The method of optoacoustics combines the specific advantages of optical tomography, which is the sensitivity to optical contrast of tissues, and ultrasound tomography, which is the large penetration depth.

The method is based on the opto-acustic effect, in which transient thermoelastic stress waves are generated by irradiation of the tissue with short laser pulses. The irradiation of the tissue with laser pulses of a few nanoseconds lead to a light distribution according to the optical properties of the tissue (fig. 1).

Beam absorption leads to a slight increase in temperature, and therefore to thermo-elastic expansion of the absorbing structure. The stress transients generated by this expansion can have frequency components up to 100~MHz (fig. 2). Due to the instantaneous stress generation this transient resembles the initial light distribution.

For their detection, piezoelectric transducers with high bandwidth were placed on the surface of the sample. From the detected temporal pressure signals, conclusions the position ofabsorbing structures as well as their optical and thermal properties can be made (fig. 3).

Fig. 4 shows a piezoelectric detector head with a ringshaped piezoelectric detector with an outer diameter of 900 µm and the fiber for irradiation in the center.

Actual focus of research is the usage of optoacoustic tomography as tool for online-monitoring of cyclophotocoagulation. This is a therapy of glaucoma, which consists of the a partly coagulation of the ciliary body by irradiation with a NIR-laser for a few seconds with 2-3 W. A destruction of this structure leads to a decreased production of aqueous humor and therefore a stable intraocular pressure. Due to the short therapeutical period, the changes induced in the tissue have to be detected in real time.

Fig. 5 shows a grayscale image of the ciliary body region of a pig eye consisting of 16 measured transients with a distance of 500 µm. Bright colors in this image belong to strongly absorbing structures, dark colors to low absorption. From this, the course of the pigmented layers surrounding the ciliary body can be can be determined, which corresponds well with the relating histological section.

For an enhanced imaging procedure a lined array of eight detectors with an overall length of less than a centimeter has been developped (fig. 6).

In fig. 7 the image of a triangular absorbing structure buried in a depth of 5 mm in the sample. The image shows only the maxima of the signals from the plane in this depth coded in gray tones. Again, bright colors indicate high absorption, dark colors low absorption. The shape of the triangle can be roughly determined. The blur of the edges is due to the large spot seperation of the single measurements.

Furthermore the changes of the tissue during the coagulation with a cw-laser diode can be detected. Fig. 8 shows transients measured before and during the coagulation of the ciliary body of a rabbit eye.