Optoacoustic tomography: time-gated measurement of pressure distributions and image reconstruction

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Abstract
Optoacoustic imaging is a potential novel medical imaging technology to image structures in turbid media to depths of several millimeters with a resolution of some tens of micrometers. Thereby short laser pulses generate thermoelastic pressure waves inside a tissue, which are detected on the surface with a wideband ultrasonic transducer. Image reconstruction has the goal of calculating the distribution of the absorbing structures in the tissue. We present a method in which the acoustic field distribution is captured as a two-dimensional snapshot at the sample surface, using an optical-reflectance-based detection principle with a detection resolution of 20 µm. A new image reconstruction is accomplished by backprojection of the detected two-dimensional pressure distributions into the sample volume by use of the delay between the laser pulse and the time the snapshot was taken. Two-dimensional pressure-wave distribution and image reconstruction are demonstrated by simulations and experiments, in which small objects are irradiated with laser pulses of 6-ns duration. The method opens the possibility to irradiate the sample hidden in a light-scattering medium directly through the detector plane, thus enabling front-surface detection of the optoacoustic signals, which is especially important if structures close to the tissue surface have to be imaged. Reconstructed tomography images with a depth resolution of 20 µm and a lateral resolution of 200 µm are presented.