1. A method to determine a magnetic resonance (MR) relaxation time in a heart muscle in a magnetic resonance examination, comprising the steps of: using a computer to operate an MR data acquisition apparatus in order to acquire MR data from an examination subject, while the examination subject is situated in the MR data acquisition apparatus, from heart muscle tissue of the left heart chamber of the examination subject; in said computer, reconstructing an image from said MR data, said image being comprised of pixels that each exhibit a signal intensity, said image depicting anatomy of the heart muscle tissue at the left heart chamber of the examination subject; in said computer, (a) implementing an automatic image segmentation of said anatomy in said image to determine an annular slice image region, comprised of pixels of said image of the heart muscle tissue of the left heart chamber so that said annular slice region has a width in a radial direction that is reduced by an inner periphery of the annular slice image region being increased by a predetermined value and an outer periphery of the annular slice image region being reduced by a predetermined value; in said computer, (b) automatically dividing said annular slice image region of said image of the heart muscle tissue of the left heart chamber into a plurality of sub-regions of said annular slice image region of said image, each sub-region comprising a sub-plurality of said plurality of said pixels of said annular slice image region of the heart muscle tissue of the left heart chamber, each of said pixels in each sub-region exhibiting said signal intensity; for each of said sub-regions, (c) automatically determining, in said computer, an MR relaxation time of the heart muscle tissue depicted in the respective sub-region from the respective signal intensities of the respective pixels thereof, thereby obtaining a plurality of MR relaxation times; and in said computer, (d) automatically quantifying a characteristic MR relaxation time, of all the heart muscle, by statistical analysis of said plurality of MR relaxation times of the heart muscle tissue depicted in the respective sub-regions, and making a representation of the quantified characteristic MR relaxation time of all the heart muscle tissue available in electronic form at an output of the computer.