Purpose

Coil embolization of cerebral aneurysms has recently been performed as a standard treatment. Periodic postoperative examination is essential after this surgery, and relatively less invasive MRI has recently been increasingly used for postoperative examination. However, new problems have been discovered with increased use of MRI. Magnetic susceptibility artifacts produced by conventional metal coils have a marked influence on the MRI findings that determine the treatment strategy, and a risk of misdiagnosis due to artifacts has been observed. Focusing on a diamagnetic metal, Au, and a paramagnetic metal, Pt, with consideration for both biocompatibility and mechanical characteristics, we investigated the composition and processing method of an alloy prepared with these metals. Our aim was to develop an alloy for coil treatment of cerebral aneurysm embolization with magnetic susceptibility equivalent to that of the human tissues and thus not produce magnetic susceptibility artifacts.

Materials and Methods

Alloys were prepared as follows: Metal pellets were placed in a zirconia crucible, heated, melted by high frequency induction heating, and casted employing tilting casting. The ingots were treated with hot forming to destroy solidified material. In addition, groove rolling was applied followed by heat treatment at 1100℃ in the atmosphere for one hour or longer. Groove rolling and heat treatment were performed two more times. Several samples with different compositions were prepared using this process with a single-phase treatment. In an artifact evaluation experiment, column-shaped (diameter 3 mm and length 8 mm) alloy samples were fixed with agarose gel in a test tube and tested (Figure 1). These experimental samples and samples prepared with the conventional PtW alloy were subjected to magnetic susceptibility measurements using SQUID Quantum Design MPMS-7 (7T-SQUID Fluxmeter, Quantum Design Japan, Tokyo, Japan) and MRI using a 1.5-T scanner (MAGNETOM Sonata, Siemens A.G., Erlangen, Germany). We developed a coil with a small-bore diameter for high-resolution for the coil receiving signals. MR images were acquired following the criteria of both the spin echo (TR:500ms, TE:20ms, pixel size:0.5×0.5mm, thickness:2mm, NEX:2) and gradient echo (TR:250 ms, TE:15 ms, pixel size:0.4×0.4mm, flip angle:30deg., thickness:2mm, NEX:2) sequence required by the US FDA standard artifact evaluation test¹. The artifact length from the sample margin was measured in the acquired images using artifact measurement software prepared by us. A coil prepared with the metal composition producing the smallest artifact and the conventional coil prepared with PtW were placed in a rabbit aneurysm model, and MRI images were acquired employing the 3D-TOF (TR:41ms, TE:9.21ms, pixel size:0.19×0.19mm, flip angle:25deg., thickness:0.8mm) sequence. The sizes of the artifacts in the images were compared.

Results and Discussion

In the in vitro experiment, the artifact produced by the newly developed alloy was smaller than that produced by the conventional alloy in images acquired by both the spin echo (Figure 2) and gradient echo sequence (Figure 3). The artifact length was correlated with the measured magnetic susceptibility of the alloy, and the artifact size decreased as the susceptibility became close to that of the surrounding tissue. Development of an alloy producing a negligible artifact was achieved. In addition, using a coil for aneurysm embolization prepared with this alloy material, magnetic susceptibility artifacts produced by the aneurysm in the body could be markedly reduced. Since small changes in the alloy preparation process may influence the magnetic susceptibility, strict control of the preparation process is important.

Conclusion

By adjusting the composition of the embolization coil in consideration of the alloy preparation method, an alloy with magnetic susceptibility equivalent to that of human tissue could be prepared that markedly reduced magnetic susceptibility artifacts. Development of MRI-compatible implantable devices for the body will be a priority.

Acknowledgements

No acknowledgement found.

References

1. ASTM F2119, Annual Book of ASTM Standards, Vol. 13.01.