Patent Application
20110172513
The present invention relates to a biological information imaging apparatus.
Recently, biological information imaging apparatuses for acquiring an image from information of a living body by using technologies such as X-rays, ultrasonic waves, and Magnetic Resonance Imaging (MRI) technologies have been widely used in a medical field. In addition, optical imaging apparatuses where a light irradiated from a light source such as a laser is allowed to propagate through a living body or the like and the propagated light and the like is detected so as to obtain information on the living body have been actively researched in the medial field. As one of the optical image technologies, a photoacoustic tomography (PAT) is proposed (for example, referred to Non-Patent Document 1).
In a photoacoustic tomography, a pulse light generated from a light source is irradiated to a living body that is a specimen, and an acoustic wave generated from a biological tissue that absorbs energy of the light propagating and diffusing through the living body is detected at a plurality of positions. In the specification, the acoustic wave is sometimes referred to as a “photoacoustic wave”. Next, the signal is analyzed, so that information of optical property values of the living body is displayed as an image. Therefore, the information of the optical property distribution of the living body, particularly, an optical energy absorption density distribution can be acquired in an easily visible form.
According to Non-Patent Document 1, in the photoacoustic tomography, an initial sound pressure P0 of a photoacoustic wave generated from a light absorber located at a specific position in the specimen due to light absorption can be expressed by the following formula (1).
[Formula 1]
P
0=Γ·μa·Φ (1)
Herein, Γ is a Grüneisen coefficient obtained by dividing a product of a thermal expansivity β and a square of a speed of sound c by a specific heat at constant pressure CP. In addition, μa is an optical absorption coefficient of the light absorber, and Φ is a light amount in a local region (a light amount irradiated to the light absorber located in a specific position; sometimes, referred to as light fluence). Since the parameter Γ is known to be substantially constant according to the tissues of the living body, the distribution of the product of the optical absorption coefficient βa and the light amount Φ, that is, optical energy absorption density distribution of the specimen can be obtained by measuring and analyzing a time change of the sound pressure P that is a magnitude of the acoustic wave at a plurality of the positions.
In the above conventional photoacoustic tomography, as understood from the formula (1), the distribution of the optical absorption coefficient μa of the specimen cannot be obtained by acquiring only the optical energy absorption density distribution through the measurement of the time change in the sound pressure P. In other words, the distribution of light amount Φ irradiated to the light absorber that generates the photoacoustic wave as well as the optical energy absorption density distribution needs to be obtained in some way.
The light irradiated to the living body is attenuated through the living body. Under the assumption that a light amount Φ0 irradiated by a light source is constant and the light is irradiated to a larger region than a propagation length of the light in the living body so that the light propagates through the living body like a plane wave, the distribution of light amount Φ of the living body can be approximated to the following formula (2).
[Formula 2]
Φ=Φ0·exp(−μeff·d1) (2)
Herein, μeff is an average effective attenuation coefficient in the living body. The term “average effective attenuation coefficient” means the “effective attenuation coefficient under the assumption that optical properties are spatially uniform in the living body”. In addition, d1 is a distance (that is, a depth) from the region (light irradiated region) of the living body which a light is irradiated from a light source to the light absorber in the living body.
In this case, the initial sound pressure P1 of the generated photoacoustic wave can be expressed by the following formula (3) based on the formula (1).
[Formula 3]
P
1=Γ·μa·Φ=Γ·μa·Φ0·exp(−μeff·d1) (3)
Therefore, the distribution of the optical absorption coefficient μa of the specimen can be acquired by obtaining the average effective attenuation coefficient μeff. Although the average effective attenuation coefficients μeff of the living body are already known with regard to some portions, the effective attenuation coefficients μeff are different among persons. In addition, the distribution of light amount Φ is exponentially changed with respect to the average effective attenuation coefficient μeff as expressed in the formula (2). Therefore, if the average effective attenuation coefficient μeff is different, the distribution of light amount Φ becomes greatly different. If there is an error in the distribution of light amount Φ, the distribution of the optical absorption coefficient μa of the specimen obtained as the result is also greatly different from the correct value. Therefore, there is a need to measure the average effective attenuation coefficient μeff of each person. In addition, as optical coefficients of the living body, there are an optical absorption coefficient μa, an equivalent scattering coefficient μs′, an effective attenuation coefficient μeff, and the like, and the following formula (4) is satisfied therebetween.
[Formula 4]
μeff=√{square root over (3μa·(μs′+μa))} (4)
The present invention has been made in view of the above issues and an object thereof is to provide an imaging apparatus for a biological image using a photoacoustic tomography capable of more accurately acquiring a distribution of an optical absorption coefficient μa of a specimen by obtaining an average effective attenuation coefficient μeff unique to the living body that is the specimen in advance.
In order to solve the above problems, there is provided an imaging apparatus having the following configuration. That is, the biological information imaging apparatus of the invention comprising:
a light source unit having a single light source or a plurality of light sources;
an acoustic wave detector that detects an acoustic wave generated from a light absorber in a living body which absorbs a portion of energy of a light irradiated to the living body by the light source unit and converts the acoustic wave to a first electrical signal;
a photo-detector that detects intensities of the light corresponding to a plurality of propagation distances of the light irradiated to the living body by the light source unit and propagates through the living body and converts the intensities of the light to a second electrical signal;
a signal processing apparatus that derives an average effective attenuation coefficient of the living body based on the second electrical signal and derives an optical property distribution of the living body based on the first electrical signal and the average effective attenuation coefficient; and
an image constructing apparatus that constructs an optical property distribution image of the living body based on the optical property distribution of the living body derived by the signal processing apparatus.
According to the biological information imaging apparatus of the invention, it is possible to more accurately obtain the average effective attenuation coefficient μeff unique to the living body that is the specimen and to more accurately acquire the distribution of the optical absorption coefficient μa of the living body.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
Preferred embodiments of the present invention will now be described in detail in accordance with the accompanying drawings.
In the biological information imaging apparatus according to the embodiment, a specimen 100 that is a living body is interposed and fixed between two fixing members 101. In addition, a first light 102 irradiated from a first light source 103 is guided to the specimen 100 through an optical unit 104 constructed with lens and the like to be irradiated to the specimen 100. At this time, energy of the first light 102 is absorbed by a light absorber 105 such as a blood vessel, so that an acoustic wave 106 is generated. The acoustic wave 106 is detected by an acoustic wave detector 107 and converted to a first electrical signal.
On the other hand, a second light 108 emitted from a second light source 109 is irradiated to the specimen 100 through a light waveguide 113. The second light 108 that propagates the specimen 100 to be emitted from the specimen 100 is detected by a photo-detector 110 that is disposed to face an irradiated portion of the second light 108 with the specimen 100 interposed therebetween and converted to a second electrical signal. The first electrical signal and the second electrical signal are analyzed by a signal processing unit 111, so that an optical property distribution of the specimen 100 is calculated from the signals. In the signal processing unit 111, image data representing the calculated optical property distribution are constructed. A display apparatus 112 displays the optical property distribution as an image by using the image data. In addition, the fixing members 101 are configured to transmit the first light 102 and the second light 108. In other words, the fixing members 101 may be made of a material of transmitting the first light 102 and the second light 108. In addition, the fixing member 101 may be configured so that the specimen 100 is exposed at the irradiated position.
Herein, an initial sound pressure of the acoustic wave is expressed by the formula (1) as described above. Therefore, under the assumption that the Grüneisen coefficient Γ is a constant value in tissues of the living body, a generation distribution of the initial sound pressure can be obtained by measuring and analyzing a time change of the sound pressure P detected at a plurality of the positions by the acoustic wave detector 107. In addition, the distribution of the product of the optical absorption coefficient μa and the light amount Φ (optical energy absorption density distribution) can also be obtained. However, only the distribution of the product of the optical absorption coefficient μa and the light amount Φ (optical energy absorption density distribution) can be obtained from the first electrical signal obtained by the acoustic wave detector 107. Therefore, in order to obtain the distribution of the optical absorption coefficient μa of the specimen, the optical energy absorption density distribution needs to be corrected with the light amount Φ.
On the other hand, in the case where the light is irradiated to a much larger region than a propagation length of the light in the specimen 100, the light amount Φ can be expressed by the following formula (2). Accordingly, since the light amount Φ can be obtained by obtaining the average effective attenuation coefficient μeff of the specimen 100, distribution of the optical absorption coefficient μa of the specimen 100 can be obtained.
In the embodiment, the second electrical signal obtained by detecting the second light 108 is used so as to obtain the average effective attenuation coefficient μeff. Herein, the photo-detector 110 scans the fixing member 101, so that the second light can be detected at a plurality of the positions. On the other hand, the second light 108 that is irradiated from the second light source 109 is irradiated to a predetermined position in a spot shape through the light waveguide 113. At this time, as shown in
In addition, the light detection is performed at a plurality of the positions, and the detected light amount is plotted according to the distance. The average effective attenuation coefficient μeff can be obtained by performing fitting to the plotted result, using the theoretical formula expressing the distribution of the optical amount distribution in the specimen 100, which depends on the shape of the specimen 100 (this is, the theoretical formula of the distribution of the intensity (in the specimen 100) of the light irradiated to the specimen 100 and propagates through the specimen 100). Although the photo-detector 110 is scanned so as to change the distance between the irradiated position of the second light 108 and the photo-detector 110 in the embodiment, as shown in
As a result, the average effective attenuation coefficient μeff of the living body is obtained, and the light amount Φ is obtained by using the obtained average effective attenuation coefficient μeff. Next, the distribution of the optical absorption coefficient μa of the specimen can be obtained by correcting the distribution of the product of the optical absorption coefficient μa and the light amount Φ (optical energy absorption density distribution) obtained from the first electrical signal with the obtained light amount Φ. More specifically, the value of the optical energy absorption density may be divided by the light amount at each local position of the specimen.
Next, an example of a fitting model is described with reference to
If the specimen in
Herein, μ is a distance from the point that faces the irradiated position 300 with the specimen 100 interposed therebetween to the photo-detector 110, and C is a coefficient depending on diffusion. In addition, ri is a distance between an i-th pseudo light source and the photo-detector 110 and is a function of ρ and a diffusion coefficient. For the approximation, the diffusion coefficient is set to an integer number.
Therefore, the light amount is detected by changing the ρ, and as shown in
Next, the embodiment is described in detail. In
In addition, although the number of the first light source 103 is one in the embodiment, a plurality of the light sources may be used. In this case, in order to increase the intensity of the light irradiated to the living body, a plurality of the light sources oscillating at the same wavelength may be used. In addition, in order to measure a difference in wavelength in the optical property distribution, a plurality of the light sources having different oscillation wavelengths may be used. In addition, if a dye laser, an optical parametric oscillator (OPO) or a titan sapphire laser, of which the oscillating wave length is convertible, can be used as the light source 103, the difference in wavelength in the optical property distribution can be measured. It is preferable that the wavelength of the first light source 103 is in a range of 700 nm to 1100 nm, where the absorbance is low in the living body. In addition, in the case where the optical property distribution of the biological tissue relatively in the vicinity of the surface of the living body is obtained, the wavelength range wider than the above wavelength range, for example, a range from 400 nm or more to 1600 nm or less may be used. The wavelength range of the second light source 109 may be the same as the above wavelength range.
The second light source is used to irradiate the light that is to be detected by the photo-detector 110. It is preferable that the second light source 109 is a light source that can generate an intensity-modulated light. The second light source 109 may generate a continuous light having a waveform different from that of the pulse light. In addition, the second light source 109 may generate a pulse light similarly to the first light source 103. More specifically, a laser is preferably used. However, instead of the laser, a photodiode or the like may be used. As an example of the laser, a semiconductor laser is preferable. However, a gas laser, a dye laser, a solid state laser, and other various lasers may be used.
The first light 102 irradiated from the first light source 103 may be irradiated to the specimen by using only the optical unit 104 or be propagated by using the light waveguide or the like. It is preferable that the light waveguide is an optical fiber. In the case where the optical fiber is used, a plurality of the optical fibers may be used for a plurality of the light sources so as to guide the light to the surface of the living body. In addition, the light beams from a plurality of the light sources may be introduced to a single optical fiber, so that all the light beams can be guided to the living body by using only one optical fiber. The optical unit 104 shown in
It is preferable that the light waveguide 113 that guides the second light 108 from the second light source 109 into the living body is an optical fiber. In addition, it is preferable that the second light 108 is irradiated to the specimen 100 in a spot shape. The light absorber 105 in the specimen 100 is a portion having a high optical absorption coefficient in the specimen 100. For example, if a human body is the object of measurement, the light absorber may be hemoglobin, a blood vessel containing a large amount of hemoglobin, or a malignant tumor. The acoustic wave detector (or probe) 107 detects the acoustic wave 106 generated from the light absorber 105 absorbing a portion of energy of the first light 102 propagating through the living body and converts the acoustic wave to the first electrical signal. The acoustic wave detector 107 may be any sound wave detector that can detect the acoustic wave signal such as a transducer using a piezo-electric phenomenon, a transducer using a resonance of light, and a transducer using a change of capacitance. In addition, an array of the transducers may be used, and a single transducer may be used.
In addition, in the embodiment, in order to detect the acoustic wave 106 at a plurality of positions, the surface of the fixing member 101 is two-dimensionally scanned by a single acoustic wave detector 107, so that the acoustic wave 106 can be detected at a plurality of the positions. Alternatively, if the acoustic wave 106 can be detected at a plurality of the positions, the same effect can be obtained. Therefore, a plurality of the acoustic wave detectors may be disposed on the surface of the fixing member 101. In addition, it is preferable that an acoustic impedance matching material such as gel or water is interposed between the acoustic wave detector 107 and the fixing member 101 so as to suppress the reflection of the acoustic wave 106.
The photo-detector 110 detects the second light 108 that propagate and transmit through the specimen (living body) 100 and converts the second light 108 to the second electrical signal. The photo-detector 110 may be any optical detector capable of detecting light such as a photodiode, an avalanche photodiode, a photomultiplier tube, and CCD. In addition, in the embodiment, in order to change the distance between the irradiated position of the second light 108 and the photo-detector 110 and to detect the light at a plurality of the position, the surface of the fixing member 101 is scanned by a single photo-detector 110. However, if the light can be detected at a plurality of the positions, the same effect can be obtained. As described above, a plurality of the photo-detectors 110 may be disposed on the surface of the fixing member 101.
In addition, if the measurement of detecting the acoustic wave 106 generated due to the irradiation of the first light 102 by the acoustic wave detector 107 is denoted by a first measurement and the measurement of detecting the light due to the irradiation of the second light 108 by the photo-detector 110 is denoted by a second measurement, it is preferable that the first measurement and the second measurement are not simultaneously performed. In this case, the first and second measurements may be alternately performed. In addition, after the one of the measurements is completed, the other may be performed.
The signal processing unit 111 analyzes the first electrical signal and the second electrical signal and calculates information on the optical property distribution of the specimen (living body) 100 by using the signals. The signal processing unit 111 calculates the optical property distribution such as the distribution of the optical absorption coefficient μa and the optical energy absorption density distribution based on the first electrical signal obtained by the acoustic wave detector 107 and the second electrical signal obtained by the photo-detector 110. In addition, the signal processing unit 111 can calculate the position and size of the light absorber 105 in the specimen (living body) 100. In addition, the signal processing unit 111 may be any unit that can store the first electrical signal and the second electrical signal and converts the electrical signals to the data of the optical property distribution by a calculation unit. For example, an oscilloscope and a computer that can analyze the data stored in the oscilloscope can be used.
In this case, by the program stored in the computer, a calculation unit (CPU) may calculate the first electrical signal and the second electrical signal and convert the signals to the data of optical property distribution. In addition, by the program, the image data that are to be displayed on the display apparatus 112 may be constructed. Alternatively, a separate memory may be provided to the signal processing unit 111, so that the first electrical signal and the second electrical signal are stored in the memory. In addition, by a program separately stored in the memory of the signal processing unit 111, the calculation unit (CPU) may calculate the first electrical signal and the second electrical signal and convert the signals to the data of optical property distribution, so that the image data can be constructed.
The signal processing unit 111 obtains a generating distribution of the initial sound pressure P0 or a distribution of a product of an optical absorption coefficient μa and a light amount Φ (optical energy absorption density distribution) from the first electrical signal. In addition, the signal processing unit 111 obtains average effective attenuation coefficient μeff from the second electrical signal by using the fitting described above. In addition, the signal processing unit 111 obtains a distribution of optical absorption coefficient μa in specimen 100 by correcting the light amount by using the obtained average effective attenuation coefficient μeff with respect to the distribution of the product of the optical absorption coefficient μa and the light amount Φ (optical energy absorption density distribution). In addition, the signal processing unit 111 generates image data that are used to display information such as the generating distribution of the initial sound pressure P0, the distribution of the product of the optical absorption coefficient μa and the light amount Φ (optical energy absorption density distribution), and the distribution of the optical absorption coefficient μa on the image display apparatus 112. The image data corresponds to the optical property distribution image of the living body in the embodiment.
The image display apparatus 112 of
In step S102, a filter process is performed on the first electrical signal obtained in step S101. If the process of step S102 is completed, the procedure proceeds to step S103.
In step S103, optical energy absorption density distribution that is the distribution of the product of the optical absorption coefficient μa and the light amount Φ is calculated from the first electrical signal after the filter process. If the process of step S103 is completed, the procedure proceeds to step S104.
In step S104, the second electrical signal is acquired by the photo-detector 110. At this time, the photo-detector 110 detects the light transmitting the specimen 100 at a plurality of positions by scanning the fixing member 101. The detection corresponds to the detection of the intensities of the light propagating through the living body (light emitted from the living body after propagating the living body) corresponding to a plurality of propagation distances of the light. If the process of step S104 is completed, the procedure proceeds to step S105.
In step S105, a fitting process is performed. More specifically, values of parameters are set so that the second electrical signal (a plurality of the values acquired at a plurality of the positions) acquired in step S104 can be fitted to the theoretical formula of the light amount Φ expressed by the formula (5). If the process of step S105 is completed, the procedure proceeds to step S106.
In step S106, the average effective attenuation coefficient μeff is calculated in the state where the second electrical signal (a plurality of the values acquired at a plurality of the positions) is best fitted to the theoretical formula of the light amount Φ expressed by the formula (5) in step S105. The value becomes the average effective attenuation coefficient μeff of the specimen (living body) 100 in the measurement. If the process of step S106 is completed, the procedure proceeds to step S107.
In step S107, the light amount Φ is obtained from the average effective attenuation coefficient μeff calculated in step S106 and the formula (2), and the distribution of the optical absorption coefficient μa is calculated by correcting the optical energy absorption density distribution with the light amount Φ. In other words, the distribution of the optical absorption coefficient μa is calculated by correcting the distribution of the product of the optical absorption coefficient μa and the light amount Φ (optical energy absorption density distribution) with the light amount Φ. If the process of step S107 is completed, the procedure proceeds to step S108.
In step S108, the image data that are to be displayed on the display apparatus 112 are constructed from the optical absorption coefficient μa obtained in step S107. If the process of step S108 is completed, the main routine is ended.
The processes for the first electrical signal and the second electrical signal are not necessarily performed in accordance with the order in the flowchart. The processes (S104 to S106) for the second electrical signal may be firstly performed, and after that, the processes (S101 to S103) for the first electrical signal may be performed. Alternatively, the acquisition processes (S101 and S104) for the first and second electrical signals may be firstly performed, and after that, the other processes may be performed.
As described hereinbefore, by using the biological information imaging apparatus according to the embodiment, it is possible to accurately obtain the optical property distribution of the living body, particularly, the distribution of the optical absorption coefficient μa and to display the distribution as an image.
In addition, in the embodiment, the light source unit is configured to include a first light source 103, an optical unit 104, a second light source 109, and a light waveguide 113. In addition, in the embodiment, the signal processing unit 111 corresponds to a signal processing apparatus and a image constructing apparatus.
In addition, in the flowchart, the process of step S101 corresponds to the acoustic wave detecting process. In addition, the process of step S103 corresponds to the absorption density distribution calculating process. The process of step S104 corresponds to the light detecting process. The process of step S106 corresponds to the average attenuation coefficient deriving process. The process of step S107 corresponds to the optical property distribution deriving process. The process of step S108 corresponds to the image constructing process. In addition, some of the steps of the flowchart may be executed by a program stored in the signal processing unit 111, and others may be executed manually.
In addition, in the case where the fixing member 101 is made of a material of transmitting the second light as described above, the photo-detector 110 may be fixed to the fixing member 101. In this case, the photo-detector 110 detects the intensity of light in the vicinity of the surface of the living body. On the other hand, in the case where the photo-detector 110 is directly mounted on the specimen 100, the photo-detector 110 detects the intensity of light in the surface of the living body.
In addition, although a blood vessel, a malignant tumor, or the like is exemplified as the light absorber 105 in the embodiment, the light absorber 105 of the invention is not limited thereto. For example, the contrast agent introduced into the living body may be treated as the light absorber 105. In addition, as described above, the effective attenuation coefficient μeff is calculated as the average optical property value of the living body, and the distribution of the optical absorption coefficient μa is obtained by using the value. Besides the effective attenuation coefficient μeff, by taking into consideration a relationship between the optical absorption coefficient μa and a scattering coefficient μs, an equivalent scattering coefficient μs′, and the like, the distribution of the optical absorption coefficient μa may be obtained by using the values of the scattering coefficient μs and the equivalent scattering coefficient μs′.
Now, a second embodiment of the invention will be described with reference to the drawings. The embodiment is an example where the second electrical signal is also detected by the first light 102 so as to obtain the effective attenuation coefficient μeff.
In the embodiment, the second electrical signal that is obtained by detecting the first light 102, which is irradiated from the first light source 103 and transmits the specimen (living body) 100, by the photo-detector 110 is used so as to obtain the average effective attenuation coefficient μeff of the specimen. In addition, the light detection is performed at a plurality of positions, and the detected light amount is plotted according to the distance between the light irradiated positions and the photo-detector 110. Similarly to the first embodiment, the average effective attenuation coefficient μeff is obtained by performing fitting to the plotted result, using the theoretical formula depending the shape of the specimen (living body) 100.
The distribution of the optical absorption coefficient μa of the specimen can be obtained by light-amount-correcting the distribution of the product of the optical absorption coefficient μa and the light amount Φ (optical energy absorption density distribution) obtained from the first electrical signal by using average effective attenuation coefficient μeff. Similarly to the first embodiment, the acoustic wave detector 107 detects the acoustic wave 106 generated from the light absorber 105 that absorbs a portion of energy of the light 102 propagating through the specimen (living body) 100 and converts the acoustic wave to the first electrical signal.
In addition, if the measurement of detecting the acoustic wave 106 generated due to the irradiation of the light 102 by the acoustic wave detector 107 is denoted by a first measurement and the measurement of detecting the light due to the irradiation of the light 102 by the photo-detector 110 is denoted by a second measurement, the first measurement and the second measurement may be simultaneously performed in the embodiment. Alternatively, the first and second measurements may be alternately performed. In addition, after the one of the measurements is completed, the other may be performed. The processes for the obtained first and second electrical signals and other components are the same as those of the first embodiment.
As described above, in the biological information imaging apparatus according to the embodiment, the first electrical signal and the second electrical signal are obtained by using the first light 102 emitted from the first light source 103. In other words, the distribution of the product of the optical absorption coefficient μa and the light amount Φ (optical energy absorption density distribution) and the average effective attenuation coefficient μeff of the living body can be obtained by using only the first light source 103.
Accordingly, the configuration of the apparatus can be simplified, so that it is possible to promote cost reduction. In addition, the first measurement and the second measurement can be simultaneously performed, so that it is possible to increase a degree of freedom of the measurement timings. In addition, the light source unit includes the first light source 103 and the optical unit 104 according to the embodiment, and the embodiment corresponds to the case where the light source unit includes a single light source.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2008-235543, filed on Sep. 12, 2008, and Japanese Patent Application No. 2009-208506, filed on Sep. 9, 2009, which are hereby incorporated by reference herein in their entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008235543 | Sep 2008 | JP | national |
| 2009-208506 | Sep 2009 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2009/066322 | 11/9/2009 | WO | 00 | 3/11/2011 |
