The protocols of automatic selecting the similarity measure based on entropy for image registration is presented. Some common similarity measures based on entropy are given firstly. The accuracy

the acutance and the celerity are put forwarded as rules to select the suit measure from these measures according to the requirement of image registration. The suit similarity measure is selected automatically based on our method for the unitmodels and the multimodels of MR

CT and PET respectively in MATLAB

which is also obtained in low resolution or noise environment. The experiment results prove the feasible and validity of these protocols. The low precision of registration caused by the unsuitable similarity measure can be avoided.