Localizing spherical fiducials in C-arm based cone-beam CT

C-arm based cone-beam CT (CBCT) imaging enables the in situ acquisition of three dimensional images. In the context of image-guided interventions, this technology potentially reduces the complexity of a procedure's workflow. Instead of acquiring the preoperative volumetric images in a separate location and transferring the patient to the interventional suite, both imaging and intervention are carried out in the same location. A key component in image-guided interventions is image to patient registration. The most common registration approach, in clinical use, is based on fiducial markers placed on the patient's skin which are then localized in the volumetric image and in the interventional environment. When using C-arm CBCT, this registration approach is challenging as in many cases the small size of the volumetric reconstruction cannot include both the skin fiducials and the organ of interest.

In this article the author shows that fiducial localization outside the reconstructed volume is possible if the projection images from which the reconstruction was obtained are available. By replacing direct fiducial localization in the volumetric images with localization in the projection images, the author obtains the fiducial coordinates in the volume's coordinate system even when the fiducials are outside the reconstructed region.

The approach was evaluated using two types of spherical fiducials, clinically used 4 mm diameter markers and a custom phantom embedded with 6 mm diameter markers that is part of a commercial navigation system. In all cases, the method localized all fiducials, including those that were outside the reconstructed volume. The method's mean (std) localization error as evaluated using fiducials that were directly localized in the CBCT reconstruction was 0.55 (0.22) mm for the 4 mm markers and 0.51(0.18) mm for the 6 mm markers.

Based on the evaluations the author concludes that the proposed localization approach is sufficiently accurate to augment or replace direct volumetric fiducial localization for thoracic-abdominal interventions. This allows the physician to position fiducials in a more flexible manner, relaxing the requirement that both the organ of interest and skin surface be contained in the volumetric reconstruction.