Why Mixed Media Markers?

Fiducial markers are useful to aid in patient positioning prior to radiation treatment.  Since organs like the prostate, lungs, and abdominal organs can move independent of the patient during treatment, implanted fiducial markers act as an internal guide to ensure the target organ is in the intended location for treatment. Another useful tool in radiation treatment targeting is MRI-CT image fusion.  Due to the enhanced visibility of soft tissue in an MRI image, these images can be fused with the CT image (used for computing the actual dosimetric treatment plan) to ensure visibility of all or most of the affected tissue.

It is useful to have fiducial markers that are visible in both the MRI image and the CT image to help facilitate accurate registration of these image sets; however, this poses a conundrum.  A CT scan relies on contrasting densities to render an image.  Therefore, an optimal marker for CT visibility will have a relatively high-density, biocompatible element (e.g. Gold) to provide contrast against the comparatively low density background patient tissue. However, the element cannot be so dense that it overly artifacts causing potentially adverse effects in treatment planning.

In contrast, an MRI relies on the response of photons in the body to a pulsed RF current in a background magnetic field. Therefore, optimal fiducial markers for MRI visibility contain elements with a magnetic susceptibility that is comparatively high relative to that of the background body tissue, which gold does not have.