Purpose: To investigate the immediate eŠects of microwave coagulation on rat tumors in various magnetic resonance (MR) images at high magneticˆeld strength using histopathological examinations as reference.Materials and Methods: Tumors implanted in rat femurs were partially thermocoagulated by microwave. Immediately after, T 1 -and T 2 -weighted images, diŠusion-weighted images (DWIs), and contrast-enhanced T 1 weighted images (CE-T 1 WIs) were acquired with a 7-tesla MR scanner. After measurements, tumors were examined histopathologically with hematoxylin-eosin (HE) staining and histochemically for acid phosphatase activity.Results: Without contrast, boundaries of coagulated areas were unclear on MR images, including apparent diŠusion coe‹cient (ADC) maps. CE-T 1 WIs clearly showed immediate contrast enhancement of untreated areas of tumor, and the area of enhancement gradually enlarged in 5 min. Quantitative analyses were conducted by classifying tumor areas by contrast enhancement results. Signal intensities of the areas in the MR images showed no signiˆcant diŠerences, but at the periphery, ADC values were signiˆcantly higher in areas with delayed enhancement than those with immediate enhancement. Compared with histopathologicalˆndings, with microwave thermocoagulation, increased ADC value seemed to derive from collection of extracellular ‰uid in the outer zone, where acid phosphatase activity was attenuated.Conclusion: ADC values in the areas with delayed enhancement of CE-T 1 WIs were higher than those in non-aŠected areas, but MR images could not show areas of coagulation within tumors. Clear detection of the boundaries of coagulated areas required contrast enhancement, even at magneticˆeld strength of 7T.