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Fibre optic sensors are well suited to measuring fluid flow in many contexts, and recently there has been burgeoning interest in their application to direct, invasive measurement of blood flow within human vasculature. Depending on the sensing method used and assumptions made, these intravascular measurements of blood flow can provide information about local blood velocity, volumetric flow, and flow-derived parameters. Fibre optic sensors can be readily integrated into medical devices, which are positioned into arteries and veins to obtain measurements that are inaccessible or cumbersome using non-invasive imaging modalities. Measurements of flow within coronary arteries is a particularly promising application of fibre optic sensing; recent studies have demonstrated the clinical utility of certain flow-based parameters, such as the coronary flow reserve (CFR) and the index of microcirculatory resistance (IMR). In this review, research and development of fibre optic flow sensors relevant to intravascular flow measurements are reviewed, with a particular focus on biomedical clinical translation.

We thank Golomb et al. ([1][1]) for their timely publication on race and percutaneous coronary intervention outcomes. As the studies included were international, it is important to distinguish what percentage were black Americans versus Africans, as they have diverging risk profiles ([2][2]).

In current-generation intracoronary (IC) imaging devices, ultrasound (US) is generated and received electrically, using piezoelectric transducers. With this technology it has been challenging to achieve both high resolution and high imaging depths, and also to provide molecular contrast. All-optical