1. A method of training a machine learning system to generate an image comparison tool, the method comprising: accessing a first medical image including first pixel data depicting first structures; accessing a second medical image including second pixel data depicting second structures; establishing a similarity value based on a comparison of the first and second pixel data; extracting a first features group from a first subset of the first pixel data, and a second features group from a second subset of the second pixel data; training a machine learning system by using the first and second features groups as inputs to the machine learning system and using the similarity value as an expected output of the machine learning system; and generating an image comparison tool based on the training.