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Specific absorption rate (SAR) and temperature increases produced inside a thorax model by an MRI apparatus equipped with a birdcage antenna operating at 64 MHz have been studied both experimentally and numerically. Considering a pacemaker (PM) equipped with a unipolar catheter inserted inside the thorax model, peak SARs averaged over 1 mg between 240 and 6400 W/kg, depending on the catheter section and length, on its position inside the phantom, and on field polarization have been obtained close to the catheter tip. On the other hand, the average SAR in the whole thorax is not influenced by the presence of the PM. Temperature increments from 0.6degC to 15degC have been obtained for 6-min MRI investigations with the lowest values when the radio-frequency (RF) magnetic field is linearly polarized along a direction perpendicular to the implant plane.