JP5645637B2 - Subject information acquisition apparatus and subject information acquisition method - Google Patents

Description

  The present invention relates to a subject information acquisition apparatus and a subject information acquisition method.

Research on an optical imaging apparatus that irradiates light to a living body from a light source such as a laser and images in-vivo information obtained based on incident light has been actively promoted in the medical field. One such optical imaging technique is Photo Acoustic Tomography (PAT).

  In photoacoustic tomography, a living body is irradiated with pulsed light generated from a light source, and acoustic waves (typically ultrasonic waves) generated from living tissue that absorbs the energy of pulsed light that has propagated and diffused in the living body are detected. . In other words, using the difference in absorption rate of light energy between the test site such as a tumor and other tissues, the elastic wave generated when the test site absorbs the irradiated light energy and expands instantaneously Is received by an acoustic wave detector (also called a probe or a transducer). By analyzing this received signal, an image proportional to the initial pressure distribution or the light absorption energy density distribution (product of the absorption coefficient distribution and the light quantity distribution) can be obtained.

  Further, these image information can be used for quantitative measurement of a specific substance in a subject, for example, hemoglobin concentration contained in blood and blood oxygen saturation, by measuring with light of various wavelengths. In recent years, preclinical research for imaging blood vessels of small animals using this photoacoustic tomography and clinical research for applying this principle to diagnosis of breast cancer and the like have been actively promoted.

  In an apparatus using the photoacoustic tomography technique, an acoustic wave measurement area may be insufficient for an area to be imaged. For example, this is a case where an acoustic wave can be received only from a specific direction, not from the entire circumference of the subject. In such a case, it is known that streak (radiation) artifacts appear on the image.

  On the other hand, Patent Document 1 discloses a method of obtaining image data of an absorption coefficient distribution by dividing the image data of the light amount distribution for each voxel or pixel.

Here, when the image data of the light absorption energy distribution obtained by the PAT apparatus of the measurement system having an insufficient acoustic wave measurement area as described above is obtained by the method of Patent Document 1, artifacts are obtained in the area where the light amount is weak. There has been a problem that an image in which is emphasized is output.
This is because the initial sound pressure image and the artifact cannot be distinguished from each other in the image, and the light amount correction is performed for both the initial sound pressure image and the artifact.

  In order to solve such a problem, in Patent Document 2, light amount correction is performed on an analog reception signal acquired from an acoustic wave probe using a variable gain amplifier, and then an absorption coefficient is obtained by image reconstruction. A method for calculating image data for a minute is disclosed.

  In this way, if the analog reception signal is corrected with a gain based on the light amount distribution of the subject, only the reception signal contributing to the contrast of the absorption coefficient distribution image can be corrected. Therefore, an image with fewer artifacts can be obtained as compared with the conventional method in which the image data itself of the initial sound pressure distribution is corrected with the image data of the light amount distribution.

JP 2010-088627 A International Publication No. 2010/024290

  However, in Patent Document 2, an analog reception signal is amplified by an amplifier in accordance with the light quantity distribution of the subject. Therefore, a variable gain amplifier capable of performing arbitrary amplification on a sound pressure time-resolved signal is provided for all reception elements. It is necessary to prepare for. As a result, there is a problem that the cost of the apparatus increases.

  In addition, in order to amplify the analog received signal itself, when the received signal that has not been corrected for light intensity is analyzed and adaptive processing is performed to obtain the light intensity distribution data of the subject, the light intensity distribution data is obtained before measurement. If not, there is a problem that the received signal cannot be corrected.

  The present invention has been made in view of the above problems, and an object thereof is to provide a technique for reducing an increase in artifacts due to light amount correction while suppressing the cost of the apparatus in photoacoustic tomography. .

The present invention employs the following configuration. That is, a light source, detects an acoustic wave generated from a subject irradiated with light from the light source, a detector comprising a receiving element for converting the signal of the time series, the light amount distribution when light is irradiated forming a memory, using the information representative of the pre-Symbol light amount distribution, a signal processing unit for correcting the signal of the time series, the subject information using the signal of the corrected time series for storing information representing to a information processing unit, wherein the signal processing unit, the light intensity distribution is converted into time data in the area in the center at a predetermined angle range of the receiving device, of the time series with the time data An object information acquiring apparatus that performs signal correction.
The present invention also employs the following configuration. That is, a light source, a detector including a receiving element that detects an acoustic wave generated from a subject irradiated with light from the light source and converts it into a time-series signal, and a light amount distribution when the light is irradiated A memory for storing information; a signal processing unit for correcting the time-series signal using information representing the light quantity distribution; and an information processing unit for forming subject information using the corrected time-series signal And the signal processing unit weights the light amount distribution based on directivity of the receiving element, converts the weighted light amount distribution into time data, and uses the time data to An object information acquiring apparatus that corrects time-series signals.

The present invention also employs the following configuration. That is, using a step of detecting an acoustic wave into a signal of the time series generated from the object irradiated with light, the information representing the light amount distribution when light is irradiated, the signal of the time series a step of correcting, using said signal corrected time series includes a step of forming the subject information, and as engineering for correcting the signal of the time series is given from the center of the receiving element An object information acquisition method comprising: converting the light amount distribution in a region within an angle range into time data; and correcting the time-series signal using the time data. is there.
The present invention also employs the following configuration. That is, using the information representing the light amount distribution when the light is irradiated, the step of detecting the acoustic wave generated from the object irradiated with light and converting it to the time-series signal, And correcting the time-series signal using the corrected time-series signal, and the step of correcting the time-series signal includes the directivity of the receiving element. Weighting based on the step, converting the weighted light amount distribution into time data, and correcting the time-series signal using the time data. This is an object information acquisition method .

  According to the present invention, in photoacoustic tomography, it is possible to reduce an increase in artifacts due to light amount correction while suppressing the cost of the apparatus.

The figure which showed typically the structure of the subject information acquisition apparatus. The flowchart explaining an example of a process of a received signal. FIG. 4 is a schematic diagram illustrating an example of a light amount distribution, a light amount distribution, and light amount correction. The schematic diagram which shows the measurement object and correction | amendment of an image in a 1st Example.

  Hereinafter, the present invention will be described in more detail with reference to the drawings. In principle, the same components are denoted by the same reference numerals, and description thereof is omitted.

(Subject information acquisition device)
The configuration of the subject information acquisition apparatus according to the present embodiment will be described with reference to FIG. The subject information acquisition apparatus of the present embodiment is an apparatus that acquires subject information related to optical characteristic values inside the subject as image data. In the present invention, the acoustic wave is typically an ultrasonic wave, and includes an elastic wave called a sound wave, an ultrasonic wave, an acoustic wave, a photoacoustic wave, and an optical ultrasonic wave. The subject information acquisition apparatus of the present invention receives acoustic waves (typically ultrasonic waves) generated in a subject by irradiating the subject with light (electromagnetic waves), and converts subject information into image data. Including a device utilizing the photoacoustic effect acquired as follows. The acquired object information includes the source distribution of acoustic waves generated by light irradiation, the initial sound pressure distribution in the object, or the optical energy absorption density distribution derived from the initial sound pressure distribution, the absorption coefficient distribution, It shows the concentration information distribution of the substances that make up the tissue. The substance concentration information distribution is, for example, an oxygen saturation distribution, an oxidized / reduced hemoglobin concentration distribution, or the like.

  The subject information acquisition apparatus according to the present embodiment includes a light source 11, an acoustic wave probe 17 as an acoustic wave detector, and a signal processing unit 19 as a basic hardware configuration. The pulsed light 12 emitted from the light source 11 is guided while being processed into a desired light distribution shape by an optical system 13 such as a lens, a mirror, an optical fiber, and a diffusing plate, and is irradiated onto a subject 15 such as a living body. When a part of the energy of the light propagating through the subject 15 is absorbed by a light absorber (such as a sound source) 14 such as a blood vessel, an acoustic wave (typically) is generated by the thermal expansion of the light absorber 14. (Ultrasonic wave) 16 is generated. This is sometimes called a “photoacoustic wave”. The acoustic wave 16 is received by the acoustic wave probe 17 and converted into an analog reception signal (electric signal). The analog reception signal is amplified or digitally converted by the signal collector 18 and then converted into image data of the subject by the signal processing unit 19. The image data is displayed as an image on the display device 20.

(Light source 11)
When the subject is a living body, the light source 11 emits light having a specific wavelength that is absorbed by a specific component among components constituting the living body. The light source may be provided integrally with the subject information acquisition apparatus of the present embodiment, or may be provided separately from the light source. As the light source, a pulse light source capable of generating pulsed light on the order of several nanometers to several hundred nanoseconds is preferable. Specifically, a pulse width of about 10 nanoseconds is used to efficiently generate photoacoustic waves. As the light source, a laser is preferable because a large output can be obtained, but a light emitting diode or the like can be used instead of the laser. As the laser, various lasers such as a solid laser, a gas laser, a dye laser, and a semiconductor laser can be used. Irradiation timing, light intensity, and the like are controlled by a light source control unit (not shown). In addition, this light source control part is normally integrated with the light source. In the present invention, the wavelength of the light source to be used is desirably a wavelength at which light propagates to the inside of the subject. Specifically, when the subject is a living body, the thickness is 500 nm or more and 1200 nm or less.

(Optical system 13)
The light 12 emitted from the light source 11 is guided to the subject while being processed into a desired light distribution shape by the optical system 13. It is also possible to propagate the light 12 using an optical waveguide such as an optical fiber. The optical system 13 is, for example, a mirror that reflects light, a lens that collects or enlarges light, or changes its shape, a diffusion plate that diffuses light, an optical fiber, or the like. Any optical component may be used as long as the light 12 emitted from the light source is irradiated on the subject 15 in a desired shape. Note that it is preferable to spread light over a certain area rather than condensing with a lens from the viewpoint of expanding the safety to the living body and the diagnostic area.

(Subject 15 and light absorber 14)
These do not constitute a part of the subject information acquisition apparatus of the present invention, but will be described below. The subject information acquisition apparatus of the present invention is mainly intended for blood vessel imaging, diagnosis of human and animal malignant tumors and vascular diseases, and follow-up of chemical treatment. Therefore, the subject 15 is assumed to be a living body, specifically, a target site for diagnosis such as breasts, fingers, and limbs of a human body or an animal. In the case of animals, in the case of a small animal such as a mouse, not only a specific part but also the whole small animal may be a target.

  The light absorber 14 inside the subject shows a relatively high absorption coefficient within the subject. Depending on the wavelength of light to be used, for example, if the human body is a measurement target, the light absorber 14 corresponds to a malignant tumor that contains a large amount of oxidized or reduced hemoglobin, blood vessels that contain them, or new blood vessels.

(Acoustic wave probe 17)
The acoustic wave probe 17 is a detector that detects acoustic waves generated on the surface of the subject and inside the subject by pulsed light. The acoustic wave probe 17 detects an acoustic wave and converts it into an electrical signal that is an analog reception signal (hereinafter also simply referred to as “analog signal”). Hereinafter, it may be simply referred to as a probe or a transducer. Any acoustic wave detector may be used as long as it can detect acoustic waves, such as a transducer using a piezoelectric phenomenon, a transducer using optical resonance, and a transducer using a change in capacitance. The acoustic wave probe 17 of the present embodiment typically has a plurality of receiving elements arranged one-dimensionally or two-dimensionally. By using such a multidimensional array element, acoustic waves can be detected simultaneously at a plurality of locations, and the measurement time can be shortened. As a result, the influence of the vibration of the subject can be reduced.
In the present invention, it is assumed that the acoustic probe 17 is installed on the subject 15 so as to receive an acoustic wave only in a specific direction.

(Signal collector 18)
The subject information acquisition apparatus of the present embodiment includes a signal collector 18 that amplifies an analog reception signal obtained from the probe 17 and converts it into a digital reception signal (hereinafter also simply referred to as “digital signal”). Is preferred. The signal collector 18 is typically composed of an amplifier, an A / D converter, an FPGA (Field Programmable Gate Array) chip, and the like. When there are a plurality of reception signals obtained from the probe, it is desirable that a plurality of signals can be processed simultaneously. Thereby, the time until the image is formed can be shortened. In the present specification, the “reception signal” is a concept including both an analog signal acquired from the probe 17 and a digital signal after AD conversion. And the received signal resulting from the photoacoustic wave from a light absorber is also called "photoacoustic signal."

(Signal processor 19)
As a main role, the signal processing unit 19 reconstructs an image using the digital reception signal obtained from the signal collector 18 and acquires optical characteristic value information inside the subject as image data. Further, as a characteristic process of the present invention, the signal processing unit 19 performs correction (hereinafter, also referred to as “light amount correction”) according to the light amount distribution on the digital reception signal obtained from the signal collector 18. As a result, an image with relatively few artifacts compared to the case where the light amount correction is not performed on the initial pressure distribution image or the light absorption energy absorption distribution image obtained when the measurement region is limited. Can be output. Note that the case where the measurement region is limited here is not from all directions surrounding the subject, but only in a specific direction as in the relationship between the subject 15 and the acoustic wave probe 17 in FIG. This indicates that the acoustic wave can be received only from.

  Usually, a computer such as a workstation is typically used for the signal processing unit 19, and reception signal processing, image reconstruction processing, and the like are performed by software that has been programmed in advance. Therefore, compared to the case where a hardware configuration such as a variable gain amplifier is added, the processing performed by the signal processing unit 19 can achieve the purpose at a low cost.

  The software used in the workstation includes, for example, a signal processing module 19a that performs noise processing and light amount correction on a received signal, an image reconstruction module 19b that performs image reconstruction, and a light amount distribution calculation that calculates a light amount distribution of the subject. It consists of a module 19d. The signal processing unit 19 of the present invention has a memory 19c for storing the light amount distribution of the subject, and the signal processing module corrects the received signal while referring to the data stored in the memory 19c. However, the signal processing module 19a, the image reconstruction module 19b, the memory 19c, and the light amount distribution calculation module 19d are normally handled as a computer such as a workstation and attached software. Therefore, the three modules and memories are often handled as one signal processing device 19 as shown in FIG.

  In the signal processing module 19a, as a basic function, the light amount correction corresponding to the light amount distribution data of the subject 15 is performed on the digital reception signal obtained from the signal collector 18. The light amount distribution data of the subject 15 is stored in the memory 19c. The stored light quantity distribution data may be calculated by the light quantity distribution calculation module 19d, or may be light quantity distribution data calculated in advance elsewhere. Here, the light quantity distribution data refers to data obtained by integrating the light quantity at each position in the subject irradiated with light.

  The light quantity distribution calculation module 19d calculates the light quantity distribution in the subject by using a calculation method capable of calculating the light quantity distribution of the subject such as a living body such as a finite element method or the Monte Carlo method. The memory 19c stores the light amount distribution data calculated by the light amount distribution calculation module 19d and the light amount distribution data calculated in advance. The light quantity distribution calculation module corresponds to the calculation unit of the present invention.

  In the image reconstruction module 19b, as a basic function, image data is formed by image reconstruction using received signal data after light amount correction obtained from the signal processing module 19a. As an image reconstruction algorithm, for example, back projection in the time domain or Fourier domain, which is usually used in tomography technology, is used. Note that when it is possible to spend a lot of time for reconstruction, an image reconstruction technique such as an iterative reconstruction method (iterative method) that performs iterative processing can also be used. As typical image reconstruction methods of PAT, there are a Fourier transform method, a universal back projection method, a filtered back projection method, a sequential reconstruction method, and the like, and any image reconstruction method is used in the present invention. It doesn't matter. The image reconstruction module corresponds to the image processing unit of the present invention.

(Display device 20)
The display device 20 is a device that displays the image data output from the signal processing unit 19, and typically uses a liquid crystal display or the like. In addition, you may provide separately from the diagnostic imaging apparatus of this invention.

(Received signal processing)
Next, an example of a digital received signal light amount correction process and an image reconstruction process performed by the signal processing unit 19, which is a characteristic process of the present invention, will be described with reference to FIGS. 2 and 3. The following numbers correspond to the process numbers in the flowchart of FIG.

  Process (1) (S200): Step of calculating the light quantity distribution in the subject

  The light amount distribution calculation module 19d calculates the light amount distribution inside the subject according to the absorption coefficient distribution, equivalent scattering coefficient distribution, light irradiation intensity distribution, and subject shape of the subject.

As the absorption coefficient distribution and the equivalent scattering coefficient distribution of the object, when physical property values are already known, such as a living body simulation phantom, the values are used. On the other hand, when the values of the absorption coefficient distribution and the equivalent scattering coefficient distribution are unknown, they can be estimated from the received signal using a known method. In this case, the digital reception signal obtained from the signal collector 18 is analyzed and an average absorption coefficient or equivalent scattering coefficient that is an optical characteristic value of the subject is estimated. When estimation from a received signal is difficult, a value can be estimated from statistical data described in literatures.
The light irradiation intensity distribution is a light intensity distribution in a region irradiated on the subject, and can be obtained in advance if the apparatus configuration and the subject shape are determined.
The subject shape can be obtained from imaging data from a camera or the like, or the shape of the subject can be shaped into a specific shape using a compression plate or the like.

  If such a data group is known, the light quantity distribution of the subject can be obtained by calculation by solving the light transport equation or the light diffusion equation. The light transport equation or the light diffusion equation can be solved by using a method already known in the numerical calculation field such as the Monte Carlo method, the difference method, or the finite element method.

  As described above, various methods can be considered for the light amount distribution calculation method of the subject. Here, an example will be described. For example, it is assumed that the shape of the subject, the average absorption coefficient, the equivalent scattering coefficient, and the light irradiation intensity are obtained in advance. Next, when the light quantity distribution data obtained by solving the light diffusion equation by the finite element method is obtained using the information, the light quantity distribution data as shown in FIG. 3A is obtained. In this example, an example in which the light 12 is incident from the opposite direction to the acoustic wave probe 17 as shown in FIG. 1 is shown.

  In FIG. 3A, the amount of light in the vicinity of the light irradiation is large, and the light attenuates and the amount of light decreases as the distance from the light irradiation increases. The light amount distribution calculation process as described above can be calculated in advance if the absorption coefficient distribution, equivalent scattering coefficient distribution, light irradiation intensity distribution, and object shape, which are set values necessary for the calculation, are determined. Therefore, it can be calculated in advance to reduce the calculation time. In this case, as shown in the signal processor 19 of FIG. 1, it is not necessary to incorporate the light amount distribution calculation module 19d in the apparatus. However, when the absorption coefficient distribution or the scattering coefficient distribution of the subject is estimated from the received signal, it is necessary to calculate after the signal is received by the signal collector 18. In that case, as shown in FIG. 1, the light quantity distribution calculation module 19d is required.

  Process (2) (S201): Step of saving the light amount distribution data calculated in process (1) in the memory

  When the signal processor 19 is a computer such as a workstation, the light quantity distribution data calculated by the above processing is stored in a file in the hard disk, and the data is stored in the memory 19c in the workstation when used. Alternatively, simultaneously with the calculation, the result is stored in the memory 19c. As long as the light quantity distribution data can be stored in the memory 19c as described above, any method may be used.

  Process (3) (S202): A step of correcting the received signal according to the light amount distribution (the amount of light amount) using the light amount distribution data stored in the memory.

  A light amount distribution profile in a direction perpendicular to the receiving surface of the receiving element corresponding to the receiving element of the acoustic wave probe 17 is calculated from the light amount distribution data stored in the memory. Next, the distance data of the light amount distribution profile is converted into time data according to the average sound speed of the subject. The influence of the light amount distribution is corrected by gradually calculating the light amount distribution profile converted into the time data with respect to the received signal data.

For example, the light amount distribution profile may be created at the center of the directivity range of the corresponding receiving element. In this case, in the case of an acoustic wave detector in which receiving elements are two-dimensionally arranged, a light amount distribution profile is created in a direction perpendicular to the receiving surface.

  The above processing will be described in detail with reference to FIG. FIG. 3B is a plot of the light quantity distribution profile in the region indicated by the dotted line a in FIG. 3A with the horizontal axis representing distance (distance from the receiving element of the ultrasonic probe 17) and the vertical axis representing light quantity. It is. The dotted line a is a direction perpendicular to the receiving surface of one receiving element included in the acoustic wave probe 17. Here, since light is irradiated from the opposite side to the receiving element through the subject, the light amount increases as the distance from the receiving element increases. At a position close to the receiving element side, the amount of light is small because the irradiated light is attenuated.

In the data of FIG. 3B, the horizontal axis is changed to time by using the average sound speed of the subject. That is, the distance may be divided by the speed of sound. The data obtained there is shown in FIG. In FIG. 3C, the X-axis in FIG. 3B is merely changed from distance to time (time from light irradiation to reception by the receiving element), and the shape of the graph does not change.
In other words, the light amount distribution profile is data indicating the amount of light on a line associated with a value (distance or time) representing the depth of the subject when a line is drawn from a certain receiving element in a direction perpendicular to the subject. is there. Therefore, the light quantity distribution profile can be created from the light quantity distribution data.

  In the above description, an example in which the light amount profile for correcting each received signal is an area perpendicular to the receiving surface of the receiving element is shown, but the light amount profile creation method is not limited to this. A dotted line b in FIG. 3A indicates a range of a predetermined angle θ that is the center of one receiving element. At this time, as the light intensity profile calculated for each receiving element, the light intensity profile obtained by integrating the light intensity values that are in the range of the angle θ from the center of the receiving element and equidistant from the center of the receiving element with the directivity. May be used. In this case, the angle range is an angle range in which the receiving element can receive an acoustic wave, and is usually determined from directivity.

  FIG. 3D is an example of a digital reception signal obtained by the receiving element of the acoustic wave probe. In the graph of FIG. 3D, the horizontal axis means that the received time is earlier as it goes to the right. Usually, the photoacoustic wave 16 generated from the light absorber 14 inside the subject 15 has an N-like shape as shown in FIG. 3D, and is in a region close to the probe. A photoacoustic wave generated from a certain light absorber is received earlier. In addition, a photoacoustic wave generated from a light absorber having the same shape and the same absorption coefficient is received with a larger sound pressure as it is closer to the receiving element and as the amount of light absorbed is larger.

Usually, attenuation (diffraction effect) of sound pressure due to propagation of sound waves of a received signal is corrected in image reconstruction and reflected in an image. On the other hand, since the light amount is not corrected in the image reconstruction, the image after the image reconstruction is an image affected by the light amount distribution. More specifically, the image obtained after reconstructing the received signal data as shown in FIG. 3D is usually the initial sound pressure distribution p ₀ (r). The initial sound pressure distribution p ₀ (r) is proportional to the product of the absorption coefficient μ _a (r) and the light quantity φ (r) as shown in the following equation (1). The product of μ _a (r) and φ (r) is called the light absorption energy density H (r).

As is clear from this equation (1), the value of the initial sound pressure distribution p ₀ (r) or the light absorption energy density H (r) is affected by the light quantity φ (r). In the conventional method, as shown in the following equation (2), in order to obtain the image data of the absorption coefficient distribution from the reconstructed image, the obtained image data p ₀ (r) or H (r) is φ Divide by (r) to obtain μ _a (r) and eliminate the effect of light quantity. In this case, if there is an artifact in the image data of H (r) or p ₀ (r), it is also corrected by the amount of light. Therefore, there is a problem that the artifact existing there is emphasized in a region where the amount of light is small.

  On the other hand, the present invention is characterized in that the digital reception signal is corrected in accordance with the amount of light emitted to each position (each part) in the subject. That is, the digital reception signal is corrected so as to reduce the difference in the amount of light in the subject. Specifically, the light amount distribution data converted into the time data of FIG. 3C is divided for each reception time with respect to the digital reception signal of FIG. For example, the data obtained by this correction process is shown in FIG. As can be seen from this figure, the signal in the reception time zone with a low light amount is emphasized, and the influence of the light amount is eliminated. By performing optical correction on the digital reception signal in this way, it is possible to perform optical correction only on the photoacoustic signal resulting from the photoacoustic wave from the light absorber.

The above will be described using equations. Here, in order to simplify the discussion, the data is handled discretely from the beginning. First, the image of the initial sound pressure distribution p ₀ (r), which is a continuous value, is discretized and treated as a matrix, and the matrix representation of the initial sound pressure distribution is P ₀ . A sensitivity matrix for converting a photoacoustic wave generated from the discretized data of the initial sound pressure distribution into acoustic wave data detected by each receiving element is Y. Y is a Jacobian matrix. Further, when the matrix expression of the sound pressure data detected by each receiving element is D, the following expression (3) is established.
D = Y · P ₀ (3)

Further, since P ₀ is expressed by the product of the absorption coefficient distribution and the light amount distribution as described above, it can be expressed as P ₀ = Ma · Φ. Here, Φ is a matrix expressing the light quantity distribution, and Ma is a matrix expressing the absorption coefficient distribution. That is, Expression (3) becomes the following Expression (4).
D = Y · M _a · Φ (4)
If an inverse matrix Φ ⁻¹ of Φ is applied to both sides, Expression (4) becomes the following Expression (5).
D · Φ ⁻¹ = Y · M _a (5)

Equation (5) calculates an inverse matrix of Φ, applies it to the sound pressure data matrix D detected by each receiving element, and reconstructs an image using the data (D · Φ ⁻¹ ), thereby obtaining an absorption coefficient. It shows that image data Ma of distribution can be obtained. At this time, it is not necessary to correct the light amount distribution on the image data as in equation (2). As a result, the light quantity distribution correction can be performed on the received signal, so that artifacts generated after image reconstruction are not increased.
Note that the inverse matrix of Φ is, for example, a light amount value equidistant from the center of the receiving element, integrated by weighting with directivity, and approximately, the light amount in a direction perpendicular to the receiving surface of the receiving element. It can be expressed by a distribution profile. In addition, although an example is given here as an inverse matrix of Φ, any correction means can be applied to the digital received signal if each received signal is corrected so as to reduce the influence of the light amount distribution in the subject. May be used. That is, any correction means may be used as long as each received signal is corrected so as to reduce the difference in the amount of light in the subject.

  Process (4) (S203): A step of performing image reconstruction processing using the digital received signal whose light amount has been corrected to form image data related to the optical characteristic value distribution of the subject.

Image reconstruction is performed using the digital reception signal with the light amount corrected shown in FIG. 3E to form image data related to the absorption coefficient distribution of the subject 15. For this process, any image reconstruction process used in normal photoacoustic tomography can be used. For example, back projection in the time domain or Fourier domain.

ここで、μ_ａ（ｒ）は吸収係数分布、Ａは比例定数、ｂ（ｒ_０，ｔ）は投影データ、d
Ω₀は任意の観測点Pに対する検出器dS₀の立体角である。この投影データを式（６）の積
分に従って逆投影することで吸収係数分布μ_ａ（ｒ）の画像データを得ることができる。 For example, when the universal back projection method, which is one of the time domain methods, is used for the calculation, image reconstruction is performed according to the following equation (6).

Here, μ _a (r) is an absorption coefficient distribution, A is a proportional constant, b (r ₀ , t) is projection data, d
Ω ₀ is the solid angle of the detector dS ₀ with respect to an arbitrary observation point P. Image data of the absorption coefficient distribution μ _a (r) can be obtained by back projecting the projection data according to the integration of Expression (6).

また、dΩ₀は、以下の式（８）である。

ここで、ｐ’は光量分布補正された検出音波データ、θは検出器と任意の観測点Pとが
なす角度である。これらの式で表したように、本発明では、通常の画像再構成とは異なり、光量分布補正された検出音波データを投影データとして用いることが特徴である。 B (r ₀ , t) is the following equation (7).

DΩ ₀ is the following formula (8).

Here, p ′ is detected sound wave data whose light quantity distribution is corrected, and θ is an angle formed by the detector and an arbitrary observation point P. As represented by these formulas, the present invention is characterized in that the detected sound wave data corrected for the light amount distribution is used as projection data, unlike normal image reconstruction.

  In the example of FIG. 1, in the image formation, the acoustic wave reception range is limited, and artifacts are generated. Even in such a case, by performing the above steps, the initial sound pressure distribution image or the light absorption energy density distribution image is not affected by the artifact without increasing the cost compared to the conventional technique for correcting the light amount. An image with little image deterioration can be provided.

<Example 1>
An example of an object information acquisition apparatus using photoacoustic tomography to which the present embodiment is applied will be described. This will be described with reference to the schematic diagram of FIG. In this example, a Q-switched YAG laser that generates pulsed light of about 10 nanoseconds at a wavelength of 1064 nm was used as the light source 11. The energy of the light pulse emitted from the pulsed light 12 is 0.6 J. The optical system 13 is set so that the pulsed light 12 is spread to a radius of about 2 cm using an optical system 13 such as a mirror and a beam expander, and then the subject can be irradiated with the light on the side opposite to the probe 17. .

A rectangular phantom as shown in FIG. 4A simulating a living body was used as the subject 15. The phantom was obtained by solidifying intralipid diluted to 1% with agar. This phantom was optically homogeneous and had an average absorption coefficient of about 0.01 mm ⁻¹ and a scattering coefficient of about 0.7 mm ⁻¹ . Moreover, the size of this phantom was set to width: 5 cm, height: 5 cm, and depth: 4 cm.
As shown in FIG. 4A, eight cylindrical rubber wires having a diameter of 0.3 mm are embedded as light absorbers 14 in the phantom as shown. Note that the absorption coefficient values of the respective light absorbers 14 are the same. Also, phantoms and acoustic wave probes are placed in a water tank filled with degassed water for acoustic matching.

  The phantom thus set was irradiated with the pulsed light 12 on the surface of the phantom opposite to the probe 17 as shown in FIG. As the acoustic wave probe 17, an ultrasonic transducer made of PZT (lead zirconate titanate) was used. This transducer is a two-dimensional array type, the number of elements is 345 (15 × 23), and the element pitch is 2 mm. The width of the element is about 2 mm.

  As shown in FIG. 4A, when the pulsed light 12 is irradiated on the phantom surface, a photoacoustic wave generated by the light being absorbed by the cylindrical light absorber 14 is generated. Those photoacoustic waves were simultaneously received by the ultrasonic transducer 17 through 345 channels. The received signal was processed using a signal collector 18 composed of an amplifier, an AD converter, and an FPGA to obtain digital data of the received signal in all channels. In order to improve the S / N ratio of the signal, the laser was irradiated 32 times, and all the received signals obtained were averaged. Thereafter, the obtained digital data was transferred to a workstation (WS) as the signal processor 19 and stored in WS.

  Next, the light quantity distribution data in the phantom was obtained by the finite element method using the optical characteristic value of the phantom, the phantom shape, and the light irradiation intensity distribution in the signal processing module which is a software program in the WS. Further, light quantity distribution data in the direction perpendicular to the receiving surface of each receiving element was calculated from the light quantity distribution data, and each received data was corrected with the light quantity. Here, a correction method is used in which received data is divided by a light amount profile corrected to time data.

  Furthermore, image reconstruction was performed by the reconstruction module 19a, which is a software program in the WS, using this correction data. Here, three-dimensional volume data is formed using a universal back projection method which is a time domain method among a plurality of image reconstruction methods. The voxel interval used at this time was 0.025 cm. The imaging range is 3.0 cm × 4.6 cm × 4.0 cm. An example of the image obtained at that time is shown in FIG. This figure shows a MIP (Maximum Intensity Projection) image obtained by projecting the maximum luminance in the direction in which all absorbers can be imaged in the three-dimensional image data.

  Next, using the received signal stored in the WS and not corrected for light quantity, the image reconstruction method employed above is used, and the same range as described above is 3.0 cm × 4.6 cm × 5.0 cm. Images were calculated. Further, the image was corrected with the light amount distribution as in the conventional technique. Here, a method of dividing each voxel value of the reconstructed image data by a voxel value of the light amount distribution data was used. An example of the image obtained at that time is shown in FIG. FIG. 4C is also an MIP image obtained by projecting the maximum luminance in the direction in which all absorbers can be imaged from the three-dimensional image data.

FIG. 4B and FIG. 4C are compared. In FIG. 4C, for the light amount correction after the image reconstruction, artifacts in the region indicated by the dotted line in the drawing, that is, the region where the light amount is weak are conspicuous. This is a result of emphasizing all images in an area with a small amount of light by light amount correction. On the other hand, in FIG. 4B, since the light amount correction is performed on the received signal, the artifact is not emphasized. That is, since only the received signal is corrected for light quantity, the artifact is not emphasized. As described above, when the acoustic wave measurement region is limited, an image with fewer artifacts than the conventional method can be obtained by correcting the light amount of the digital reception signal.

<Example 2>
An example of an object information acquisition apparatus using photoacoustic tomography to which the present embodiment is applied will be described. In this example, the same phantom and measurement system as in Example 1 were used. However, in this embodiment, the optical characteristic value of the subject is obtained by analyzing the received signal.

First, as in FIG. 1, the photoacoustic wave is received by irradiating light from the opposite direction to the probe 17. Next, although not shown in FIG. 1, the photoacoustic wave is received by irradiating light from the probe direction. Image reconstruction is performed for each received signal, and two initial sound pressure distribution images are acquired. Next, these ratios were taken, and the average absorption coefficient and equivalent scattering coefficient of the subject were determined so that the ratio between the ratio data and the light distribution obtained by calculation was minimized. Next, the light quantity distribution of the subject was obtained by the light quantity distribution calculation module 19c using the obtained average absorption coefficient and equivalent scattering coefficient of the subject, the subject shape, and the irradiation intensity distribution. Next, the light amount of the received signal was corrected with this light amount distribution data in the same manner as in Example 1, and image reconstruction was performed using the corrected received signal to obtain an image.
The image obtained by such a method is the same as that shown in FIG. 4B, and the artifacts are reduced compared to FIG. 4C, which is an image obtained by the conventional technique.

  From the above, even if the received signal is adaptively analyzed to obtain the optical constant of the subject and the received signal is optically corrected using the value, an image with fewer artifacts than the conventional method can be obtained.

<Example 3>
An example of an object information acquisition apparatus using photoacoustic tomography to which the present embodiment is applied will be described. In this example, the same phantom and measurement system as in Example 1 were used.

First, the light quantity distribution in the phantom was calculated in the same manner as in Example 1. Next, a light quantity profile was calculated by integrating the values of the light quantity distribution within 30 degrees in all angles from the center of each receiving element and equidistant from the center of the receiving element, weighted by the directivity of the detecting element. Using this light quantity profile, each received signal data was corrected in the same manner as in Example 1, and corrected received signal data was created. Using this corrected received signal data, image reconstruction was performed to calculate a relative absorption coefficient distribution image.
As a result, it was possible to obtain an absorption coefficient distribution image in which artifacts were reduced as compared with the prior art and quantitative characteristics were improved as compared with FIG.

  From the above, it is possible to obtain an optical characteristic value distribution image in which artifacts are reduced as compared with the prior art while maintaining quantitativeness by correcting the received signal with a light amount profile considering the directivity of the probe. .

  11: Light source, 17: Acoustic wave probe, 19: Signal processing unit, 19a: Signal processing module, 19b: Image reconstruction module, 19c: Memory, 19d: Light quantity distribution calculation module

Claims (18)

A light source;
A detector including a receiving element that detects an acoustic wave generated from a subject irradiated with light from the light source and converts the acoustic wave into a time-series signal ;
A memory for storing information representing the Kino light amount distribution and the light is irradiated,
Using the information representative of the pre-Symbol light amount distribution, a signal processing unit for correcting the signal of the time series,
An information processing unit which forms the subject information using the signal of the corrected time series,
Have
The signal processing unit converts the light amount distribution in a region within a predetermined angle range from the center of the receiving element into time data, and corrects the time-series signal using the time data. Subject information acquisition apparatus. A converter that converts the time-series signal converted by the receiving element into a time-series digital signal;
The signal processing unit corrects the time-series digital signal using the time data.
The object information acquiring apparatus according to claim 1, wherein: The signal processing unit converts the light amount distribution in a region within the predetermined angle range into the time data representing a total value of light amounts at positions equidistant from the receiving element corresponding to each reception time.
The subject information acquiring apparatus according to claim 1 or 2, wherein The signal processing unit weights the light amount distribution in the region within the predetermined angle range based on the directivity of the receiving element, and converts the weighted light amount distribution into the time data.
4. The subject information acquisition apparatus according to claim 1, wherein The predetermined angle range is an angle range determined from directivity of the receiving element.
5. The subject information acquisition apparatus according to claim 1, wherein A light source;
A detector including a receiving element that detects an acoustic wave generated from a subject irradiated with light from the light source and converts the acoustic wave into a time-series signal;
A memory for storing information representing a light amount distribution when light is irradiated;
Using the information representing the light amount distribution, a signal processing unit for correcting the time-series signal;
An information processing unit that forms subject information using the corrected time-series signal;
Have
The signal processing unit weights the light amount distribution based on directivity of the receiving element, converts the weighted light amount distribution into time data, and corrects the time-series signal using the time data. I do
A subject information acquisition apparatus characterized by the above. A converter that converts the time-series signal converted by the receiving element into a time-series digital signal;
The signal processing unit corrects the time-series digital signal using the time data.
The object information acquiring apparatus according to claim 6. The signal processing unit converts the weighted light amount distribution into the time data representing a total value of light amounts at positions equidistant from the receiving element corresponding to each reception time.
The object information acquiring apparatus according to claim 6 or 7, characterized in that Divide the time series signal by the time data
The object information acquiring apparatus according to claim 1, wherein the object information acquiring apparatus is an object information acquiring apparatus. The said signal processing part correct | amends the said time-sequential signal so that the difference in the light quantity irradiated to each position in the said subject may be reduced. The any one of Claim 1 to 9 characterized by the above-mentioned. The subject information acquisition apparatus described. The signal processing unit, pre-Symbol object information acquiring apparatus according to any one of claims 1 to 10, characterized in that it comprises a calculator for calculating a light amount distribution. The arithmetic unit, the using object of the optical characteristic value and shape, the object information acquiring apparatus according to claim 11, characterized in that to calculate the light intensity distribution in the subject. The object information acquiring apparatus according to claim 12 , wherein the calculation unit obtains an optical characteristic value of the object by analyzing the time-series signal. The signal processing unit on the basis of the previous SL light amount distribution, in a direction perpendicular from the reception surface before Ki受 signal elements, to create a light intensity distribution profile showing a relationship between the depth and light intensity of the subject, the light intensity distribution The subject information acquisition apparatus according to any one of claims 1 to 13 , wherein the time data is created using a profile and a sound velocity in the subject. The information processing unit generates image data based on the subject information.
The object information acquiring apparatus according to claim 1, wherein: Detecting an acoustic wave generated from the subject irradiated with light and converting it into a time-series signal ;
Using the information that represents the light amount distribution when light is irradiated, the step of correcting the signal of the time series,
Forming the subject information using the signal of the corrected time series,
Have
As Engineering for correcting the signal of the time series,
Converting the light amount distribution in a region within a predetermined angle range from the center of the receiving element into time data;
And a step of correcting the time-series signal using the time data . Detecting an acoustic wave generated from the subject irradiated with light and converting it into a time-series signal;
Correcting the time-series signal using information representing a light amount distribution when light is irradiated; and
Forming subject information using the corrected time-series signal;
Have
The step of correcting the time-series signal includes:
Weighting the light quantity distribution based on the directivity of the receiving element;
Converting the weighted light quantity distribution into time data;
Correcting the time-series signal using the time data.
2. A method for acquiring subject information, characterized in that: Converting the time-series signal into a time-series digital signal;
In the step of correcting the time series signal, the time series digital signal is corrected using information representing the light amount distribution.
The object information acquiring method according to claim 16 or 17, characterized in that:

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