Maintaining an animal’s body temperature during magnetic resonance imaging (MRI) poses great challenges, as many temperature measuring devices and warming systems are incompatible with the MRI machine. The aim of this study was to examine body temperature changes and evaluate the impact of using a hot water bottle and a cloth blanket on rectal temperature during magnetic resonance imaging in cats. We included in this study 30 cats from different breeds that underwent magnetic resonance imaging for 60 min that were randomly divided into a passively insulated group (G1) covered with a blanket (n = 15) and a positively heated group (G2) using a silicone hot water bottle under the abdomen and the same cloth blanket over the cat (n = 15). The body temperature was measured before premedication, before induction of anesthesia, and after the MRI examination. Body temperature decreased slightly but significantly (p < 0.05) after premedication. At the end of the MRI, body temperature had decreased more in G1 than G2 (p = 0.033) to 37.0 (36.5–37.5) °C and 38 (37.9–38.2) °C, respectively. This study provides clinical evidence that cats needing magnetic resonance imaging of the head can be protected from hypothermia by using a hot water bottle placed underneath their abdomen and a cloth blanket covering their full body.