Increasing registration accuracy by sub-volume based mutual information registration

Purpose: In order to take full advantage of the many imaging modalities available today such as CT, MRI, PET etc., accurate image registration is essential. Mutual information (MI) registration is a commonly used and robust method as it is based on the complete image information instead of fiducial markers or anatomical landmarks. However, MI registration is often dominated by the patient external contour and bony structures. This is problematic in cases when an organ of interest does not have a fixed position in relation to bony anatomy; a typical example is the prostate. To be able to ensure proper registration with the prostate in focus it is necessary to restrict the registration volume to include the prostate with a small margin, i.e. a sub-volume based registration (SVR). In the present study, the accuracy and precision for sub-volume based MI registrations in the prostate area was investigated and compared to full-volume registrations (FVR).