Effects  of  kVp  and  mAs  on  image  spa4al  resolu4on,  contrast,  dose,  

and  noise    Vibha  Chaswal,  Ph.D.  

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

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  

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  

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  

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  

Dose  and  contrast  versus  kVp  

For  screen  film  radiography  

Ref: Bushberg

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  

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.

Hounsfield  Unit  

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  

Digital  Radiography  

•  Enhance  contrast  digitally  using  Window  and  Level  and    

Ref: Bushberg

Digital  Radiography  

•  Post-­‐processing  using  the  Edge  enhancement  filter  

Ref: Bushberg

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)    

Spa4al  resolu4on  of  different  imaging  systems  -­‐  MTF  

Ref: Bushberg

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.  

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  

Thank  You!