Tomographic reconstruction of a particle field in fluid flow, T. Clark

See http://www.mathworks.com/matlabcentral/newsreader/view_thread/254747
for a discussion.

The problem can be solved via lsqlin in MATLAB:
lb = zeros (n,1) ;
x = lsqlin (A, b, [ ], [ ], [ ], [ ], lb) ;