Improved drug delivery control during chemotherapy is a major concern to increase their therapeutic index. Drug accumulation in solid tumor can be visualized using MRI contrast agent such as iron oxide nanoparticles encapsulated in liposomes. Once accumulated in tumor, the combination of a thermosensitive composition with an external source of activation allows local release of drug. MRI guided-High intensity focused ultrasound (HIFU) represents a non invasive technique to generate local hyperthermia for drug release of thermosensitive magnetoliposomes (TSM). In this study we performed encapsulation of ultrasmall superparamagnetic iron oxide nanoparticles (USPIO) in thermosensitive liposomes to obtain TSM. Magnetic behavior of this MRI contrast agent was observed during TSM membrane permeabilization. For this, measurement of transverse and longitudinal relaxivities on MRI, and real time experiments were performed on TSM samples loaded with USPIO during heating using a water bath or HIFU. Results showed significant differences for MRI signal enhancement and relaxivities ratios before and after heating, which were absent for non-thermosensitive liposomes and free nanoparticles used as controls. Thus, incorporation of USPIO as MRI-contrast agents into thermosensitive liposomes should, besides TSM tumor accumulation, allows the visualization of TSM membrane permeabilization upon temperature elevation. In conclusion, HIFU under MR image guidance in combination with USPIO loaded thermosensitive liposomes as drug delivery system has the potential for a better control of drug delivery and to increase the therapeutic index of chemotherapy.Read less <