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Proton resonance frequency shift-based MR thermometry (MRT) is hampered by temporal magnetic field changes. Temporal changes in the magnetic susceptibility distribution lead to nonlocal field changes and are, therefore, a possible source of errors. The magnetic volume susceptibility of tissue is temperature dependent. For water-like tissues, this dependency is in the order of 0.002 ppm/°C. For fat, it is in the same order of magnitude as the temperature dependence of the proton electron screening constant of water (0.01 ppm/°C). For this reason, proton resonance frequency shift-based MR thermometry in fatty tissues, like the human breast, is expected to be prone to errors. We aimed to quantify the influence of the temperature dependence of the susceptibility on proton resonance frequency shift-based MR thermometry. Heating experiments were performed in a controlled phantom set-up to show the impact of temperature-induced susceptibility changes on actual proton resonance frequency shift-based temperature maps. To study the implications for a clinical case, simulations were performed in a 3D breast model. Temperature errors were quantified by computation of magnetic field changes in the glandular tissue, resulting from susceptibility changes in a thermally heated region. The results of the experiments and simulations showed that the temperature-induced susceptibility changes of water and fat lead to significant errors in proton resonance frequency shift-based MR thermometry. Magn Reson Med 64:1360-1372, 2010. V C 2010 Wiley-Liss, Inc. Key words: susceptibility; temperature; PRFS-based MR thermometry; HIFU; fat; glandular tissue Next to excellent soft-tissue contrast, MRI also offers methods to acquire temperature maps, which allow for monitoring and guiding of thermal interventions. Proton resonance frequency shift (PRFS)-based MR thermometry (MRT) is the most commonly used temperature monitoring technique. It is based on the temperature dependence of the proton resonance frequency (PRF) of protons in water molecules and is thus applicable in water-containing tissues. The technique does not work for protons in fat molecules and for this reason adequate fat suppression is essential for accurate thermometry in tissues containing both water and fat.PRFS-based MRT is hampered by temporal field changes. In the application of this technique, the contribution of field changes, which originate from temperature-induced magnetic volume susceptibility changes, is commonly ignored. This disregards the fact that the magnetic volume susceptibility x (which will hereinafter be referred to as ''susceptibility'') of water and water-containing tissue is temperature dependent (1,2). Several studies have suggested that this may lead to PRFS-based temperature errors in water and water-containing tissues (1-4). This has been shown for water (3) and brain tissue (2), which both have a temperature dependent susceptibility in the order of dx/dT ¼ 0.002 ppm/ C.In tissues containing both water and fat, more considerable susceptibility-relat...

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