Wellcome / EPSRC Centre for Interventional and Surgical Sciences


Development of ultrasensitive sensor arrays for deep tissue photoacoustic imaging

Photoacoustic imaging is a novel hybrid imaging modality that relies on the generation of ultrasound waves by the absorption of short laser pulses in biological tissue. Its fundamental advantage derives from the fact that it encodes tissue optical absorption on to ultrasonic waves which are minimally scattered in soft tissues. It thus provides both the high spectrally selective contrast of optical imaging techniques and the high resolution of ultrasound. Photoacoustic imaging is particularly well suited to visualising vascular anatomy due to the strong absorption exhibited by haemoglobin. As a consequence, it provides significantly higher label-free vascular contrast than existing imaging modalities such as ultrasound. This offers new opportunities for delineating tumour margins to aid cancer treatment planning, identifying major blood vessels to help guide fetal or laparoscopic surgery and monitoring minimally invasive ablative therapies used in cardiovascular medicine.

Whilst the advances in PA imaging are encouraging, there exist several significant instrumentation related challenges. One of these relates to the detection of PA signals. Piezoelectric receivers are most widely used but have several shortcomings. Achieving the high sensitivity required to detect the extremely weak PA signals generated at cm scale depths in tissue requires large element sizes and resonant material compositions leading to narrow directivity and poor frequency response characteristics. These factors negatively impact on image quality by introducing artefacts, blurring and distortion. 

Optical ultrasound sensors offer an alternative that can address these limitations. One method that has shown particular promise is the use of a polymer film Fabry Perot (FP) etalon as an ultrasound sensor, an approach pioneered at UCL. This can provide exquisite image quality, which is a consequence of the very small effective acoustic element size (<50μm) and uniform broadband frequency response it provides. However, limited sensitivity makes it challenging to achieve penetration depths beyond 5-10mm. This is insufficient for a number of important potential clinical applications such as visualising tumour margins deep within the liver to guide surgical excision, identifying non superficial tumours within the breast to aid pre-treatment planning or assessing cancerous nodes in the neck in some patient groups. 

The aim of the project is to address these limitations by developing a novel instrument that exploits a new type of ultrasound sensor based on an optical microresonator. By virtue of an extremely high Q factor, this type of sensor offers the prospect of two orders of magnitude higher sensitivity than the FP etalon sensor enabling penetration of depths of 2-3cm to be achieved. This would represent a step change in PA imaging performance and in doing so pave the way for in vivo high resolution human imaging at depths currently unattainable thereby extending clinical applicability. The project will involve the fabrication of novel polymer optical microresonator sensors, the development of advanced parallelised optical read-out schemes for real-time image acquisition and engineering a prototype imaging instrument for use in clinical studies.

The project is largely experimental and will suit students interested in working with optical sensors, pulsed and CW lasers, fibre optics, ultrasonic devices, electronic control and time-resolved optical readout systems and a desire to see their work translated to practical application in medicine.


Professor Paul Beard

Dr Andrew Plumb

Dr James Guggenheim