JP2017086173A - Subject information acquisition device and control method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a device for satisfactorily acquiring an optical coefficient of the background of a subject in photoacoustic tomography.SOLUTION: A subject information acquisition device includes an element 203 for receiving an acoustic wave generated when light is irradiated to a subject 204 and outputting an electric signal, and an information acquisition part 208 for acquiring an optical coefficient inside the subject using the electric signal. The information acquisition part generates intensity distribution data corresponding to an initial sound pressure distribution inside the subject using the electric signal and ultrasonic attenuation characteristics of the subject, acquires a plurality of signal intensities according to a light propagation distance inside the subject from the value included in the intensity distribution data, acquires a light quantity distribution inside the subject, and acquires the optical coefficient inside the subject using the plurality of signal intensities and the light quantity distribution.SELECTED DRAWING: Figure 2

Description

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

  In recent years, photoacoustic tomography has been proposed as one of optical imaging techniques. An apparatus using photoacoustic tomography irradiates a test object with pulsed light and propagates and diffuses the test object. And the acoustic wave (photoacoustic wave) generated from the absorber which absorbed light energy is detected, and signal processing is performed. As a result, the characteristic information related to the optical characteristic value inside the inspection object can be acquired and imaged.

  In order to obtain the absorption coefficient distribution, which is an optical characteristic inside the object to be inspected, from the photoacoustic wave, it is necessary to obtain the distribution of the amount of light irradiated to the absorber. However, since the light introduced into the object to be inspected is absorbed and diffused, it is difficult to estimate the amount of light irradiated to the absorber. Therefore, in photoacoustic tomography, a light energy absorption density distribution obtained by multiplying the absorption coefficient distribution by the amount of light may be imaged.

  The apparatus disclosed in Patent Document 1 extracts two or more regions having known absorption coefficients from a photoacoustic image, and obtains optical characteristics (such as initial sound pressure) in the regions. Next, the light amount distribution is calculated using preset optical coefficients of the subject background (average absorption coefficient, scattering coefficient, etc. inside the subject). Then, the optical coefficient of the subject background is estimated by calculation using the light quantity distribution and the optical characteristics.

JP2012-223283A

  However, in photoacoustic tomography, values related to the calculation of light quantity distribution such as optical coefficients are necessary elements for obtaining characteristic information, and therefore a method for obtaining these values better is required. Yes.

  The present invention has been made in view of the above problems. An object of the present invention is to satisfactorily acquire the optical coefficient of the background of a subject in photoacoustic tomography.

The present invention employs the following configuration. That is,
An element that receives an acoustic wave generated by irradiating the subject with light and outputs an electrical signal;
An information acquisition unit that acquires an optical coefficient inside the subject using the electrical signal;
Have
The information acquisition unit
Using the electrical signal and the ultrasonic attenuation characteristics of the subject to generate intensity distribution data corresponding to the initial sound pressure distribution inside the subject,
From a value included in the intensity distribution data, obtain a plurality of signal intensity according to the propagation distance of the light inside the subject,
Obtaining a light intensity distribution inside the subject;
An object information acquiring apparatus that acquires the optical coefficient inside the object using the plurality of signal intensities and the light amount distribution.

The present invention also employs the following configuration. That is,
An element that receives an acoustic wave generated by irradiating the subject with light and outputs an electrical signal;
An information acquisition unit that acquires an optical coefficient inside the subject using the electrical signal;
A method for controlling a subject information acquisition apparatus comprising:
The information acquisition unit
Generating intensity distribution data corresponding to an initial sound pressure distribution inside the subject using the electrical signal and the ultrasonic attenuation characteristics of the subject;
Obtaining a plurality of signal intensities according to the propagation distance of the light inside the subject from values included in the intensity distribution data;
Obtaining a light amount distribution inside the subject;
Obtaining the optical coefficient inside the subject using the plurality of signal intensities and the light amount distribution;
A control method for a subject information acquiring apparatus.

  According to the present invention, in the photoacoustic tomography, the optical coefficient of the background of the subject can be acquired satisfactorily.

The figure explaining the principle in Example 1 Photoacoustic apparatus in Embodiment 1 Flowchart in the first embodiment The figure explaining signal strength distribution in Example 1. Photoacoustic apparatus in Embodiment 2 The figure which scans the light irradiation position in Example 2 Diagram for examining absorber position and damping characteristics

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. However, the dimensions, materials, shapes, and relative arrangements of the components described below should be appropriately changed depending on the configuration of the apparatus to which the invention is applied and various conditions. Therefore, the scope of the present invention is not intended to be limited to the following description.

  The present invention relates to a technique for detecting acoustic waves propagating from a subject, generating characteristic information inside the subject, and acquiring the characteristic information. Therefore, the present invention can be understood as a subject information acquisition apparatus or a control method thereof, a subject information acquisition method, or a signal processing method. The present invention can also be understood as a program that causes an information processing apparatus including hardware resources such as a CPU and a memory to execute these methods, and a storage medium that stores the program.

  The subject information acquisition apparatus of the present invention receives an acoustic wave generated in a subject by irradiating the subject with light (electromagnetic waves), and acquires the subject's characteristic information as image data. Includes devices that use. In this case, the characteristic information is characteristic value information corresponding to each of a plurality of positions in the subject, which is generated using a reception signal obtained by receiving a photoacoustic wave.

The characteristic information acquired by photoacoustic measurement is a value reflecting the absorption rate of light energy. For example, a generation source of an acoustic wave generated by light irradiation, an initial sound pressure in a subject, a light energy absorption density or absorption coefficient derived from the initial sound pressure, and a concentration of a substance constituting a tissue are included. Further, the oxygen saturation distribution can be calculated by obtaining the oxygenated hemoglobin concentration and the reduced hemoglobin concentration as the substance concentration. In addition, glucose concentration, collagen concentration, melanin concentration, fat and water volume fraction, and the like are also required. In addition, a two-dimensional or three-dimensional characteristic information distribution is obtained based on the characteristic information of each position in the subject. The distribution data can be generated as image data. The characteristic information may be obtained not as numerical data but as distribution information of each position in the subject. That is, distribution information such as initial sound pressure distribution, energy absorption density distribution, absorption coefficient distribution, and oxygen saturation distribution may be used as the subject information. These are collectively referred to as optical characteristic information distribution.

  The acoustic wave referred to in the present invention is typically an ultrasonic wave and includes an elastic wave called a sound wave or an acoustic wave. An electric signal converted from an acoustic wave by a probe or the like is also called an acoustic signal. However, the description of ultrasonic waves or acoustic waves in this specification is not intended to limit the wavelength of those elastic waves. An acoustic wave generated by the photoacoustic effect is called a photoacoustic wave or an optical ultrasonic wave. An electrical signal derived from a photoacoustic wave is also called a photoacoustic signal.

<Examination>
Consider a case where there are a first absorber 701 and a second absorber 702 at the same depth from the surface 704 of the subject, with the same conditions such as size and absorption coefficient, as shown in FIG. There is a third absorber 703 between the surface 704 and the first absorber 701. On the other hand, there is no absorber between the surface 704 and the second absorber 702. In this case, the amount of light applied to the second absorber 702 is a value after the amount of light applied to the subject surface is attenuated by being absorbed and scattered according to the background optical coefficient. On the other hand, the amount of light applied to the first absorber 701 is affected by the reduction due to light absorption by the third absorber 703 in addition to the reduction according to the background optical coefficient. As a result, the signal intensity of the photoacoustic wave generated from the first absorber 701 is smaller than that of the second absorber 702.

  Next, consider a case where absorbers having similar conditions are arranged at different depths. In this case, due to the attenuation of the acoustic wave, the signal from the absorber at the deep position becomes weaker than the signal from the absorber at the shallow position. Moreover, since the amount of attenuation of light increases as the depth increases, the intensity of the photoacoustic wave is further decreased. From these, it can be seen that in order to accurately calculate the optical coefficient, it is necessary to acquire the acoustic attenuation characteristic of the subject by some method.

[Example 1]
(principle)
The principle of the present invention will be described. Inspected objects 102a and 102b shown in FIGS. 1A and 1B respectively have media having different optical coefficients. This figure schematically shows a state in which photons propagate through the medium when the same amount of measurement light 101 is incident on each of the inspection objects 102a and 102b. The optical coefficient includes an absorption coefficient, a scattering coefficient, an anisotropy factor related to an angle at the time of scattering, and the like. The optical coefficient of the medium does not appear as a strong signal in the photoacoustic image, unlike an absorber such as a blood vessel.

After the incidence of light, photons propagate through the material while being scattered by the scatterer 103. Also, if there is light absorption, the photon disappears. In general, when the absorption coefficient is high, the distance that propagates through the substance becomes short. Also, if the scattering coefficient (μ _s ) is high, the mean free path (which can be approximated to the inverse of the scattering coefficient) is shortened, and the number of scattering increases. Conversely, if the scattering coefficient is low, the mean free path becomes longer and the number of times of scattering decreases.

FIG. 1A shows a case where the scattering coefficient is high. FIG. 1B shows a case where the scattering coefficient is relatively low. Both distances until the photon is absorbed are the same, but the number of scattering until then is different. That is, in FIG. 1A, after the photon is scattered three times, it is absorbed at a distance that reaches the fourth scatterer. On the other hand, in FIG. 1B, photons are absorbed by the third scatterer after being scattered twice. The measurement light 101 is irradiated in parallel with the z-axis that is the depth direction. In addition, the scattering angle shown here was one of zero degrees, 45 degrees, and -45 degrees ahead of the traveling direction.

  The measurement light 101 has a large number of photons. The direction of scattering and the occurrence of absorption for one photon are determined with a certain probability. Therefore, the overall light amount distribution can be statistically processed. Further, after entering the medium, the light is scattered not only in the z direction but also in various directions such as the xy direction. Therefore, in the case of FIG. 1A where the number of scattering is large, the amount of light reaching a deep position in the z direction is relatively small. On the other hand, in the case of FIG. 1B, since the number of times of scattering is relatively small, the probability that light spreads in the xy direction is small. As a result, a relatively large amount of light reaches a deep position in the z direction.

  When the optical coefficient is known, the light amount distribution is obtained by the Monte Carlo method, the method of analytically solving the diffusion equation, the finite element method, or the like based on the light amount at the time of irradiation or the shape of the subject. If the optical coefficient is calculated as a variable, various light intensity distributions can be obtained.

Γは弾性特性値であるグリューナイセン（Ｇｒｕｎｅｉｓｅｎ）係数であり、体積膨張係数（β）と音速（ｃ）の二乗の積を比熱（Ｃｐ）で割ったものである。μ_ａｏは吸収体での吸収係数である。Φは局所的な領域での光量（吸収体に照射された光量）である。 The sound pressure (P) generated when the absorber is irradiated with light is expressed by Equation (1).

Γ is a Gruneisen coefficient that is an elastic characteristic value, and is obtained by dividing the product of the square of the volume expansion coefficient (β) and the speed of sound (c) by the specific heat (Cp). μ _ao is an absorption coefficient in the absorber. Φ is the amount of light in the local region (the amount of light irradiated to the absorber).

Φ_０は、表面での入射光である。したがって式（２）は、光が深さ方向に進行するにつれ指数関数的に減衰することを示している。なお、μ_ｅｆｆは、媒体内での平均的な等価減衰係数である。等価減衰係数は、散乱係数や吸収係数を反映したものとして示される。ここで、生体が被検査物の場合、血管（動脈や静脈）のように、グリューナイセン係数や吸収係数が同程度の吸収体を複数含んでいる。このような被検査物を光音響トモグラフィーによって画像化すると、吸収体の位置における光量の分布を測定できる。計算結果と比較することにより光学係数を推定することができる。 The amount of light Φ can be expressed by, for example, Expression (2) using the depth function z.

Φ ₀ is the incident light at the surface. Thus, equation (2) indicates that light decays exponentially as it travels in the depth direction. Note that μ _eff is an average equivalent attenuation coefficient in the medium. The equivalent attenuation coefficient is shown as reflecting the scattering coefficient and the absorption coefficient. Here, when the living body is an object to be inspected, it includes a plurality of absorbers having the same Gruneisen coefficient and absorption coefficient, such as blood vessels (arteries and veins). When such an inspection object is imaged by photoacoustic tomography, the distribution of the light quantity at the position of the absorber can be measured. The optical coefficient can be estimated by comparing with the calculation result.

ここで、α：減衰係数、Ｐ_ｉ：初期音圧、ｆ：送信周波数、Ｌ：伝搬距離である。このように光音響波は指数的に減衰する。そのため、光学係数を算出するときの精度を上げるためにはこの減衰を補正する必要がある。典型的には、タイムゲインコントロール（ＴＧＣ）のような方法が用いられる。 Further, the photoacoustic wave is attenuated in the process of propagation after generation until reaching the probe. Therefore, it is preferable to perform the correction. In other words, the initial sound pressure generated in the absorber is attenuated before reaching the probe as shown in Equation (1). Equation (3) is the initial sound pressure (P _i )
And the sound pressure (P _d ) detected by the detector.

Here, α is an attenuation coefficient, P _{i is an} initial sound pressure, f is a transmission frequency, and L is a propagation distance. Thus, the photoacoustic wave attenuates exponentially. Therefore, it is necessary to correct this attenuation in order to increase the accuracy when calculating the optical coefficient. Typically, a method such as time gain control (TGC) is used.

(Photoacoustic device)
As an example of the photoacoustic apparatus of the present invention, an apparatus using a handheld probe will be described. FIG. 2A shows the arrangement of the probe and the light irradiation unit in the handheld probe. A line-shaped light irradiation unit 201 is in the center, and two-dimensional probes 202 are arranged on both sides thereof.

  FIG. 2B shows the configuration of the photoacoustic apparatus. The apparatus includes a photoacoustic probe 203, an optical control unit 205, an ultrasonic control unit 206, an apparatus control unit 207, an information acquisition unit 208, and a display unit 209. The photoacoustic probe 203 is arranged so that the probe surface is in contact with the inspection object 204. In this photoacoustic apparatus, the photoacoustic measurement can be performed by synchronizing the reception timing of the probe 202 with the light irradiated from the light irradiation unit 201. In addition, if ultrasonic waves are transmitted and received by the probe 202, ultrasonic measurement becomes possible. Note that separate probes may be prepared for photoacoustic use and ultrasonic echo measurement.

  The device control unit 207 issues a command related to control of the entire device such as light source and probe reception control. In addition, the device control unit has a user interface (UI), and based on instructions from the operator, change measurement parameters, start / end measurement, select an image processing method, save patient information and images, Data analysis can be performed. The information acquisition unit performs information processing such as image reconstruction. The obtained image is displayed on the display unit 209.

(Light irradiation part)
The light irradiation unit 201 is a line-shaped portion that irradiates pulse light that irradiates the inspection object 204. The pulsed light is guided from the light source to the light irradiation unit 201 by a bundle fiber. That is, a plurality of point light sources are arranged in a line, resulting in a line light source. In addition, the structure of an irradiation part is not restricted to this. Light may be magnified by a lens or the like, and a line-shaped light source may be formed by a slit. Here, the irradiation shape is a line shape in order to create a two-dimensional tomographic image, but it may be configured to irradiate light over a wide area of the subject.

  As the light source, a laser light source is desirable in order to obtain a large output. However, a light emitting diode or a flash lamp may be used. When a laser is used, various lasers such as a solid laser, a gas laser, a dye laser, and a semiconductor laser can be used. The timing, waveform, intensity, and the like of light irradiation are controlled by the light control unit 205.

  Further, in order to effectively generate the photoacoustic wave, it is necessary to irradiate light in a sufficiently short time according to the thermal characteristics of the inspection object. When the inspection object is a living body, the pulse width of the pulsed light generated from the light source is preferably about 10 to 50 nanoseconds. Further, it is desirable that the wavelength of the pulsed light is a wavelength at which the light propagates to the inside of the inspection object. Specifically, in the case of a living body, it is 700 nm or more and 1100 nm or less. Here, a titanium sapphire laser, which is a solid laser, is used, and the wavelengths are 760 and 800 nm. If it is the structure which can irradiate the light of several wavelengths, a substance density | concentration can be calculated using the difference in the degree of absorption for every wavelength.

(Probe)
The two-dimensional probe 202 is an element that receives photoacoustic waves and transmits / receives ultrasonic waves, and is also called a transducer. Examples of such elements include PZT (piezoelectric ceramics) and CMUT (capacitive micromachine probe). The handheld probe 202 of this embodiment is composed of, for example, 64 × 10 elements on one side. The element receives an acoustic wave and outputs an electrical signal.

The signal converted into an electrical signal by the probe is transmitted to the ultrasonic control unit 206, amplified by an amplifier, converted into a digital signal by an A / D converter, and sent to the device control unit 207. Note that the reception timing of the acoustic wave is controlled by the apparatus control unit so as to be synchronized with the light irradiation. The band of the probe 202 is, for example, 2 MHz-5 MHz. The sampling frequency is 50 MHz and 2048 sampling is performed. The data is signed 12 bits. Further, when generating an ultrasonic image, time gain control for compensating for attenuation according to the depth may be performed.

(Information acquisition unit)
The information acquisition unit 208 generates a photoacoustic image inside the subject by image reconstruction using a photoacoustic signal derived from a photoacoustic wave. In addition, it has information processing ability necessary for light quantity calculation and optical coefficient acquisition. Further, when obtaining the ultrasonic attenuation characteristic by ultrasonic measurement, an ultrasonic signal derived from the ultrasonic echo is processed. The information acquisition unit further performs a desired process such as signal correction. The information acquisition unit can be configured by an information processing apparatus including a processor and a memory. Each function of the information acquisition unit can be realized by each module of the program operating on the processor. Further, the information acquisition unit may be configured by an information processing device that is common to the light control unit and the device control unit.

(Signal processing)
The signal processing of the present invention will be described with reference to the flowchart of FIG.
In step S1, measurement is started. In this state, the surgeon holds the photoacoustic probe 203 and brings the probe 202 into contact with the object to be inspected via the acoustic matching gel.

  In step S2, ultrasonic measurement is performed. This process is performed in order to acquire the ultrasonic attenuation characteristic inside the inspection object used for signal correction or the like. However, a general value may be used according to the characteristics of the object to be inspected (for example, age, sex, or part in the case of a living body), or a value obtained by prior measurement may be used. Such a value can be acquired by storing in advance in a memory (not shown) or receiving input from the UI. In that case, this step is unnecessary.

  After transmitting the ultrasonic wave, the probe 202 receives a reflected signal from the inspection object. At this time, it is preferable to perform beam forming by appropriately setting the focal position and the like. Moreover, you may set a required frequency according to the site | part to measure. The information acquisition unit generates a B-Scan image in the direction parallel to the line-shaped light irradiation unit 201 as an ultrasonic image. Since the probe is arranged two-dimensionally, a three-dimensional ultrasonic image can be obtained. Here, advanced correction is not always necessary. However, when the photoacoustic apparatus also serves as an ultrasonic imaging apparatus, an ultrasonic image is presented to the user in addition to the photoacoustic image. In that case, correction such as TGC may be performed.

  Subsequently, the information acquisition unit extracts an area where scattering is uniform from the ultrasonic image, and acquires the ultrasonic attenuation characteristic represented by Equation (3) from the luminance attenuation in the depth direction. The ultrasonic attenuation may be calculated from the TGC value. Moreover, you may measure separately in the position suitable for measuring an ultrasonic attenuation characteristic. Note that a typical value is 0.5 dB / cm MHz when using a general ultrasonic attenuation characteristic of a part of the inspection object.

In step S3, photoacoustic measurement is performed. The light irradiation unit 201 emits pulsed light, and the probe 202 receives a photoacoustic wave in synchronization therewith. By performing photoacoustic measurement at different wavelengths, arteries, veins, tumors, and the like can be selectively imaged. The information acquisition unit generates a photoacoustic image by a known reconstruction method such as a universal back projection method or a phasing addition method. In the universal back projection method, the initial sound pressure distribution p (r) is expressed by Equation (4).

At this time, the term b (r _d , t) corresponding to the projection data is shown in Equation (5). Here, p _d (r ₀ ) is a photoacoustic signal detected by the detection element, r ₀ is the position of each detection element, t is time, and Ω _d is the solid angle of the probe.

  When performing reconstruction, ultrasonic attenuation is compensated according to the distance from the desired voxel position (reconstruction target position) to the probe 202. It is also preferable to correct the sensitivity to depth and frequency. The generation of the photoacoustic image in this step may be performed with the same degree of accuracy as the generation of the photoacoustic image finally presented to the user, or may be performed by a relatively simple process. Simple processing includes processing for thinning out data and speeding up calculation. Further, the processing may be simplified by reducing the measurement position during photoacoustic measurement. In that case, the photoacoustic image of this process may be used only for acquiring the optical coefficient, and the photoacoustic wave for generating the final presentation image may be acquired separately.

  The photoacoustic image acquired in this step reflects the initial sound pressure distribution inside the subject. As described above, the initial sound pressure distribution may be used as the photoacoustic image, or an energy absorption density distribution defined by the initial sound pressure and the absorption coefficient may be used. Since the initial sound pressure distribution or energy absorption density distribution is a set of signal intensity values for each position, it can also be called signal intensity distribution data.

  In this step, since the image accuracy changes depending on the location and orientation (angle) of the blood vessel inside the subject, it is preferable to correct it. For example, when the probe is arranged in a hemispherical container, the resolution at the time of imaging is high near the center of the hemisphere, and the resolution is lower toward the periphery. Therefore, when generating a photoacoustic image, it is preferable to correct the peripheral region by using a plurality of measurement data. In the case of a hemispherical container as shown in FIG. 5A, a blood vessel extending in the Z direction is less likely to appear in the image than a blood vessel extending in the X direction or the Y direction. Therefore, the angle dependency may be reduced by performing processing for enhancing the signal intensity of the absorber extending in the Z direction.

  In step S4, the information acquisition unit extracts a blood vessel from the photoacoustic image. In this step, an absorber having a desired shape can be extracted even if there are absorbers of different shapes such as tumors and blood vessels. Here, a blood vessel whose thickness is within a certain range is extracted as a desired absorber. The purpose of extracting a blood vessel of a desired shape is to reduce the influence of the frequency band included in the photoacoustic wave depending on the shape of the absorber, and the sensitivity of the probe 202 having frequency characteristics. . Here, blood vessels whose thickness is within a certain range (0.5 mm to 1.0 mm) are extracted. A general method can be used for blood vessel extraction. For example, a threshold value of a pixel value is determined and binarized, and a place where there is a signal is a blood vessel. Furthermore, only a blood vessel having a desired thickness is extracted by using an image filter such as a band-pass filter.

  In step S5, the information acquisition unit creates a signal intensity distribution. Here, blood vessels having the same (or similar) absorption coefficient are extracted from the blood vessels extracted in step S4. Specifically, for example, either an artery or a vein is selected and extracted.

Here, the creation of the signal intensity distribution will be described. FIG. 4A is a MIP image (Maximum Intensity Projection image: maximum value projection image) generated based on a three-dimensional photoacoustic image. When generating the MIP image, the information acquisition unit first sets the direction in which the line-shaped light irradiation unit extends as the Z axis (direction perpendicular to the paper surface), and generates a two-dimensional tomographic image at a plurality of positions in the Z direction. . Then, the signal intensity at the same position (pixel) is compared between a plurality of two-dimensional tomographic images to obtain the maximum intensity. By performing this process at all positions of the two-dimensional tomographic image, an MIP image that is a kind of signal intensity distribution data is obtained. It should be noted that when an image is generated by the maximum value projection, correction such as exclusion of abnormal values may be performed.

  Subsequently, in the three-dimensional photoacoustic image, the information acquisition unit sets the line-shaped light irradiation unit to the Z axis (direction perpendicular to the paper surface), the distance from the light source to R, and the angle from the axis in the depth direction (X axis). Set a cylindrical coordinate system in which is θ. In the MIP image, this coordinate system is represented by polar coordinates in which the distance R from the origin is the moving radius and the angle is θ.

  Then, the information acquisition unit selects a pixel having the maximum signal intensity I for each pixel group having the same distance R, and plots the signal intensity logarithmically. Thereby, a graph as shown in FIG. 4B is obtained. For example, in FIG. 4A, a pixel group having the same distance R as the absorber 403d is on the dotted line 404. The maximum intensity on the dotted line 404 corresponds to the lowest peak in FIG. In this way, the maximum value of the signal is obtained for each position group (pixel group) that is equidistant from the light irradiation unit 201 in the XY plane by projecting the maximum signal intensity on the R axis.

  In the example of FIG. 4, since a linear light source is used, a plot using the distance R was performed in the MIP image. On the other hand, when light is irradiated over a wide range, a plot using the depth z may be performed. In other words, the pixel having the maximum value is selected for each pixel having the same or similar light propagation distance inside the subject. When the subject is irradiated with light in a line shape as shown in FIG. 4 and the light source is shown as a dot in the projection image, the propagation distance of the light from the incident position is represented by R. The same applies to a point light source. On the other hand, the propagation distance when light is irradiated over a wide area in a planar shape is represented by a depth z. Therefore, if z is replaced with R, Equation (2) can be applied to FIG.

  Subsequently, the information acquisition unit creates an optical coefficient calculation function 405 so that the signal intensity does not intersect with other signals on the graph in which the signal intensity is indicated by the logarithmic axis as shown in FIG. The type of optical coefficient calculation function is not limited. For example, a linear function or curve that passes through the vertices of two peaks protruding in a logarithmic plot may be created. Moreover, you may create an envelope based on the vertex of each peak. A polynomial or the like may be used by fitting. When creating an optical coefficient calculation function, it is preferable to limit the depth range in order to avoid the influence of a strong signal generated near the surface.

  Subsequently, the information acquisition unit selects 406 and 407 that are regions corresponding to the peak in contact with the optical coefficient calculation function, and generates a signal intensity distribution as shown in FIG. The region portions 406 and 407 correspond to the reference numerals 403b and 403c in FIG. It can be estimated that the absorbers in the region portions 406 and 407 are of the same type. This is because the types of absorbers are limited to arteries, veins, tumors, etc., and it is known in advance how much intensity each absorber generates with respect to the wavelength of the selected irradiation light. Because. It is more preferable to select a region in which the position of an artery or vein is anatomically known. Further, the signal strength may be normalized by the maximum signal strength. Thereby, it is not necessary to consider the influence of the Gruneisen coefficient or the absorption coefficient of the absorber.

  In addition, the kind of absorber can also be separated by another method. For example, photoacoustic images of 760 nm and 800 nm are created, and oxygen saturation is calculated from these photoacoustic images. Then, the artery and vein are separated from the difference in oxygen saturation. As a result, it is possible to acquire a photoacoustic image made of the same kind of absorber such as only an artery or only a vein.

  In step S6, the information acquisition unit performs light amount distribution calculation. This light quantity distribution calculation process can be said to be substantially equivalent to the signal intensity distribution calculation process. This is because the parameters such as the absorption coefficient are the same among the same type of absorbers, and the signal intensity is proportional to the light quantity distribution. Therefore, under the conditions of this flow, obtaining the light amount distribution results in obtaining the signal intensity distribution. Therefore, comparison is possible by converting either the light amount or the signal intensity into the other.

  The information acquisition unit calculates the light amount distribution using the absorption coefficient, scattering coefficient, and anisotropy factor values as variables at the central positions of the absorbers 406 and 407 obtained in step S5. In addition, you may calculate in the arbitrary positions on the optical coefficient calculation function instead of the position of an absorber. For example, a calculation method such as a Monte Carlo method, an analytical method, or a finite element method can be used. In order to remove common parameters, it is preferable to perform a normalization process.

  In step S7, the information acquisition unit compares the measurement value of the signal intensity at the positions of the absorbers 406 and 407 selected from the signal intensity distribution in step S5 with the result of the light amount distribution calculation obtained in step S6 at those positions. . At this time, the comparison may be performed after converting the result of light amount distribution calculation into signal intensity, or conversely, the signal intensity may be converted into light intensity. Or you may compare by another parameter (for example, normalized value).

  Subsequently, the information acquisition unit compares both values at a plurality of positions, and proceeds to step S8 if the difference is within a predetermined set value. On the other hand, if the difference is larger than the set value, the process returns to step S6. For example, in the case of FIG. 4C, the signal intensity at the center of the two selected absorbers is compared with the light amount, and an affirmative determination is made when both of the comparison results are within the threshold value. Moreover, when there are many comparison positions (for example, 10 places), you may give priority to the comparison result in the position near a light source. This is because the absorber closer to the light source has less noise such as the influence of other absorbers, and therefore, it is considered that the numerical values of the light amount and the signal intensity are high. In addition, any method that statistically processes the difference value at each comparison position and compares it with the threshold value can be applied to this step.

  Then, the information acquisition unit changes the value of at least one of the absorption coefficient and the scattering coefficient, and performs calculation and determination again. This process is performed until the comparison result converges within a threshold value. As described above, when the comparison result is converged while changing the set values of the absorption coefficient and the scattering coefficient, and finally the optical coefficient is obtained, an iterative method such as the Newton method can be used.

  In step S8, the optical coefficient is determined. If the comparison result is within the set value in step S7, the value is calculated as a desired optical coefficient. And a process is complete | finished by S9 process. If the obtained optical coefficient is not in the expected range, the photoacoustic measurement may be performed again by changing the measurement location.

  Although details will be described later, the background optical coefficient of the subject thus obtained can be used for calculation of the light amount distribution. The light quantity distribution can be used when calculating the absorption coefficient distribution from the initial sound pressure distribution. The initial sound pressure distribution used at this time may be acquired in step S3, or may be acquired by performing photoacoustic measurement again. Or you may utilize the acquired optical coefficient for correction | amendment of the photoacoustic image already produced | generated. The acquired optical coefficient may be stored in the memory in association with the subject information.

As described above, according to the present invention, it is possible to calculate the background optical coefficient indicating light scattering and absorption inside the subject by calculation using the photoacoustic image. That is, the processing of the present invention can be easily realized by the mechanism provided in the photoacoustic apparatus. Further, when the photoacoustic signal or photoacoustic image used for acquiring the optical coefficient is reused for imaging inside the subject, the processing can be made efficient. Furthermore, in the present invention, the optical coefficient is calculated by comparing the signal intensity and the light amount value of the photoacoustic image, based on a mathematical expression indicating the relationship between the optical coefficient and the light attenuation while changing the scattering coefficient and the absorption coefficient. Therefore, information based on the actual state inside the subject can be acquired. As a result, the reconstruction accuracy of the photoacoustic image is also improved.

[Example 2]
(Photoacoustic device)
FIG. 5 shows a photoacoustic apparatus for breast measurement in this example. FIG. 5A is a cross-sectional view of a holding member for an inspection object and an acoustic wave measuring device in the photoacoustic apparatus. FIG. 5B is a plan view of the probe as seen from above through the holding member.

  Regarding the measuring instrument, 512 probes 502 are arranged in a spiral shape along the inner surface of the hemispherical container 501. Furthermore, a space through which measurement light from the light irradiation unit 503 passes is provided at the bottom of the hemispherical container 501. Then, the measurement light is irradiated from the negative direction of the z axis to the inspection object. The inspection object is disposed on the holding member 505. The holding member is preferably made of a material that transmits light and acoustic waves, such as polyethylene terephthalate. The inside of the hemispherical container 501 and the inside of the holding member 505 are filled with an acoustic matching material (for example, water or castor oil) as necessary.

  The relative positional relationship between the hemispherical container 501 and the inspection object varies depending on the XY stage (not shown). Then, substantially parallel pulsed light 506 is irradiated at each position where the hemispherical container 501 is scanned by the XY stage. The probe 502 is an element that detects photoacoustic waves. As the data acquired by the probe 502 is reconstructed by the information acquisition unit, a three-dimensional photoacoustic image can be acquired. Note that the ultrasonic echo measurement used when acquiring the acoustic characteristics inside the subject is performed by the linear ultrasonic probe 504. The linear ultrasonic probe 504 can scan.

(Light irradiation part)
In order to generate photoacoustics effectively, it is necessary to irradiate light in a sufficiently short time according to the thermal characteristics of the inspection object. When the inspection object is a living body, the pulse width of the pulsed light generated from the light source is preferably about 10 to 50 nanoseconds. Here, a titanium sapphire laser which is a solid-state laser is used. In addition, two wavelengths of light of 760 nm and 800 nm are used for measuring the oxygen saturation. A tunable laser may be used as the laser device, or a plurality of laser devices capable of irradiating light of different wavelengths may be combined.

(Photoacoustic probe)
The probe 502 is an element that receives photoacoustic waves. Here, a CMUT (capacitive micromachine probe) is used. The probe is a single element, has an opening of φ3 mm, and has a bandwidth of 0.5 MHz to 5 MHz. By including a low frequency in the band, a good image can be obtained even for a blood vessel having a thickness of about 3 mm. That is, it is difficult for a situation in which the inside of the blood vessel is removed to look like a ring. The sampling frequency is 50 MHz and 2048 sampling is performed. The data is signed 12 bits.

(Linear ultrasonic probe)
The linear ultrasonic probe 504 can transmit and receive ultrasonic waves and obtain a morphological image. PZT (piezoelectric ceramics) is used as such an element. The number of elements is 256, and the band is 5 MHz to 10 MHz. The sampling frequency is 50 MHz and 2048 sampling is performed. The data is signed 12 bits.

(Signal processing flow)
With reference to the flowchart of FIG. 3, a part different from the first embodiment will be particularly described. At the start of measurement in step S1, the breast is placed on the holding member 505. In the ultrasonic measurement in the step S2, the linear ultrasonic probe 504 is scanned in the x direction. As a result, a B-scan image parallel to the zy plane can be obtained. An information acquisition part calculates | requires an ultrasonic attenuation characteristic from this image. However, the step S2 may be changed to a general reading process of attenuation characteristic values from the memory.

  In step S3, photoacoustic measurement is performed. FIG. 6 is a schematic diagram for performing photoacoustic measurement while scanning the incident position of the irradiation light 506 on the inspection object. The incident position of the traveling light sequentially moves from 506a to 506c. The light is hardly attenuated in the acoustic matching material and is almost parallel. However, after the light is incident on the inside of the subject stored in the holding member 505, the light diffuses inside the subject according to the scattering coefficient. Then, after traveling a certain distance, it is absorbed according to the absorption coefficient.

  The probe 502 receives a photoacoustic wave in synchronization with the irradiation of pulsed light. The information acquisition unit generates a three-dimensional reconstructed image by a desired method. At this time, the influence of the ultrasonic attenuation characteristic obtained in the previous process is taken into consideration. Here, the photoacoustic image with respect to irradiation for every pulse is used.

  In step S4, the information acquisition unit extracts a blood vessel from the photoacoustic image. At that time, a blood vessel having a thickness of 0.5 mm to 3 mm is selected using a filter or the like. In step S5, the information acquisition unit creates a signal intensity distribution. Here, as in the first embodiment, signal intensities at a plurality of positions made of the same type of absorber are obtained.

  In step S6, the information acquisition unit calculates a signal intensity distribution. Here, the light quantity distribution is calculated assuming the values of the absorption coefficient, scattering coefficient, and anisotropy factor at a plurality of positions obtained in step S5. In step S7, the information acquisition unit compares the signal intensity distribution calculation result with the measured value at the position selected by the signal intensity distribution. If the difference is within the set value, the process proceeds to step S8. On the other hand, if it is larger than the set value, the process returns to step S6, and either one of the absorption coefficient and the scattering coefficient is changed and the calculation is performed again.

  In step S8, the information acquisition unit determines an optical coefficient. If the difference is within the set value in step S7, the value is calculated as a desired optical coefficient. By calculating at the light irradiation position, it is possible to obtain a two-dimensional distribution of the absorption coefficient and scattering coefficient of the medium to be inspected with respect to the xy plane. Note that, by narrowing the calculation range for signal processing, changes in the absorption coefficient and scattering coefficient of a local medium such as a tumor can be captured. In step S9, the process ends.

  According to the present invention, since the distribution of the optical characteristic information inside the subject can be imaged using the optical coefficient calculated from the photoacoustic image, internal observation such as diagnosis can be performed with high accuracy. Since the photoacoustic image can be generated by a normal function of the photoacoustic apparatus, no special configuration is required.

(Other examples)
The present invention supplies a program that realizes one or more functions of the above-described embodiments to a system or apparatus via a network or a storage medium, and one or more processors in a computer of the system or apparatus read and execute the program This process can be realized. It can also be realized by a circuit (for example, ASIC) that realizes one or more functions.

  202: Probe, 208: Information acquisition unit

Claims (18)

An element that receives an acoustic wave generated by irradiating the subject with light and outputs an electrical signal;
An information acquisition unit that acquires an optical coefficient inside the subject using the electrical signal;
Have
The information acquisition unit
Using the electrical signal and the ultrasonic attenuation characteristics of the subject to generate intensity distribution data corresponding to the initial sound pressure distribution inside the subject,
From a value included in the intensity distribution data, obtain a plurality of signal intensity according to the propagation distance of the light inside the subject,
Obtaining a light intensity distribution inside the subject;
An object information acquiring apparatus that acquires the optical coefficient inside the object using the plurality of signal intensities and the light amount distribution. 2. The object information acquisition according to claim 1, wherein the information acquisition unit acquires the optical coefficient using a mathematical formula indicating a relationship between a propagation distance of light inside the object and attenuation of the light amount. apparatus. The information acquisition unit extracts signal intensity based on the same type of absorber from the intensity distribution data at a plurality of positions, and acquires the optical coefficient by comparing the signal intensity and the light amount at the plurality of positions, respectively. The object information acquiring apparatus according to claim 2, wherein: The object information acquisition apparatus according to claim 3, wherein the information acquisition unit performs comparison at each of the plurality of positions after converting either the signal intensity or the light amount into the other. The subject according to claim 3 or 4, wherein the information acquisition unit acquires the optical coefficient by converging the comparison results at the plurality of positions within a predetermined threshold in the mathematical expression. Information acquisition device. The optical coefficient is an equivalent attenuation coefficient;
6. The information acquisition unit according to claim 5, wherein the information acquisition unit converges the comparison result within the predetermined threshold value by changing one of the absorption coefficient and the scattering coefficient inside the subject. Subject information acquisition apparatus. The object information acquiring apparatus according to claim 3, wherein the information acquiring unit extracts a blood vessel as the absorber. The object information acquiring apparatus according to claim 7, wherein the information acquiring unit extracts the blood vessel having a constant thickness. The object information according to claim 1, wherein the information acquisition unit uses the initial sound pressure distribution or an energy absorption density distribution inside the object as the intensity distribution data. Acquisition device. The information acquisition unit generates three-dimensional intensity distribution data, generates two-dimensional intensity distribution data by maximum value projection using the three-dimensional intensity distribution data, and generates the two-dimensional intensity distribution data from the two-dimensional intensity distribution data. 10. The object information acquiring apparatus according to claim 1, wherein the signal intensity is acquired. The information acquisition unit acquires the plurality of signal intensities by selecting a maximum value by comparing values included in the intensity distribution data for each propagation distance of the light inside the subject. The object information acquiring apparatus according to any one of claims 1 to 10. The object information according to claim 1, wherein the information acquisition unit acquires an optical characteristic information distribution inside the object using the electrical signal and the optical coefficient. Acquisition device. The object information acquiring apparatus according to claim 12, wherein the information acquiring unit uses the same electrical signal when generating the intensity distribution data and acquiring the optical characteristic information distribution. The object information acquiring apparatus according to claim 12, wherein the information acquisition unit uses the different electrical signals when generating the intensity distribution data and acquiring the optical characteristic information distribution. . The said information acquisition part produces | generates the said intensity distribution data using the said ultrasonic attenuation characteristic acquired by transmitting / receiving an ultrasonic wave inside the said test object, The any one of Claim 1 thru | or 14 characterized by the above-mentioned. 2. The object information acquiring apparatus according to item. The object information acquiring apparatus according to claim 15, wherein the element is also used for transmitting and receiving ultrasonic waves for acquiring the ultrasonic attenuation characteristics. The object information acquiring apparatus according to claim 15, further comprising an ultrasonic probe for transmitting and receiving ultrasonic waves inside the object. An element that receives an acoustic wave generated by irradiating the subject with light and outputs an electrical signal;
An information acquisition unit that acquires an optical coefficient inside the subject using the electrical signal;
A method for controlling a subject information acquisition apparatus comprising:
The information acquisition unit
Generating intensity distribution data corresponding to an initial sound pressure distribution inside the subject using the electrical signal and the ultrasonic attenuation characteristics of the subject;
Obtaining a plurality of signal intensities according to the propagation distance of the light inside the subject from values included in the intensity distribution data;
Obtaining a light amount distribution inside the subject;
Obtaining the optical coefficient inside the subject using the plurality of signal intensities and the light amount distribution;
A method for controlling a subject information acquiring apparatus, comprising:

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