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

Abstract

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 T1 in soft silicone polymers can lead to temperature-dependent changes of MRI intensity acquired with T1 weighting. We propose the use of silicone filaments inserted in tissue for measuring temperature during MRI-guided cryoablations. The temperature dependence of T1 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°C to + 20°C. T1 -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, 4mm in diameter, inserted in animal tissue were conducted to assess the practical feasibility of the method. Measurements of nuclear relaxation times of protons in soft silicone polymers show a monotonic, nearly linear, change with temperature (R2 >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<-0.98 and p<0.01). The MRI experiments show that a temperature change of 3°C can be resolved in a distance of about 2.5mm. Based on MRI images and external sensor calibrations for a sample with a gradient in temperature, temperature maps with 3°C isotherms are created for a bulk phantom. Experiments demonstrate that these changes in MRI intensity with temperature can also be seen in 4mm silicone rods embedded in ex vivo animal tissue. 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.