In a method to determine at least one sodium value describing the 23Na+ content in at least one region of interest in a target region in the body of a patient, at least one sodium image data set of the target region is acquired with a magnetic resonance imaging device using sodium-23 imaging, the sodium image data set including image data dependent on the occurrence of sodium. The at least one region of interest is defined for which the sodium value is to be determined in the sodium image data set. The sodium value is determined by comparison of the image data in the region of interest with reference image data of at least one subject with a defined 23Na+ content, the reference image data having been acquired with the same sequence. A local coil can be used to implement the method that has a phantom integrated therein that allows the sodium image data set and the reference image data to be acquired together.