Development of electro-optically tuneable Fabry-Pérot ultrasound sensors for high speed biomedical photoacoustic imaging

• Applicants: Professor Jan Laufer, Ph.D.; Professor Dr. Sigurd Schrader
• Subject Area: Medical Physics, Biomedical Technology
• Term: from 2015 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 283368314

Photoacoustic (PA) imaging is an emerging biomedical imaging modality that relies on the absorption of short optical pulses to generate ultrasound waves within the tissue. These waves propagate to the skin where time-resolved PA signals are detected by transducer arrays. High resolution (tens of microns) 3-D images are then obtained using image reconstruction algorithms. PA imaging combines a number of powerful attributes, such as multiscale imaging capabilities ranging from single cell resolution (using PA microscopy) to micron resolution using PA tomography (tens of microns at mm depths to hundreds of microns at cm depths) and strong contrast in vascularised soft tissues where other modalities such as MRI, x-ray CT, and ultrasound lack sensitivity. It is non-invasive and combines the advantages of purely optical imaging modalities, i.e. spectral specificity, with those of conventional ultrasound imaging, i.e. high spatial resolution. While piezoelectric ultrasound detectors are the most widely used, they have distinct drawbacks when applied to superficial, high resolution PA imaging since the required small active element size results in low acoustic sensitivity. By contrast, optical ultrasound detectors such as Fabry-Pérot interferometer (FPI) sensors have been shown to provide very small, diffraction-limited element sizes, high acoustic sensitivity, near-uniform frequency response (dc-100MHz), and optical transparency for efficient backward mode PA imaging. The all-optical PA scanner based on the FPI sensor has arguably set the standard for high resolution, 3-D imaging to cm depths. Its imaging performance has enabled the acquisition of compelling images of the vasculature in the brain, the skin, and tumours in preclinical studies. However, the current design of the scanner has one significant limitation: low imaging speed compared to state-of-the-art scanners based on piezoelectric detector arrays. This project aims to overcome this limitation by developing electro-optically tuneable FPI sensors for fast biomedical PA imaging by meeting the following objectives: 1) development and synthesis of electro-optically (EO) tuneable polymer spacers, 2) development of methods for the fabrication of EOFPI sensors, 3) instrumentation development for EOFPI control and parallelised readout, and 4) demonstration of high frame rate PA imaging using parallelised detection. This is expected to provide a step change in imaging speed achievable with the FPI sensing concept. Ultimately, this technology could provide real time volumetric, high resolution PA imaging, which would open the door to a broad range of preclinical and clinical applications, such as neuro-functional imaging and blood flow imaging. In this project, the aim is to demonstrate the proof-of-principle of EOFPI-based PA imaging and its attendant increase in imaging speed.

• DFG Programme: Research Grants