1. A method for grayscale registration of a first medical image and a second medical image, comprising the steps of: spatially registering the first medical image and the second medical image relative to each other; generating at least one pixel value histogram based on pixel values of the first and second medical images; generating a lookup table based at least in part on the at least one pixel value histogram, a first image acquisition method for the first medical image and a second image acquisition method for the second medical image; and applying the lookup table to the pixel values of the first medical image to generate a third medical image, the third medical image being transformed from the first medical image and registered to the second medical image.