Positron-electron annihilation in living organisms occurs in about 30% via the formation of a metastable ortho-positronium atom that annihilates into two 511 keV photons in tissues because of the pick-off and conversion processes. Positronium (Ps) annihilation lifetime and intensities can be used to determine the size and quantity of defects in a material's microstructure, such as voids or pores in the range of nanometers. This is particularly true for blood clots. Here we present pilot investigations of positronium properties in fibrin clots. The studies are complemented by the use of SEM Edax and micro-computed tomography (μCT) to evaluate the extracted thrombotic material's properties. μCT is a versatile characterization method offering in situ and in operando possibilities and is a qualitative diagnostic tool. With μCT the presence of pores, cracks, and structural errors can be verified, and hence the 3D inner structure of samples can be investigated.