Wendler, T.; Lasser, T.; Ziegler, S. I.; Scheidhauer, K.
Freehand SPECT/US: new approach for intraoperative hybrid imaging of sentinel nodes
AIM: Development and evaluation in phantom setup of system for hybrid imaging of sentinel nodes based on freehand SPECT and 2D US. MATERIALS & METHODS: A freehand SPECT system (capable of 3D intraoperative SPECT in small body regions) was extended by a module for real-time positioning of the US probe relative to the freehand SPECT system (optical positioning system) and synchronized input of the U S video. US calibration using single wall calibration method was performed with a calibration error of less than 2mm. Optical positioning data allowed calculation of oblique plane of US in coordinates of 3D freehand SPECT imaging. The information was then used to generate a real-time fused visualization of the 2D US with overlaid freehand SPECT data (radioactivity overlaid in color on US grayscale B-mode image, similarly to Doppler images). In a first phantom experiment 3 radioactive nodes (each 250kBq, 5mm diameter spheres) and 3 n on-radioactive nodes (5mm diameter spheres) were implanted in a phantom at 1-4 cm depth (phantom size 15x15x6cm3). After freehand SPECT imaging (6min acquisition covering 4 sides, 5x5x5mm3 voxels, 20i of adapted MLEM for freehand SPECT), the phantom was scanned with US probe. The centroids of the radioactive nodes in the US plane and in the freehand SPECT images were compared in several screenshots. RESULTS: The fused visualization allowed intuitive localization of radioactive nodes and easy differentiation of non-radioactive nodes. No artifacts were present. The deviation of the centroids was below 5.2mm for all 3 radioactive nodes (average 3.4mm, SD 1.4mm). CONCLUSIONS: This preliminary experience shows that hybrid imaging is feasible with freehand SPECT/US system. Evaluation of the system in humans can be then performed as a next step in a preoperative setup. This kind of devices may open new alternatives in sentinel node biopsy in particular in complex anatomies like pelvis, abdomen and head and neck area. ACKNOWLEDGEMENTS: This research was partly funded by the DFG’s SFB-824, TUM's GSISH and %NOP% SurgicEye %NOP% GmbH.