PET Imaging - negative distance on a sinogram?

[BobP]

What is meant by a negative distance on a sinogram in PET imaging?
http://frcrphysics.com/wp-content/uploads/2014/05/PET-sinogram.jpg

In this image displacement can be +ve or -ve. What would a -ve distance be?

Thanks

 

[mfb]

In A or B I don't see a negative displacement.
C or D are projections, so I would expect the displacement to be the other direction.

A specific question about the plotting style could fit better in the medical science forum.

 

[BobP]

In A or B I don't see a negative displacement.
C or D are projections, so I would expect the displacement to be the other direction.

A specific question about the plotting style could fit better in the medical science forum.

OK thanks I will re-post there. I have found an example if a graph with negative displacements though?