Due to an accumulated error from the pair-wise registration, the stitched image may be blurred or have a gap when a loop is encountered. In order to remove the accumulated error, we identify a closed loop where a first image frame overlaps with a second image frame, the second image frame being captured earlier in a scanning sequence than the first image frame; register the first image frame with the second image frame; and apply a global optimization to adjust registration parameters for the plurality of pair-wise registrations of image frames within the closed loop using global constraints.