Applying Magnetic Resonance Thermometry to Engineering Flows

Measuring three-dimensional temperature fields simultaneously with three-dimensional three-component velocity fields by means of Magnetic Resonance Imaging (MRI) methods is the goal of a DFG-funded joint research project at Technische Universitat Darmstadt and the University Medical Center Freiburg. This paper summarizes the basics needed to understand the principles of measuring temperature and velocity with MRI. A brief outline of former research applying MR Thermometry (MRTh) on engineering flows (e.g. Sun and Hall, 2001 or Elkins et al., 2004) is given. Based on the interaction of atomic nuclei with surrounding magnetic fields (Haacke et al., 1999), MR Thermometry is a medical tool for monitoring tissue temperature during minimally invasive thermal therapies, such as hyperthermia (43-45°C, treatment of cancer cells) or high-temperature thermal ablation (50-80°C, induce cell necrosis). The advantage of MRI-guiding during these medical therapies is to map the local tissue temperature with spatial resolution and in real-time. The most advanced and promising technique is PRF Thermometry, which has a minimal resolvable temperature of slightly less than 1K. PRF Thermometry uses the temperature dependent properties of water molecules ranging from liquid to crystal state (Jeffrey, 1997). In order to understand the link between magnetic and temperature-dependent properties of water molecules an appropriate understanding of hydrogen bonding is necessary. A basic discussion is given in the full paper. However, until now MRTh has not become a conventional instrument for medical diagnostics. The acceptance of MRTh still suffers from non-reliable temperature measurements in non-aqueous tissue, such as fat, as well as artifacts due to patient movement or heat-induced tissue mutation (Rieke et al., 2008). These disadvantages are not present in the environment of a stationary flow model. In comparison to other temperature measurement techniques, MRTh has certain advantages and disadvantages. Especially in cases of accuracy and measurement dimension MRTh is competitive to standard techniques, whereas the temperature range, the restriction to use water as measurement fluid, the missing transient capability and the response time summarize MRTh’s weaker points. In order to use MRTh properly a reference temperature has to be taken from inside the field-of-view (FOV). The paper summarizes which measurement techniques can be applied within the MR environment for calibration and verification of MRTh measurements. Most applicable are fiber optical sensors, which are not influenced by the scanners magnetic field (thermographic phosphor) or can easily be calibrated (gallium arsenide crystals) for the use inside the MR scanner (Buchenberg et al., 2014). In a preliminary experiment temperature difference maps where achieved using MRTh for straight pipe flows at different constant fluid temperatures and different flow rates. Good agreement was found with simultaneously achieved temperature measurements using MR applicable fiber optical probes. A discussion on difficulties that can be faced when attempting to set up and measure temperature distributions using MRTh is given. They arise from changes of the magnetic susceptibility of different materials within the FOV. Different susceptibilities lead to artifacts in the measured image that need to be compensated for. Often errors due to susceptibility changes are in the same order as the measured temperature differences. Procedures to compensate for such artifacts or to avoid them will have to be developed.