[Gate-users] Potential bug with short time slice / low activity

[Guillem Pratx]

Dear GATE users and developers,

I have noticed a strange behavior with PET simulations that use low counts and short time slices. In my set-up, I am simulating a point source inside the Inveon PET scanner. The activity of the point source is varied from 1 to 1000 Bq,the total run time is 100 sec, the the time slice is 0.2 s (so 500 slices).

For 1 Bq, one would expect ~ 7 counts on average but in fact I observed 23 (average over 250 realizations). For higher activity, the error is smaller but still statistically significant (see attached plot). The issue disappears around 1000 Bq or if the number of time slices is reduced to ~ 10 (instead of 500). Note that if the activity of the source is 0 Bq, then no counts are observed, so this is not a background issue.

So my question is is there an explanation for this behavior, or could this be a bug in how counts are generated within each time frame?

Thank you for your help in solving this issue.

Guillem Pratx, PhD
_______________________________________________

   Assistant Professor			Office A247
   Radiation Oncology			1050 Arastradero Rd
   Stanford University			 Palo Alto, CA 94304
   http://pratxlab.stanford.edu           (650) 724-9829
_______________________________________________

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.opengatecollaboration.org/mailman/private/gate-users/attachments/20140311/a6fcfa09/attachment-0001.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: time_slice_bug.png
Type: image/png
Size: 20164 bytes
Desc: not available
URL: <http://lists.opengatecollaboration.org/mailman/private/gate-users/attachments/20140311/a6fcfa09/attachment-0001.png>