Case study: a 3D ultrasound organ map that stays consistent when the patient moves

Summary: A reference sensor and a second coordinate transform let 2D ultrasound slices captured before and after patient movement be registered into one organ map instead of discarded.

The challenge

Clinicians building a 3D map of an internal organ sweep a handheld ultrasound probe across the body and register each 2D slice into a shared frame using a six-degree-of-freedom electromagnetic navigation system. The navigation system reports positions relative to its own transmitter, but the map must live in the patient's coordinate system. If the patient moves mid-scan, slices acquired before and after the movement no longer line up and the map is corrupted. Attaching the transmitter to the body is usually impractical.

What we did

A reference sensor on the patient tracks the body's six-degree-of-freedom pose relative to the transmitter. Two transforms are then maintained: Tbody, from body to transmitter, and Tprobe, from probe to transmitter. Every scanned point is converted from probe coordinates into body coordinates through both transforms rather than stored relative to the transmitter. While the patient is still, Tbody is constant; when the patient moves, Tbody changes and the updated transform brings pre- and post-movement coordinates into the same frame. The post describes this as a differential correction.

For the 3D tooling around such work, we compared the licensed Avizo with the open-source 3D Slicer, explored 3D Slicer's real-time capabilities and developed a real-time 3D Slicer extension that uses the platform's segmentation and tracking algorithms as the base for customer-specific algorithms, alongside CT/MRI image fusion and segmentation.

Outcome

Scans that would previously have been ruined by patient movement can be recovered and used, so a sweep does not have to be restarted. The 3D Slicer work produced a working real-time extension.

Why it matters for your project

Registration errors are a common reason image-guided systems fail in practice rather than in the lab. If your device fuses tracked 2D data into a 3D model, or you are choosing between a custom engine and a platform such as 3D Slicer for a proof of concept, this is a decision we have already worked through.

FAQ

Does this require a new navigation system? No. It adds a reference sensor and a second transform on top of a standard six-degree-of-freedom electromagnetic tracker.

Why 3D Slicer rather than a licensed platform? 3D Slicer is open source, built for medical image analysis and visualization, and suited to research and clinical use; Avizo is licensed with annual costs and more extensive documentation. The right choice depends on the project.

Contact: info@vision-elements.com