In this project the potential of photoacoustic imaging using optical detection in non-contact mode for clinical diagnostics will be evaluated. The novelty of our approach consists in the acquisition of photoacoustically generated data using holographic-optical methodology. By parallelizing the data acquisition, the imaging process is extremely accelerated, thus enabling for the first time an in vivo application in clinical practice.

Photoacoustic imaging is based on the emission of pressure waves, which are emitted by absorbers due to thermoelastic expansion as a result of the absorption of pulsed laser radiation. In principle, photoacoustics thus enables the imaging of absorbing structures in less strongly absorbing environments, e.g. blood vessels in the skin. To contrast vessels, a wavelength strongly absorbed by haemoglobin and slightly absorbed by the dermis is chosen. The pressure transients are usually detected with piezoelectric transducers (point scan or line array) on the skin surface and evaluated analogous to the known ultrasound image acquisition. Photoacoustic tomography is used to generate images that are not based on the usually small acoustic impedance differences, but on the much more specific optical contrast. However, high-resolution images, as shown in Fig. 1, are based on acquisition times of several minutes, so that these have so far only been achievable on anaesthetised animals and under laboratory conditions.

The difference between our approach and existing methods lies in the type of pressure wave detection. We measure the transient surface deformation of the skin initiated by excitation laser pulses using a modified holographic-optical technique (Fig. 2). For this purpose, the speckle displacement during surface movement is recorded in parallel and repetitively by a high-speed camera using a coherent double-pulse technique. This shortens the recording time by several orders of magnitude compared to other methods. From the topographic change detected over time, the absorbing structure can be reconstructed three-dimensionally tomographically using numerical time-reversal techniques.

Own publications, peer reviewed:
1. Buj C, Münter M, Schmarbeck B, Horstmann J, Hüttmann G, Brinkmann R. Noncontact holographic detection for photoacoustic tomography. Journal of biomedical optics 2017; 22(10):1-14.
2. Horstmann J, Spahr H, Buj C, Münter M, Brinkmann R. Full-field speckle interferometry for non-contact photoacoustic tomography. Physics in Medicine and Biology 2015; 60(10):4045-4058.