A shielded Overhauser marker for MR tracking of interventional devices

Improvements to an active MR tracking technique are described. Real-time position monitoring of interventional procedures can be realized by incorporating a small marker that emits an NMR signal into the tip of an interventional device, and the marker's emitted NMR signal is enhanced by use of the Overhauser phenomenon. A significant advance over prior designs has achieved by making the marker have a cylindrical shape and by confining the saturation energy to the marker's interior. The performance of the improved active marker was verified in the laboratory and in vitro. The experiments demonstrated that the marker was visible in MR images when inserted in different excised tissues, and even in air, with positive contrast and with various imaging sequences. The tissue magnetization was minimally perturbed, and the marker emitted a variable but enhanced signal in all orientations in the magnetic field. The marker can potentially be used to mark locations on the body for frameless stereotaxy or to identify inserted devices.