Do Respiration and Cardiac Motion Induce Magnetic Field Fluctuations in the Breast and Are There Implications for MR Thermometry?

Purpose:To assess the distribution of respiration and cardiac motion-induced field fluctuations in the breast and to evaluate the implications of such fluctuations for proton resonance frequency shift (PRFS) MR thermometry in the breast. Materials and Methods: Gradient echo MR field maps were made to study the effect of regular respiration, maximum capacity respiration, and cardiac motion on the stability of the local magnetic field in four healthy female volunteers. Field fluctuations (in parts-per-million [ppm]) were averaged over a region of interest covering both breasts. Results: The average field fluctuation due to regular respiration was 0.13 ppm, due to maximum capacity respiration 0.16 ppm and <0.03 ppm due to cardiac motion. These fluctuations can be misinterpreted as temperature changes of 13, 16, and 3 degrees C when PRFS-based MR thermometry is used during thermal treatment of breast cancer. Conclusion: Respiration causes significant field fluctuations in the breast. If MR thermometry were to be safety used in clinical practice, these fluctuations should be taken into account and should probably be corrected for.