Thoracic Cartilage Ultrasound-CT Registration using Dense Skeleton Graph

TL;DR

This paper presents a novel graph-based non-rigid registration method that accurately maps planned ultrasound paths from a CT atlas to individual patients' thoracic cartilage structures, improving US imaging guidance.

Contribution

It introduces a dense skeleton graph approach that explicitly considers subcutaneous bone features for improved registration accuracy in thoracic ultrasound applications.

Findings

Effective mapping of CT to US trajectories demonstrated

Hausdorff distance achieved was 9.48 mm on average

Path transferring error was 2.21 mm on average

Abstract

Autonomous ultrasound (US) imaging has gained increased interest recently, and it has been seen as a potential solution to overcome the limitations of free-hand US examinations, such as inter-operator variations. However, it is still challenging to accurately map planned paths from a generic atlas to individual patients, particularly for thoracic applications with high acoustic-impedance bone structures under the skin. To address this challenge, a graph-based non-rigid registration is proposed to enable transferring planned paths from the atlas to the current setup by explicitly considering subcutaneous bone surface features instead of the skin surface. To this end, the sternum and cartilage branches are segmented using a template matching to assist coarse alignment of US and CT point clouds. Afterward, a directed graph is generated based on the CT template. Then, the self-organizing map using geographical distance is successively performed twice to extract the optimal graph representations for CT and US point clouds, individually. To evaluate the proposed approach, five cartilage point clouds from distinct patients are employed. The results demonstrate that the proposed graph-based registration can effectively map trajectories from CT to the current setup for displaying US views through limited intercostal space. The non-rigid registration results in terms of Hausdorff distance (Mean$\pm$SD) is 9.48$\pm$0.27 mm and the path transferring error in terms of Euclidean distance is 2.21$\pm$1.11 mm.