Euclidean means the Euclidean distance, i.e., the mean squared difference between the gray values of model and reference.

The more the images are close to each other, the more the metric value will approach 0.

Correlation measures the correlation ratio of the registered images.

It takes values between 0 and 1. The case correlation ratio = 1 correspond to the case of two identical and registered images.

The Mutual information metrics, especially the normalized one, are recommended when images from different modalities, e.g., CT and MRT, are to be registered.