Surface Referencing Against Deformable Soft Bodies: A low-cost, optical based solution for positional tracking of medical device contact along human skin.

Introduction

Purpose: demonstrate feasibility of an optical positioning system based on surface referencing of a deformable soft body Application: handheld (radiometer) scanner for breast cancer screening need a method locating the point of contact on the skin Requirements:

• low cost: <$400
• good accuracy: +/- 0.5mm
• real-time

Radiometer vs Existing Scanners

Standard approaches — X-ray mammography and ultrasound imaging — produce images of internal tissue structure. Radiometer-based scanner only produces a heat map and no discernible ‘landmarks’. Localization is necessary for biopsy.

X-ray Mammogram

Image of internal tissue structure can be used for localization of wire or needle for biopsy

Ultrasound

Image of internal tissue structure can be used for localization of wire or needle for biopsy

Radiometer

Heat map image does not produce discernible information for localization.

Motivation for Radiometer Approach

Keypoints for answering some FAQs

Previous Localization Approaches

Previous approaches produced noisy/unstable localization as well as inadequate accuracy.

Inertial Measurement Unit

• significant measurement drift
• Magnetometer compensator is susceptible to interference with nearby metal

Structured Light 3D Scan

• From a 3rd person perspective, this method can produce a 3D scan & rendering of the entire scene.
• Unfortunately, the breast is not a rigid structure so a 3D snapshot is not a reliable solution.

Resistive Film

A resistive film is applied to the skin surface. Device contact happens on the film. Conformal to skin surface. Localization is adaptive to skin deformation. However, measurement is easily skewed by additional contact points (e.g. hand of user). Difficult to achieve high accuracy.

Insights from Previous Attempts

Previous attempts were revealing as to how to design a better position tracking system.

Adaptive

Measurement approach should be adaptive to elastic nature of skin

Vision Based

Vision-based approach achieves high-resolution at low cost

Geometry

Leverage physical proportions to achieve “high gain” measurement

Concept: Camera On-Board

Observing key lessons learned from previous attempts, I produced concept sketches of a camera-on-board solution. Camera is mounted directly onto the device, and looks down on fiducial markers along the surface on which the device sits. Direct referencing: Close coupling between essential parts of the system results in high robustness. High gain geometry: Small physical perturbations correspond to large pixel movement.

Concept sketch for a camera-on board system

Interpolating between multiple fiducial markers on the skin surface enables position estimation along the surface even under strain.

Simple interpolation technique

Components

Custom made components for the purpose of demonstrating the experimental system.

Breast Phantom

Breast phantom is cast from gelatin with a similar formulation as ballistic gel — giving an elastic behavior analogous to human flesh.

Device, Camera, and Fiducial Markers

Shown is a carefully dimensioned wooden block, representing the radiometer device. A camera is mounted on top as the observer. In the foreground are various sizes of fiducial markers. The smallest of which are used on the breast phantom.

System in Action

The system fully assembled and in action. Left: view of the system from a 3rd person perspective. Right: perspective from the on- board camera.

Direct Referencing

Mounting camera on-board produces position measurements that are closely coupled between device body & patient skin. Small physical perturbations correspond to large pixel movement.

Low cost; High resolution

Even low-cost webcams are capable of sub-millimeter measurements when geometry of the system is appropriately leveraged. Cost of webcam, Logitech C920: $60

Results

Purpose: demonstrate feasibility of an optical positioning system based on surface referencing of a deformable soft body

Requirements:
• low cost: <$400
• good accuracy: +/- 0.5mm
• real-time

Results:
• cost of demo: $80
• accuracy: +/- 0.5mm can be significantly improved by further tuning
• real-time: fiducial markers require minimal run-time computation
• works on a deformable surface
• low cost of integration
• total time of demo completion: 4hrs

Appendix

A few tidbits for the observant audience.

Additional Markers

Additional markers were placed on top of the device as one of the ways to extract ground truth in the experimental system.

Rigid Foam Form

In demonstration of how the system captures the unperturbed skin surface, a rigid foam form was used because it has a simple geometry which lends well to developing a ground truth model.

Related Work

Here is a product from 2009, by Anoto. The pen uses a small on-board camera to track fiducial markers on special paper in order to record handwriting.

Related Work, continued

The digital pen demonstrates the feasibility of miniaturization for the experimental method used in this project.