Electromagnetic tracking for thermal ablation and biopsy guidance: clinical evaluation of spatial accuracy

Abstract

Purpose: To evaluate the spatial accuracy of electromagnetic needle tracking and demonstrate the feasibility of ultrasonography (US)-computed tomography (CT) fusion during CT- and US-guided biopsy and radiofrequency ablation procedures.

Materials and methods: The authors performed a 20-patient clinical trial to investigate electromagnetic needle tracking during interventional procedures. The study was approved by the institutional investigational review board, and written informed consent was obtained from all patients. Needles were positioned by using CT and US guidance. A commercial electromagnetic tracking device was used in combination with prototype internally tracked needles and custom software to record needle positions relative to previously obtained CT scans. Position tracking data were acquired to evaluate the tracking error, defined as the difference between tracked needle position and reference standard needle position on verification CT scans. Registration between tracking space and image space was obtained by using reference markers attached to the skin ("fiducials"), and different registration methods were compared. The US transducer was tracked to demonstrate the potential use of real-time US-CT fusion for imaging guidance.

Results: One patient was excluded from analysis because he was unable to follow breathing instructions during the acquisition of CT scans. Nineteen of the 20 patients were evaluable, demonstrating a basic tracking error of 5.8 mm +/- 2.6, which improved to 3.5 mm +/- 1.9 with use of nonrigid registrations that used previous internal needle positions as additional fiducials. Fusion of tracked US with CT was successful. Patient motion and distortion of the tracking system by the CT table and gantry were identified as sources of error.

Conclusions: The demonstrated spatial tracking accuracy is sufficient to display clinically relevant preprocedural imaging information during needle-based procedures. Virtual needles displayed within preprocedural images may be helpful for clandestine targets such as arterial phase enhancing liver lesions or during thermal ablations when obscuring gas is released. Electromagnetic tracking may help improve imaging guidance for interventional procedures and warrants further investigation, especially for procedures in which the outcomes are dependent on accuracy.

Trial registration: ClinicalTrials.gov NCT00102544.

Figures

Figure 1

Diagram of research work station, tracking equipment with field generator (FG), and US scanner in the CT suite for tracked needle-based interventional procedures.

Figure 2

Photograph of an interventional procedure in the CT suite. The electromagnetic field generator (FG) is mounted on an articulated arm, which is connected to the CT gantry. Both the generator and arm are covered with a sterile cover.

Figure 3

(a) S4-1 US transducer with a 6-df electromagnetic position sensor attached. (b) Tracked stylet-sheath combination. (c) Tracked three-hole needle guide.

Figure 4

Photograph of passive (white arrows) and actively tracked (black arrows) fiducials attached to the skin. The passive fiducials are 15 mm in diameter and have a central divot in which to place the needle during registration. The active fiducials are 11 mm in diameter and contain a 5-df sensor coil.

Figure 5

Screenshot of the custom navigation software shows multiplanar reconstructions (MPRs) of the preprocedural CT scan relative to the tracked needle position (top row), needle angle and position relative to the target site (bottom left), and the US-CT fusion display with variable transparency of the US scan (bottom right). In this image, the fusion display shows the US scan with high opacity, revealing only some of the rib structures of the underlying CT scan (white arrows). Figure 9 shows an enlargement of this fusion display, with a corresponding CT-only view for easier comparison with the US scan.

Figure 6

(a) MPR of a preprocedural CT scan shows the target lesion (white arrow) with the virtual needle (black arrow) superimposed on the basis of the needle’s tracking position and registration immediately before a confirmation CT scan was obtained. (b) The same MPR in the confirmation scan shows the image of the 19-gauge tracked needle (white arrow) and the superimposed virtual needle (black arrow). The thick end of the needle in the CT scan of the needle indicates the location of the integrated 9 × 1-mm sensor coil. (c) Magnified image of the needle tip corresponding to the inset in b. The corresponding tracking error is indicated as the distance between the three-dimensional positions of the virtual needle tip and needle image.

Figure 7

Flow chart of the basic and advanced registration methods.

Figure 8

Diagram of the transformation chain used to fuse the real-time US scan with the corresponding CT MPR. The two-dimensional US scan, the tracking sensor attached to the US probe, the tracking system, and the volumetric CT scan each have their own coordinate system CUS, Csensor, Ctracking, and CCT, respectively. Rigid body transformations T transform the coordinates from one coordinate system to another.

Figure 9

(a) Interventional US scan and (b) corresponding multiplanar CT reconstruction calculated and displayed in real time by using a registration based on skin fiducials only. The “virtual” needle, positioned at the edge of the lesion, is superimposed on both images, in the position given by the tracking device and registration transformation. These views can be generated by changing the US transparency in the fusion display depicted in Figure 5.

Figure 10

Chart shows the tracking error (mean ± standard deviation) as a function of the number of previous needle positions used for the advanced registration method described in the text and in Figure 7.