CT colonography interpretation is difficult and time-consuming because fecal residue or fluid can mimic or
obscure polyps, leading to diagnostic errors. To compensate for this, it is normal practice to obtain CT data
with the patient in prone and supine positions. Repositioning redistributes fecal residue and colonic gas; fecal
residue tends to move, while fixed mural pathology does not.
The cornerstone of competent interpretation is the matching of corresponding endoluminal locations between
prone and supine acquisitions. Robust and accurate automated registration between acquisitions should lead
to faster and more accurate detection of colorectal cancer and polyps. Any directional bias when registering
the colonic surfaces could lead to incorrect anatomical correspondence resulting in reader error. We aim to
reduce directional bias and so increase robustness by adapting a cylindrical registration algorithm to penalize
inverse-consistency error, using a symmetric optimization.
Using 17 validation cases, the mean inverse-consistency error was reduced significantly by 86%, from 3.3 mm
to 0.45 mm. Furthermore, we show improved alignment of the prone and supine colonic surfaces, evidenced by a
reduction in the mean-of-squared-differences by 43% overall. Mean registration error, measured at a sparse set
of manually selected reference points, remained at the same level as the non-symmetric method (no significant
differences). Our results suggest that the inverse-consistent symmetric algorithm performs more robustly than
non-symmetric implementation of B-spline registration.