3D transcranial ultrasound as a novel intra-operative imaging technique for DBS surgery: a feasibility study.

PURPOSE: Intra-operative image guidance during deep brain stimulation (DBS) surgery is usually avoided due to cost and overhead of intra-operative MRI and CT acquisitions. Recently, there has been interest in the community towards the usage of non-invasive transcranial ultrasound (TCUS) through the preauricular bone window. In this work, we investigate, for the first time, the feasibility of using 3D-TCUS for imaging of already implanted DBS electrodes. As a first step towards this goal, we report imaging methods and electrode localisation errors outside of the operating room on eight previously operated DBS patients.

METHODS: We evaluate the feasibility of using 3D-TCUS by registering volumes to pre-operative T1-MRI. US-MRI registration is achieved through a two-step point-based approach. First, a rough surface scan of the subjects' skin surface in 3D-TCUS space is registered to a segmented skin-surface point cloud from MRI. Next, we perform a refinement using rigid registration of multiple pairs of manually marked anatomical landmarks. We validate against post-operative CT scans which are also registered to pre-operative MRI.

RESULTS: Qualitative results are given in form of 3D reconstruction examples at 2.5 and 3.5 MHz TCUS image frequency, overlaid on pre-operative T1-MRI and post-operative CT. Quantitative evaluation is performed by reporting the accuracy of electrode tip localisation at 2.5 and 3.5 MHz after our US-MRI approach. As a baseline, we also report RMSE errors for pairs of anatomical landmarks in pre-operative MRI and 3D-TCUS.

CONCLUSION: Multiple image examples show the appearance and quality of 3D-TCUS scans, depending on the bone window. Overall accuracy of anatomic point pairs lies on the order of 3.2 mm, using our registration approach. Compared to this baseline, electrode tip localisation in 3D-TCUS has a mean accuracy on the order of 4.8 mm and a precision on the order of 2.3 mm. While insufficient at first glance, we argue why these results are promising nonetheless. Our work motivates further future work in improved TCUS scanning, advanced TCUS-MRI registration and computer-aided electrode detection in 3D-TCUS.