Cerebral aneurysm clips often cause artifacts and image distortion in magnetic resonance imaging (MRI) and computed tomography (CT). For MRI with rather strong magnetic fields B, which are near 2 tesla currently and may go higher, there are further concerns in terms of the interactions between the clips and the magnetic field. These interactions are materials dependent. Ferromagnetic materials have extremely high magnetic susceptibility χ and exhibit magnetic hysteresis. When they are placed in a magnetic field not parallel with the magnetization, a torque would be exerted on them to cause deflection. Paramagnetic materials exhibit low magnetic susceptibility, and develop an induced magnetization in direct proportion to the applied field. This magnetization is always in the direction of the applied field, hence no torque would be expected. However, magnetic force always prevails as F = mχB(dB/dz), where m is the clip mass and dB/dz is the field gradient.
Large forces can lead to clip movement, resulting in more risk to the patient. To reduce these complications, new aneurysm clips made of light-weight titanium metal have been developed. These clips have much-improved characteristics over the cobalt-alloy products currently being used in neurosurgery. They have significantly smaller magnetic susceptibility and lower density. Consequently, they are subjected to less magnetic force and produce much reduced artifact and image distortion in MRI and CT environments. Test results also include those from scanning electron micrographic and mechanical analyses.
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