[gate-users] Very low number of scatter in GE Discovery

[Schmidtlein, Ross/Sloan-Kettering Institute]

Hi Mikhail,

Is this the discovery ST or LS?  I have done some simulations with each
and in 3D mode I have gotten agreement within ~1-2% with NEMA94 and 2001
SF.  My first guess is that you have not set your phantom as a sensitive
volume and as a result few or no Compton hit's are scored.

Also, looking over your digitizer I have noticed a few values that may
be off.  If you are using detector blocks in a cylindrical PET type
system then in readout set depth should be 2.  In addition the energy
resolution are given in the literature for the DLS is about 20% and DST
is 17%.  The energy window you are using may be a little wide.  However,
these are minor issues and will only alter your SF by a few percent.

Cheers,

ross

-----Original Message-----
From: gate-users-bounces at lphe1pet1.epfl.ch
[mailto:gate-users-bounces at lphe1pet1.epfl.ch] On Behalf Of Mikhail
Shilov
Sent: Tuesday, March 07, 2006 10:59 AM
To: gate-users at lphe1pet1.epfl.ch
Subject: [gate-users] Very low number of scatter in GE Discovery


Dear Gate group,

I was investigating the fraction of scatter in GE discovery in normal
and in 3D mode ( no septa). I ran a simple simulation (see the attached
file) with 3 cylindrical F-18 sources inside a 20 cm radius and 30 cm
height cylindrical phantom and recorded gateSingles.dat and
gateCoincidences.dat files. I then looked at the number of recorded
coincidences with at least one Compton scatter inside the phantom. It
turned out that this number is ~0.5% (~0.8% in 3D) of total recorded
coincidences. This is not what I would expect from the literature (
~30%). The fraction of singles is what I expected ~ 30%

I was wondering if anyone looked as this issue before and if my result
is reasonable.

Here is my digitizer script:

#       A D D E R
/gate/digitizer/Singles/insert adder
#       R E A D O U T 
/gate/digitizer/Singles/insert readout
/gate/digitizer/Singles/readout/setDepth 1
#      E N E R G Y   B L U R R I N G
/gate/digitizer/Singles/insert blurring
/gate/digitizer/Singles/blurring/setResolution 0.15
/gate/digitizer/Singles/blurring/setEnergyOfReference 511. keV
#       E N E R G Y   C U T
/gate/digitizer/Singles/insert thresholder
/gate/digitizer/Singles/thresholder/setThreshold 300. keV
/gate/digitizer/Singles/insert upholder
/gate/digitizer/Singles/upholder/setUphold 650. keV
#       D E A D    T I M E 
/gate/digitizer/Singles/insert deadtime
/gate/digitizer/Singles/deadtime/disable
/gate/digitizer/Singles/deadtime/setMode nonparalysable
/gate/digitizer/Singles/deadtime/chooseDTVolume block
#       C O I N C     S O R T E R 
/gate/digitizer/Coincidences/setWindow 12.5 ns
  

Mikhail A. Shilov
The Johns Hopkins University
Department of Radiology
Division of Medical Imaging Physics
Tel:   (410)502-3773
Fax:  410-614-1977
mailto: mshilov1 at jhmi.edu