Non-invasive temperature mapping using MRI: comparison of two methods based on chemical shift and T1-relaxation

Purpose: To implement and evaluate the accuracy of non-invasive temperature mapping using MRI methods based on the chemical shift (CS) and T₁ relaxation in media of various heterogeneity during focal (laser) and external thermal energy deposition.Materials and Methods: All measurements were performed on a 1.5 T superconducting clinical scanner using the temperature dependence of the water proton chemical shift and the T₁ relaxation time. Homogeneous gel and heterogeneous muscle phantoms were heated focally with a fiberoptic laser probe and externally of varying degree ex vivo by water circulating in a temperature range of 20–50°C. Magnetic resonance imaging data were compared to simultaneously recorded fiberoptic temperature readings.Results: Both methods provided accurate results in homogeneous media (turkey) with better accuracy for the chemical shift method (CS: ±1.5°C, T₁: ±2.0°C). In gel, the accuracy with the CS method was ±0.6°C. The accuracy decreased in heterogeneous media containing fat (T₁: ±3.5°C, CS: +5°C). In focal heating of turkey muscle, the accuracy was within 1.5°C with the T₁ method.Conclusion: Temperature monitoring with the chemical shift provides better results in homogeneous media containing no fat. In fat tissue, the temperature calculation proved to be difficult.