1. A method implemented by a computing device comprising at least one processor and memory for grayscale registration of a first medical image and a second medical image, the method comprising: spatially registering, by the computing device, the first medical image and the second medical image relative to each other; generating, by the computing device, at least one joint pixel value histogram based on pixel values of the first and second medical images; generating, by the computing device, a lookup table based at least in part on the at least one joint 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, by the computing device, 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; wherein the step of generating the lookup table includes: selecting a parametric transform function from a plurality of predetermined, parametric transform functions based on the first and second medical image acquisition methods for the first and second medical images, respectively; and statistically fitting parameters of the selected parametric transform function to the at least one pixel value histogram, the statistically fitting determines values of the parameters, wherein the lookup table is generated using the selected parametric transform function and the values of the fitted parameters; and wherein each of the first and second medical images is one of a digitized analog image and a digitally acquired image and wherein the plurality of predetermined parametric transform functions includes: (1) a function for registering a digitized analog image to another digitized analog image, (2) a function for registering a digitally acquired image to another digitally acquired image, (3) a function for registering a digitally acquired image to a digitized analog image, and (4) a function for registering a digitized analog image to a digitally acquired image.