JPWO2007088709A1 - Photoacoustic tomography apparatus and photoacoustic tomography method - Google Patents

Abstract

A light source 10 that generates a near-infrared light pulse, means for condensing the pulsed light and irradiating the object 15 to be measured, a laser scanning unit 11 that scans the focal position of the pulsed light, and multiphotons of the pulsed light An acoustic transducer 18 for detecting an acoustic wave generated from the measured object 15 by absorption is provided, and three-dimensional data indicating a material distribution inside the measured object 15 is acquired based on the intensity information and the generated position information of the acoustic wave. Then, a tomographic image on an arbitrary plane is displayed on the display unit 23. As a result, by utilizing the nonlinear optical effect of the near-infrared pulsed light, an acoustic wave can be generated locally from the focal position inside the measured object 15 and the deep region of the measured object 15 can be observed with high resolution. It becomes possible to do. Further, by using the excitation light in the near infrared region, attenuation of the excitation light in the living body can be prevented and the penetration depth can be increased.

Description

  The present invention relates to a photoacoustic tomography apparatus and a photoacoustic tomography method for visualizing internal information of a measurement object using a photoacoustic effect.

  Conventionally, X-ray computed tomography (X-ray computed tomography) (X-ray computed tomography) and magnetic resonance imaging (MRI) have been used as methods for observing information inside a measurement object, for example, tissues and organs inside a living body. Techniques such as Resonance Imaging) and ultrasonic tomography are known, and are widely used in medical settings. However, although these methods can observe the deep structure of the object to be measured over a depth of several tens of centimeters, observation with a spatial resolution of 1 mm or less is very difficult, and sufficient information is not always obtained depending on the purpose of diagnosis. There was no case.

  On the other hand, as a living body measurement method with high spatial resolution, an optical coherence tomography (OCT) method has recently attracted attention. This is a measurement technique that uses the coherence of light. The reflected light from the sample interferes with the reflected light from the reference mirror, and a tomographic image is obtained based on the reflection position information inside the sample obtained from the interference information. Is to be generated. According to this method, internal information of a biological sample can be visualized with a high spatial resolution of about several μm. However, as described above, in the OCT method, it is necessary to reciprocate light from the surface of the living body to the site to be observed in order to irradiate the living body with light and detect the light reflected in the living body. Since it is a scattering material, it is difficult to propagate in the living body for a long distance. Therefore, the OCT method has a problem that only a thin object such as an intima of blood vessels or a relatively transparent object such as an eye can be observed.

  Therefore, in recent years, a technique called a photoacoustic tomography (PAT) method has been developed as an imaging technique for living body tomographic images, and its usefulness is becoming clear (see, for example, Patent Document 1). This is because the atoms or molecules in the sample are excited by irradiating the measurement object with pulsed light, and the material distribution information in the measurement object is obtained by detecting the acoustic wave generated by the non-radiative transition of the atoms or molecules. It is acquired and imaged. 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 region than the OCT method.

Japanese National Patent Publication No. 11-514549

However, in the conventional photoacoustic tomography apparatus, a photoacoustic wave by linear (one-photon) absorption of visible short pulse light having a pulse width on the order of nanoseconds (10 ⁻⁹ seconds) is used as excitation light. Therefore, as shown in FIG. 2 (a), when the sample (object to be measured) is irradiated with laser light, absorption occurs in all parts through which the light passes, and acoustic waves are also generated from regions other than the focal position. To do. Therefore, there is a problem that the spatial resolution is lowered when observing the deep region of the sample.

  Therefore, the problem to be solved by the present invention is to provide a photoacoustic tomography apparatus and a photoacoustic tomography method capable of photographing the deep structure of the measurement object 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 a substance distribution inside a measurement object using a photoacoustic effect.
a) a pulsed laser irradiation means for inducing multiphoton absorption by irradiating a measured object with a pulsed laser;
b) an acoustic wave detecting means for detecting an acoustic wave generated from the measurement object by the multiphoton absorption;
c) signal processing means for imaging substance distribution 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, it is possible to induce multiphoton absorption 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 absorption.

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

  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. Alternatively, a combination of both may be used.

The photoacoustic tomography apparatus of the present invention having the above configuration induces multiphoton excitation by condensing and irradiating a measured object with a pulsed laser as excitation light, whereby FIG. 5 (details will be described later). 2), an acoustic wave that increases nonlinearly with respect to the energy applied to the measurement object can be obtained. As a result, as shown in FIG. To generate an acoustic wave. Therefore, according to the photoacoustic tomography apparatus of the present invention, it is possible to prevent the generation of an acoustic wave from an unnecessary region, and it is possible to observe a deeper region of the sample while maintaining high 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. 3, 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 advantages of the present invention can be further exhibited. 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. It is a figure explaining the generation | occurrence | production area | region of the acoustic wave by the focused irradiation of pulsed light, (a) shows the one-photon excitation photoacoustic wave by a visible pulse laser, (b) is the multiphoton excitation light by a near-infrared pulse laser. Indicates acoustic waves. 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. (Source: Oregon Medical Center website <URL: http://omlc.ogi.edu/spectra/>). The block diagram which shows another structural example of the photoacoustic tomography apparatus which concerns on this invention. The figure which shows the incident pulse energy dependence of the photoacoustic wave intensity | strength in the test example 1 which concerns on this invention. The figure which shows the depth direction dependence of the two-photon absorption and the one-photon absorption in Test Example 2 according to the present invention. The figure which shows the depth direction dependence of the two-photon absorption excitation (a) and the one-photon absorption excitation (b) photoacoustic signal waveform in Test Example 3 according to the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Near-infrared pulse light source 11 ... Laser scanning part 12 ... Microscope 13 ... Reflection mirror 14 ... Objective lens 15 ... Measurement 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

  Hereinafter, the best mode 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 comprises a processing unit 22 and a control unit 21 for controlling the respective units. 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 the personal computer 20 includes a display unit 23 having a monitor. It is connected.

  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 an Nd: 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 signal processing unit 22 includes the intensity information of the acoustic wave transmitted from the signal amplification unit 19 and the generation position information of the acoustic wave, that is, the focal points of the pulsed light transmitted from the laser scanning unit 11 in the X-axis and Y-axis directions. The 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 and digitized, and a predetermined calculation is performed based on these information, whereby the substance distribution inside the measurement object 15 is determined. The three-dimensional image data shown is generated. 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 due to multiphoton absorption by the near-infrared pulse inside the measured 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 and the photoacoustic tomography method using the same as described above, acoustic waves are generated locally from the focal region by multiphoton absorption by the near-infrared light pulse. Therefore, it is possible to prevent the generation of photoacoustic signals from a non-target region and to observe a deep region without reducing the spatial resolution.

  Further, by using near-infrared light with little bioabsorption as the excitation light, the excitation light can penetrate into the deep part of the living body. In particular, when such a near-infrared light pulse having a wavelength capable of specifically exciting a blood component by two-photon absorption is used, the blood distribution in the living body can be visualized.

  Furthermore, if a multiphoton absorbing material excited by near-infrared light is introduced into the measurement object, it acts as a contrast agent in the living body, and the contrast of the measurement object image can be improved. The following test examples show the result that a multiphoton excitation acoustic wave can be efficiently detected by putting a two-photon absorption substance in an aqueous solution in which hemoglobin as a main component of blood is dissolved.

  (Test Example 1) A solution containing a blood component as a measurement object was prepared as follows. A two-photon absorber, rhodamine B (Rhodamine B: manufactured by SIGMA) was mixed with an aqueous solution of hemoglobin (manufactured by SIGMA) (solution 1). Here, rhodamine B is a substance that causes two-photon absorption by light having a wavelength of 1064 nm. Using this solution 1 as a measurement object, laser irradiation was performed to detect acoustic waves. The wavelength of the laser used here is 1064 nm. FIG. 5 shows the incident pulse energy dependence of the detected acoustic wave. As shown here, the output of the photoacoustic wave is nonlinear with respect to the pulse energy, and the acoustic wave is generated by multiphoton absorption. This means that the photoacoustic wave can be detected only from the focal portion where the light intensity is strong, and the deep part of the living body can be visualized with high resolution.

  (Test Example 2) Further, the improvement in resolution of photoacoustic tomography by using multiphoton excitation was estimated. The HE-stained rat liver slice (thickness of several tens of μm) that causes two-photon absorption in the near-infrared wavelength region is applied with a 800-nm near-infrared pulse laser (two-photon absorption) and a 488-nm visible laser (one-photon absorption) Condensing with a double objective lens, the position dependency of the absorption amount of two-photon and one-photon in the optical axis direction was measured (FIG. 6). Here, the amount of absorption was measured by fluorescence intensity. The measurement results show that two-photon absorption occurs only near the focal point compared to one-photon absorption. There is a difference of several times in the half-value width of the position dependency of the one-photon absorption and the two-photon absorption in the optical axis direction. By using multiphoton absorption, the resolution can be improved several times in the depth direction.

  (Test Example 3) Two-photon absorber Rhodamine B (Rhodamine B: manufactured by SIGMA) Chloroform solution (Solution 2) is placed in a 1 mm-thick glass cell and placed in a water tank, and 1064 nm and 532 nm pulse lasers are focused and irradiated. did. The photoacoustic wave generated from the solution 2 was measured with a 10 MHz acoustic transducer (10K6.4I: manufactured by Japan Probe Co., Ltd.) which was also installed in the water tank. Here, rhodamine B is a substance that produces two-photon absorption and one-photon absorption, respectively, with light having wavelengths of 1064 nm and 532 nm. The glass cell containing the solution 2 was moved with respect to the optical axis direction (depth direction), and the change in the photoacoustic signal waveform was measured (FIG. 7). As can be seen from FIG. 7, the one-photon absorption excitation photoacoustic wave generated by 532 nm light is generated even when the object to be measured is not at the focal position, but the two-photon absorption excitation photoacoustic wave generated by 1064 nm light is measured. Occurs only when the body is near focus. Thus, it is shown that the spatial resolution is improved in the depth direction by using the multiphoton absorption excitation photoacoustic wave as compared with the conventional one-photon excitation photoacoustic tomography.

  Therefore, the photoacoustic tomography apparatus and the photoacoustic tomography method of the present embodiment can be suitably used particularly for imaging of capillaries and small arteriovenous 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.

  As described above, the best mode for carrying out the photoacoustic tomography apparatus and the photoacoustic tomography method of the present invention has been described using the embodiments. However, the present invention is not limited to the above embodiments, and the present invention is not limited thereto. Within this range, appropriate changes are allowed.

  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.

  The operation of the photoacoustic tomography apparatus as described above will be described taking as an example the case of performing the above-described brain function imaging. 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 and the photoacoustic tomography method of the present invention are not limited to the observation of a living body as described above, and are applied to, for example, nondestructive inspection of various samples such as product inspection of semiconductor elements. Is possible. In this case, it is desirable to use an appropriate wavelength according to the multiphoton absorption of the substance to be detected.

Claims (13)

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 absorption by irradiating a measured object with a pulsed laser;
b) an acoustic wave detecting means for detecting an acoustic wave generated from the measurement object by the multiphoton absorption;
c) signal processing means for imaging substance distribution information inside the measurement object based on the detection result by the acoustic wave detection means;
A photoacoustic tomography apparatus comprising:   The photoacoustic tomography apparatus according to claim 1, wherein the pulse laser is any one of a nanosecond pulse laser, a picosecond pulse laser, and a femtosecond pulse laser.   Furthermore, it has laser scanning means for two-dimensionally or three-dimensionally scanning the focal position of the pulse laser, and the signal processing means is based on the intensity information of the acoustic wave generated from each focal position. The photoacoustic tomography apparatus according to claim 1 or 2, wherein internal two-dimensional or three-dimensional material distribution information is acquired and imaged.   The photoacoustic tomography apparatus according to claim 1, wherein the pulse laser irradiation unit irradiates a near-infrared pulse laser.   5. The photoacoustic tomography apparatus according to claim 4, wherein the near-infrared pulse laser has a wavelength capable of specifically exciting a blood component by two-photon absorption.   The pulse laser irradiation means can irradiate a near-infrared pulse laser having a wavelength that can specifically excite oxyhemoglobin by two-photon absorption and a wavelength that can specifically excite reduced hemoglobin by two-photon absorption. The signal processing means measures the blood oxygen concentration inside the measurement object based on the result of detecting the acoustic wave at each wavelength by the acoustic wave detection means, and images the distribution information. The photoacoustic tomography apparatus according to claim 5. In the photoacoustic tomography method that visualizes the substance distribution inside the measurement object using the photoacoustic effect,
a) irradiating an object to be measured with a pulse laser to generate an acoustic wave by multiphoton absorption only at the focal position of the laser;
b) detecting the acoustic wave;
c) imaging the substance distribution information inside the measurement object based on the detection result of the acoustic wave;
A photoacoustic tomography method comprising:   8. The photoacoustic tomography method according to claim 7, wherein the pulse laser is any one of a nanosecond pulse laser, a picosecond pulse laser, and a femtosecond pulse laser.   The object to be measured is irradiated while scanning the focal position of the pulse laser two-dimensionally or three-dimensionally, and an acoustic wave generated at each focal position is detected. Based on the detection result, the inside of the object to be measured is detected. 9. The photoacoustic tomography method according to claim 7, wherein two-dimensional or three-dimensional material distribution information is acquired and imaged.   10. The photoacoustic tomography method according to claim 7, wherein a near infrared pulse laser is used as the pulse laser.   The photoacoustic tomography method according to claim 10, wherein the near-infrared pulse laser has a wavelength capable of specifically exciting a blood component by two-photon absorption.   The object to be measured is irradiated with a near-infrared pulse laser having a wavelength that can specifically excite oxyhemoglobin by two-photon absorption and a wavelength that can specifically excite reduced hemoglobin by two-photon absorption as the pulse laser, The photoacoustic tomography method according to claim 11, wherein blood oxygen concentration distribution information inside the subject is acquired and imaged based on detection results of acoustic waves generated at each wavelength.   13. The photoacoustic tomography method according to claim 7, wherein a substance whose multiphoton absorption is induced by irradiation with the near infrared pulse laser is used as a contrast agent.

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