[Gate-users] results problem

Nicolas Karakatsanis knicolas at mail.ntua.gr
Tue Mar 10 15:07:29 CET 2009

Dear Mario,

this problem has been encountered before, when GATE was used to quantify 
the positron range of a source.

In such a case a suitable and simple simulation set up to define the 
positron range would involve the use of a point source inside a sphere 
phantom. The point source could be F-18 within a water solution and the 
phantom can be a simple sphere of water.

In this case, if you define one single water sphere volume in GATE that 
will cover both the source water solution and the phantom water sphere 
you will get in the ROOT results a positron annihilation distance 
histogram with two peaks.The first (higher) peak, located at almost-zero 
distance, is the expected one, while the second (lower) peak is unexpected.

In order to deal with this problem, you can define two daughter water 
volumes (which have the same mother volume, e.g. world) that are 
attached each other at the borders of the point source distribution.
Normally one of the volumes has the same position and shape as the 
source spatial distribution and the other volume begins where the 
previous volume ends, representing the attenuating phantom volume.
The latter volume surrounds the former and a border between the two 
volumes is created at the interface between them.

If this interface is at a distance shorter than 0.5mm from the center of 
the point source, then the second peak in the annihilation distance 
histogram will no longer exist.

These two volumes can either be cylindrical or spheres.In my example I 
have used cylindrical water volumes. In order to define them in GATE, 
one can set the radius of the  first smaller volume as follows:
internal_radius_1 = zero, external_radius_1 = very small value, 
depending on the radius of the point-like source, e.g. 0.01mm

A second cylindrical volume can be created at the same position with the 
previous volume and the following radius:
internal_radius_2  > external_radius_1 , e.g. 0.011mm and 
external_radius_2 = the external radius of the water phantom  in your 
simulated experiment.

Also the height of the second cylinder is very larger than the height of 
the point-like source distribution

Thus, the second volume almost surrounds the first volume. The use of 
sphere volumes would ensure that the second volume completely surrounds 
the point source.

Generally, I believe that the presence of interfaces between materials 
at a distance shorten than 0.5mm from the positron generation point 
solves effectively the observed problem at the positron range histogram

Now, if you use a line source instead of point source, you would employ 
a similar set-up and probably prefer the two volumes to be cylindrical 
instead of spheres.

For more complex source-phantom geometries (e.g. voxelized realistic 
source-phantom distributions) the problem remains but it's effect is 
negligible judging from my experience. One reason is the presence of 
material interfaces at a short distance from each positron-generation 
point in space.

Please let us know if the above suggestions were helpful or not

Best regards

Nicolas A. Karakatsanis, PhD Candidate
Biomedical Simulations and Imaging Technologies Laboratory
School of Electrical and Computer Engineering
National Technical University of Athens

E-mail: knicolas at mail.ntua.gr
URL: http://users.ntua.gr/knicolas


mario bachaalany wrote:
> hi all,
> I'm trying to simulate a pet system with four detectors made of LYSO 
> crystals, a fluorine-18 as a source, and a sphere of water as a 
> phantom, and I'm getting some kind of two peaks in the positron 
> annihilation distance histogram in the root output. In general it must 
> be a 'decreasing  exponential function'. i already tried to change the 
> geometry and  material of the phantom but still getting the same 
> results. Does  anyone have the same problem and what could be the 
> cause.......
> thanks in advance.
> Mario
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