1. A method to process complex image data, comprising: providing, to a processor, complex image data acquired from an examination subject during a plurality of echo times in a magnetic resonance (MR) data acquisition using a plurality of acquisition units selected from the group consisting of individual reception coils and respective portions of a multi-coil reception array, according to at least one MR parameter that affects signals, representing said complex image data, acquired in said MR data acquisition, said examination subject comprising a first spin species and a second spin species differing from said first spin species with a phase difference between precessing spins of said first spin species and precessing spins of said second spin species being different in at least two of said echo times; and in said processor, determining a value of said at least one MR parameter with spatial resolution for a plurality of image points in said complex image data by causing a function, which depends on said at least one MR parameter and a corresponding image point in said complex image data during said at least two of said echo times, to satisfy an extreme condition, and making said value of said at least one MR parameter available in electronic form at an output of said processor.