Imaging for Safer Surgery – Michael Ketcha’s Algorithm for Targeting the Deformed Spine

A recent paper by Michael Ketcha and coauthors at the I-STAR Lab reports a method for accurately targeting vertebrae in surgery under conditions of strong spinal deformation. Previous research showed a method by which target vertebrae defined in preoperative CT or MRI can be accurately localized in intraoperative radiographs via the “LevelCheck” algorithm for 3D-2D image registration. While LevelCheck was shown to provide accurate localization over a broad range of clinical conditions, the underlying registration model is rigid, meaning that it does not account for strong changes in spinal curvature occurring between the preoperative image and the intraoperative scene. Such deformation can be considerable, for example, in scenarios where preoperative images are acquired with the patient in a prone position, but intraoperative images are acquired with the patient laying supine – and sometimes kyphosed or lordosed to improve surgical access. Ketcha’s algorithm extends the utility of the LevelCheck algorithm to such scenarios by developing a “multi-scale” registration process – called msLevelCheck. The multi-scale method begins with a (rigid) LevelCheck initialization and proceeds in a region-of-interest pyramid to successively smaller segments and, in the final stage, about individual vertebrae. The resulting effect is a deformable transformation of vertebral labels from the preoperative 3D image to the intraoperative 2D image. Ketcha’s paper shows the algorithm to be accurate and robust in laboratory phantom studies across a broad range of spinal curvature and includes the first clinical testing of the msLevelCheck approach in images of actual spine surgery patients.

Previous research to address such deformation relies on segmentation of structures in the 3D preoperative image – a potentially time-consuming process that introduces additional workflow and potential source of error – to effect a “piece-wise rigid” registration of individual segmented structures. The msLevelCheck approach operates without such segmentation, operating instead directly on image intensities and gradients in an increasingly “local” registration through the multi-scale process to effect a global deformation of the vertebral labels. The algorithm was shown to accurately label vertebrae within a few mm of expert-defined reference labels, offering a potentially useful tool for safer spine surgery.

Read the full paper here.
M D Ketcha, T De Silva, A Uneri, M W Jacobson, J Goerres, G Kleinszig, S Vogt, J-P Wolinsky, and J H Siewerdsen, “Multi-stage 3D–2D registration for correction of anatomical deformation in image-guided spine surgery,” Phys Med Biol 62: 4604–4622 (2017).