Maneas, Efthymios; (2019) Multimodal and multiscale imaging of the human placental vasculature. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Minimally invasive fetal interventions, such as those used for therapy of twin-to- twin transfusion syndrome (TTTS), require accurate image guidance to optimise patient outcomes. Photoacoustic imaging can provide molecular contrast based on the optical absorption of the haemoglobin, and in this dissertation, it was proposed as a novel technique to image the human placental vasculature. Normal term and in utero TTTS treated placentas were imaged post-partum using two novel photoacoustic imaging systems. With PA imaging, vasculature was resolved to a depth of approximately 7 mm from the chorionic placental surface; the photocoagulated tissue provided a negative contrast and the ablation depth of the scar was visualised. Complementary imaging of the placental vasculature in a microscopic size scale was performed with a handheld incident dark field illumination video microscope in fresh and formalin-fixed term placentas. Real time visualisation of the villus tree down to the terminal villi level was achieved without any contrast injection or extensive tissue preparation. Additionally, the novel application of photoacoustic imaging to guide minimally invasive fetal interventions motivated the development of tissue-mimicking placental phantoms for bench-top system validation and for clinical training. Ideally, phantoms for this modality comprise materials with optical and acoustic properties that can be precisely and independently controlled, which are stable over time, and which are non-toxic and low-cost. Gel wax was proposed as a novel tissue-mimicking material (TMM) that satisfies these criteria, and that it can be used to represent various soft tissues and fabricate heterogeneous phantoms with structures based on patient-specific anatomy. This dissertation sets the stage for the development of miniaturised photoacoustic imaging probes for intraoperative guidance, and new methods of understanding the placental vascular anatomy in health and disease. Gel wax has strong potential to become a next generation TMM for evaluation, and standardisation of imaging systems, and for clinical training.

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