The discovery of the piezoelectric effect, by Pierre and Jacques Curie in 1880(Cady, 1964), had triggered many important studies and applications.  It has been discovered that in the 32 crystal classes, which conventionally separated into 7 crystal systems, only 20 classes possesses piezoelectric crystals (Nye, 1972).  Only the crystal classes that possess no center of symmetry are the ones have piezoelectricity.  The dipoles form groups of atoms in the lattice are the cause of the asymmetric crystal to have the piezoelectric effects.

        The piezoelectric material possesses the property that it can generate charges on the surface electrodes under deformation, and a mechanical strain will be induced as an electrical field is applied to a pair of electrodes.  These are the direct and converse piezoelectric effects.  This is the reciprocal relationship of the piezoelectric material, which can be expressed by the figure shown below. 

        The positive and negative sign on the piezoelectric material are the orientations of the dipoles inside the solid.  Note that the directions of the current that flow into the piezoelectric solid and the generated charges on the covered electrodes are opposite in sign with respect to the dipoles.  The piezoelectric solid extend as external voltage or tension forces are applied.  The generated charges on the electrodes and the direction of current flow current are identical, but the applied and induced voltages are opposite to each other, and vice versa. 

 

　

　

　

　

　

　

　

　

　

　

　

　

     By applying analog signal to piezoelectric transducer at the tip of IVUS catheter, a beam of acoustic can be generated, this is the converse piezoelectric effect.  As the acoustic waves bounced back from different layers of blood vessel, piezoelectric transducers deformed by these waves and generate electrical signal to decoding system, which is the direct piezoelectric effect.  This is the principle physic makes IVUS works and becomes the gold standard of diagnosis and guide for vascular diseases.

                                                                                                 Next principle of IVUS：

                                                                                                 Acoustic wave