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

Mon Aug 27 17:09:43 CEST 2007

Hi,

This effect is not something new. One or two years ago someone already 
found this and reported here.
At that time I was also curious about this so I spent some time looking 
into it. After reading the source
code, my conclusion is that this result is caused by the GATE code about 
how it processing the energy
deposit from hits.

When two gamma ray photons travel inside the material, GATE records all 
the energy deposit in an array. 
Then in the "adder" process, GATE will add up all the energy deposits from 
the same gamma photon and 
stored in one of the cells in the same array. During this step, the gamma 
photon with a higher total deposited
energy will always stored in front of the other gamma photon (or after, I 
can remember clearly). So in 
the output file the photon1 and photon2 are just a label, it does not 
represents the original photon1 or
photon2.

So if you really want the spectrum of the coincidences, just add the 
photon1 and photon2 together.

Yuxuan

Irène Buvat <buvat at imed.jussieu.fr>
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08/26/2007 09:19 AM

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

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Subject: FW: Difference in energy spectrum for first and second event in
        cylindrical PET system
Date: Fri, 24 Aug 2007 16:54:28 -0500
Thread-Topic: Difference in energy spectrum for first and second event in
    cylindrical PET system
Thread-Index: Acfmlfu0s1jG3B4WQ1WCyGHMNxLkBQAAzp2w
From: "Andrew Goertzen" <AGoertzen2 at exchange.hsc.mb.ca>
To: <gate-users-owner at lists.healthgrid.org>
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Thanks,
Andrew

Hi All,
I am simulating a cylindrical PET system in GATE 3.0.0 with the geometry 
of a microPET R4 and I am hoping someone can help explain the behaviour I 
am seeing.  My source is a 1.25 MBq line source at the centre of the FOV. 
Data is written to an ASCII text file.  When I create an energy histogram 
from the coincidence data output file I notice that the events identified 
as 'single 1' in a coincidence event (i.e. columns 1-21 of a coincident 
event in ascii format as described in section 10.1 of the user manual) 
have a different energy spectrum than events identified as 'single 2'.  I 
have included an example of this in the attached jpg image.  I do not 
think it is time related because I get this behaviour with and without 
1.5ns of time blurring.  Also, the timestamp for single 1 is not always 
less than for single 2.  I do not think it is energy related because the 
energy of single 1 is neither always greater than or always less than 
single 2.
So I am hoping that someone can answer the following questions:
How does GATE determine the order of singles that make up a coincident 
event?
Has anyone else observed this sort of difference between the first and 
second single?
I have included the .mac file I am using.
Regards,
Andrew Goertzen
Department of Radiology
University of Manitoba
<<EnergyHistogram.jpg>> <<microPETR4.mac>>

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Irene Buvat, PhD
Equipe "Imagerie de la perfusion et des échanges moléculaires"
U678 INSERM
CHU Pitié Salpêtrière
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email : buvat at imed.jussieu.fr
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