Absolute MR thermometry using time-domain analysis of multi-gradient-echo magnitude images.

MRI allows for absolute temperature measurements in substances containing two spectral resonances of which the frequency difference Delta f(T) is related to absolute temperature. This frequency difference can be extracted from spectroscopic data. An image-based MR technique that allows for the acquisition of spectroscopic data at high temporal and spatial resolution is the multi-gradient-echo sequence. In this work, the application of the multi-gradient-echo sequence for MR thermometry purposes was further developed. We investigated the possibility of postprocessing the multi-gradient-echo data into absolute temperature maps, using time-domain analysis of the magnitude of the multi-gradient-echo signals. In this approach, instead of an indirect computation of Delta f(T) from separately found frequencies, Delta f(T) is a direct output parameter. In vitro experiments were performed to provide proof of concept for retrieving absolute temperature maps from the time-domain analysis of multi-gradient-echo magnitude images. It is shown that this technique is insensitive to both field drift and local field disturbances. Furthermore, ex vivo bone marrow experiments were performed, using the fat resonance as a reference for absolute temperature mapping. It is shown that the postprocessing based on the magnitude signal in the time domain allows for the determination of Delta f(T) in bone marrow.