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 cause the processor to: acquire baseline magnetic resonance data; reconstruct a first image using the baseline magnetic resonance data; acquire undersampled magnetic resonance data, wherein the undersampled magnetic resonance data is undersampled in k-space in comparison to the baseline magnetic resonance data; reconstruct a second image using the undersampled magnetic resonance data 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.