Nonlinear imaging takes advantage of the localized nature of the interaction to achieve high spatial resolution, optical
sectioning, and deeper penetration in tissue. However, nonlinear contrast (other than fluorescence or harmonic
generation) is generally difficult to measure because it is overwhelmed by the large background of detected illumination
light. Especially challenging to measure is the nonlinear refractive index - accessing this quantity would allow the
extension of widely employed phase microscopy methods to the nonlinear regime. We have developed a technique to
suppress the background in these types of measurements by using femtosecond pulse shaping to encode nonlinear
interactions in background-free regions of the frequency spectrum. Using this individual pulse shaping based technique
we have been able to measure self-phase modulation (SPM) in highly scattering environments, such as biological tissue,
with very modest power levels. Using our measurement technique we have demonstrated strong intrinsic SPM signatures
of glutamate-induced neuronal activity in hippocampal brain slices. We have also extended this measurement method to
cross-phase modulation, the two-color analogue to SPM. The two-color approach dramatically improves the
measurement sensitivity by reducing undesired background and associated noise. We will describe the nonlinear phase
contrast measurement technique and report on its application for imaging neuronal activity.
The determination of the center of rotation for projection is a very important step for the establishment of a
X-ray two-dimensional computed tomography imaging system. The error of the center of rotation may lead to
artifact in computed tomography image. In this work, the current popular measurement methods for the center of
rotation are described and their advantages and disadvantages are reviewed, and a new method is thus put
forward through analysis. The proposed algorithm is based on the laws: 'A particle is scanned a circle by
computed tomography scanner, then the integral of the particle's projection address under each projection view
equals to zero' and 'The sum of the coordinates of crossing points of projection sine curves of two random
particles equals to zero'. The center of rotation for projection is determined by calculating the mean of projection
address of the X-ray beam penetrating object. Compared with the current methods, the proposed algorithm needs
no phantom and geometry parameter. The algorithm is real-time and has high measurement precision. The
feasibility of the method is validated using the experiment results.
Nonlinear microscopies (most commonly, two-photon fluorescence, second harmonic generation, and coherent
anti-Stokes Raman scattering (CARS)) have had notable successes in imaging a variety of endogenous and exogenous targets
in recent years. These methods generate light at a color different from any of the exciting laser pulses, which makes the
signal relatively easy to detect. Our work has focused on developing microscopy techniques using a wider range of
nonlinear signatures (two-photon absorption of nonfluorescent species, self phase modulation) which have some specific
advantages - for example, in recent papers we have shown that we can differentiate between different types of melanin
in pigmented lesions, image hemoglobin and its oxygenation, and measure neuronal activity. In general, these signatures
do not generate light at a different color and we rely on the advantages of femtosecond laser pulse shaping methods to
amplify the signals and make them visible (for example, using heterodyne detection of the induced signal with one of the
co-propagating laser pulses). Here we extend this work to stimulated Raman and CARS geometries. In the simplest
experiments, both colors arise from filtering a single fs laser pulse, then modulating afterwards; in other cases, we
demonstrate that spectral reshaping can retain high frequency resolution in Raman and CARS geometries with
femtosecond laser pulses.
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