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

•Relaxation times of agarose gel are measured at fields of 50μT–3T and at temperatures of 5–45°C.•Interestingly, the effect of temperature on agarose gel T1 relaxation is reversed around 5mT.•The relaxation time data are used to form temperature maps measured at 50μT.•The paper presents the first temperature maps measured at 50μT. 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 50μT–52mT and at temperatures 5–45°C. Measurements with a 3T 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 3T; the effect of temperature on T1 is reversed at approximately 5mT. 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 50μT reproduced the temperature gradient with good contrast.