Magnetic resonance thermometry and its biological applications – Physical principles and practical considerations

Abstract

Most parameters that influence the magnetic resonance imaging (MRI) signal experience a temperature dependence. The fact that MRI can be used for non-invasive measurements of temperature and temperature change deep inside the human body has been known for over 30 years. Today, MR temperature imaging is widely used to monitor and evaluate thermal therapies such as radio frequency, microwave, laser, and focused ultrasound therapy. In this paper we cover the physical principles underlying the biological applications of MR temperature imaging and discuss practical considerations and remaining challenges. For biological tissue, the MR signal of interest comes mostly from hydrogen protons of water molecules but also from protons in, e.g., adipose tissue and various metabolites. Most of the discussed methods, such as those using the proton resonance frequency (PRF) shift, T 1 , T 2 , and diffusion only measure temperature change, but measurements of absolute temperatures are also possible using spectroscopic imaging methods (taking advantage of various metabolite signals as internal references) or various types of contrast agents. Currently, the PRF method is the most used clinically due to good sensitivity, excellent linearity with temperature, and because it is largely independent of tissue type. Because the PRF method does not work in adipose tissues, T 1 -and T 2 -based methods have recently gained interest for monitoring temperature change in areas with high fat content such as the breast and abdomen. Absolute temperature measurement methods using spectroscopic imaging and contrast agents often offer too low spatial and temporal resolution for accurate monitoring of ablative thermal procedures, but have shown great promise in monitoring the slower and usually less spatially localized temperature change observed during hyperthermia procedures. Much of the current research effort for ablative procedures is aimed at providing faster measurements, larger field-ofview coverage, simultaneous monitoring in aqueous and adipose tissues, and more motioninsensitive acquisitions for better precision measurements in organs such as the heart, liver, and kidneys. For hyperthermia applications, larger coverage, motion insensitivity, and simultaneous aqueous and adipose monitoring are also important, but great effort is also aimed at solving the problem of long-term field drift which gets interpreted as temperature change when using the PRF method.

T2 mapping as a predictor of nonperfused volume in MRgFUS treatment of desmoid tumors

“…However, visualization of the intraprocedural ablated volume remains a large problem in focused ultrasound therapy. Thermal dose maps from MR thermometry are used during the treatment to visualize the treated volume, but fail to reliably predict the extent of ablation [10][11][12][13]. Post-treatment contrast enhanced MR imaging allows assessment of the nonperfused volume (NPV), the gold standard assessment of the degree of tumor ablation.…”

Multinuclear absolute magnetic resonance thermometry

“…Studies have shown that the volume of magnetic susceptibility changes linearly with temperature 52 , and its effect on the 1 H resonance frequency shift is roughly an order of magnitude smaller than the electrical shielding effect 18 , 52 , 53 . As a result, calibration of the absolute thermometry method on the sample includes the sample-specific magnetic susceptibility shielding information for both sodium and proton.…”

Systematic review of pre-clinical and clinical devices for magnetic resonance-guided radiofrequency hyperthermia

“…Several different approaches for noninvasive thermometry, like impedance tomography, active and passive microwave imaging, CT, laser, infrared, ultrasound, MR-techniques, have been investigated in the last decades. Among all these different techniques, MR thermometry (MRT) has been elaborated most extensively and validated in clinical and experimental settings [20,21]. In addition, the gantry of the current MRI systems provides the possibility for mechanical integration with hyperthermia applicators [22].…”