Mattes et al. vs Viola and Wells Mutual Information


Can someone please explain the difference between Mattes et al Mutual Information MMI and Viola and Wells Mutual Information VMI?

From the description in the links, I understand that MMI uses Parzen histogram with a user-defined number of pins ( it is not clear what is the difference between Parzen window and Parzen histogram?). MMI uses 3rd-order BSpline kernel for only the moving image (in Viola and Wells it uses Gaussian kernel for both images).