Imaging three-dimensional biological

For the application of QDs to three-dimensional biological imaging, a large two-photon absorption cross section is required to avoid cell damage by light irradiation. For application to optoelectronics, QDs should have a large nonlinear refractive index as well as fast response. Two-photon absorption and the optical Kerr effect of QDs are third-order nonlinear optical effects, which can be evaluated from the third-order nonlinear susceptibility, or the nonlinear refractive index, y, and the nonlinear absorption coefficient, p. Experimentally, third-order nonlinear optical parameters have been examined by four-wave mixing and Z-scan experiments. 

Chen, H., Sedat. J. W., and Agard. D. A. (1989). Manipulation, display, and analysis of three-dimensional biological images. In Handbtwk of Biological Confocal Microscopy (J. Pawley, ed.), 1st ed., pp. 127-135. IMR Press. Madison, Wl. 

Protein Data Bank (Section 27 20) A central repository in which crystallographic coordinates for biological mole cules especially proteins are stored The data are accessi ble via the Worldwide Web and can be transformed into three dimensional images with appropriate molecular modeling software... 

Diaspro A, Robello M (2000) Two-photon excitation of fluorescence for three-dimensional optical imaging of biological structures. J Photochem Photobiol B 55 1-8... 

Noise can be also introduced by biochemical heterogeneity of the specimen. This can be a major cause of uncertainty in biological imaging. The high (three-dimensional) spatial resolution of fluorescence microscopy results in low numbers of fluorophores in the detection volume. In a typical biological sample, the number of fluorophores in the detection volume can be as low as 2-3 fluorophores for a confocal microscope equipped with a high NA objective at a fluorescent dye concentration of 100 nM. This introduces another source of noise for imaging applications, chemical or molecular noise, related to the inherent randomness of diffusion and the interaction of molecules. 

At present, most PET scanners can acquire in both a two-dimensional as well as a three-dimensional mode, whereas SPECT cameras measure in a three-dimensional mode. The physical property of the dual-positron gamma-rays emission lends itself to mathematical reconstruction algorithms to produce three-dimensional images in which the calculations are much closer to exact theoretical ones than those of SPECT. This is, in part, due to the two-photon as opposed to single-photon approach. PET can now achieve resolutions, for example in animal-dedicated scanners, in the order of 1 or 2 mm. The resolution is inherently limited theoretically only by the mean free path or distance in which the positron travels before it annihilates with an electron, e.g. those in biological water 2-8 mm. SPECT, although achieving millimeter resolution with the appropriate instrumentation, cannot quite achieve these levels. 

Campagnola, P. J., Millard, A. C., Terasaki, M., Hoppe, P. E., Malone, C. J., and Mohler, W. A. 2002. Three-dimensional high-resolution second-harmonic generation imaging of endogenous structural proteins in biological tissues. Biophys. J. 82 493-508. 

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