A multimodality imaging phantom is disclosed which is useful for calibrating devices for imaging vascular conduits. The phantom is compatible with X-ray, ultrasound and magnetic resonance imaging techniques. It allows testing, calibration, and inter-modality comparative study of imaging devices, in static or dynamic flow conditions. It also provides a geometric reference for evaluation of accuracy of imaging devices. The tissue-mimicking material is preferably an agar-based solidified gel. A vessel of known desired geometry runs throughout the gel and is connected to an inlet and outlet at its extremities for generating a flow circulation in the vessel. Said phantom also contains fiducial markers detectable in the above-mentioned modalities. The markers are preferably made of glass and are embedded in a layer of agar gel containing a fat component. The markers are implanted at precise known locations to allow identification and orientation of plane views, and they can be used for calibration, resealing and fusion of 3D images obtained from different modalities, and 3D image reconstruction from angiographic plane views. Also disclosed is a process for manufacturing said phantom.