[Gate-users] a question about the 3d sinogram format

[yuxuan.zhang at di.mdacc.tmc.edu]

Hi, Dear Kris and STIR users,

I have a question about the format of 3d sinogram. When I was trying to 
bin my raw data into
a 3d sinogram, I don't know what's the exactly definition of positive and 
negative ring difference.
For example, if there's a crystal pair between crystal_i and crystal_j 
with the axial position of z_i and z_j.
>From the raw data, I could get two combinations of the same coincidence 
pair as (C_i, C_j) or (C_j, C_i).
Obviously these two type of combinations should goes to the same oblique 
sinogram. But how to calculate
the ring difference in this case? Ring difference could be either 
(z_i-z_j) or (z_j-z_i). Which one is correct?

If I choose one of them, for example, z_i-z_j, and with the value of 
z_i=1, z_j=2, then the ring_difference = -1,
if I choose the other, the ring_difference = +1. 
I think the difference of these two definitions are the same as switch the 
cells in the Michelogram between up-left corner 
and bottom-right corner. I have tried to bin the sinogram in both way and 
do the reconstruction by OSEM program, 
and the results looks almost the same. Does that mean the absolute 
direction of the ring difference is not important? 
or there's something else I misunderstood?

Thanks.

Yuxuan

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