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Optical fibre based endoscopes are increasingly used for imaging and sensing within the human body without navigational guidance of the miniaturised fibre probe. Meanwhile, other medical device placement is a standard procedure in clinic. We demonstrate successful imaging of optical device location with centimetre resolution in clinically relevant models, in a realistically lit environment, achieved through the detection of early arriving photons with a time resolved single photon detector array. This prototype has been developed within the UK-EPSRC Proteus project, moving advanced research technologies towards clinical implementation.
Short (~100ps) laser pulses are transmitted from the tip of the endoscope at 785nm in the “optical window” where attenuation is less severe in clinical scenarios. Most of the photons that pass through tissue undergo much scattering from the disordered tissue structures providing only low accuracy determination of the location of the light source. However, some photons probabilistically undergo less scattering, travelling through the medium in an almost straight line without a much extended path. Such photons exit the body sooner than the highly scattered light.
A camera based upon a 32 × 32 array of Single Photon Avalanche Diodes (SPADs) made with CMOS technology is used to image the small number photons exiting the tissue. The time resolution capabilities of such a single photon detector (50ps time bin resolution, 200ps jitter) allow observation of the photon arrival times simultaneously for all 1024 pixels of the imaging array. Photon arrival statistics distinguish the early arriving photons from the highly scattered light, revealing the endoscope location. Scattered photon arrivals peak at delays of multiple nanoseconds due to the thick tissue samples. The progression of light through complex scattering structures can be observed.
Normal fluorescent room lighting has distinct emission peaks. Appropriate choice of operating wavelength between these spectral features, combined with aggressive filtering, allows operation in normal fluorescent lighting. This compact packaged system is demonstrated in a normally lit room to determine optical endomicroscope location in a whole ventilated ovine lung as well as tissue models including bone structure. At the limit of capabilities of this prototype, demonstration through an entire human torso is shown to be possible.
System improvements and the potential of the next generation prototype in development will be discussed. This offers the potential for real time (sub second) imaging of device location with a portable system for application in standard medical procedures, such as catheter insertion. The avoidance of the need to confirm device placement with X-ray imaging has potential to decrease disruption to procedures throughout clinical practice.
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