Editing STIR FAQ (section)

=== A lot of tests fail ===
Most likely this due to a bug in the "incremental backprojector" which 
crops up depending on compiler/optimisation settings etc. An extensive discussion
of this bug appeared on the STIR users lists. You might be able to see this 
[https://sourceforge.net/search/index.php?group_id=42522&type_of_search=mlists&limit=25&q=back-projector+incremental+: using this link].

For example, a known system that has this problem is Ubuntu 64bit using gcc 4.5 in 64 bit mode (and -O3).
 
There are several ways to check: 
* run recon_test_pack with the --nointbp flag to remove tests that use this projector 

* display the sensitivity image that you're getting. Most likely you'll see a hot spot in the middle or some 45 degree lines 
 
You could always compile as 32bit even on 64bit linux of course (use 
EXTRA_CFLAGS=-m32  EXTRA_LINKFLAGS=-m32, and don't forget to either 
change DEST or make clean). However, we do not recommend using the 
incremental backprojector for the iterative algorithms anyway as 
there's no corresponding forward projector.