  Subtraction of aligned images is a means to assess changes in a wide variety
of clinical applications. In this paper we explore the information theoretical
origin of Mutual Information (MI), which is based on Shannon's entropy.However,
the interpretation of standard MI registration as a communication channel
suggests that MI is too restrictive a criterion. In this paper the concept of
Mutual Information (MI) is extended to (Normalized) Focussed Mutual Information
(FMI) to incorporate prior knowledge to overcome some shortcomings of MI. We
use this to develop new methodologies to successfully address specific
registration problems, the follow-up of dental restorations, cephalometry, and
the monitoring of implants.
