In medical imaging different techniques have been developed to gain information from inside a tissue. Optoacoustics is a method to generate tomography pictures of tissue using Q-switched laser pulses. Due to thermal and pressure confinement, a short light pulse generates a pressure distribution inside tissue, which mirrors absorbing structures and can be measured outside the tissue. Using a temporal back-projection method, the pressure distribution measured on the tissue surface allows us to gain a tomography picture of the absorbing structures inside tissue. This study presents a novel computational algorithm, which, at least in principle, yields an exact reconstruction of the absorbing structures in three-dimensional space inside the tissue. The reconstruction is based on 2D pressure distributions captured outside at different delay times. The algorithm is tested in a simulation and back-projection of pressure transients of a small absorber and a single point source.

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A Markov-mean-field model is developed to describe growth-induced polarity in singlecomponent organic crystals formed by dipolar molecules. Results of an analytical theory agree well with corresponding Monte Carlo simulations. Polarity formation is analyzed in terms of three basic energy differences, two of them resulting from the interaction of functional groups (synthons) and a third one accounting for the lateral interaction. Basic packing types are discussed with respect to polarity formation. The Markov model provides a general description of the phenomenological behavior when moving energetically from centric to polar structures. Keeping synthon interactions within reasonable limits, the range for designing lateral coupling is limited to a few kilojoules per mole for a square lattice: Between a gap of about -2 kJ/mol e ∆E ⊥ e 3-4 kJ/mol (300 K), the stochastic process of orientational disorder is either significantly increasing (centric structures) or decreasing (polar structures) polarity. Outside of these borders, orientational disorder represents only a small perturbation to centric or polar structures. With respect to the existence of structure types, the model predicts that a molecular packing where the lateral energy difference between a parallel and an antiparallel alignment of molecules is larger than about 3 kJ/mol (square lattice, 300 K) most likely results in a centric structure featuring a low level of orientational disorder. A fundamentally new behavior for the growth of polar crystals is predicted: As a consequence of the Markov model, one of the two inequivalent growth directions of a polar axis is metastable against a 180°flipping of most of the dipoles. This type of a continuous process of twinning is different from geometrical twinning. A first set of data on real crystals is given, demonstrating polarity formation in crystal structures for which the X-ray analysis has reported only centric space groups.

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We present a new display mode of the results of an Octopus visual field examination, which allows for an easy and reliable assessment of the local and diffuse components of the disturbances of a visual field. Essentially, this display consists of the cumulative distribution of the local defect values. Application to typical cases is discussed.