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Thermal dose in tumor tissue is a key factor for regional hyperthermia (HT) combined with chemotherapy and for drug delivery using thermosensitive liposomes (TSL). It influences therapy outcome, affects the accumulation of liposomes, and triggers the content release from TSL in the target tissue. For the development and clinical application of TSL, noninvasive visualization is of critical importance. For this purpose, TSL loaded with MRI contrast agent (CA) have been developed. With increase in temperature, the CA is released from TSL at the phase transition temperature Tm resulting in a relaxation time change, which allows MRI monitoring. The purpose of this study was to examine the feasibility of an in vivo application and MR characterization of Gd-DTPA-BMA-loaded phosphatidylglyceroglycerol-TSL (Gd-TSL) at mild HT conditions in tumor tissue using a clinically relevant setting.Gd-TSL were characterized in vitro with varying thermal doses between 37°C and 45°C and distinct solvents by MR at 0.5 T and 1.5 T. In vivo studies were performed in C57BL/6 mice bearing BFS-1 fibrosarcomas at 1.5 T. One tumor-bearing leg was immersed in a temperature-controlled water bath (T). Gd-TSL (Tm = 43.5 ± 0.2°C) were injected either intratumorally or intravenously at T = 37.3 ± 0.1°C or T = 42.5 ± 0.3°C. As a control, nonliposomal Gd-DTPA-BMA was injected intravenously at T = 43.1 ± 0.3°C. A second tumor on the contralateral limb, which remained unheated, served as a control. CA release was monitored by T1-weighted spin-echo.The in vitro characterization demonstrated at heated and unheated samples a strong increase in T1-relaxivity of Gd-TSL solutions from 0.4 mM−1 s−1 (37.5°C) to 4.2 mM−1 s−1 (43.3°C) at 0.5 T. Thermal dose and solvent affected the rate of relaxation time change significantly. A fast and complete release was observed in samples with serum, whereas Gd-TSL in glucose was only partially released within 1 hour. A dedicated experimental setup was developed for standardized in vivo investigation. Tumor signal intensity changes were detectable in all animals. After intratumoral injection of Gd-TSL, the signal increased heterogeneously (max., +52% ± 25%) within 3 minutes after temperature increase and decreased strongly thereafter, whereas after i.v. injection, the signal increased homogeneously (+19% ± 3%) within 2 minutes persisting thereafter. The unheated control tumors on the contralateral legs showed a 10% ± 3% signal increase within 2 minutes. Injection at 37°C showed a continuous signal increase in “heated” and unheated tumors of up to 8% to 10%. Nonliposomal CA injection demonstrated that tumors were well perfused during HT.HT-induced CA release from Gd-TSL was monitored and characterized by MRI after i.v. injection in tumor-bearing mice. Higher temperatures resulted in higher signal changes. Immediately after i.v. injection, heated tumor tissue was distinguishable from unheated tumor tissue. The Gd-TSL appears to be suitable for MR monitoring of HT tumor treatment in a clinical MRI setting independent of field strength.