JP2011045514A - Photoacoustic tomography apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve such a problem that a contrast of tomographic images is reduced conventionally because of mixing of photoacoustic signals by one-photon excitation in a photoacoustic tomography apparatus employing multiphoton excitation. <P>SOLUTION: The photoacoustic tomography apparatus includes a light source 10 for generating near-infrared light pulses, an irradiation optical system for irradiating a body 15 to be measured by focusing the pulse light, a laser scanning unit 11 for scanning the position of a focal point of the pulse light, an acoustic transducer 18 for selectively detecting an acoustic wave generated from the body 15 by multiphoton excitation of the pulse light, a signal amplification unit 19, a signal processing unit 22, and a display unit 23 which acquires three-dimensional data indicating the distribution of substances inside the body 15 based on generation position information and displays a tomographic image of any plane. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

      The present invention relates to a photoacoustic tomography apparatus for visualizing internal information of a measurement object using photoacoustic effect by multiphoton excitation and frequency filtering of a photoacoustic signal.

      In recent years, the need for tomographic imaging of living tissue has increased. However, with conventional MRI and CT, the resolution is insufficient (resolution: ˜1 mm), and it is difficult to observe deep portions with an optical microscope. Therefore, a technique called a photoacoustic tomography (PAT) method has been developed as an imaging technique for living body tomographic images for deep observation at high resolution, and its usefulness is becoming clear. This is because the absorption material in the sample is excited by irradiating the measurement object with a light pulse, and the absorption information in the measurement object is acquired and imaged by detecting the photoacoustic wave generated by the thermoelastic expansion of the absorption material. To do. As described above, the photoacoustic tomography method generates an acoustic wave with little attenuation in a living body by irradiating the measurement object with light, and acquires information inside the measurement object by detecting the acoustic wave. Therefore, it is possible to detect a signal from a deeper portion. However, in the conventional photoacoustic tomography apparatus, it is difficult to accurately capture the cross-sectional structure of the absorbing material because the resolution of the optical axis method (depth direction) is determined by the properties of sound waves and it is difficult to achieve high spatial resolution. . Further, a substance having a large absorption coefficient has a large effect of absorption on the surface, and information on the deep part of the absorbing substance cannot be captured.

      Therefore, the resolution of the optical axis method (in the depth direction) is improved by using the property of light called multiphoton excitation instead of the property of sound waves, and as a method of accurately capturing information inside the absorbing material, multiphoton excitation is used. A photoacoustic tomography apparatus has been developed (see Patent Document 1). In this method, multi-photon excitation is caused by condensing and irradiating a pulse laser, and absorption information in the measurement object is imaged by detecting an acoustic wave generated by the multi-photon excitation. Multiphoton excitation uses a wavelength twice the absorption wavelength of the object to be measured to excite the absorbing material, so that it is possible to acquire not only the surface of the absorbing material but also deep information. In addition, since photoacoustic wave generation occurs only in a very small region where light is collected, high-resolution imaging is possible.

Japanese Patent Application No. 2007-556801

        However, the probability of occurrence of multiphoton absorption is smaller than the probability of occurrence of single photon absorption, and there is a problem that photoacoustic waves due to single photon absorption are mixed when performing multiphoton excitation photoacoustic tomography.

      Therefore, the problem to be solved by the present invention is to provide a photoacoustic tomography apparatus capable of measuring deep part information (structure) of a measurement object having high absorption with high resolution.

The photoacoustic tomography apparatus of the present invention, which has been made to solve the above problems, is a photoacoustic tomography apparatus that visualizes the absorption substance distribution inside the measurement object using the multiphoton excitation photoacoustic effect.
a) a pulsed laser irradiation means for inducing multiphoton excitation by irradiating a measured object with a pulsed laser;
b) means for detecting only acoustic waves generated from the object to be measured by the multiphoton excitation;
c) signal processing means for imaging the absorbing substance information inside the measurement object based on the detection result by the acoustic wave detection means;
It is characterized by having.

      Here, the pulse laser may be any laser capable of performing multiphoton excitation. In general, a multi-photon excitation can be induced by using a laser that oscillates twice the absorption wavelength of the object to be measured, and condensing and incident pulsed light having a high peak power on the object to be measured. As such a pulse laser, a picosecond pulse laser or a femtosecond pulse laser can be preferably used. Further, even a nanosecond pulse laser having a high peak power can induce multiphoton excitation.

      Further, the means for detecting the acoustic wave generated from the measurement object by the multiphoton excitation separately from the photoacoustic wave generated by the one-photon excitation is included in the one-photon acoustic wave and the multi-photon photoacoustic wave. A combination of two or more acoustic transducers having different frequency bands is used to selectively detect photoacoustic waves by multiphoton excitation depending on the difference in frequency components. Alternatively, a single broadband transducer may be used, the frequency analysis of the acoustic signal may be performed by the signal processing unit, and the contribution rate (intensity information) of each frequency component may be calculated.

      The signal processing means for imaging the absorption substance information inside the measurement object based on the detection result by the acoustic wave detection means is the calculation of the signal intensity from two or more acoustic transducers, or the respective frequency bands by frequency analysis. The information on the amount of substance at the laser condensing position can be acquired based on the contribution rate (intensity information) of the signal component.

      The photoacoustic tomography apparatus of the present invention is provided with laser scanning means for scanning the focal position of the pulse laser two-dimensionally or three-dimensionally, and the acoustic wave generated from each focal position by the signal processing means. It is desirable to obtain and image two-dimensional or three-dimensional substance distribution information inside the measurement object based on the intensity information.

      The laser scanning means may be one that moves the object to be measured while fixing the focal position of the laser light, or may be one that moves the focal position of the laser light while fixing the object to be measured. Or a combination of both.

      The photoacoustic tomography apparatus of the present invention having the above-described configuration induces multiphoton excitation by condensing and irradiating a measured object with a pulse laser as excitation light, and the photoacoustic generated by the multiphoton excitation. Waves can be selectively detected. As a result, it is possible to selectively detect only acoustic waves that are locally excited only in the focal region. Therefore, the photoacoustic tomography apparatus of the present invention can prevent the generation of acoustic waves from regions other than the region to be measured (for example, a signal from the surface of a structure made of a material having a large absorption coefficient). It is possible to observe a deeper region of the sample while maintaining the resolution.

      In addition, as said pulse laser irradiation means, it is desirable to use what can irradiate the near-infrared light pulse with a wavelength of 700 nm-2500 nm. As shown in Fig. 2, near-infrared light is less absorbed by melanin and water and is not easily attenuated in the living body, so that the penetration depth of excitation light can be increased and high-resolution observation of deep regions is possible. The above-described advantages of the present invention can be further exhibited (FIG. 2: Oregon Medical Center website: http://omlc.ogi.edu/spectra/). Here, for example, when a near-infrared pulse laser having a wavelength of 800 nm is focused and irradiated on a living body, a substance having absorption near 400 nm only in the focal region (for example, oxygenated hemoglobin or Reduced hemoglobin) is excited.

The schematic diagram which shows one Example of the photoacoustic tomography apparatus which concerns on this invention. The graph which shows the wavelength characteristic of the absorption molar coefficient of oxyhemoglobin, reduced hemoglobin, and melanin, and the wavelength characteristic of the absorption coefficient of water. Absorption spectra of (A) 1-photon absorption dye (IRA980BT) and (B) 2-photon absorption dye (Rhodamine B) used in Test Example 1 according to the present invention. Photoacoustic signal time waveforms of (A) 1-photon absorption dye (IRA980BT, 1/512 of a saturated solution) and (B) 2-photon absorption dye (Rhodamine B) obtained by simulation in Test Example 1 according to the present invention and Power spectrum. The power spectrum of the one-photon absorption dye (IRA980BT, 1/512 of a saturated solution) and the two-photon absorption dye (Rhodamine B) obtained by experiment in Test Example 1 according to this study. 1-photon excitation dye (IRA980BT) (top), 2-photon excitation dye (rhodamine B) (middle) in Test Example 2 according to the present invention (bottom) mixed with both one-photon absorption dye and two-photon absorption dye The figure which shows the example of a measurement of the cross-sectional image using the frequency filter of the filled glass capillary. The schematic diagram which shows one Example of the photoacoustic tomography apparatus which concerns on this invention.

        Hereinafter, modes for carrying out the present invention will be described based on examples. FIG. 1 is a diagram showing a schematic configuration of a photoacoustic tomography apparatus according to an embodiment of the present invention. The photoacoustic tomography apparatus according to this embodiment includes a near-infrared pulse light source 10, a laser scanning unit 11, an irradiation optical system for irradiating a measurement object 15 with laser light, an acoustic transducer 18, a signal amplification unit 19, and a signal. It comprised with the process part 22 and the control part 21 for controlling said each part. The signal processing unit 22 and the control unit 21 are embodied by a personal computer 20 (abbreviated as “PC” in the drawing) equipped with predetermined software, and a display unit 23 having a monitor is connected to the PC 20. ing.

        As the near-infrared pulse light source 10, for example, a light source capable of generating near-infrared light pulses such as a titanium sapphire laser or a YAG laser is used. The irradiation optical system includes an objective lens 14 for condensing pulsed light and irradiating the measurement object 15, and a reflecting mirror for causing the pulsed light emitted from the near-infrared pulse light source 10 to enter the objective lens 14. And a microscope 12 with 13. The microscope 12 is further provided with a stage 16 for placing the measurement object 15 and a stage drive unit 17 for driving the stage 16, and the stage 16 is moved up and down by the stage drive unit 17. Thus, the focal position of the laser beam can be scanned in the optical axis direction (that is, the Z-axis direction in the figure).

        The laser scanning unit 11 is attached to the microscope 12. The laser scanning unit 11 drives the movable mirror (not shown) provided inside the laser scanning unit 11 to emit pulsed light irradiated on the measurement target 15. This is for scanning in a plane orthogonal to the optical axis direction (that is, the X-axis and Y-axis directions in the figure). Instead of providing such a laser scanning unit 11, the stage 16 can be moved in the XY axis direction so that the focal position of the pulsed light with respect to the measured object 15 can be scanned in the XY plane. May be.

        The acoustic transducer 18 is composed of a piezoelectric element that collects an acoustic wave emitted from the inside of the measurement object 15 by absorption of pulsed light and converts it into an electrical signal. The electrical signal from the acoustic transducer 18 is received by a signal amplifying unit 19. The signal is amplified and converted into a digital signal by the signal processing unit 22.

        The acoustic transducer 18 may be a combination of acoustic transducers having various frequency bands. Electric signals from various acoustic transducers are amplified by the signal amplifier 19 and converted into digital signals by the signal processor 22.

        The signal processing unit 22 includes the intensity information and frequency information of the acoustic wave transmitted from the signal amplifying unit 19 and the generation position information of the acoustic wave, that is, the X axis and the Y axis of the pulsed light transmitted from the laser scanning unit 11. Direction focal position information and the focal position information in the Z-axis direction of the pulsed light transmitted from the stage drive unit 17 are received, digitized, and subjected to a predetermined calculation based on these information to be measured. 15 generates three-dimensional image data indicating the substance distribution inside. Further, the signal processing unit 22 generates a two-dimensional image (tomographic image) representing an arbitrary cross section of the measurement object 15 based on the generated three-dimensional image data, and displays the two-dimensional image on the monitor of the display unit 23. The

        When taking a tomographic image of a living body using the photoacoustic tomography apparatus having the above-described configuration, first, pulsed light is emitted from the near-infrared pulse light source 10 at a predetermined interval to irradiate the measurement object 15. At this time, the pulsed light emitted from the near-infrared pulse light source 10 is reflected by the reflecting mirror 13 provided in the microscope 12 through the laser scanning unit 11, condensed by the objective lens 14, and placed on the stage 16. The irradiated object 15 is irradiated. As a result, an acoustic wave is generated only in the focal region by multiphoton excitation by the near-infrared pulse inside the measurement object 15. The acoustic wave generated at the focal position propagates through the living body and is detected by the acoustic transducer 18, and the detection signal is sent to the signal processing unit 22 through the signal amplification unit 19.

        Here, by irradiating the pulsed light and detecting the acoustic wave as described above while scanning the focal position of the pulsed light in the X-axis and Y-axis directions using the laser scanning unit 11, A two-dimensional image of the XY plane at a predetermined depth position can be taken, and the stage drive unit 17 is used to change the focal position of the pulsed light in the Z-axis direction (that is, the depth direction of the measured object 15). By capturing a plurality of such two-dimensional images, the three-dimensional data inside the measured object 15 can be acquired.

        According to the photoacoustic tomography apparatus of the present embodiment as described above, acoustic waves can be generated locally from the focal region by multiphoton excitation by near-infrared light pulses, and can be selectively detected. Thus, it is possible to prevent the generation of photoacoustic signals from a non-target region and to observe the deep region without reducing the spatial resolution.

        As shown in Test Example 1 below, the frequency components contained in the photoacoustic signals for multiphoton excitation and single photon excitation are different. Therefore, by analyzing the frequency of the photoacoustic signal, the photoacoustic signal based only on multiphoton excitation can be obtained. It is possible to take it out. As a result, it is possible to efficiently remove a signal due to one-photon absorption that occurs strongly on the surface, and it is possible to directly extract information on the deep part of the living body with higher contrast.

        In addition, if a two-photon absorption substance excited by near-infrared pulsed light is introduced into the object to be measured, it can be visualized even in the case of the object to be measured that does not contain a substance excited by two-photon absorption by near infrared light. It becomes possible.

      (Test Example 1) (A) The difference between the frequency component of the photoacoustic wave due to one-photon excitation and the difference between the frequency component of the photoacoustic wave due to (B) two-photon excitation were simulated (FIG. 4). Absorbing dyes ((A) IRA980BT and (B) rhodamine B (absorption spectrum shown in FIG. 3 (Source: Oregon Medical Center website: http://omlc.ogi.edu/spectra/, Exiton website)) ))) The solution was assumed to have a use wavelength of 1064 nm. In this calculation, the waveform of the sound wave generated by the depth dependency of the light absorption amount is described, and the spectrum of the sound wave is calculated by Fourier transform of the sound wave waveform. As for the frequency component of the photoacoustic wave generated by two-photon excitation, the spectrum of the acoustic wave is calculated assuming that the laser beam is a Gaussian beam and that the photoacoustic wave is generated by the square of the light intensity. As can be seen from this result, (A) in the case of a photoacoustic wave by one-photon excitation (for example, when there is an absorption peak of one photon at 1064 nm as in IRA980BT) and (B) in the case of a photoacoustic wave by two-photon excitation Comparing (for example, when there is an absorption peak at 532 nm as in rhodamine B), it can be seen that the photoacoustic wave generated by two-photon excitation generates higher frequency components. It can be seen that the frequency components of the photoacoustic wave generated by the one-photon excitation and the two-photon excitation obtained by the experiment shown in FIG. 5 have the same tendency as the simulation result.

      (Test Example 2) A cross-section of a glass capillary filled with an absorbing dye (rhodamine B and IRA 980BT) solution as an object to be measured (manufactured by TOHO, outer diameter 3 mm, inner diameter 2.4 mm) is a multiphoton excitation photoacoustic tomography that is a conventional method. Measurement was performed by an imaging method (selection of all frequencies) and a multiphoton excitation photoacoustic tomography method (with frequency selection) using frequency analysis according to the present invention (FIG. 6). The wavelength used is 1064 nm. In the case of the ethanol solution of the dye IRA980BT (manufactured by Exiton) that causes the one-photon absorption in the upper stage, in the case of the ethanol solution of the dye rhodamine B (manufactured by Sigma) that causes the two-photon absorption in the upper stage, The cross-section of a glass capillary containing both rhodamine B) and a dye (IRA980BT) exhibiting one-photon absorption was measured. The left column shows the case where an image is created using all frequency components (conventional method), and the middle column shows the image using a frequency of 1-10 MHz when an image is created using a frequency of 0-1 MHz. The case where it created is shown. As can be seen from the comparison between the upper and middle stages of FIG. 6, in the case of two-photon excitation photoacoustics, it can be seen that the cross section of the glass capillary is captured with good contrast. It can also be seen that the cross-section is captured with better contrast when the image is constructed from only high-frequency components. By performing frequency analysis in this way, contrast can be improved, and even when 1-photon and 2-photon absorption dye are mixed (lower part of FIG. 6), improvement in contrast can be obtained by selecting the frequency.

      Thus, it is derived from the theoretical calculation that the frequency component of the photoacoustic wave due to the one-photon excitation is different from that due to the multiphoton excitation, and only the photoacoustic wave due to the multiphoton excitation is extracted by extracting the high-frequency component in the experiment. The contrast can be improved.

      Therefore, the photoacoustic tomography apparatus of the present embodiment can be suitably used particularly for imaging of capillaries and small arteries and veins, measurement of deep structures of tissues and organs, and the like.

      The target object may be any object as long as it is multiphoton excited by a near-infrared pulse laser, but a blood component is particularly desirable. For example, by using oxyhemoglobin and deoxyhemoglobin as the objects of interest, the oxygen concentration distribution in the brain is measured using the difference in the two-photon absorption peak between oxyhemoglobin and deoxyhemoglobin (see FIG. 3). It is possible to perform functional imaging that looks like Specifically, when a laser having a wavelength twice the one-photon absorption peak wavelength of oxyhemoglobin is scanned in the XYZ directions and the position dependency of the generated photoacoustic wave intensity is measured, a signal is obtained from blood rich in oxyhemoglobin. Since it occurs strongly, only blood vessels containing blood with a high oxygen concentration are visualized. Similarly, when measurement is performed using a laser having a wavelength twice that of one photon absorption of reduced hemoglobin, only blood vessels containing a lot of blood having a low oxygen concentration can be visualized. By performing measurement while changing the wavelength in this way, it is possible to form an image of a blood vessel including information on the oxygen concentration in the blood vessel, and for example, it is possible to observe an activated site in the brain. As the near-infrared pulse light source, it is desirable to use a laser whose wavelength is variable in the near-infrared region (700 nm to 2500 nm) capable of two-photon excitation of oxyhemoglobin and deoxyhemoglobin. The near-infrared pulse laser may be any laser capable of performing multiphoton excitation. Generally, a nanosecond pulse laser, a picosecond pulse laser, or a femtosecond pulse is used. A laser is used.

      Since the photoacoustic tomography apparatus of the present embodiment can efficiently detect an acoustic wave generated only from the focal position, it can accurately capture a cross section of a substance that is highly absorbed, such as a blood vessel. For example, vasoconstriction is known to change when myocardial ischemia occurs, and vasoconstriction is associated with various diseases. Therefore, visualization of the vasoconstriction movement in the deep part of the living body is very important.

      As described above, the best mode for carrying out the photoacoustic tomography apparatus of the present invention has been described using the embodiments. However, the present invention is not limited to the above-described embodiments, and may be appropriately changed within the scope of the present invention. Is acceptable.

      The photoacoustic tomography apparatus of the present invention is not limited to the configuration using the microscope as described above. For example, the laser light irradiation means and the acoustic wave detection means are in direct contact with the surface of the measurement object. It is good also as what performs irradiation of a laser beam, and reception of an acoustic wave. An example of the configuration of such a photoacoustic tomography apparatus is shown in FIG. In addition, the same code | symbol is attached | subjected about the structure similar to FIG. 1, and description is abbreviate | omitted suitably. Here, the pulsed light guided from the near-infrared pulse light source 10 is condensed on the surface of the probe 30 used in contact with the measured object 15 (the surface that is in contact with the measured object 15). A light irradiating unit 31 for irradiating 15, an acoustic transducer 18 composed of a piezoelectric element such as PVDF (polyvinylidene fluoride resin) for detecting an acoustic wave generated in the measurement object 15 by absorption of the pulsed light, and Is provided. The light irradiation unit 31 can change the focal position of the pulsed light in the optical axis direction. Further, the probe scans the focal position of the pulsed light by the light irradiation unit 31 in a plane orthogonal to the optical axis. It is desirable to provide a scanning unit 32 for obtaining the two-dimensional or three-dimensional material distribution information in the measurement object 15. In addition, it is good also as a structure which scans pulsed light by arranging many light irradiation parts 31 and the acoustic transducers 18 as mentioned above on the surface of the probe 30, and switching and operating them. Moreover, it is good also as a structure which simplifies the signal processing of the frequency selection in the signal processing part 22 by arranging the transducer from which a frequency sensitivity curve differs.

      The operation of the photoacoustic tomography apparatus as described above will be described using the example of brain function imaging described above. Since the blood oxygen concentration in the brain is closely related to the brain activity, the active site of the brain can be observed by acquiring the concentration distribution information and imaging it. First, the probe 30 is brought into contact with the head of the measurement object 15, and pulsed light having a wavelength capable of specifically exciting oxyhemoglobin by two-photon excitation is emitted from the light irradiation unit 31 and focused by the two-photon excitation. An acoustic wave generated from the position is detected by the acoustic transducer 18. At this time, the light irradiation unit 31 and the scanning unit 32 scan the focal position of the pulsed light in a three-dimensional manner, whereby irradiation of the pulsed light and detection of the acoustic wave signal are performed for each part of the brain. Subsequently, in the same manner, pulsed light irradiation and acoustic waves are detected at a wavelength capable of specifically exciting reduced hemoglobin by two-photon excitation, and acoustic waves derived from oxyhemoglobin or reduced hemoglobin obtained as described above are detected. Based on the signal and the focal position information of the pulsed light transmitted from the probe 30, the signal processor 22 performs a predetermined calculation. Thereby, the blood oxygen concentration in each part of the brain is calculated, and three-dimensional image data indicating the distribution of the blood oxygen concentration is generated. Further, the signal processing unit 22 generates a three-dimensional image or a two-dimensional image showing an arbitrary cross section of the brain based on the generated three-dimensional image data, and displays it on the monitor of the display unit 23.

      The photoacoustic tomography apparatus of the present invention is not limited to the observation of a living body as described above, and can be applied to, for example, nondestructive inspection of various samples such as product inspection of semiconductor elements. In this case, it is desirable to use an appropriate wavelength according to the multiphoton absorption of the substance to be detected.

DESCRIPTION OF SYMBOLS 10 ... Near-infrared pulse light source 11 ... Laser scanning part 12 ... Microscope 13 ... Reflective mirror 14 ... Objective lens 15 ... Measuring object 16 ... Stage 17 ... Stage drive part 18 ... Acoustic transducer 19 ... Signal amplification part 20 ... Personal computer 21 ... Control part 22 ... Signal processing part 23 ... Display part 30 ... Probe 31 ... Light irradiation part 32 ... Scanning part

Claims (4)

In the photoacoustic tomography apparatus that visualizes the substance distribution inside the measurement object using the photoacoustic effect,
a) a pulsed laser irradiation means for inducing multiphoton excitation by irradiating a measured object with a pulsed laser;
b) an acoustic wave detecting means for detecting only an acoustic wave generated from the measurement object by the multiphoton excitation;
c) signal processing means for imaging the absorbing substance information inside the measurement object based on the detection result by the acoustic wave detection means;
A photoacoustic tomography apparatus comprising:       Only acoustic waves generated from the object to be measured by multiphoton excitation are detected from the acoustic waves generated by irradiating the object to be measured with a pulse laser by using frequency filtering means for the acoustic waves. The photoacoustic tomography apparatus according to claim 1.         3. The frequency filtering unit detects and uses a contribution rate of a frequency component higher than a threshold frequency or a contribution rate of a frequency component lower than a threshold frequency with respect to all frequency components of the acoustic wave. The photoacoustic tomography apparatus described.         3. The frequency filtering unit detects and uses a contribution rate of frequency components within a range between a threshold frequency 1 and a threshold frequency 2 with respect to all frequency components of the acoustic wave. Photoacoustic tomography device.