A number of other intensity-based cost functions have been implemented and used for intramodality registration. These include stochastic sign crossing [12,15], sum of absolute differences [5,12] and mutual information (see the chapter "Across-Modality Registration using Intensity-Based Cost Functions"). A study by Lin et al.  compared a variety of cost functions for registering PET fluoro-dopa images and found the sum of absolute differences cost function to produce results similar to those obtained with the ratio uniformity and least squares methods.
Variants of these intensity-based cost functions have also been used for intermodality registration. This topic is discussed in detail in the chapter "Across-Modality Registration using Intensity-Based Cost Functions."
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