An X-ray image sensor having scintillating material embedded into wave-guide structures fabricated in a CMOS image sensor (CIS). After the CIS has been fabricated, openings (deep pores) are formed in the back side of the CIS wafer. These openings terminate at a distance of about 1 to 5 microns below the upper silicon surface of the wafer. The depth of these openings can be controlled by stopping on a buried insulating layer, or by stopping on an epitaxial silicon layer having a distinctive doping concentration. The openings are aligned with corresponding photodiodes of the CIS. The openings may have a shape that narrows as approaching the photodiodes. A thin layer of a reflective material may be formed on the sidewalls of the openings, thereby improving the efficiency of the resulting waveguide structures. Scintillating material (e.g., CsI(Tl)) is introduced into the openings using a ForceFill™ technology or by mechanical pressing.