Coherence Gated Doppler Blood Flow Sensor

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Researchers at the University of Maryland have developed a low-cost fibre sensor for detecting blood flow. It has a diameter of just 125 microns, meaning it could be incorporated into a surgical instrument to alert the operator that they are approaching a blood vessel. It combines features of laser Doppler flowmetry and low coherence interferometry to provide a measurement with high spatial localisation, but without many of the complications of a full Doppler optical coherence tomography (OCT) system.

The device was reported in the open access journal Biomedical Optics Express1. At its core it’s similar to a time domain OCT system, but with modifications to make it more suited to this task. Most obviously, there’s no lateral scanning – the probe itself is simply a single mode fibre coupled to a GRIN fibre for focusing. And whereas an OCT imaging system would typically have a depth resolution of around 10 microns, the resolution of this sensor is around 190 microns. This is necessary because there’s no depth scanning, so what we would normally think of as the ‘resolution’ is really the probing range.

Time domain OCT systems need some form of phase modulation so that the interference pattern can be detected over the constant ‘d.c.’ signal. This device doesn’t have any way of generating phase shifts, so it can’t ‘see’ static objects. The phase modulation is instead generated by moving scatterers. As in Doppler OCT, the frequency of the interference pattern depends on the velocity of these scatterers. But only the component of the velocity parallel to the probe’s axis is measured, so quantitative measurements of the true velocity can’t be made.

The team tested their device on a capillary phantom, as well as on rat femoral vessels and a live sheep’s brain. They were able to distinguish between the response from arteries and veins, as well as identifying bulk motion of the tissue or probe. The signal can easily be converted to sound, which may make the device more practical as a surgical tool. Having said that, more work will be needed to determine the sensitivity and specificity of the measurement before its potential applications can really be assessed.

References

  1. C. Liang, Y. Wu, J. Schmitt, P. Bigeleisen, J. Slavin, M. Jafri, C. Tang, and Y. Chen, “Coherence-gated Doppler: a fiber sensor for precise localization of blood flow,” Biomed. Opt. Express  4, 760-771 (2013).

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