Assessment of ultra-high-field Magnetic Resonance Imaging safety via temperature increase monitoring with Magnetic Resonance Thermometry

Patient safety during Magnetic Resonance Imaging (MRI) examinations is currently guaranteed through the compliance with the limitations on the Specific Absorption Rate (SAR) exposure provided by the current regulations. SAR limits are implemented by scanner vendors in a strictly conservative way to nullify the risk of patient hazard, even at the cost of limiting sometimes the diagnostic power of the exam. Nonetheless, both in the case of ultra-high field applications (i.e. with static magnetic field of 7 T and above) and at clinical field strengths for example in the presence of metallic prosthetic implants, the inhomogeneities in the RF excitation field distribution may cause a local and subject-specific SAR increase. In those cases, local tissue temperatures could reach damaging levels. To directly quantify the local temperature increase caused by the more commonly used RF acquisition sequences in MRI of the human extremities, we have implemented Magnetic Resonance Thermometry (MRT) techniques on a 7 T MRI scanner. We demonstrated that temperature increase maps are easily and fast obtainable with MRT techniques, which are sensitive enough to detect potential hazardous tissue heating.