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

Optoacoustic imaging is a potential novel medical imaging technology to ima ge structures in turbid media to depths of several millimeters with a resol ution of some tens of micrometers. Thereby short laser pulses generate ther moelastic pressure waves inside a tissue, which are detected on the surface with a wideband ultrasonic transducer. Image reconstruction has the goal o f calculating the distribution of the absorbing structures in the tissue. W e present a method in which the acoustic field distribution is captured as a two-dimensional snapshot at the sample surface, using an optical-reflecta nce-based detection principle with a detection resolution of 20 mum. A new image reconstruction is accomplished by backprojection of the detected two- dimensional pressure distributions into the sample volume by use of the del ay between the laser pulse and the time the snapshot was taken. Two-dimensi onal pressure-wave distribution and image reconstruction are demonstrated b y simulations and experiments, in which small objects are irradiated with l aser pulses of 6-ns duration. The method opens the possibility to irradiate the sample hidden in a light-scattering medium directly through the detect or plane, thus enabling front-surface detection of the optoacoustic signals , which is especially important if structures close to the tissue surface h ave to be imaged. Reconstructed tomography images with a depth resolution o f 20 Lm and a lateral resolution of 200 mum are presented. (C) 2001 Optical Society of America.