Measurement of sub-zero temperatures in MRI using T1 temperature sensitive soft silicone materials: Applications for MRI-guided cryosurgery

Purpose One standard method, proton resonance frequency shift, for measuring temperature using magnetic resonance imaging (MRI), in MRI-guided surgeries, fails completely below the freezing point of water. Because of this, we have developed a new methodology for monitoring temperature with MRI below freezing. The purpose of this paper is to show that a strong temperature dependence of the nuclear relaxation time T-1 in soft silicone polymers can lead to temperature-dependent changes of MRI intensity acquired with T-1 weighting. We propose the use of silicone filaments inserted in tissue for measuring temperature during MRI-guided cryoablations. Methods The temperature dependence of T-1 in bio-compatible soft silicone polymers was measured using nuclear magnetic resonance spectroscopy and MRI. Phantoms, made of bulk silicone materials and put in an MRI-compatible thermal container with dry ice, allowed temperature measurements ranging from -60 degrees C to + 20 degrees C. T-1-weighted gradient echo images of the phantoms were acquired at spatially uniform temperatures and with a gradient in temperature to determine the efficacy of using these materials as temperature indicators in MRI. Ex vivo experiments on silicone rods, 4 mm in diameter, inserted in animal tissue were conducted to assess the practical feasibility of the method. Results Measurements of nuclear relaxation times of protons in soft silicone polymers show a monotonic, nearly linear, change with temperature (R-2 > 0.98) and have a significant correlation with temperature (Pearson's r > 0.99, p < 0.01). Similarly, the intensity of the MR images in these materials, taken with a gradient echo sequence, are also temperature dependent. There is again a monotonic change in MRI intensity that correlates well with the measured temperature (Pearson's r p < 0.01). The MRI experiments show that a temperature change of 3 degrees C can be resolved in a distance of about 2.5 mm. Based on MRI images and external sensor calibrations for a sample with a gradient in temperature, temperature maps with 3 degrees C isotherms are created for a bulk phantom. Experiments demonstrate that these changes in MRI intensity with temperature can also be seen in 4 mm silicone rods embedded in ex vivo animal tissue. Conclusions We have developed a new method for measuring temperature in MRI that potentially could be used during MRI-guided cryoablation operations, reducing both procedure time and cost, and making these surgeries safer.

Automated summary

This study demonstrates a strong correlation between the nuclear relaxation time T-1 in soft silicone polymers and temperature, which can be used to monitor temperature changes in MRI. The results show that temperature-dependent changes in MRI intensity can be clearly observed and potentially applied in MRI-guided cryoablation procedures.