An electromagnetic "Tracker-in-Table" configuration for X-ray fluoroscopy and cone-beam CT-guided surgery.

PURPOSE: A novel electromagnetic tracking configuration was characterized and implemented for image-guided surgery incorporating C-arm fluoroscopy and/or cone-beam CT (CBCT). The tracker employed a field generator (FG) with an open rectangular aperture and a frame enclosure with two essentially hollow sides, yielding a design that presents little or no X-ray attenuation across the C-arm orbit. The "Window" FG (WFG) was characterized in comparison with a conventional "Aurora" FG (AFG), and a configuration in which the WFG was incorporated directly into the operating table was investigated in preclinical phantom studies.
METHOD: The geometric accuracy and field of view (FOV) of the WFG and AFG were evaluated in terms of target registration error (TRE) using an acrylic phantom on an (electromagnetic compatible) experimental bench. The WFG design was incorporated in a prototype operating table featuring a carbon fiber top beneath, which the FG could be translated for positioning under the patient. The X-ray compatibility was evaluated using a prototype mobile C-arm for fluoroscopy and CBCT in an anthropomorphic chest phantom. The susceptibility to EM field distortion associated with surgical tools (e.g., spine screws) and the C-arm itself was investigated in terms of TRE, and calibration methods were tested to provide robust image-world registration with minimal perturbation from the rotational C-arm.
RESULTS: The WFG demonstrated mean TRE of 1.28 ± 0.79 mm compared to 1.13 ± 0.72 mm for the AFG, with no statistically significant difference between the two (p = 0.32 and n = 250). The WFG exhibited a deeper field of view by ~10 cm providing an equivalent degree of geometric accuracy to a depth of z ~55 cm, compared to z ~45 cm for the AFG. Although the presence of a small number of spine screws did not degrade tracker accuracy, the mobile C-arm perturbed the electromagnetic field sufficiently to degrade TRE; however, a calibration method was identified to mitigate the effect. Specifically, the average calibration between posterior-anterior and lateral orientations of the C-arm was found to yield fairly robust registration for any C-arm pose with only a slight reduction in geometric accuracy (1.43 ± 0.31 mm in comparison with 1.28 ± 0.79 mm, p = 0.05). The WFG demonstrated reasonable X-ray compatibility, although the initial design of the window frame included suboptimal material and shape of the side bars that caused a level of streak artifacts in CBCT reconstructions. The streak artifacts were of sufficient magnitude to degrade soft-tissue visibility in CBCT but were negligible in the context of high-contrast imaging tasks (e.g., bone visualization).
CONCLUSION: The open frame of the WFG offers a potentially valuable configuration for electromagnetic trackers in image-guided surgery applications that are based on X-ray fluoroscopy and/or CBCT. The geometric accuracy and FOV are comparable to the conventional AFG and offers increased depth (z-direction) FOV. Incorporation directly within the operating table offers a streamlined implementation in which the tracker is in place but "invisible," potentially simplifying tableside logistics, avoidance of the sterile field, and compatibility with X-ray imaging.