Potential and limitations of magnetic resonance imaging for real-time monitoring of interstitial laser phototherapy

Rationale and Objectives.: Magnetic resonance (MR) imaging has been suggested as a method to monitor interstitial laser phototherapy (ILP) in deep tissues. Unfortunately, a reliable relation between temperature and MR parameters has not yet been demonstrated. In this study, we examined whether such a relation exists and whether MR imaging can measure absolute temperature or temperature changes. Methods.: We evaluated, in the range of 21°C to 80°C, the temperature dependence of the MR imaging signal and T1 in samples of liver, water, CuSO₄, and oil. Spin-echo and fast low-angle shot (FLASH) sequences were used. Results.: The MR imaging signal of liver, CuSO₄, and water continuously decreased when the temperature was increased from 21°C to 80°C. By contrast, the MR imaging signal of the oil increased with increasing temperature up to 40-50°C and then decreased at higher temperatures. We observed a reliable linear relation only between T1 and temperature in a range of 30-60°C for oil and CuSO₄. Conclusion.: MR imaging has the potential to measure thermal variations with an uncertainty of approximately ±10°C. However, the use of MR imaging to monitor the real-time thermal effect induced in biologic tissues during laser irradiation requires further investigation before it can be applied clinically.