Electromagnetic positioning aids interventional guidance

Interventionalists still rely on some "educated guesswork" in determining the location of catheters, biopsy needles, or ablation probes in a patient's body. As a result, researchers at the National Institutes of Health have developed a "mini-global positioning system" that could improve the guidance and safety of interventional procedures.

Electromagnetic tracking has been proposed in the past, but the new technology departs from fluoroscopy-based concepts. A small field generator near the patient tracks the location of tiny electromagnetic sensors placed on biopsy needles or ablation probes. The sensors' signals are transmitted to a computer, where special software provides real-time location of the probes in correlation with preprocedural imaging.

The technology "glues" together CT or MR images or even PET data sets with the electromagnetic mapping of the region of interest. This enables accurate navigation within the patient, said coauthor Dr. Bradford J. Wood, senior clinical investigator at the NIH, who described the approach at the 2004 RSNA meeting.

Wood and colleagues used phantom models to test the accuracy of the mini-GPS. They performed semiautomatic registration of the magnetic space to preprocedural image coordinates provided by CT and gauged the margin of error between the probe tip and the target between insertions. They found that the electromagnetic tracking of RFA probes allowed for multimodality navigation within a 1.6-mm range of error. A study of electromagnetic tracking of guidewires, catheters, and biopsy needles with PET and MR imaging correlation performed on animals (JVIR 2005;16:493-505) confirmed previous results, as have ongoing tests with human subjects.

The system provides accurate navigation coordinates not only at the time of placing the needles but also when the interventionalist needs to reposition the probes. This capability can be particularly helpful when a tumor cannot be completely visualized by conventional imaging, or, as often occurs with ultrasound guidance, the tumor "gasses out" when it's being cooked, Wood said.

The technique is intended not to replace current guidance technologies but rather to be combined with imaging data that otherwise would not be available. It makes probe placement or repositioning easier, reduces procedural time and radiation exposure (when intervention takes place under CT guidance), adds options for patients who react negatively to contrast agents, and may help improve patient outcomes, he said.

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

The tracking device shows promise but needs further testing. A key factor to consider is the effect of respiratory motion artifacts on ablation targets, which phantoms cannot reproduce, said Dr. Daniel B. Brown, an assistant professor of radiology at the Mallinckrodt Institute of Radiology.

"In tumors of the liver, kidneys, and lungs, the viscera move with each respiration, so correlation may be more challenging," Brown said.