Simultaneous fat‐referenced proton resonance frequency shift thermometry and MR elastography for the monitoring of thermal ablations

PURPOSE: Simultaneous fat-referenced proton resonance frequency shift (FRPRFS) thermometry combined with MR elastography (MRE) is proposed, to continuously monitor thermal ablations for all types of soft tissues, including fat-containing tissues. Fat-referenced proton resonance frequency shift thermometry makes it possible to measure temperature even in the water fraction of fat-containing tissues while enabling local field-drift correction. Magnetic resonance elastography allows measuring the mechanical properties of tissues that are related to tissue structural damage. METHODS: A gradient-echo MR sequence framework was proposed that combines the need for multiple TE acquisitions for the water-fat separation of FRPRFS, and the need for multiple MRE phase offsets for elastogram reconstructions. Feasibility was first assessed in a fat-containing gelatin phantom undergoing moderate heating by a hot water circulation system. Subsequently, high intensity focused ultrasound heating was conducted in porcine muscle tissue ex vivo (N = 4; 2 samples, 2 locations/sample). RESULTS: Both FRPRFS temperature maps and elastograms were updated every 4.1 seconds. In the gelatin phantom, FRPRFS was in good agreement with optical fiber thermometry (average difference 1.2 +/- 1 degrees C). In ex vivo high-intensity focused ultrasound experiments on muscle tissue, the shear modulus was found to decrease significantly by 34.3% +/- 7.7% (experiment 1, sample 1), 17.9% +/- 10.0% (experiment 2, sample 1), 55.1% +/- 8.7% (experiment 3, sample 2), and 34.7% +/- 8.4% (experiment 4, sample 2) as a result of temperature increase (DeltaT = 22.5 degrees C +/- 4.2 degrees C, 14.0 degrees C +/- 2.8 degrees C, 14.7 degrees C +/- 3.7 degrees C, and 14.5 degrees C +/- 3.0 degrees C, respectively). CONCLUSION: This study demonstrated the feasibility of monitoring thermal ablations with FRPRFS thermometry together with MRE, even in fat-containing tissues. The acquisition time is similar to non-FRPRFS thermometry combined with MRE.