Masato Ohmi

In vitro simultaneous measurement of refractive index and thickness of biological tissue by the low coherence interferometry

The authors proposed and demonstrated in vitro simultaneous measurement of refractive index and thickness of biological tissue. The technique is based on the low coherence interferometry combined with precise translation stages. Refractive indices were determined with the accuracy of less than 1% for tissue samples of a few hundred micron thickness, including chicken tissue, human tooth and nail. Simultaneous measurement of refractive index and thickness of multilayer tissue are also demonstrated.

Low-coherence interferometer system for the simultaneous measurement of refractive index and thickness

We have developed a low-coherence interferometer system used for the simultaneous measurement of refractive index n and thickness t of transparent plates. Both the phase index n(p) and group index n(g) can be determined automatically in a wide thickness range of from 10 microm to a few millimeters. Two unique techniques are presented to measure n(p), n(g), and t simultaneously. One allows us to determine n(p), n(g), and t accurately by using a special sample holder, in which the measurement accuracy is 0.3% for the thickness t above 0.1 mm. In the other technique the chromatic dispersion delta n of index is approximately expressed as a function of (n(p) - 1) on the basis of measured values of n(p) and n(g) for a variety of materials, and then the simultaneous measurement is performed with a normal sample holder. In addition, a measurement accuracy of less than 1% is achieved even when the sample is as thin as 20 microm. The measurement time is also 3 min or more.

High-speed simultaneous measurement of refractive index and thickness of transparent plates by low-coherence interferometry and confocal optics

We propose and demonstrate a novel measurement technique for determination of n and t, where the measurement time is only 1 s or less for a thickness of <1 mm. Such a high-speed measurement is successfully achieved by simultaneous detection of coherence-gate and confocal signals with a single scanning of the sample. A measurement accuracy of 0.3% or less was successfully attained for a thickness of <1 mm. A multi-point measurement result is also presented for the index and thickness distributions of a radial-graded-index rod lens.

Dynamic observation of sweat glands of human finger tip using all-optical-fiber high-speed optical coherence tomography

High-speed optical coherence tomography (OCT) makes it possible to perform a time-sequential imaging of biological tissue and small organs. In this paper, we demonstrate in vivo observation of dynamics of sweat glands of human finger tip using high-speed OCT with push–pull driven fiber-optic PZT phase modulators. Movement of a sweat droplet through a micro spiral duct can be tracked clearly. An interesting function of sweat glands is found out in time-sequential OCT imaging.

Imaging of in vitro chicken leg using time-resolved near-infrared optical tomography

Near-infrared optical imaging gains much attention because of its noninvasiveness and deep penetration depths into tissue. Here, we report near-infrared optical tomographic imaging of an in vitro chicken leg from time-resolved measurements. The in vitro chicken leg, dipped in a cylindrical container filled with diluted Intralipid-10% solution, was imaged with a multichannel time-resolved imaging system. A two-dimensional reconstruction algorithm based on a modified generalized pulse spectrum technique has been developed to reconstruct the images of both the absorption and reduced scattering coefficients simultaneously and quickly. The results demonstrate that a simultaneous reconstruction of absorption and reduced scattering coefficients from time-resolved measurement has a potential to reveal the changes in the optical properties associated with not only the physiological information but also the anatomical structure of the organ.

Novel optical fingerprint sensor utilizing optical characteristics of skin tissue under fingerprints

We have developed an optical fingerprint sensor for personal identification. Conventional sensors detect contact between the convex parts of fingerprints and the input surface of the sensor, however, we have devised a novel sensor that utilizes the optical characteristics of the skin tissue under fingerprints. We obtained tomographic images from under fingerprints by optical coherence tomography (OCT), and discovered that the reflected and scattered light from the skin tissue under the concave parts of fingerprints was lower than the convex parts. In other words, the concave parts had a higher light transmittance than the convex parts. Moreover, even when there were wrinkles in a fingerprint, the same optical characteristics were present. Based on this, we made an experimental sensor that detected fingerprint patterns using light transmittance dispersion in the skin tissue. This sensor consists of light emitting diodes (LED) that irradiate red light from the side of a fingernail and an image formation system that forms an image onto an imaging device, by using the light that penetrated the finger. Using this sensor, we obtained fingerprint pattern images in which the concave parts were brighter than the convex parts. These results showed good agreement with the transmittance dispersion described above. Consequently, it has been demonstrated that a fingerprint sensor utilizing the optical can efficiently increase the recognition of fingerprint patterns of wrinkled or wet fingers, which conventional sensors have difficulty recognizing.

Quasi in-focus optical coherence tomography

We propose here a unique method for in-focus imaging over the entire cross-sectional area of interest. This is the so-called quasi in-focus optical coherence tomography (OCT) or multiple OCT in which OCT images are obtained by shifting the focal plane of an objective, followed by piling up of these OCT images. A preliminary experiment was made using chicken tissue as a sample; as a result, a stripe pattern of fibrous muscle was clearly observed over a depth of more than 3 mm. In in-vitro tomographic imaging of the human stomach wall, quasi in-focus OCT can provide a very clear image of the muscularis mucosae, which is a bending film like tissue of a few tens of microns thickness, showing that our method is useful for the early-stage diagnosis of stomach cancer.

Calcium detection of human hair and nail by the nanosecond time-gated spectroscopy of laser-ablation plume

We demonstrate the nanosecond time-gated spectroscopy of plume in laser ablation of biological tissue, which allows us to detect calcium (Ca) with high sensitivity by the use of either a UV or a near-IR laser pulse. Clear and sharp peaks of Ca+ appear in the luminescence spectrum of laser-ablation plume although the Ca content is only 0.1 percent in human hair and nail. Luminescence peaks of sodium atom (Na) and ionized carbon are also detectable. This specific spectroscopy is low invasive because a single low-energy laser pulse illuminates the tissue sample, and it does not require any poisonous sensititizers like fluorescence dye. This method, therefore, is a promising candidate for optical biopsy in the near future. In particular, Ca detection of human hair may lead to new diagnosis, including monitor of daily intake of Ca and a screening diagnosis of osteoporosis.

In-situ tomographic observation of tissue surface during laser ablation

In laser ablation of tissues, tomography of the tissue surface is necessary for measurement of the crater depth and observation of damage of the surrounding tissue. We demonstrate here OCT images of craters made by UV laser ablation of different tissues. The maximum depth of a crater is found among several OCT images, and then the ablation rate is determined. The conventional OCT of the spatial resolution of 15 μm was used in our experiment, but OCT of the resolution of the order of 1 μm is required because the ablation rate is usually a few microns per pulse. Such a high-resolution OCT is also demonstrated in this paper, where the light source is a halogen lamp. Combination of laser ablation and OCT will lead to in situ tomographic observation of tissue surface during laser ablation, which should allow us to develop new laser surgeries.

Optical reflection tomography along the geometrical thickness

Very recently, we proposed and demonstrated a novel optical reflection tomography along the geometrical thickness, reflecting a real cross-sectional structure of an object. This technique is based on simultaneous measurement of refractive index n and thickness t of a sample using the combination of a low coherence interferometer and confocal optics. The interferometer provides optical coherence tomography (OCT) of the dimension of the optical thickness (=n x t) along the optical axis, while the confocal optics gives us another type of reflection tomography, having the thickness dimension of nearly t/n along the optical axis. This tomography can be called confocal reflection tomography (CRT) and has not yet been demonstrated, to our knowledge. Simple image processing of OCT and CRT results in desired reflection tomographic image, showing 2D refractive index distribution along the geometrical thickness. In this paper, we present the validity of our proposed method using the concave glass plate as well as the application for in vivo measurement of biological tissue.