Temperature dependence of relaxation times and temperature mapping in ultra-low-field MRI

•Relaxation times of agarose gel are measured at fields of 50 μT-3 T and at temperatures of 5-45 °C.•Interestingly, the effect of temperature on agarose gel T1 relaxation is reversed around 5 mT.•The relaxation time data are used to form temperature maps measured at 50 μT.•The paper presents the first temperature maps measured at 50 μT.

Ultra-low-field MRI is an emerging technology that allows MRI and NMR measurements in microtesla-range fields. In this work, the possibilities of relaxation-based temperature measurements with ultra-low-field MRI were investigated by measuring T1 and T2 relaxation times of agarose gel at 50 μT-52 mT and at temperatures 5-45 °C. Measurements with a 3 T scanner were made for comparison. The Bloembergen-Purcell-Pound relaxation theory was combined with a two-state model to explain the field-strength and temperature dependence of the data. The results show that the temperature dependencies of agarose gel T1 and T2 in the microtesla range differ drastically from those at 3 T; the effect of temperature on T1 is reversed at approximately 5 mT. The obtained results were used to reconstruct temperature maps from ultra-low-field scans. These time-dependent temperature maps measured from an agarose gel phantom at 50 μT reproduced the temperature gradient with good contrast.

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