Real-time navigation device promises increased precision during interventions

A "mini-global positioning system" could improve the conventional guidance of radiofrequency ablation and the safety of image-guided interventional procedures.

The system, composed of tiny electromagnetic sensors placed on single and cluster RFA probes, functions within an electromagnetic field. The sensors' signals are interpreted by a computer, whose software identifies and tracks the real-time locations of the probes in the context of a 3D model based on imaging data obtained prior to the procedure.

The technology "glues" together CT, PET, or MR imaging data sets with the electromagnetic mapping of the region of interest, allowing accurate navigation within the patient, according to Dr. Bradford J. Wood, an interventional radiologist in the Imaging Sciences Program at the National Institutes of Health Clinical Center.

"Whereas once catheters, needles, and guidewires were followed with x-ray, ultrasound, or CT, now they can be followed with multiple modalities and updated in real-time without actually exposing the patient to (additional) radiation," Wood said.

His research interests focus on image-guided oncology and robotics.

The NIH filed a U.S. Provisional Patent Application on the device in November 2004. A month later, the NIH published the device's availability for licensing in the federal government's official mouthpiece, the Federal Register. The application describes the tracking system as well as an ablation device coupled to it.

Funding for further development might be obtained under a Cooperative Research and Development Agreement administered by the NIH Office of Technology Transfer. The CRADA could involve collaborative research with the inventors.

Wood and colleagues initially tested the accuracy of the device using phantom models. They performed semi-automatic registration of the magnetic space to preprocedural image coordinates provided by CT, gauging the margin of error of probe tip to target between insertions. They found that the electromagnetic tracking of RFA probes allowed for multimodality navigation within a 1.6-mm range of error. Recent tests on animals and three patients confirmed the early results.

The mini-GPS provides accurate navigation coordinates at the time the probes are placed and also when the interventionalist needs to reposition them, according to Wood. This ability can be particularly helpful when a tumor cannot be completely visualized. This often happens under ultrasound guidance, when the tumor is irradiated with RF, because tissue, burned during thermal ablation, releases gas that obscures the real-time ultrasound image.

Electromagnetic tracking allows accurate multimodality navigation, adding 3D information that otherwise would not be available during RFA, Wood said. Accurate probe placement or repositioning reduces the time needed to complete the procedure and cuts radiation exposure to the patient when intervention takes place under CT guidance.

It has the potential to improve patient outcomes, although proving such a benefit will take time, Wood said.

Wood noted that electromagnetic tracking is not intended to replace current guidance technologies but to enhance and facilitate procedures. It can add further options for patients who may react negatively to contrast agents.

"This just adds more meaning to the term 'image-guided' for image-guided therapy," he said.