1. A medical apparatus comprising: a magnetic resonance imaging system for acquiring magnetic resonance data; a processor for controlling the medical apparatus; and a memory containing machine readable instructions for execution by the processor, wherein execution of the instructions causes the processor to: acquire a first set of magnetic resonance data; reconstruct a first image using the first set of magnetic resonance data; acquire an undersampled magnetic resonance data set, wherein the undersampled magnetic resonance data set is undersampled in k-space in comparison to the first set of magnetic resonance data; reconstruct a second image using the undersampled magnetic resonance data set and the first image, wherein the second image is reconstructed using an image ratio constrained reconstruction algorithm; and calculate a temperature map using the second image.