Our purpose is to spectrally probe the main brain absorbers. The determination of their spatial distribution remains a challenge. According to anatomical data, the proposed 3D model of the rat pial-cortical vascular networks is divided into three parts: (1) the pial vessels could be approximated by a dense layer of around 250 micrometers depth; (2) the penetrating vessels repartition is described as periodic hexagonal prisms with three modules; (3) the capillary network is modelized using a periodic tiling of polyhedron with a density of 817mm.mm-3 and a branching pattern of 10000mm-3. With anaesthetized rats under stereotaxic conditions, in vivo time-resolved brain spectroscopy experiments are presented. The setup is designed to allow broadband time-resolved spectroscopy using a streak camera. A femtosecond white light continuum is produced by focusing 800nm pulses (0.5mJ, 1kHz, 150fs) in an adapted third order non linear medium. In the case of water, the spectrum expands over 380-780nm with an efficiency of 20 percent. Mathematical homogenization techniques could be applied to the radiative transfer equation with this geometrical vascular architecture and might be useful to analyze in depth time-resolved spectroscopy of such complex media.