Interaction of artificial metallic objects with biosusceptometric measurements

In human subjects, metallic objects cause distortions of the magnetic fields used by magnetic resonance imaging (0.5 - 3.0 T) or by SQUID biomagnetic liver susceptometry (0.1 - 30 mT) and may lead to artifacts in the measurement of the relaxation rate or the magnetic susceptibility. In biosusceptometry, the measured signal will depend not only on the magnetic susceptibility of the object, but also on its distance to the sensor assembly, and in case of ferromagnetic objects, on the direction of its remanent field. The magnetic susceptibility of a vascular access port-a-cath and of surgical clips have been measured by a SQUID biosusceptometer. Additionally, the impact from port-a-caths and dental braces on liver iron concentration (LIC) measurements was measured in vivo with respect to their radial distance from the gradiometer center axis. For the port-a-cath, a mean magnetic volume susceptibility of (83.5 +/- 0.3).10(-6) SI-units was found, which may be compared with the magnetic susceptibility of titanium at room temperature of (180 +/- 2).10(-6) SI demonstrating the absence of ferromagnetic contamination. At a radial distance of 5 cm from the gradiometer center axis, the voltage amplitude is similar to the signal generated by a normal liver. Modern surgical clips have nearly no impact on LIC measurements. However, dental braces although further away from the center axis, often superimpose the signal even from an iron overloaded liver. Depending on the Ni-content, these objects reveal ferromagnetic properties and contribute in first order with a one parameter reciprocal distance function to the measured liver iron signal.