Reduced effects of spherical aberration on penetration depth under two-photon excitation

Characterizing refractive index and thickness of biological tissues using combined multiphoton microscopy and optical coherence tomography

We present a noninvasive method for characterizing the refractive index (RI) and thickness distribution in biological tissues using a combined multiphoton microscopy (MPM) and optical coherence tomography (OCT) system. Tissue layers are distinguished by the MPM and OCT images, and the RI and thickness of each layer are determined by analyzing the co-registered MPM and OCT images. The precision of this method is evaluated on four standard samples which are water, air, immersion oil and cover glass. Precision of within ~1% error compared to reference values is obtained. Biological tissue measurement is demonstrated on fish cornea. Three layers are detected, which are identified as the epithelium and stroma I and II of the cornea. The corresponding RI of each layer is measured to be ~1.446-1.448, 1.345-1.372, and 1.392-1.436, respectively. The difference of RI in the three layers correlates with the tissue compositions including cells in epithelium, large collagen fiber bundles in stroma I, and small collagen fibers in stroma II. The combined MPM/OCT technique is shown to be able to distinguish tissue layers through biochemically specific contrasts and measure RI and thickness of tissue layers at different depths.

Methanol immersion reduces spherical aberration of water dipping lenses at long wavelengths used in multi-photon laser scanning microscopy

Dipping objectives were tested for multi-photon laser scanning microscopy, since their large working distances are advantageous for thick specimens and the absence of a coverslip facilitates examination of living material. Images of fluorescent bead specimens, particularly at wavelengths greater than 850 nm showed defects consistent with spherical aberration. Substituting methanol for water as the immersion medium surrounding the beads corrected these defects and produced an increase in fluorescence signal intensity. The same immersion method was applied to two representative biological samples of fixed tissue: mouse brain labeled with FITC for tubulin and mouse gut in which the Peyer's patches were labeled with Texas Red bilosomes. Tissue morphology was well preserved by methanol immersion of both tissues; the two-photon-excited fluorescence signal was six times higher than in water and the depth of penetration of useful imaging was doubled. No modification of the microscope was needed except the provision of a ring to retain a sufficient depth of methanol for imaging.

Measurement and correction of in vivo sample aberrations employing a nonlinear guide-star in two-photon excited fluorescence microscopy

We demonstrate that sample induced aberrations can be measured in a nonlinear microscope. This uses the fact that two-photon excited fluorescence naturally produces a localized point source inside the sample: the nonlinear guide-star (NL-GS). The wavefront emitted from the NL-GS can then be recorded using a Shack-Hartmann sensor. Compensation of the recorded sample aberrations is performed by the deformable mirror in a single-step. This technique is applied to fixed and in vivo biological samples, showing, in some cases, more than one order of magnitude improvement in the total collected signal intensity.

Adaptive optics in microscopy

The imaging properties of optical microscopes are often compromised by aberrations that reduce image resolution and contrast. Adaptive optics technology has been employed in various systems to correct these aberrations and restore performance. This has required various departures from the traditional adaptive optics schemes that are used in astronomy. This review discusses the sources of aberrations, their effects and their correction with adaptive optics, particularly in confocal and two-photon microscopes. Different methods of wavefront sensing, indirect aberration measurement and aberration correction devices are discussed. Applications of adaptive optics in the related areas of optical data storage, optical tweezers and micro/nanofabrication are also reviewed.

Spatial localization of cardiac optical mapping with multiphoton excitation

Depth and radius of regions interrogated by cardiac optical mapping with a laser beam depend on photon travel inside the heart. It would be useful to limit the range of depth and radius interrogated. We modeled the effects of a condensing lens to concentrate laser light at a target depth inside the heart, and near infrared excitation to increase penetration and produce two-photon absorption. A Monte Carlo simulation that incorporated a 0.55-NA lens, and absorption and scattering of 1064- or 488-nm laser light in 3-D cardiac tissue indicated the distribution of excitation fluence inside the tissue. A subsequent simulation incorporating absorption and scattering of transmembrane voltage-sensitive fluorescence (wavelength 669 nm) indicated locations from which fluorescence photons exiting the tissue surface originated. The results indicate that mapping at depths up to 300 microm in hearts can provide significant improvement in localization over existing cardiac optical mapping. The estimated interrogation region is sufficiently small to examine cardiac events at a cellular or subcellular scale and may allow mapping at various depths in the heart.

Three-dimensional coherent transfer function for reflection confocal microscopy in the presence of refractive-index mismatch

The three-dimensional (3-D) coherent transfer function for reflection confocal microscopy of high-numerical-aperture objectives is derived and calculated in the presence of refractive-index mismatch when a laser beam is focused into a medium of refractive index different from its immersion medium. This aberrated coherent transfer function is then used to estimate the readout efficiency of 3-D data bits recorded in a thick medium. It is shown that the readout efficiency of confocal microscopy for 3-D bit data storage is decreased with the focal depth of an objective in a recording medium. However, a high readout efficiency can be maintained if the tube length of a reading objective is linearly altered to compensate for the spherical aberration caused by the refractive-index mismatch.