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

yuxuan.zhang at di.mdacc.tmc.edu yuxuan.zhang at di.mdacc.tmc.edu
Wed Aug 29 00:13:20 CEST 2007

First, let me copy my old email about this question in 2004, 


I spent some time read the source code of GATE recently and found out why 
spectrum looks different.

The GATE program first generates two gamma photons, let's say S0 and S1. 
Then GATE will traces down the two photons respectively, and generates
a Hits list. After that, the program will generates Adder_list and 
one after another before generates the Single_list. 

If you check the ASCII output files, you will find that in the 
Singles_Adder file,
for each photon, if a Compton backscatter event happened, then the event 
of energy
deposit after backscatter will be listed in front of all other events in 
the Adder_list. 
So for each photon-pair with one  Compton backscatter event, we should 
have two 
possible event lists as:

Case 1:
(1). S0 backscatter in detector B (or A), but deposits energy (0.17MeV) in 
detector A (or B)
(2). S0 deposits remained energy (0.34MeV) in detector B (or A)
(3). S1_photoelectric_absorption (0.511MeV) in detector A (or B)

Case 2:
(1). S0_photoelectric_absorption (0.511MeV) in detector A (or B)
(2). S1 backscatter in detector B (or A), but deposits energy (0.17MeV) in 
detector A (or B)
(3). S1 deposits remained energy (0.34MeV) in detector B (or A)

when GATE process the Adder_lsit, in Case1, it will add the energy from 
enevt3 to enevt1 and
update the other information of event1 with event3, and keep the event2, 
then generates the
Readout_list with two events:
(1). S1 (energy 0.17+0.511MeV) in detector A (or B)
(2). S0_remained_energy (0.34MeV) in detector B (or A)

for Case2, GATE will add the energy from enevt2 to enevt1 and
keep the original information of event1, then generates the
Readout list as:
(1). S0 (energy 0.17+0.511MeV) in detector A (or B)
(2). S1_remained_energy (0.34MeV) in detector B (or A)

Therefore, no matter the Compton backscatter event was happened with 
photon S0 or S1, or in 
detector A or detector B, in the Singles_Readout List, the high energy 
Single always comes 
first. Then later, this Single will appears in the first column in the 
coincidence list. 
Which means that the first and the second photons in the coincidence pairs 
do not always 
correspond with the original two gamma photons S0 and S1.


2. When GATE processing the data it will in the order of digitizer chain.
   and it always like this:
   hits -> adder -> readout -> (blur) -> coincidence
                                      -> delay -> delaycoincidence

 As what I explained in the old email, all these steps is happened in the 
process of 
"adder" when GATE was processing the Compton back scatter events. So 
whenever the 
"coincidence" or "delay" module began to process the singles list, the 
sequence of 
the singles was already fixed in the singles list, and cannot be changed.

I don't know what's your meaning about "sort by detector or detector pair". Whenever
you sort one detector, for example, detector0, it will appear both in the 
column and second column, and you don't know which event belong to the 
gamma1 and
which event belong to the gamma2. So all you can do is just add up all the 
from column1 and column2 (and divide by 2 if you like), and that's the 
spectra for detector0.

Dr. Yuxuan ZHANG 
Dept. Experimental Diagnostic Imaging
Univ. Texas, MD Anderson Cancer Center
1515 Holcombe Blvd, Unit 217
Houston, TX 77030-4095

Tel: +1-713-745-1671 
Fax: +1-713-745-1672

Paul Vaska <vaska at bnl.gov>
08/27/2007 12:00 PM

        To:     yuxuan.zhang at di.mdacc.tmc.edu
        cc:     Irène Buvat <buvat at imed.jussieu.fr>, 
gate-users-bounces at lists.healthgrid.org, gate-users at lists.healthgrid.org, 
Wonho Lee <wlee at bnl.gov>, Sudeepti Southekal <southekal at bnl.gov>
        Subject:        Re: [Gate-users] Fwd: FW: Difference in energy spectrum for first and 
second event in         cylindrical PET system

Hi Andrew,

I don't really understand this answer.  We have found a different 
pattern concerning the ordering in the coincidence output.  Maybe the 
code has changed.  From looking at time difference spectra for a 
given detector pair (i.e. time of det b minus time of det a, _not_ 
time of column 2 minus time of column 1) it appears that true and 
random coincidences are treated differently.  For trues, it seems 
that the order can be either way (don't know what determines the 
order) but for randoms, the 2nd single is always the later one. 
Given this, it is still disconcerting to me that the energy spectra 
are different.  True and random coincidences can have different 
energy spectra (due to different constraints on geometry) but I don't 
see how this could explain your effect.   Hopefully if you sort by 
detector or detector pair instead of the position of the data in the 
file, the energy and time spectra would make sense.


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 27, 2007, at 11:09 AM, yuxuan.zhang at di.mdacc.tmc.edu wrote:

> 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>
> Sent by: gate-users-bounces at lists.healthgrid.org
> 08/26/2007 09:19 AM
>         To:        gate-users at lists.healthgrid.org
>         cc:
>         Subject:        [Gate-users] Fwd: FW: Difference in energy 
> spectrum for first and second event in         cylindrical PET system
> X-Ids: 165
> X-Greylist: delayed 857 seconds by postgrey-1.21 at ns2919.ovh.net;
>   Sat, 25 Aug 2007 00:17:05 CEST
> X-IronPort-AV: E=Sophos;i="4.19,305,1183352400";
>  d="mac'?jpg'145?scan'145,208,217,145";a="310266"
> 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>
> X-OriginalArrivalTime: 24 Aug 2007 21:54:28.0267 (UTC)
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> The following message was rejected by the address 'gate- 
> users at lists.healthgrid.org'.  Could you please post it on my behalf?
> 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>>
> Content-Type: image/jpeg;
>       name="EnergyHistogram.jpg"
> Content-Description: EnergyHistogram.jpg
> Content-Disposition: attachment;
>      filename="EnergyHistogram.jpg"
> Content-Type: application/octet-stream;
>   name="microPETR4.mac"
> Content-Description: microPETR4.mac
> Content-Disposition: attachment;
>        filename="microPETR4.mac"
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