Ultrasound imaging is widely used for guiding minimally invasive procedures, including fetal surgery. Visualisation of medical devices such as medical needles is critically important and it remains challenging in many clinical contexts. During in-plane insertions, a needle can have poor visibility at steep insertion angles and at large insertion depths. During out-of-plane insertions, the needle tip can have a similar ultrasonic appearance to the needle shaft when it intersects with the ultrasound imaging plane. When the needle tip is not accurately identified, it can damage critical structures, with potentially severe consequences, including loss of pregnancy. In this paper, we present a tracking system to directly visualise the needle tip with an ultrasonic beacon. The waves transmitted by the beacon were received by an external ultrasound imaging probe. Pairs of co-registered images were acquired in rapid succession with this probe: a photoacoustic image obtained with the system in receive-only mode, and a conventional B-mode ultrasound image. The beacon comprised a custom elastomeric nanocomposite coating at the distal end of an optical fibre, which was positioned within the lumen of a commercial 22 gauge needle. Delivery of pulsed light to the coating resulted in the photoacousticgeneration of ultrasonic waves. The measured tracking accuracies in water in theaxial and lateral dimensions were 0.39±0.19 mm and 1.85±0.29 mm,respectively. To obtain a preliminary indication of the clinical potential ofthis ultrasonic needle tracking system, needle insertions were performed in anin vivo fetal sheep model. The results demonstrate thatultrasonic needle tracking with a fibre-optic transmitter is feasible in aclinically realistic fetal surgery environment, and that it could be useful toguide minimally invasive procedures by providing accurate visualisation of themedical device tip.
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