how can i connect betwee eigenvalues and image ?

in my work i have several fMRI brain images for Alzheimer Disease. and i get these images and compare between each part in these images to get the parts that are different between positive and negative images. how can i get these parts? i want to use PCA to get eigenvalues that they are the principle component of these images but how can i say this eigenvalue for this part in this image?