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Ballistic photons reveal endoscope location during surgery

05 Sep 2017

Edinburgh project could help doctors follow progress of probes within the human body.

Endoscopes are valuable tools for both diagnosis and surgery, but keeping track of exactly where the probe is located within the patient's body can present difficulties, especially as advances in endoscope design enable them to reach anatomical regions that were previously out of reach.

The exact position of the endoscope's illuminated distal tip can be estimated from its starting point using a surgeon's good judgment, but a more precise method would be preferable.

A project at Heriot-Watt University and the University of Edinburgh, carried out as part of the Proteus Interdisciplinary Research Collaboration, may offer a solution.

It found that the location of a fiber-optic endoscope can be observed by using the small fraction of photons from the endoscope tip that escape the tissue without experiencing scattering - called ballistic photons - and those that are only scattered slightly, known as snake photons. The research was published in Biomedical Optics Express.

The breakthrough employs a time-resolved single-photon detector array, sensitive enough to detect both ballistic and snake photons, and also record the time taken for the photons to pass through the tissues. This enables the photons of interest to be separated from those that are significantly scattered, and the exact location of the endoscope to be calculated.

Tests on tissue models of birds and the lungs of sheep confirmed that the position of the endoscope tip could be imaged to a much higher degree of precision than was otherwise possible, although the team noted that the timing statistics - rather than just the signal amplitude - were important in determining the least-scattered photon arrivals.

The human body represents a more stringent test than animal models, but trials using the human hand and torso found that the technique was able to locate the endoscope tip in those scenarios too, despite the lower signal to noise ratio.

Interdisciplinary approach
"The success of this technique depends critically on counting sufficient photons exiting the tissue to provide good photon statistics," noted the team in its published paper. "Unsurprisingly, as larger or denser biological models are used, greater photon scattering is introduced, adding greater delay to the most scattered photons and offering a clearer demonstration of the advantages of a time-resolved approach."

A wavelength of 785 nm was used in this study, selected in order to reduce tissue absorption and avoid overlap with ambient light sources. However, a different wavelength may ultimately enable the signal to noise performance to be enhanced further, and allow deeper imaging depths.

The technique can at present locate the endoscope tip to approximately centimeter accuracy, as compared to the millimeter positional accuracy currently possible when wired coils in medical devices are detected by electromagnetic systems. However, the new technique's compatibility with existing optical endomicroscopy fibers makes it a potentially valuable technique for clinicians, and the Edinburgh team now plans to investigate the limits of the accuracy it can deliver.

"My favorite element of this work was the ability to work with clinicians to understand a practical healthcare challenge, then tailor advanced technologies and principles that would not normally make it out of a physics lab to solve real problems," commented Michael Tanner of Heriot-Watt. "I hope we can continue this interdisciplinary approach to make a real difference in healthcare technology."

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