[Gate-users] about attenuation and sensitivity in GATE

Tue Apr 29 19:23:48 CEST 2008

Dear Susana and GATE users,

Thank you very much for your solutions.

I have tested Susana's ideas by following 5 quick simulations:
1. air + back-to-back(object+source): 3409/2/20/20 
(true/scatter/random/delay)
2. vacuum + back-to-back: 3483/0/20/23
3. water + back-to-back: 2208/383/17/18
4. no object + back-to-back: 29472/0/11/16
5. no object + F-18: 5818/0/19/28

All simulations are applied on ECAT 962, the object (if have one) is same 
shape to the source (cylinder with 4 cm radius and 3.88 cm long). Activity 
is 0.01 mCi and scan time is 1 second. (see the attached .mac)

Obviously, simulation 4 is definitely wrong!! By using the absolute 
sensitivity of this system (about 9 cps/kBq according to 
Daube-Witherspoon, J Nucl Med 2002, 43: 1398-1409 ), the recorded true 
counts should be around 3300. Does this mean that we cannot use this 
combination for simulations?

Simulation 5 is also questionable. When there is no medium around the 
source, how can Gate simulate annihilations without electrons? And the 
total counts is also too high. Simulation 1 & 2 are reasonable from my 
point of view.

Did I do something wrong, or miss something here ? Any comments are very 
welcome. Thank you very much.

Sincerely,
Hongwei

"Susana Branco Silva" <susana.silva at fc.ul.pt> 
04/25/2008 07:14 PM

To
"'Hongwei Ye'" <HYe at tmriusa.com>, <gate-users at lists.healthgrid.org>
cc

Subject
RE: [Gate-users] about attenuation and sensitivity in GATE

Hi Hongwei,

If you want to “turn off” the attenuation information you don’t need to 
define the phantom! You just need to define the source (shape; type of 
source; distribution of the emission; placement of the source…).
And, if you want to “turn off” the physical effects (positron range, 
scattering and photon accolinearity) you may use the gamma/gamma emission 
source.

This choice (of these 2 turn off) allow to produce simulated data 
representing an “acquisition best case scenario”.

Susana Branco Silva

Faculdade de Ciências da Universidade de Lisboa
Campo Grande, 1749-016 Lisboa, Portugal
Phone: (+351) 21 750 01 77 ext.: 20515
Fax:  (+351) 21 750 00 30
email: susana.silva at fc.ul.pt
url: www.ibeb.fc.ul.pt

From: gate-users-bounces at lists.healthgrid.org 
[mailto:gate-users-bounces at lists.healthgrid.org] On Behalf Of Hongwei Ye
Sent: sábado, 26 de Abril de 2008 0:14
To: gate-users at lists.healthgrid.org
Subject: [Gate-users] about attenuation and sensitivity in GATE

Dear All, 

I'm wondering if there is any way to turn off the attenuation in GATE 
(because I'd like to use a simulation without attenuation in 
reconstruction). 

What I did right now is to use "Air" to replace "Water" (or something 
else) for the object material. But I met a problem here: 

I have tested two simulations: same objects with same activity, same scan 
time, but simulation 1 is made by "Air" and simulation 2 is made by 
"Water". Guess what I got? Simulation 2 (13920 counts) had more 
coincidences than simulation 1 (8646 counts) even if simulation 2 was 
supposed to be attenuated more by water. (I attached .mac file here) 

Can anyone explain this phenomena or guide me how to turn off the 
attenuation ? Should I use "Vacuum" instead of "Air" and set up "back to 
back gamma source"? 

One more question is about the sensitivity: what is its range for ECAT 962 
system? By roughly calculating, I got 20~50 cps/kBq depending on different 
objects. It seems to me those number are too high. 

Thanks a lot. 

Regards, 
Hongwei 

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