The accurate tracking of medical devices is a key clinical requirement that currently requires the use of ionising X-ray radiation and / or contrast agents. These essential procedures have potential long term detrimental effects, especially on babies, and also causes significant disruption (and therefore cost) due to both the need to protect staff and waiting for the availability of, or transport to, X-ray equipment. There are therefore significant clinical drivers to develop alternative tracking methods. Very recently, we have demonstrated a ground breaking approach to tracking medical devices located deep in tissues using single photon imaging [1]. Our approach exploits the fact that if a point source of light is placed inside the body, a tiny fraction of the light will emerge from the body with a close to line-of-sight path. Crucially, these line-of-sight photons (particles of light) hold precise information about the spatial location of the point source inside the tissue, but extracting this information is not trivial. The key to accessing it is the fact that the line-of-sight photons exit the body with a shorter transit time than the more diffuse photons - a fact that allows us to exploit a technique known as time-correlated single-photon counting (TCSPC) to detect and distinguish them from more diffuse photons. In contrast to "normal" cameras, which do not record the arrival time of the photons on the detector array, TCSPC-based imaging relies on using a source of light that produces short pulses of light at precisely known times, together with a single-photon sensitive detector array that can record the arrival times of individual photons. In this manner, TCSPC imaging allows us to design an imaging system that can selectively detect and image the location of the emerging line-of-sight photons before the diffuse photons start to emerge, and this allows us to locate the precise position of the source. Although we have now demonstrated the potential of this technique for medical device tracking, the clinical translation has been hampered by the low fill-factor (how much of the detector array is light-sensitive) of commercially available TCSPC detector arrays. This low fill-factor (~1%) effectively means that we lose 99% of the light reaching the detector array, limiting the maximum frame rate to ~0.05 Hz - too low to provide adequate feedback to the clinician during catheter placement. Recently, through STFC funding, we have demonstrated that so-called "photonic lantern" transitions provide a new and powerful route to addressing the low fill-factor of commercially available SPAD arrays [2]. The overarching goal of this project will therefore be to work with our commercial partners, Photon Force, to exploit this capability, and develop a TCSPC system capable of tracking catheters with video frame rates. We will then work with clinician scientists to translate the technology towards clinical exploitation by demonstrating the tracking capability using relevant models. The results of this project will then be used to support translational clinical studies, and to work with Photon Force to develop a TCSPC tracking system suitable for the medical market. [1] M. G. Tanner et al, Biomed. Opt. Express 8, 4077-4095 (2017) [2] H. K. Chandrasekharan et al. Nat. Commun. 8, 14080 (2017).