[Gate-users] Projection data in SPECT interfile

[Albert Hirtl]

Dear Gate users,

We are starting to simulate a SPECT system and noticed, that we do not 
clearly understand how the projections are stored in the interfile output.

Basically we have two opposing and rotating (around the z-axis) SPECT 
heads with a point source at (0,0,0) and the chosen output projection is 
YZ (/gate/output/projection/projectionPlane YZ).  To my understanding, 
the source should not move in the simulated image, since it is in the 
centre of the device.

However, if I open the .sin file with imageJ and check the image, the 
activity source does move - in the case discussed here - to the right 
from projection to projection. Therefore my question: What exactly  is 
meant by the YZ projection?

1. Are the counts on the detector projected onto YZ of a fixed 
coordinate system (defined by the world) which is not moving with the 
detector head?
2. Or is the coordinate system describing the YZ projection in a 
coordinate system moving with the detector head?

If 1 is true, then I could understand the moving counts in the image. If 
2 is true, then the counts should always be centred and static.

It would be greatly appreciated if someone could clarify the situation 
for me. Probably I simply do not understand correctly how things are done...

Thanks in advance!

Cheers,
Albert

-- 
Medizinische Universität Wien - Allgemeines Krankenhaus Wien
Universitätsklinik für Nuklearmedizin
Währinger Gürtel 18-20
1090 Wien
Austria

Tel.: +43 (0) 1 / 40400 5545

Web:
http://www.meduniwien.ac.at/user/albert.hirtl
http://www.meduniwien.ac.at/nuklear/forschunglehre/physik/index.html

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