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Image Contrast, Noise, Resolution

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Image Contrast, Noise, Resolution

  1. 1. Effects  of  kVp  and  mAs  on  image   spa4al  resolu4on,  contrast,  dose,   and  noise     Vibha  Chaswal,  Ph.D.  
  2. 2. Peak  Voltage  (KVp)  of  an  X-­‐ray  tube   •  Highest  X-­‐ray  energy   is  determined  by  peak   voltage  applied  across   the  x-­‐ray  tube.     •  With  filtra4on:                Eave  =  (1/2-­‐1/3)Emax   •  KVp  and  filtra,on  =   Quality  of  the  X-­‐ray   beam   Ref: Bushberg
  3. 3. Milli  Ampere  Second  (mAs)  tube   current   •  Tube  current  is  the  rate  of  electron  flow  from   the  cathode  to  anode,  measured  in   milliamperes  (mA)   •  1  mA  =  6.24  x  1015   •  S  =  exposure  4me,  dura4on  of  x-­‐ray   produc4on   •  Indicates  quan4ty:  Number  of  photons  is   propor4onal  to  mAs  
  4. 4. Rules  of  Thumb   •  •  •  •  •  kVp  and  exposure:   Exposure  α  (kVp)2   For  a  fixed  exposure  technique:   (kVp1/kVp2)5  =  mAs2/mAs1   kVp  determines  quan0ty,  quality,  and   transmission  through  the  object  whereas  mAs   determines  quan0ty  
  5. 5. Image  Contrast   •  Medical  Imaging  is  the  Process  of   Conver4ng  Tissue  Characteris4cs   into  a  Visual  Image   •  Contrast:    Difference  in  the  image   gray-­‐scale  between  closely  adjacent   regions  on  the  image.   Contrast sensitivity: imaging system's ability to translate physical object contrast into image contrast Increasing  Contrast  Sensi4vity   Increases  Image  Contrast  and  the   Visibility  of  Objects  in  the  Body  
  6. 6. Different  defini4ons  of  contrast   •  Subject  contrast:  difference  in  some  aspect  of   the  signal  prior  to  its  being  recorded   (x-­‐ray  operators  use  different  kVp  and  mAs  to   control  subject  contrast)   •  Displayed  contrast:  digital  imaging   (CT  (x-­‐ray  tomography)  imaging  uses  mAs  for   increasing  contrast  resolu4on  (contrast  to  noise   ra4o)  kVp  dependence:Once  kVp  is  set,  out  of   sight  out  of  mind  
  7. 7. Dose  and  contrast  versus  kVp   For  screen  film  radiography   Ref: Bushberg
  8. 8. Computed  Tomography:  3  steps   Scan:  produces  image  data     Reconstruc,on:  produces  digital  image  =>  a  matrix  of  pixels  with  CT  numbers     Digital  to  analog  conversion:  produces  visible  analog  image  represented  by   different  shades  of  gray  
  9. 9. Hounsfield  Unit   X-ray attenuation depends on both the density and atomic number (Z) of materials and the energy of the x-ray photons. For CT imaging a high KV (like 120-140) and heavy beam filtration is used. This minimizes the photoelectric interactions that are influenced by the Z of a material. Therefore, CT numbers are determined by the density of the tissues or materials.
  10. 10. Displayed  Contrast:  CT   •  Defined  by  difference  in  gray  scale  values  of   closely  lying  adjacent  structures   •  Gray  scale  values  assigned  to  pixels  during  DI   to  analog  conversion  of  DI   •  Visible  contrast  can  be  controlled  by  window,   level  and  zoom  or  post-­‐processing  techniques   •  CT  imaging  uses  a  high  KV  (like  120-­‐140  kVp)   and  heavy  beam  filtra@on  =>    minimizes  the   photoelectric  interac@ons  
  11. 11. Digital  Radiography   •  Enhance  contrast   digitally  using   Window  and  Level   and     Ref: Bushberg
  12. 12. Digital  Radiography   •  Post-­‐ processing   using  the   Edge   enhancement   filter   Ref: Bushberg
  13. 13. Spa4al  Resolu4on   •  Ability  of  an  image  system  to  dis4nctly  depict   two  objects  as  they  become  smaller  and  closer   together   •  Directly  related  to  mAs  =  quan,ty  of  photons   making  the  image   •  kVp  set  for  a  technique   •  LOTS  of  other  factors  that  affect  spa4al   resolu4on   •  Quan4fied  using  MTF  (cycles/mm)    
  14. 14. Spa4al  resolu4on  of  different   imaging  systems  -­‐  MTF   Ref: Bushberg
  15. 15. Noise   •  Local  varia4ons  in  contrast  due  to  a  background   texture  called  noise  that  does  not  represent  the   ahenua4on  in  pa4ent   •  Random:  e.g.,  caused  by  random  varia4ons  in  x-­‐ ray  photons  interac4ng  in  the  4ssue   •  Screen-­‐film  radiography:  Visual  percep,on  of   noise  is  reduced  when  the  detected  x-­‐ray   photons  increase.   •  Increasing  mAs  and  kVp  decrease  noise  as  the  #   of  detected  photons  increase,  so  does  pa,ent   dose.  
  16. 16. Signal-­‐to-­‐Noise  Ra4o   •  •  •  •  If  N  =  photons/pixel  then     SNR  =  √N   Noise  and  Dose:    to  increase  the  SNR  by  2  the  dose  to  the   pa4ent  (N)  has  to  be  increased  by  4  
  17. 17. Thank  You!  

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