The tumor location, evaluation of its shape, its position and its interactions with close brain structures, etc., are medical acts very important and in great demand. Clinical Tools brain's observation areas are now numerous, like X-ray scanner, magnetic resonance imaging (MRI), computed tomography and scintigraphy. Among these techniques, MRI is the increasingly used. This non-invasive technique exploits the magnetic properties of tissues and allows the observation of brain anatomy. The resulting image with high resolution allows the precise location of the various structures. Actually MRI technology allows to have digital images with image restoration "of the material" necessary for the implementation of processing and analysis algorithms that helps in the diagnosis of brain tumors. The real challenge is to accurately detect the tumor area. In this article we present a new method for detecting and segmenting tumor regions. This method consists of two steps: a) study and histogram analysis of the MRI image to find a threshold of intensity which can help us to extract tumor; b) tumor detection and extraction. Our method will be completed by a characterization of the tumor area by determining its geometric properties.