Method for measuring changes in light absorption of highly scattering media

• US 6,381,018 B1
• Filed: 07/27/1999
• Issued: 04/30/2002
• Est. Priority Date: 07/28/1998
• Status: Expired due to Fees

• Current Assignee: Los Alamos National Security LLC (Government of the United States of America)
• Original Assignee: Regents of the University of California (University of California)
• Inventors: Bigio, Irving J., Mourant, Judith R., Johnson, Tamara M.
• Primary Examiner(s): Pham, Hoa Q.
• Application Number:
  US09/363,270
• Time in Patent Office: 1,008 Days
• Field of Search: 356/73, 356/432-435, 356/338-343, 356/445, 356/39, 356/446, 356/40, 356/318, 356/326, 385/12, 385/115, 385/116, 385/117, 600/310, 600/316, 600/476, 600/478, 731/7-3-9, 733/7, 734/2, 733/912, 250/227.27
• US Class Current: 356/432
• CPC Class Codes: G01N 21/31 Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry G01N21/72 takes precedence
• CPC Class Codes: G01N 21/49 within a body or fluid

Abstract

The noninvasive measurement of variations in absorption that are due to changes in concentrations of biochemically relevant compounds in tissue is important in many clinical settings. One problem with such measurements is that the pathlength traveled by the collected light through the tissue depends on the scattering properties of the tissue. It is demonstrated, using both Monte Carlo simulations and experimental measurements, that for an appropriate separation between light-delivery and light-collection fibers, the pathlength of the collected photons is insensitive to scattering parameters for the range of parameters typically found in tissue. This is important for developing rapid, noninvasive, inexpensive, and accurate methods for measuring absorption changes in tissue.

Claims

1. An apparatus for measuring absorption of light by a material in small volume of highly scattering medium, which comprises in combination:

(a) a source of light having a wavelength which is absorbed by said material;

(b) a first optical fiber, having a first end for receiving the light from said source of light, and a second end for directing the light into the surface of the highly scattering medium, such that the light enters said highly scattering medium;

(c) a second optical fiber, having a first end located approximately 1.7 mm from the second end of said first optical fiber on the surface of said highly scattering medium, for collecting and receiving a portion of the light scattered from the highly scattering medium in a direction opposite to the light entering the highly scattering medium, and a second end, whereby the dependence of the pathlength of the light traversing the highly scattering medium taken to reach the first end of said second fiber on the scattering parameters of the highly scattering medium is minimized; and

(d) a light detector for detecting light exiting the second end of said second optical fiber, such that a measurement of the light absorbed by a portion of the material is obtained.

2. The apparatus for measuring absorption of light by a material in a small volume of highly scattering medium, as described in claim 1, wherein the diameter of said first optical fiber and the diameter of said second optical fiber are less than 400 μ

m.

3. The apparatus for measuring absorption of light by a material in a small volume of highly scattering medium, as described in claim 1, further comprising a first mirror for receiving light emerging from the second end of said first optical fiber and for directing the light onto the surface of the highly scattering medium, such that the light enters said highly scattering medium, and a second mirror for collecting and receiving a portion of the light scattered from the highly scattering medium in a direction opposite to the light entering the highly scattering medium and for directing the received light onto the first end of said second optical fiber.

4. The apparatus for measuring absorption of light by a material in a small volume of highly scattering medium, as described in claim 1, wherein the second end of said first optical fiber and the first end of said second optical fiber are placed in direct optical contact with the surface of the highly scattering medium in order to reduce reflections from the surface from entering the first end of said second optical fiber.

5. The apparatus for measuring absorption of light by a material in a small volume of highly scattering medium, as described in claim 1, wherein said source of light is a monochromatic source of light.

6. The apparatus for measuring absorption of light by a material in a small volume of highly scattering medium, as described in claim 1, wherein said source of light is a polychromatic source of light and said apparatus further comprises a spectrograph for receiving the light exiting the second end of said second optical fiber and selecting a chosen wavelength therefrom, such that the chosen wavelength is detected by said light detector.

7. The apparatus for measuring absorption of light by a material in a small volume of highly scattering medium, as described in claim 6, wherein said polychromatic source of light is selected from the group consisting of xenon and tungsten arc lamps.

8. The apparatus for measuring absorption of light by a material in a small volume of highly scattering medium, as described in claim 1, wherein means are provided for measuring the intensity of said source of light.