SAR MONITORING AT 7T USING PRF SHIFT THERMOMETRY

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Introduction: As RF power deposition increases with field strength, SAR limitations and heating are a major concern at high field. FDA regulations state that the maximum temperature increase in any gram of tissue has to be within 1¿C. Common SAR monitoring schemes use the deposited power to infer the resulting temperature change based on thermal modeling using the bio-heat equation. However, the actual temperature change is often not measured directly or only at isolated points. Methods and Discussion: In this study, we investigate a method based on the temperature sensitivity of the proton resonance frequency (PRF) that allows direct measurement of the induced heat in the object. Thermal maps are used to measure the local absorbed power distribution in a spherical phantom at 7T. To induce RF heating, the GRE pulse sequence was interspersed with off-resonance high power RF pulses. Only a single slice was imaged, to minimize the SAR heating caused by the temperature monitoring sequence itself. To correct for errors caused by main magnetic field drift, four vial of oil were placed around the phantom for phase reference. Because the PRF of oil is independent of temperature, it is not affected by phase changes from RF power deposition.