[Gate-users] Strange pile up effects with Cylindrical PET system

[Simon Stute]

Hi,

I have not really an idea to explain this physically speaking. But maybe
some suggestions about your simulation design.
Do your crystals are totally included in their mother volume ? I mean when
you make them grow to 17 mm, did you anticipate the size of the mother
volume.
In case of a child volume bigger than its mother volume, results are not
predictable at all !
Same thing when volumes are overlapping.
So maybe it comes from that.

Cheers,
Simon

On Thu, Feb 25, 2010 at 6:36 PM, K B <kd8had at gmail.com> wrote:

> Hello Everyone,
>
>  I am noticing a strange thing when using cylindrical PET. I am
> simulating a very high activity using mono-energy 511 keV source(and
> therefore expect pileups) and when my detector is thinner than 16 mm
> (radial thickness), I see events past the 511 keV photopeak as expected
> and this is a continuous spread past 511 keV. However, when I change my
> crystal thickness to 17 mm or above, there the continuous spectrum is
> replaced by one small peak at 681 kev (511 + 170.3). This seems wrong to
> me. There cannot exist a hard threshold for detection. This is a singles
> energy spread.
>
>  An example of the spectrum with 16 mm and 17 mm crystals is attached
> and is online at http://img15.imageshack.us/img15/4026/imageshackzw.png
>
>  I am using a cylindrical PET and the readout is set at depth 1. The
> same digitizer is used for both the 16 mm and 17 mm crystals.
>
> Thanks to anyone who can shed some light on this.
>
> KB
>
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