Causes	
  of	
  Image	
  Noise	
  in	
  PET	
  
Vibha	
  Chaswal,	
  Ph.D.	
  
Positron	
  Emission	
  Tomography	
  
PET	
  imaging:	
  Ideal	
  case	
  
= Annihilation event
Positron
emitting
nucleus

ϒ1	
  

= event of detection
of photo...
Data	
  arrangement	
  

ϒ1	
  
θ

Angle (θ)

Sinogram

ϒ2	
  
t

(t, θ)
Position (t)

Count on a Line of Response gets ma...
DeviaEons	
  from	
  Ideal	
  case	
  –	
  scaGer	
  
events	
  
True counts
ϒ1	
  
e+	
  
Detector
Ring

True Line of
Res...
DeviaEons	
  from	
  Ideal	
  case-­‐	
  Random	
  
counts	
  
True counts
ϒ1	
  
+	
  
Detector
Ring

False (LOR),
contri...
PET:	
  Noise	
  
•  Background	
  noise:	
  ScaGer	
  events	
  map	
  
misplaced	
  counts	
  to	
  the	
  sinogram	
  a...
Factors	
  affecEng	
  counts	
  
•  Trues	
  
↑	
  as	
  radio-­‐nuclide	
  concentraEon	
  ↑	
  (the	
  good	
  stuff)	
  ...
PET	
  improvements	
  leading	
  to	
  
improved	
  SNR	
  
•  Faster	
  electronics	
  
•  Faster	
  scinEllators:	
  sh...
Resources
	
  
•  DW	
  Townsend,	
  ‘Physical	
  principles	
  and	
  
technology	
  of	
  clinical	
  PET	
  imaging’,	
...
Thank	
  You!	
  
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Causes of Noise in PET imaging

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Causes of Noise in PET imaging

  1. 1. Causes  of  Image  Noise  in  PET   Vibha  Chaswal,  Ph.D.  
  2. 2. Positron  Emission  Tomography  
  3. 3. PET  imaging:  Ideal  case   = Annihilation event Positron emitting nucleus ϒ1   = event of detection of photon in the detector ring e+   Detector Ring Line of Response (LOR) ϒ2   Two simultaneous (within 6-12 ns time difference) events in the detector make a line of response.
  4. 4. Data  arrangement   ϒ1   θ Angle (θ) Sinogram ϒ2   t (t, θ) Position (t) Count on a Line of Response gets mapped to the corresponding position in a sinogram.
  5. 5. DeviaEons  from  Ideal  case  –  scaGer   events   True counts ϒ1   e+   Detector Ring True Line of Response (LOR) ϒ2   ϒ2`   = Annihilation event Detected scatter event’s misplaced LOR = event of detection of photon in the detector ring
  6. 6. DeviaEons  from  Ideal  case-­‐  Random   counts   True counts ϒ1   +   Detector Ring False (LOR), contributed from two separate annihilation events ϒ2   = Annihilation event = event of detection of photon in the detector ring
  7. 7. PET:  Noise   •  Background  noise:  ScaGer  events  map   misplaced  counts  to  the  sinogram  and   Random  events  map  false  or  spurious  counts   •  ScaGer  -­‐  Supress  with  collimaEon  and   detectors  with  beGer  energy  resoluEon   •  Randoms  -­‐  Supress  by  smaller  sampling   window  of  coincidence,  i.e.,  faster  scinEllator   detectors;  and  CollimaEon  
  8. 8. Factors  affecEng  counts   •  Trues   ↑  as  radio-­‐nuclide  concentraEon  ↑  (the  good  stuff)   ↓  as  paEent  size  ↑  (absorpEon  and  scaGer  effects)   •  Randoms   ↑↑  as  count  rate  ↑  (varies  as  square  of  count  rate)    Effects  dominate  image  noise  at  high  injected  acEviEes    Reduce  by  faster  electronics,  faster  crystals   •  Sca/ers   ↓  with  collimaEon   (about  15%  for  2D  PET  and  50%  for  3D  PET)   Reduce  with  collimaEon   (energy  selecEon  not  efficient  in  PET)  
  9. 9. PET  improvements  leading  to   improved  SNR   •  Faster  electronics   •  Faster  scinEllators:  shorter  coincidence  Eming   window,  reduced  dead  Eme,  improved  energy   resoluEon  window.     •  PET/CT  technology,  ACFs  from  CT  scan  (reduced   paEent  movement  related  noise  factors  from  long   transmission  scans),  total  scanning  Eme  reduced   to  10-­‐15  minutes.   •  Fourier  re-­‐binning  and  staEsEcally-­‐based   algorithms  
  10. 10. Resources   •  DW  Townsend,  ‘Physical  principles  and   technology  of  clinical  PET  imaging’,  BJR,  March   2004,  Vol  33  (2)   •  Bushberg  book   •  J  A  Anderson,  ‘IntroducEon  to  PET/CT’,  CRCPD   winter  2004  meeEng  presentaEon  
  11. 11. Thank  You!  

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