Photoacoustic imaging is a non-invasive imaging modality which allows structural, functional and molecular imaging. The method relies on the photoacoustic effect which describes conversion between light and acoustic waves due to absorption of electromagnetic waves and localized thermal excitation. The principle is depicted in figure 1: short pulses of electromagnetic radiation, mostly short laser pulses, are used to illuminate a sample. The local absorption of the light is followed by rapid heating, which subsequently leads to thermal expansion. Finally, broadband acoustic waves are generated. By recording the outgoing ultrasonic waves with adequate ultrasonic transducers outside of the sample the initial absorbed energy distribution can be recovered. Thus, photoacoustic imaging is a hybrid technique making use of optical absorption and ultrasonic wave propagation. Thereby the advantages of both techniques are combined: the high contrast of optical imaging and the high resolution of ultrasonic imaging.
Figure 1) Principle of photoacoustic imaging.
One can distinguish between two major implementations: photoacoustic tomography (PAT) and photoacoustic microscopy (PAM); both are depicted in figure 2.
In PAT a semitransparent sample is illuminated by an expanded laser beam, thus illuminating the whole sample volume (Fig 2.a). The spatial varying local absorption leads to generation of ultrasonic waves which are recorded by an ultrasonic transducer (Fig. 2b). By moving the transducer around the sample, or by using an array of transducers, a dataset of pressure curves is acquired. By using adequate reconstruction algorithms the absorption of light within the sample (= image information) can be reconstructed. The resolution of PAT is determined by the duration of the excitation laser pulse and the bandwidth of the transducers, and is typically below 100µm.
In PAM (Fig. 2c) the laser beam is focused into a rather small volume, thus launching ultrasonic waves only in this localized volume. Therefore, the axial resolution can be as good as the optical resolution, i.e. below 1µm. The depth information can be obtained by the runtime of the acoustic waves. For 3D imaging the sample is scanned in two dimensions. For high speed imaging, scanning mirrors are used to optically raster scan the sample.
Figure 2) Schematics of photoacoustic tomography (a, b) and photoacoustic microscopy (c)
Photoacoustic Imaging at the Recendt
Major developments and research highlights of the photoacoustic imaging group:
Novel integrating line detectors: Novel ultrasonic detectors for photoacoustic imaging were developed. So called “integrating line detectors” (ILD) are realized by utilizing glass-optical or polymer-optical fibers. Being part of an interferometer these fibers are used for detection of ultrasonic pressure variations. Currently, an array of these ILD which allows fast imaging is under development.
Remote photoacoustic imaging: To allow acoustic coupling between sample and transducers the examined specimen is usually immersed into a water bath or a coupling agent is used to allow propagation of the ultrasonic waves. This can be a severe limitation for industrial applications, or for medical applications where coupling agents are prohibited, e.g. when examining open wounds or during brain surgery. Therefore, remote photoacoustic imaging (rPAI) was developed, which uses optical means to measure ultrasonic waves directly on the specimen surface without the need for a coupling agent.
Femtosecond photoacoustic microscopy: Using an excitation laser with ultra-short pulses and high repetition rate enables high-speed imaging at high resolution. The goal of the ongoing project is real-time photoacoustic imaging with high resolution (~1x1x1µm³). The photoacoustic microscope will be combined with a multi-photon microscope to allow multimodal imaging.
Image reconstruction and theoretical work: Besides experimental work the photoacoustic imaging group works on the development and enhancement of image reconstruction algorithms. These algorithms allow, e.g., reconstruction for acoustic heterogeneous specimens (e.g. tissue including bone). Further works concern the characterization and compensation of ultrasonic attenuation, which allows better imaging quality and higher resolution. Theoretical studies deal with optimal detector geometries by taking stochastic processes into account.
Find more information about this technology in our project sheet!
Dipl.-Ing. Dr. Thomas Berer
Tel.: +43 (732) 2468 - 4650