Coolant quality for magnetic resonance imaging systems

As radiologists demand increased power, speed and duty cycle from their magnetic resonance imaging (MRI) systems, thermal management of the gradient sub-system becomes more challenging. A heat exchanger in the MRI system cools heat-generating components by pumping water through hollow copper tubing, which also carries high electrical currents. Water is used as a coolant because of its high specific heat capacity and a high thermal conductivity. Since the coolant is in contact with high electrical currents, leakage currents must be accounted for by maintaining a minimum coolant resistivity of 2.5 k Ohm-cm to prevent injuries to customers and service personnel. The use of completely deionized water can mitigate the risk of leakage currents; however, it is very corrosive and would be incompatible with the materials in contact with the water. Plain water is conductive and pure deionized water is corrosive, so experiments were conducted to investigate the use of deionized, low-conductivity water as a coolant. Water quality was determined by its pH level, resistivity level, amount of copper corrosion and amount of biological activity present. The optimal pH range to inhibit metallic corrosion was defined as pH 8-9. To further prevent corrosion and bacterial growth, a corrosion inhibitor and biocide were added to the water. The addition of corrosion inhibitor and biocide limits the maximum achievable resistivity level of the mixed water solution. Additional experiments were performed to study the effects of coolant quality over time and to develop a proactive system for coolant maintenance. A periodic deionization and filtering process was designed to maintain proper water quality in order to ensure safe operation of the system and efficient cooling. The deionization and filtering process acts to remove ions from the water via ion exchange. This process occurs as needed on a bypass line, which allows the main system to continue running. The deionization process; however, w- as found to remove the corrosion inhibitor and requires that it must be replaced.