Fundamental Principles of Magnetic Resonance Temperature Imaging

Since both the chemical environment and relaxation properties of the nuclei that are the source of the signal in magnetic resonance (MR) are sensitive to Brownian motion and the associated molecular tumbling rates, MR imaging (MRI) techniques are intrinsically sensitive to temperature. Of the many MR parameters that can provide temperature- sensitive contrast, the temperature dependence and sensitivity of several parameters in particular, have proven useful for monitoring temperature changes in soft tissue during delivery of hyperthermia or thermal therapies: the apparent diffusion constant of water (D), the spin-lattice relaxation time (T1), and the water proton resonance frequency (PRF). The temperature sensitivities associated with each of these parameters are large enough to allow temperature-dependent changes to be observed quantitatively using either direct or indirect measurements using standard MRI devices over a range of temperatures relevant for thermal therapy. The development of these techniques to noninvasively measure temperature changes in tissue has brought renewed interest in using these techniques to enhance the guidance of thermal therapy treatments.