QUANTIFICATION OF OPTICAL ABSORPTION COEFFICIENTS USING ACOUSTIC SPECTRA IN PHOTOACOUSTIC TOMOGRAPHY

• US 20130199299A1
• Filed: 04/08/2011
• Published: 08/08/2013
• Est. Priority Date: 04/09/2010
• Status: Active Grant

• Current Assignee: Washington University In St Louis
• Original Assignee: Washington University In St Louis
• Inventors: Wang, Lihong, Guo, Zijian
• Granted Patent: US 9,086,365 B2
• Time in Patent Office: Days
• US Class Current: 73/655
• CPC Class Codes: A61B 2576/00 Medical imaging apparatus involving image processing or analysis A61B1/00009, A61B6/52 and A61B8/52 take precedence; image processing per se G06T; details of algorithms for analysing medical images G06T7/0012
• CPC Class Codes: A61B 5/0095 by applying light and detecting acoustic waves, i.e. photoacoustic measurements
• CPC Class Codes: A61B 5/14542 for measuring blood gases A61B5/14551 takes precedence
• CPC Class Codes: A61B 5/14546 for measuring analytes not otherwise provided for, e.g. ions, cytochromes
• CPC Class Codes: A61B 5/417 the bone marrow
• CPC Class Codes: A61B 5/7257 using Fourier transforms
• CPC Class Codes: A61B 8/13 Tomography A61B8/10, A61B8/12 take precedence; tomography for radiation diagnosis A61B6/02
• CPC Class Codes: A61B 8/5215 involving processing of medical diagnostic data
• CPC Class Codes: G01N 21/1702 with opto-acoustic detection, e.g. for gases or analysing solids

Abstract

Accurately quantifying optical absorption coefficient using acoustic spectra of photoacoustic signals. Optical absorption is closely associated with many physiological parameters, such as the concentration and oxygen saturation of hemoglobin, and it can be used to quantify the concentrations of non-fluorescent molecules. A sample is illuminated by, for example, a pulsed laser and following the absorption of optical energy, a photoacoustic pressure is generated via thermo-elastic expansion. The acoustic waves then propagate and are detected by a transducer. The optical absorption coefficient of the sample is quantified from spectra of the measured photoacoustic signals. Factors, such as system bandwidth and acoustic attenuation, may affect the quantification but are canceled by dividing the acoustic spectra measured at multiple optical wavelengths.

Claims

1. A method of determining an optical absorption coefficient of an object, said method comprising:

focusing a first light pulse emitted by a light source into a volume of the object, wherein the first light pulse has a first wavelength;

detecting a first photoacoustic spectrum using an acoustic transducer, wherein the first acoustic spectrum is emitted by the object in response to receiving the first light pulse;

focusing a second light pulse emitted by the light source into the volume of the object, wherein the second light pulse has a second wavelength;

detecting a second acoustic spectrum using an acoustic transducer, wherein the second acoustic spectrum is emitted by the object in response to receiving the second light pulse; and

determining the absorption coefficient of the volume of the object as a function of the first acoustic spectrum and the second acoustic spectrum.

2. The method of claim 1 wherein detecting the first acoustic spectrum comprises receiving at the acoustic transducer a first acoustic wave emitted by the object in response to receiving the first light pulse and performing a Fourier transformation on the received first acoustic wave to generate the first acoustic spectrum.

3. The method of claim 1 wherein detecting the second acoustic spectrum comprises receiving at the acoustic transducer a second acoustic wave emitted by the object in response to receiving the second light pulse and performing a Fourier transformation on the received second acoustic wave to generate the second acoustic spectrum.

4. The method of claim 1 wherein determining the absorption coefficient of the volume of the object comprises dividing the first acoustic spectrum by the second acoustic spectrum frequency by frequency to generate a normalized acoustic spectrum, and fitting the normalized acoustic spectrum to an absorption coefficient curve.

5. The method of claim 1 wherein the light source comprises a first light source for emitting the first light pulse and a second light source for emitting the second light pulse.

6. The method of claim 1 wherein the first wavelength is different from the second wavelength.

7. The method of claim 1 further comprising disregarding a portion of the detected first acoustic spectrum and the second acoustic spectrum outside of a system bandwidth, wherein the system bandwidth is at least partially determined as a function of a bandwidth of the acoustic transducer.

8. The method of claim 1 further comprising determining at least one of HbT, HbO₂, HbR, and sO₂ values as a function of the determined absorption coefficient.

9. A system for determining an optical absorption coefficient of a volume of an object, said system comprising:

a light source for emitting a first light pulse having a first wavelength and a second light pulse having a second wavelength;

a focusing element for focusing the first light pulse and the second light pulse into a volume of the object;

an acoustic transducer for detecting a first acoustic spectrum emitted by the object in response to the volume of the object absorbing the first light pulse and a second acoustic spectrum emitted by the object in response to the volume of the object absorbing the second light pulse; and

a controller for determining the absorption coefficient of the volume of the object as a function of the first acoustic spectrum and the second acoustic spectrum.

10. The system of claim 9 wherein:

the acoustic transducer detects the first acoustic spectrum by receiving a first acoustic wave emitted by the object in response to receiving the first light pulse and performing a Fourier transformation on the received first acoustic wave to generate the first acoustic spectrum; and

the acoustic transducer detects the second acoustic spectrum by receiving a second acoustic wave emitted by the object in response to receiving the second light pulse and performing a Fourier transformation on the received second acoustic wave to generate the second acoustic spectrum.

11. The system of claim 9 wherein the controller determines the absorption coefficient of the volume of the object comprises by dividing the first acoustic spectrum by the second acoustic spectrum, frequency by frequency, to generate a normalized acoustic spectrum and fits the normalized acoustic spectrum to an absorption coefficient curve.

12. The system of claim 9 wherein the light source comprises a first light source for emitting the first light pulse and a second light source for emitting the second light pulse, and wherein the first wavelength is different from the second wavelength.

13. The system of claim 9 wherein the controller disregards a portion of the detected first acoustic spectrum and the second acoustic spectrum outside of a system bandwidth, wherein said system bandwidth is at least partially determined as a function of a bandwidth of the acoustic transducer.

14. The system of claim 9 wherein the controller determines at least one of HbT, HbO₂, HbR, and sO₂ values as a function of the determined absorption coefficient.

15. A system for generating an image of an object and determining an optical absorption coefficient of a volume of an object, said system comprising:

a light source for emitting a first light pulse having a first wavelength and a second light pulse having a second wavelength;

a focusing element for focusing the first light pulse and the second light pulse into a volume of a plurality of volumes of the object;

an acoustic transducer for detecting a first acoustic spectrum produced by the object in response to the volume of the object absorbing the first light pulse and a second acoustic spectrum produced by the object in response to the volume of the object absorbing the second light pulse;

a controller for determining the absorption coefficient of the volume of the object as a function of the first acoustic spectrum and the second acoustic spectrum and for reconstructing an image from acoustic spectra detected from the plurality of volumes of the object; and

a scanner for altering a spatial relationship between the object and the focusing element after the first acoustic spectrum and the second acoustic spectrum are detected by the acoustic transducer for the volume such that the first light pulse and the second light pulse are focused into another volume of the plurality of volumes of the object.

16. The system of claim 15 wherein:

the acoustic transducer detects the first acoustic spectrum by receiving a first acoustic wave emitted by the object in response to receiving the first light pulse and performing a Fourier transformation on the received first acoustic wave to generate the first acoustic spectrum; and

the acoustic transducer detects the second acoustic spectrum by receiving a second acoustic wave emitted by the object in response to receiving the second light pulse and performing a Fourier transformation on the received second acoustic wave to generate the second acoustic spectrum.

17. The system of claim 15 wherein the controller determines the absorption coefficient of the volume of the object comprises by dividing the first acoustic spectrum by the second acoustic spectrum, frequency by frequency, to generate a normalized acoustic spectrum and fits the normalized acoustic spectrum to an absorption coefficient curve.

18. The system of claim 15 wherein the light source comprises a first light source for emitting the first light pulse and a second light source for emitting the second light pulse, and wherein the first wavelength is different from the second wavelength.

19. The system of claim 15 wherein the controller disregards a portion of the detected first acoustic spectrum and the second acoustic spectrum outside of a system bandwidth, wherein said system bandwidth is at least partially determined as a function of a bandwidth of the transducer.

20. The system of claim 15 wherein the controller determines at least one of HbT, HbO₂, HbR, and sO₂ values as a function of the determined absorption coefficient.