The radiopacity of Ni-Ti alloys: A fundamental approach

Binary Ni-Ti (Nitinol) alloys have been widely used in medical devices due to their superelastic and shape memory effects, in conjunction with their excellent biocompatibility. Despite these properties, there remain problems with the visibility of Nitinol devices during in-vivo deployment using e.g. an x-ray fluoroscope. Binary Nitinol possesses inferior radiopacity when compared to stainless steel and other major implantable metals and alloys. Our group has attempted to develop a fundamental approach towards a proper understanding of what constitutes the radiopacity of Ni-Ti. Here we calculate the radiopacity of Ni-Ti from first principles using Beer-Lamberts law of x-ray absorption, taking into account that x-ray fluoroscopy is performed with a poly-energetic x-ray under certain filtration conditions. We found that the photon distribution of the x-ray equipment, beam hardening from filtration, and the presence of absorption edges are the determining factors in defining the effective radiopacity of Ni-Ti. We then considered the addition of a ternary element to binary Ni-Ti to improve its radiopacity. The improvement of radiopacity due to the addition of Platinum, Tantalum and Tungsten are calculated using the same methods. We then compared our calculations with x-ray fluoroscopy experiments, carried out in the ambient environment and under a simulated body attenuation. Excellent agreement between the calculated and observed radiopacity was found.