Carr, Elizabeth; (2023) Fibre Optic Thermal Sensing for Direct Intravascular Flow Measurements: Bench-top and Pre-Clinical Studies. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The ability to measure blood flow directly and continuously is of significant interest in many medical specialities. In cardiology, these measurements can be of critical importance to determine whether coronary stenting should be performed. However, pressure measurements are frequently used in lieu of flow measurements, and this equivalency is limited in the presence of other diseases. Instead, coronary flow can be measured using cold saline thermodilution with downstream temperature measurements. Due to the manual nature of this technique, results can be unreliable, time-consuming to obtain, and user-dependent, thereby limiting broader clinical uptake. A robust flow measurement method could therefore be transformative. This work develops an all-optical intravascular flow sensor based upon a novel optical analogue to thermodilution; heating is performed optically upstream using laser pulses, and the temperature change is recorded downstream. The sensor comprises two optical fibres. The first is capped with a thermally-expansive polymer that enables temperature sensing; the second delivers diffuse laser light. The recorded temperature changes are then related to the volumetric flow rate. To understand this novel paradigm, an analytical model is developed and the simulated temperature changes are compared to experimental results. Bench-top experiments were performed to measure flow rates comparable to those found in healthy coronary arteries. Both parametric and neural network methods were used to measure the flow rates. The bench-top flow experiments were followed by a proof-of-concept pre-clinical swine study. This was, to the authors’ knowledge, the first in vivo study in which time-of-flight flow measurements were performed optically. Reproducible temperature changes were observed within the coronary arteries, with laser pulses time-gated to the ECG signal. This work concludes that optical time-of-flight measurements of coronary flow are feasible. This work represents a significant step towards a reliable and direct method for invasively measuring blood flow.

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