. scatter radiation
    • reduced by:
    • grid
    • air gap
    • collimator
    • filter
  43. grid
    • alternating strips of lead and spacer material
    • placed betweeen the patient and the film
    • sole purpose of the grid is to improve the quality of the radiograph by absorbing scatter
    • increases patient dose of radiation
  44. grid ratio
    height of the lead strips to the distance between the strips

    chiros use 12:1
  45. Air-gap technique
    • 6-10" gap between patient and film
    • lateral cervical view
    • distance between body part and film: scatter radiation diverge
    • reduces patient dose but distorts/magnifies the image
  46. Collimation
    • limits the size of the beam to the size of the body part
    • best way to reduce scatter radiation
  47. filtration
    • removal of low energy photons
    • made of aluminum
    • stops weak photons from entering patients body- reducing scatter radiation
  48. RAD: radiation absorbed dose
    • used to identify irratiation of patients
    • measures radiation energy absorbed
  49. REM: Radiation equivalent man
    • occupationally exposed people
    • identifies the effectivness of radiation absormed
    • badge worn on collar
    • 1 RAD= 1 REM
  50. MPD: maximum permissible dose
    • the number of REMS a person can get in a lifetime w/o getting rad posioning
    • MPD= 5 (age-18)/ yr
    • 5 REMS is the safe limit per year
  51. Bremsstrahlung (braking radiation)
    • majority of the useful beam
    • interacts with the target nucleus
  52. characteristic
    • high intensity electron hits an inner shell electron, knocked off position
    • outer shell then fills its spot
  53. compton scatter
    • primary form of scatter
    • moderate energy knocks an outer shell
    • causes a loss of energy
  54. classical scatter (thompson scatter) (Coherent scatter)
    • low energy xray changes direction w/ no energy loss
    • contributes to film fog
  55. photoelectric effect
    low energy photon absorbed by the subject creating a leatent image
  56. quantum mottle (radiographic noice)
    • mottle caused by fluctuation of number of photons absorbed by the intensifying screens
    • faster screens produce more quantum mottle
    • use hight MAS low KVP decreases radiographic noise
  57. processing
    • solutions that are too hot= increase density
    • solutions that are too low= insufficient density
  58. processing steps
    • 1) developer: brings out latent image developing silver halide
    • 2) rinse:
    • 3) fixer: hardens the image, making it permanant
    • 4) washing: removes developer and fixer
  59. reducer developer
    produces shades of grey (phenidone/hydroquione)
  60. acceleraor developer
    • swelling of emulsion
    • na+ carbonate
  61. preservative
    • controls oxidation
    • na+ sulfite, K sulfite
  62. restrainer
    • protects unexposed crystals
    • potassium bromide/iodine
  63. hardener
    • controls swelling of emulsion
    • glutaraldehyde
  64. fixer acidifier/activator
    • nuetralizes the developer/ stop bath
    • acetic acid
  65. clearing agent
    removes undeveloped ag halide/ hypo solution
  66. hardener
    • stiffens/shrinks the emulsion
    • potassium alum
  67. preservative
    maintains ph of fixer
  68. dark films
    light leak, developer too hot, overexposure
  69. light films
    developer treatment low, chemicals need replacing
  70. brown films
    inadequate fixer
  71. milky films
    inadequate fixer
  72. greasy films
    insufficient washing
  73. increased fog
    old film, hot developer, contaminated chemicals
  74. soft emulsion
    insufficient fixing
  75. store films
    vertically to avoid pressure damage
  76. static electricity
    • produces when removing film quickly
    • black and branching lightening lines
  77. rectifier
    changes AC to DC
  78. transformer
    steps up or down the power
  79. transducer
    one form of energy to another
  80. attenuation
    beam of radiation loses energy
  81. green blue tinit
    added tot he base of film to reduce glare
  82. static electricity
    black and branching
  83. nail marks on film
    • appear as creases
    • MC reason for creases
  84. lateral skull view
    sella turcica best seen here
  85. PA caldwell
    • frontal sinus
    • 15 * caudal
  86. AP Townes
    • foramen magnum
    • 35* caudad
  87. waters film
    maxillary sinuses
  88. neutral lateral cervical
    air gap technique, 72" FFD
  89. APLC
    15* caudad tilt
  90. APOM
    • dens and atlas best viewed
    • used for jefferson burst frx
  91. cervical obliques
  92. Cerv articular pillar
    • articular processes and apophyseal jts
    • pt is PA
  93. swimmers view
    standing, arm closest to bucky is raised overhead, other arm at pt's side
  94. chest views
    • PA
    • deep breath in and hold
    • 72" FFD
  95. lateral lumbar
  96. L5/S1 spot (ferguson's projection) sacral base tilt
    • 25* cephalic
    • dont breath dont move
  97. lumbar obliques
    • pars
    • 40 FFD
  98. AP sacrum
    15* tube tilt
  99. AP Coccyx
    10* tube tilt caudal
  100. lateral coccyx
    no tub e tilt
  101. AP angulated SI jts
    tube 30*/35* cephalic
  102. AC jts w/wo weights
    • tube 5* cephalad
    • AP
  103. External rotation of shoulder
    views greater tuberosity
  104. internal rotation of shoulder
    views lesser tuberosity
  105. Axial clavicle
    tube tilt 15* caudal for PA position, 15* cephalic for AP position
  106. AP elbow
    hand supinated
  107. lateral elbow
    fat pad sign
  108. PA ulnar deviation
    scaphoid and lunate
  109. AP pelvis
    supine or standing, feet turned in 15 *
  110. tunnel projection
    • intercondylar fossa, osteochondritis desicans
    • tube tilt 45* caudal
    • prone patient w/ knee flexed to 45*
  111. tangential (sunrise) patella
    patella and patellofemoral jt space
  112. AP Dorsoplantar foot
    tube tilt 10* cephalic