Accurate and fast temperature mapping during ohmic heating using proton resonance frequency shift MRI thermometry

Proton resonance frequency (PRF) shift MRI thermometry was incorporated into a fast low angle shot imaging sequence to acquire two-dimensional temperature maps of two model liquid-particulate mixtures undergoing ohmic heating process. The samples used consisted of two phases, namely whey gel and NaCl solution, and were enclosed in an in-house made ohmic heating device driven by an 143 V AC power supply. Salt was added to the gel and solution to alter their electrical conductivity. The PRF shift was used as a temperature indicator, which was linearly and reversibly proportional to the temperature change. The specific PRF shift for the whey gel was determined as -0.0098 and -0.0097 ppm/°C for the two samples. For the NaCl solution, the values were -0.0096 and -0.0102 ppm/°C, respectively. Detailed temperature maps with spatial resolution of 0.94 mm and temporal resolution of 0.64 s were obtained. The temperature uncertainties were about ±1 °C for the whey gel and ±2 °C for the solution. The ease to interpret, the speed, and the accuracy of this new technique justify its applications in dynamic food process. The temperature maps provided details for the investigation of uneven heating of a liquid-particulate mixture undergoing ohmic heating and this temperature mapping technique is a powerful tool for ohmic model development and validation.