[Gate-users] Re: Difference in energy spectrum for first and second event in cylindrical PET system

Paul Vaska vaska at bnl.gov
Wed Aug 29 18:05:36 CEST 2007

Perhaps Yuxuan and I are both correct about the ordering of events in  
the coincidence output.  According to Yuxuan, for true coincidences,  
the higher energy single is listed first (single 1).  That is why  
there is no fixed time order for true events and why the energy  
spectra for single 1 and single 2 differ.  On the other hand, for  
random coincidences, the event that is earlier in time is listed  
first.  Note that the coincidence output corresponds to the prompt  
time window and thus includes both true and random coincidences  (and  
the energy spectra for single 1 and single 2 will be a mixture).  For  
the delayed coincidence output, because the order depends on event  
time, I agree with Andrew that the energy spectra should not differ.   
However, this may be an artificial problem.  My point about sorting  
by detector or detector pair is so that you avoid binning your data  
based on artificial properties such as the order that events are  
written to an output file.  What really matters is the energy  
spectrum for a given detector, not the spectrum for a given column in  
an output file.  Therefore, I'm not too worried that single 1 and  
single 2 have different spectra (because 1 and 2 do not correspond to  
any physical property), but would be very concerned if detector a and  
detector b had different spectra (because this is not physically  


Paul Vaska, Ph.D.
Medical Department, Building 490
Brookhaven National Laboratory
Upton, NY 11973
vaska at bnl.gov (631)344-6228 fax:(631)344-4146

On Aug 28, 2007, at 6:44 PM, Andrew Goertzen wrote:

> Hi Paul and Yuxuan,
> Following your suggestions, I took a closer look at the energy  
> histogram
> of the delayed coincidences by looking at the delays.dat output file.
> As Paul said, the order of the singles in a random coincidence is  
> always
> that the early single is the first half and single 2 (the later  
> one) is
> always the second half.  In the delays file, the two singles should  
> be,
> by definition, just singles and thus they should exhibit an identical
> energy spectrum.  This is not what I see when I look at the data (see
> attached energy histogram).  In my data, single 1 has a different  
> energy
> spectrum than single 2.  The only real difference between the two
> singles is that single 1 is 1 microsecond earlier than single 2.  I  
> have
> no idea why the two singles in the delays file should have different
> spectra, so any additional insight you could offer into this would be
> appreciated.
> Cheers,
> Andrew
> Andrew Goertzen
> Department of Radiology
> University of Manitoba
> <DelaysEnergyHistogram.jpg>
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