Editing STIR FAQ (section)

== Should I change the default projectors used by the reconstruction? ==
For STIR 2.x, the default for the iterative reconstructions was to use a ray tracing forward projector and an incremental interpolating backprojector. This turned out to be a bad choice as these projectors are not matched, which creates problems (even if the incremental backprojector does work on your system, see the known problems). It's probably best to use matched projectors, and the easiest way to do that is to use a "matrix". The fastest is the ray tracing matrix. This is the default projector since STIR 3.0.

However, this is still not ideal as this defaults to using only 1 ray per bin in the sinogram. This can create discretisation artefacts in high count situations. For the iterative algorithms, you should therefore probably use something like this in your .par file:
<pre>
projector pair type:= Matrix
Projector Pair Using Matrix Parameters:=
  Matrix type:= Ray Tracing
  Ray tracing matrix parameters:=
    number of rays in tangential direction to trace for each bin:= 10
  End Ray tracing matrix parameters:=
End Projector Pair Using Matrix Parameters:=
</pre>
For the analytic algorithms, using a ray tracer as backprojector can create (other) discretisation artefacts. You can alleviate this by using more rays, but in 3D, the ray tracing matrix doesn't take a Jacobian into account (as you don't have to for iterative reconstructions). Either stick to the default interpolating backprojector, or use its matrix equivalent. See the User's Guide.