JP6012202B2 - Acoustic signal receiver - Google Patents

Description

  The present invention relates to an acoustic signal receiving apparatus.

  Imaging devices using X-rays, ultrasound, and MRI (nuclear magnetic resonance) are often used in the medical field. On the other hand, research on devices using optical imaging technology that obtains in-vivo information by propagating light emitted from a light source such as a laser into a subject such as a living body and detecting the propagating light in the medical field It is being actively promoted.

  As one of such optical imaging techniques, photoacoustic tomography (PAT: photoacoustic tomography) has been proposed. In PAT, an acoustic wave (hereinafter also referred to as a photoacoustic wave) is generated from a living tissue that has absorbed energy of light propagated and diffused in the subject by irradiating the subject with pulsed light generated from a light source. This photoacoustic wave can be detected at a plurality of locations, and the signals can be analyzed and information related to the optical characteristic values inside the subject can be visualized. Thereby, the optical characteristic value distribution in the subject, particularly the light energy absorption density distribution, can be obtained with high resolution.

  As an acoustic wave detector, a transducer using a piezoelectric phenomenon is generally used. In addition, a transducer using a change in electrostatic capacity is generally provided.

  In recent years, a detector using light resonance has been reported (Non-Patent Document 1). This is a technology that detects acoustic waves based on the principle of a Fabry-Perot interferometer (hereinafter, also referred to as FP method), and has a wide-band reception performance and is capable of obtaining high-definition images. is there.

  However, the FP method has a problem that the measurement time is long. For example, in Non-Patent Document 1, when acquiring two-dimensional distribution data of photoacoustic waves, measurement light for evaluating light reflectance is scanned by a galvanometer. That is, in order to acquire one volume data, the resonance position of light is raster-scanned and data is acquired at each position. At the same time, in order to set an optimum wavelength for each measurement position, data is acquired while changing the measurement wavelength for each position. Therefore, it has been reported that it takes 10 minutes or more to obtain a three-dimensional image of several millimeters square.

  In general, it is important from a practical point of view that a measuring instrument acquires data in as short a time as possible. In particular, when the object to be measured is a living body or the like, the state of the subject is sequentially changed by body movement, so that it is impossible to obtain a correct image if it takes time to acquire data.

  Therefore, in order to collectively acquire a two-dimensional distribution of elastic waves, an example has been reported in which the sound pressure of an ultrasonic wave acquired by an FP-type receiving element is detected using a CCD camera as a two-dimensional array sensor ( Non-patent document 2).

JP 2001-354732 A

E. Zang, J. et al. Laufer, and P.M. Beard, "Backward-mode multiwavelength photoacoustic scanner using a planar Fabry-Perot polymer film ultrasound sensor for high-resolution three-dimensional imaging of biological tissues", Applied Optics, 47,561-577 (2008) M.M. Lamont, P.M. Beard, “2D imaging of ultrafield fields using CCD array to map output of Fabry-Perot polymer film sensor”, Electronics Letters, 42, 3, (200).

  As described above, in the FP-type sound wave detection apparatus, it is very effective to collect the light detection in the two-dimensional plane using a CCD or the like in order to shorten the measurement time. However, when the present inventors made extensive studies with a view to practical application, problems that are not described in Non-Patent Document 2 have been clarified.

  The FP method is a method in which an acoustic wave is received by a receiving film, and a slight change in the thickness of the receiving film when the sound pressure reaches is monitored by light to detect the sound pressure. In other words, if the film thickness of the formed receiving film is slightly different from the design value, the sound pressure cannot be measured correctly. In general, there are process variations in the formation of the receiving film, so there are many variations in film thickness even on a single substrate, but this film thickness variation is within the design tolerance of the product. It can be used practically.

  However, according to the calculation of the design tolerance by the present inventors, it is clear that the FP acoustic wave detector affects the reception sensitivity even when a film thickness distribution of several nanometers exists. It became. In other words, if a signal is to be received correctly in a two-dimensional plane, it is necessary to suppress the film thickness distribution within a few nanometers. However, it is extremely difficult to perform such precise control in a practical film forming process.

  The present invention has been made in view of the above problems, and an object of the present invention is to realize higher sensitivity characteristics even when a film thickness distribution exists in the receiving film in acoustic wave detection. There is.

  An acoustic signal receiving device according to the present invention includes a first mirror on which measurement light is incident, a second mirror that is disposed opposite to the first mirror and on which an acoustic wave from a subject is incident, An acoustic wave receiving layer provided between the first mirror and the second mirror and capable of modulating the refractive index, and the first mirror according to deformation of the acoustic wave receiving layer due to incidence of the acoustic wave And a detector for detecting a change in the optical path length between the second mirror and the refractive index distribution of the acoustic wave receiving layer, the variation in the optical path length due to the film thickness distribution of the acoustic wave receiving layer Is compensated for.

  The acoustic signal receiving device according to the present invention includes a first mirror on which measurement light is incident, a second mirror that is disposed opposite to the first mirror and on which an acoustic wave from a subject is incident, the first mirror An acoustic wave receiving layer provided between the mirror and the second mirror, and an optical path between the first mirror and the second mirror according to deformation of the acoustic wave receiving layer due to the incidence of the acoustic wave An acoustic signal receiving device comprising a detector for detecting a change in length, wherein the acoustic wave receiving layer has a function of modulating a refractive index, and a control unit for controlling the refractive index modulation amount And the control unit changes the refractive index of the acoustic wave receiving layer so as to have a value corresponding to the variation in the optical path length due to the film thickness distribution of the acoustic wave receiving layer.

  According to the present invention, even when a film thickness distribution exists in the receiving film in acoustic wave detection, it is possible to realize higher sensitivity characteristics.

The figure which shows an example of a structure of the conventional Fabry-Perot type | mold interferometer. The figure which shows an example of the reflective characteristic of a Fabry-Perot type | mold interferometer. The figure which shows an example of a structure of the Fabry-Perot type | mold interferometer of this invention. The figure which shows an example of the structure of the Fabry-Perot type probe which can apply this invention. The figure which shows an example of a structure of the biological information imaging device which can apply this invention. 4A and 4B show an example of a device manufacturing process in an embodiment of the present invention. The figure which shows an example of the control method in embodiment of this invention. The figure which shows an example of the time chart in embodiment of this invention. The figure which shows an example of the electrode structure in embodiment of this invention. The schematic diagram of the molecular structure of the acoustic wave receiving layer used for the Example of this invention.

[Basic configuration of receiving element]
Next, embodiments of the present invention will be described with reference to the drawings.

  In addition, the measurement light in this invention refers to the light used for the measurement by a Fabry-Perot (FP) type interferometer. The incident light incident on the FP interferometer and the reflected light reflected by the FP interferometer and guided to the array optical sensor are all included in the measurement light.

  First, an acoustic wave detecting element using resonance of light that has been conventionally reported will be described with reference to FIG. As shown in this figure, a structure that resonates light between parallel reflectors is called an FP interferometer. In the future, an acoustic wave detector using this FP interferometer will be referred to as an FP probe.

  A polymer film 104 having a thickness d is sandwiched between the first mirror 101 and the second mirror 102 to form the resonator 103. As shown in the figure, the first mirror 101 and the second mirror 102 are arranged to face each other so as to form a cavity. The interferometer is irradiated with incident light 105 from the first mirror 101. At this time, the light amount Ir of the reflected light 106 is expressed by the following equation (1).

  Here, φ is expressed by the following equation (2).

Here, Ii is the amount of incident light 105, R is the reflectance of the first mirror 101 and the second mirror 102, λ ₀ is the wavelength of the incident light 105 and the reflected light 106, d is the distance between the mirrors, and n Is the refractive index of the polymer film 104. φ corresponds to the phase difference when reciprocating between two mirrors.

  An example of a graph of the reflectance Ir / Ii as a function of φ is shown in FIG. A periodic drop in the amount of reflected light Ir occurs, and the reflectance is lowest when φ = 2mπ (m is a natural number).

  When the acoustic wave 107 is incident on the FP probe, the distance d between the mirrors changes due to deformation of the probe. As a result, φ changes, so that the reflectance Ir / Ii changes. The incident acoustic wave 107 can be detected by measuring the change in the amount of reflected light Ir with a photodiode or the like. In such a detection principle, the greater the change in the amount of reflected light, the greater the intensity of the incident acoustic wave 107.

In order for the reflected light amount Ir to change greatly when the acoustic wave 107 is incident, it is necessary to increase the rate of change of the reflectance Ir / Ii with respect to the change of φ. In Figure 2, the phi _m, most change rate is large, that is because the slope is steep, it can be said most sensitive good elements in phi _m.

Is a graph of a reflectance Ir / Ii as a function of the lambda ₀ is shown in FIG. 2 (b). Matching the wavelength to λ _m having the highest rate of change of the reflectance Ir / Ii corresponds to matching the phase difference to φ _m and maximizes the sensitivity.

Thus, in the FP type probe, an optimum film thickness is required if the measurement wavelength to be used is determined. For example, referring to this figure, the film thickness variations when using a light source having a single wavelength lambda _A, it is generally necessary to suppress within 0.05% ±. In order to achieve this numerical value, a considerably high accuracy is required as a film forming technique.

If the receiving surface of the FP probe is the xy plane, the film thickness at each position is d (x, y), and φ _m representing the optimum sensitivity at each position is φ _m (x, y), It is expressed as (3).

Based on the above, the feature of the present invention is that even when the film thickness d (x, y) varies depending on the location, φ _m takes a constant value regardless of the location. Therefore, in the present invention, in order to cancel out the in-plane distribution of the film thickness d, the refractive index n has an in-plane distribution so that φ _m becomes a substantially constant value in all xy coordinates. In other words, the optical path length can be expressed by the product of the refractive index n and the physical thickness d, and n is also distributed in the xy plane. Therefore, the present invention has a function of modulating the refractive index of the film used for the receiving layer. As a result, it is possible to make the optical path length constant throughout the element by introducing the concept represented by the equation (4).

Hereinafter, the refractive index modulation type FP acoustic wave detection element of the present invention will be described with reference to FIG. The basic configuration is almost the same as that of FIG. The FP interferometer 303 shown in FIG. 3 is sandwiched between two mirrors 301 and 302 arranged to face each other. In FP interferometer 303, the refractive index of the layer 304 for receiving acoustic waves 307 _{n r,} the film thickness is _{d r.} At this time, the phase difference is expressed by Expression (5).

  This also shows the same characteristics as in FIG. When the acoustic wave 307 enters the FP-type probe, the inter-mirror distance d changes. As a result, φ changes, and the reflectance Ir / Ii changes. The incident acoustic wave 107 can be detected by measuring the change in the amount of reflected light Ir with a photodiode or the like. The greater the change in the amount of reflected light, the greater the intensity of the incident acoustic wave 107.

Here, if the wavelength λ _A of the measurement light is a fixed value, it is necessary to adjust other parameters in order to adjust the phase difference to φ _m . In general, the refractive index is a physical property value determined by a material, and the thickness d is a parameter determined by a manufacturing process. On the other hand, since the present invention has a function of modulating the refractive index of the material used for the receiving layer, n _r can be adjusted as appropriate. Thus, it is possible to compensate for variations in optical path length caused by the distribution of d _r caused by variations in the manufacturing process. Here, the compensation for the variation only requires that the variation in the optical path length of the film thickness distribution of the receiving layer is reduced as compared with that before the adjustment of the refractive index distribution of the receiving layer.

If the manufacturing variation cannot be absorbed only by the modulation of the refractive index n _r , the wavelength of the measurement light is increased by one, the FP probe region is divided, the wavelength λ _A is used in one region, and another region Then, by using the wavelength λ _B , a wide area can be compensated.

  Depending on the degree of variation, the number of wavelengths may be further increased. In this case, a laser whose wavelength is continuously variable can be used.

  Since the FP type probe measures the change in the amount of reflected light only at the position where the incident light 105 (305), which is the measurement light, is incident, the spot area of the incident light is an area having reception sensitivity. Therefore, two-dimensional distribution data of acoustic waves can be obtained by raster scanning incident light with a galvanometer or the like, and an image is obtained using this.

  In the case of the present invention, since it is possible to use a single wavelength as incident light, it is possible to irradiate the entire surface of the element simultaneously with incident light and to use reflected light at a high speed without raster scanning. Images can also be acquired. For example, a CCD or CMOS image sensor for a digital camera can be used.

FIG. 4 is a diagram illustrating a cross-sectional structure of the FP probe according to the present embodiment.
As a material of the first mirror 401 and the second mirror 402, a dielectric multilayer film or a metal film can be used. An acoustic wave receiving layer 403 exists between the mirrors.
The acoustic wave receiving layer 403 preferably has a large distortion when an elastic wave is incident on the FP probe. For example, an organic polymer film is used.

  The acoustic wave receiving film 403 has a function of modulating the refractive index. For example, as the film having such a function, a structure in which a side chain type polymer liquid crystal or a low molecular liquid crystal described in Patent Document 1 or the like is stabilized with a polymer matrix can be used. Note that the acoustic wave receiving layer 403 is not limited to an organic film but an inorganic film as long as this condition is satisfied as long as the film is deformed when receiving sound waves and has a function of refractive index modulation. It doesn't matter.

  For example, when a side chain type polymer liquid crystal material is used, the acoustic wave receiving layer 403 is formed as follows. First, a uniaxially oriented alignment film is disposed on a substrate having electrodes, and a liquid crystal material is applied thereon. At that time, when the liquid crystal material is applied on the substrate in a high temperature state with low viscosity, for example, in a temperature range showing isotropic property, controllability is good.

  As a coating method, a spin coating method, a screen printing method, a color development method, a dip coating method, or the like can be used. Thereafter, by slowly cooling to a liquid crystal phase, a side chain type polymer liquid crystal layer in which the direction of the side chain is controlled can be obtained.

  When the alignment film is not used, it is possible to align the direction of the liquid crystal molecules in the side chain by applying a magnetic field in a certain direction when slowly cooling from the isotropic phase to the liquid crystal phase.

Next, an electrode for applying a voltage is formed on the liquid crystal layer thus obtained. At that time, it is preferable to form the film at a low temperature so as not to break the alignment state of the liquid crystal. Note that an electrode may be formed by applying an ink-jet method in which metal fine particles such as gold are dispersed in a solvent.
By doing so, it is possible to obtain a configuration in which the liquid crystal layer is disposed on the electrode on the substrate and the electrode is further formed thereon.

  Here, when the dielectric constant anisotropy of the side chain of the side chain type polymer liquid crystal, that is, the side chain exhibiting liquid crystallinity (mesogen group) is positive, an electric force is applied by applying a voltage between these electrodes. Orientation deformation occurs so that mesogenic groups are arranged in a direction parallel to the line. Depending on the arrangement direction of the mesogenic groups, the refractive index when observed from the substrate normal direction changes.

  Further, the amount of change in the refractive index can be controlled depending on the voltage. This makes it possible to continuously modulate the optical path length when polarized light is applied in the uniaxial alignment treatment direction of the present liquid crystal element.

  On the other hand, when the dielectric anisotropy of the mesogen group is negative, by using a vertical alignment film as the alignment film disposed on the substrate, the optical path length can be continuously changed by applying voltage as described above. Can do.

  In addition, for example, when a polymer-stabilized blue phase liquid crystal is used, the refractive index can be modulated regardless of the direction of polarization. It is also possible to use polymer dispersed liquid crystal. As described above, by using the low-molecular liquid crystal and the polymer matrix in combination, it is possible to maintain the elasticity necessary for ultrasonic reception at the polymer portion and to control the optical path length with the low-molecular liquid crystal. . As a result, even if the receiving layer is a single layer, it is possible to achieve both an ultrasonic wave reception function and a refractive index modulation function. That is, the acoustic wave receiving layer also functions as a compensation layer.

In addition, since it can be used as the acoustic wave receiving layer of the present invention as long as it has a function of modulating the refractive index, a liquid crystalline material is not necessarily used. For example, when a charged substance having a different refractive index depending on the concentration is used, it is possible to control the refractive index by applying a concentration gradient from the outside using electrophoresis.
In addition, in order to modulate the refractive index of the acoustic wave receiving layer from the outside, an electrode and a driving device are arranged.

In FIG. 4, the entire FP probe is protected by a protective film 405. As the protective film 405, an organic polymer film such as parylene, or an inorganic film such as SiO ₂ is used.
As the substrate 404, glass or acrylic can be used. At this time, in order to reduce the influence of light interference in the substrate 404, the substrate 404 is preferably wedge-shaped.
Furthermore, in order to avoid reflection of light on the surface of the substrate 404, it is preferable to perform an AR coating treatment 406.

[Basic configuration of system configuration]
FIG. 5 illustrates a configuration example of the imaging apparatus according to the present embodiment.
The imaging apparatus of this embodiment includes a light source 504 that emits light 503 that irradiates a subject 501 and generates a photoacoustic wave 502. When the subject 501 is a living body, an internal light absorber such as a tumor or a blood vessel or a surface light absorber can be imaged. The photoacoustic wave 502 is generated by the light absorber in the inside or the surface of the subject 501 absorbing part of the light energy. An FP probe 505 for detecting the photoacoustic wave 502 is provided.

  The FP probe 505 is provided with a receiving layer capable of changing the refractive index in accordance with the above-described film thickness distribution, and can be controlled from the outside. A refractive index control unit 517 is provided for controlling this.

  The sound pressure can be detected by irradiating the measurement light 506 to the FP probe 505. A measurement light source 507 for generating the measurement light 506 is provided. In addition, a control unit 508 for controlling the measurement light source 507 is provided. The measurement light source 507 may be a single wavelength light source or a light source whose wavelength can be switched. A light source capable of continuously changing the wavelength may be used. The control unit 508 performs wavelength switching and light irradiation on / off.

  Furthermore, an array type optical sensor 509 for measuring the amount of reflected light of the measurement light 506 incident on the FP probe 505 and converting it into an electrical signal is provided. The acoustic signal receiving apparatus is configured as described above.

  An imaging device is configured by further configuring a signal processing unit 510 and an image display unit 511 in the acoustic signal receiving device. That is, the imaging apparatus according to the present embodiment includes an image display unit 511 that analyzes an electrical signal obtained by the array type optical sensor 509 in the signal processing unit 510 and displays the obtained optical characteristic value distribution information.

  The measurement light emitted from the measurement light source 507 is magnified by the lens 512, reflected by the FP probe 505, and then incident on the array type optical sensor 509. Thereby, the reflection intensity distribution on the FP probe 505 can be obtained. A mirror 513 or a half mirror 514 is used as the optical system. The optical system only needs to be configured so as to be able to measure the reflectance of the FP probe 505, and can employ a configuration using a polarizing mirror and a wave plate instead of the half mirror 514, or a configuration using an optical fiber. By this optical system, the position on the FP probe 505 and the pixel on the array type optical sensor 509 are associated with each other.

  As the array type optical sensor 509, a two-dimensional array type or a one-dimensional array type optical sensor can be used. For example, a CCD sensor or a CMOS sensor can be used. Any other array type optical sensor can be used as long as it can measure the amount of reflected light of the measurement light 506 when the photoacoustic wave 502 enters the FP probe 505 and convert it into an electrical signal.

  Since the distance between the mirrors of the FP probe 505 varies depending on the position in the substrate surface, the refractive index of the receiving layer is adjusted at each position (each pixel associated with the array type photosensor 509). The optical path length is made constant in the element plane.

  The light 503 irradiated to the subject 501 uses light having a characteristic wavelength that is absorbed by a specific component among the components constituting the subject 501. As the light 503, pulsed light can be used. The pulsed light is on the order of several pico to several hundreds of nanoseconds, and when the subject is a living body, it is preferable to employ pulsed light of several nanometers to several tens of nanoseconds. A laser is preferable as the light source 504 that generates the light 503, but a light emitting diode, a flash lamp, or the like may be used instead of the laser.

  As the photoacoustic wave generating laser, various lasers such as a solid-state laser, a gas laser, a dye laser, and a semiconductor laser can be used. If a oscillating wavelength-convertible dye, OPO (Optical Parametric Oscillators), or TiS (Titanium Sapphire) is used, a difference in optical characteristic value distribution depending on the wavelength can be measured. Regarding the wavelength of the light source to be used, a region of 700 nm to 1100 nm with less absorption by living tissue is preferable. When the observation region is close to the surface of a subject other than a living body or a living body, the wavelength region is wider than the above wavelength region, for example, a wavelength region of 400 nm to 1600 nm, and further an ultraviolet region, terahertz wave, microwave, radio, etc. The use of the wave domain is also possible.

  In FIG. 5, the light 503 is irradiated to the subject 501 from a direction that does not become a shadow of the FP probe 505. However, if the FP probe 505 is configured to transmit the wavelength of the light 503, the light 503 can be irradiated through the FP probe 505.

  In order to efficiently detect the photoacoustic wave 502 generated from the subject 501 with the FP probe 505, it is desirable to use an acoustic coupling medium between the subject 501 and the FP probe 505. In FIG. 5, water is used as the acoustic coupling medium, and the subject 501 is arranged in the water tank 515. However, there is an acoustic coupling medium between the subject 501 and the FP probe 505. It only has to be through. For example, a matching gel for ultrasonic diagnosis may be applied between the subject 501 and the FP probe 505.

  The FP-type probe 505 absorbs a part of the energy of the light 503 when the subject 501 is irradiated with the light 503, and generates a photoacoustic wave (ultrasonic wave) 502 generated from the inside of the subject. This is detected as a reflected light amount change at 506. The detected light amount is converted into an electrical signal by the array type optical sensor 509. The distribution of the electric signal in the array type optical sensor 509 represents the intensity distribution of the photoacoustic wave 502 that reaches the FP probe 505. Thereby, the pressure distribution of the photoacoustic wave 502 that reaches the FP probe 505 can be obtained.

  Further, the signal processing unit 510 performs optical processing such as the position and size of the light absorber in the subject 501, the light absorption coefficient, or the light energy deposition amount distribution based on the extracted electrical signal distribution in the array type optical sensor 509. Calculate the characteristic value distribution.

  As a reconstruction algorithm for obtaining an optical characteristic value distribution from the obtained electric signal distribution, universal back projection, phasing addition, model-based image reconstruction, or the like can be employed. It should be noted that an area in which the film thickness is remarkably abnormal due to the presence of a foreign substance in the acoustic wave receiving layer 403 of the FP type probe cannot be used as data in advance, and the image reconstruction processing is performed. At this time, it is also possible to correct the missing data portion and form an image.

  Note that the signal processing unit 510 stores a temporal change distribution of an electrical signal representing the intensity of the photoacoustic wave 502, and any signal can be used as long as it can be converted into optical characteristic value distribution data by a calculation means. It may be used.

  Note that light having a plurality of wavelengths can be used as the light 503. In this case, the optical coefficient in the living body is calculated for each wavelength, and the concentration distribution of the substance that composes the living body is imaged by comparing those values with the wavelength dependence specific to the substance constituting the living tissue. It is also possible to do. Examples of substances constituting a living tissue include glucose, collagen, oxidized / deoxidized hemoglobin, and the like.

  In the embodiment of the present invention, it is desirable to include an image display unit 511 that displays image information obtained by signal processing.

[Embodiment 1]
Next, various embodiments of the present invention will be described focusing on matters relating to refractive index modulation. Here, a method of using the refractive index modulated while receiving by the FP sensor will be described.

  FIG. 6 shows an example of a manufacturing process of an element used in this embodiment. An element can be obtained by performing film formation in the following order. Here, the transparent electrode is formed over the entire reception effective area.

<Production of element>
1: A dielectric multilayer mirror (602) is formed on a glass substrate (601).
2: A transparent electrode (603) is formed on the dielectric multilayer mirror (602), a horizontal alignment film (604) is further formed thereon, and an alignment treatment (605) is performed.
3: A side chain polymer liquid crystal layer (606) as an acoustic wave receiving layer is formed on the horizontal alignment film (604).
4: A gold electrode (607) having both a light reflection function and a voltage application function is formed on the side chain polymer liquid crystal layer (606).
5: A protective film (608) is formed on the gold electrode (607).
6: An electrode wire (not shown) is taken out from the transparent electrode (603) and the gold electrode (607) and connected to an AC voltage source (not shown) to obtain the FP element 609.

In this device, the optical path length of the liquid crystal layer can be changed by modulating the voltage. Therefore, when the reflection characteristics are measured using this device and the horizontal axis is plotted with the voltage and the vertical axis is plotted with the reflectance, the same as FIG. Can be obtained. Thereby, n _c (x, y) in Equation 5 can be modulated by the voltage.

In the liquid crystal material of a general display applications, the minimum value of n _c is about 1.5 the refractive index n _⊥ of the liquid crystal molecules uniaxial, the maximum value is the refractive index of the liquid crystal molecular long axis direction n _〃 About 1.6 is used. A value of about 0.1 is widely used as the numerical value of the refractive index anisotropy Δn. Some materials have been developed in which Δn shows a numerical value of 0.3 or more. In the present invention, any liquid crystal material may be used because the layer thickness may be adjusted appropriately so as to satisfy the optimum conditions.

<Adjustment of compensation amount>
The FP element (609), which is a cell obtained by the above-described process, has a distribution in the element plane in the optical path length at the time of optical interference due to the film thickness distribution of the side chain polymer liquid crystal layer (606). In order to compensate for this distribution, the amount of voltage applied to the liquid crystal is adjusted.

  Further, the value of this element varies depending on the polarization axis of the measurement light due to the influence of the refractive index anisotropy of the liquid crystal. Therefore, by using a polarizing plate in advance, the polarization axis direction of the measurement light and the alignment processing direction (abnormal light refractive index direction) of the liquid crystal are matched. By doing so, the liquid crystal molecule major axis direction and the polarization axis can be matched, so that the optical path length can be changed by applying a voltage to the liquid crystal layer.

  The distribution amount is preferably measured for each pixel of the array photosensor (509), but may be measured for each of a plurality of pixels.

The FP probe (505) is irradiated with measurement light having a predetermined wavelength, and the reflected light is measured by the array type optical sensor (509). At this time, the amount of light incident on the sensor is measured while changing the AC applied voltage, and the voltage-reflectance profile is measured. Thereby, a voltage value giving φ _m can be obtained.

  By performing this for all pixels, a look-up table (LUT) is created and stored in a storage medium. If the characteristics of the liquid crystal to be used are likely to change with temperature, a similar LUT is created by changing the temperature.

<< Use of refractive index modulation type FP probe >>
In the LUT, acoustic wave data is acquired for each group, with pixels having the same optimal voltage value as one group. In this case, it is possible to measure an acoustic wave even if the voltage is not exactly the same as the voltage giving φ _m . Therefore, when the required accuracy of the apparatus is not high, voltage values that fall within the valley of the voltage-light quantity curve may be regarded as the same group and measured.

That is, if control over the voltage value to be set, always measurable with phi _m condition generally becomes a good sensitivity and measurement time according to the voltage number of division is increased. Conversely, if the voltage value is controlled roughly, the sensitivity variation increases, but the measurement time is short. In consideration of such a trade-off relationship, it is preferable to design the device under optimum conditions.

A measurement sequence will be described with reference to FIG.
FIG. 7A is a top view of the FP probe. The circular dotted line in the figure is a contour line related to the optical path length in the FP probe. The central portion has a film thickness distribution in which the optical path length is long, that is, the film thickness is large, and the optical path length is short at the end part, that is, the film thickness is thin. A voltage application group is set according to the contour line.
FIGS. 7B to 7E are cross-sectional views of FIG.

  The first photoacoustic signal generating laser is irradiated, and the photoacoustic wave obtained there is imaged by pixels corresponding to the first group. That is, the area indicated by the circled number 1 in FIGS. 7A and 7B is imaged. Hereinafter, the circled number 1 is described as (1). Similarly, the numbers 2 to 4 are also described as (2) to (4).

  The alignment state of the liquid crystal layer shows the same alignment state as in (1) in the regions other than (1), but it can be ignored because it is not used for image processing. That is, when the light intensity is read from the upper surface of the element by the array type optical sensor and image processing is performed, only the pixel information corresponding to (1) is used, and the information of (2) to (4) is not used.

  Next, the second photoacoustic signal generating laser is irradiated, and the photoacoustic wave obtained there is imaged by pixels corresponding to the second group. That is, the region shown in (2) of FIGS. 7A and 7C is imaged. The alignment state of the liquid crystal layer shows the same alignment state as in (2) in the regions other than (2), but it can be ignored because it is not used for image processing. In other words, only the pixel information corresponding to (2) is used when the light intensity is read from the upper surface of the element by the array type optical sensor and image processing is performed, and the information of (1), (3), and (4) is not used.

  Similarly, by imaging in the areas (3) and (4), it is possible to receive an acoustic wave with the optical path length set to be almost optimal in each of the areas (1) to (4).

  In this way, the acoustic wave reception is performed a plurality of times corresponding to the alignment amount of the liquid crystal layer for each of the plurality of regions, in other words, corresponding to the compensation amount of the optical path length. These are combined during data analysis to obtain a signal for the entire element surface.

  In this example, the area is divided into four areas. However, it is sufficient to divide the area into an arbitrary number of areas (N areas) and acquire data by the same method.

  If the pulse repetition frequency of the laser for photoacoustic signal generation is f (Hz) and the response time of the liquid crystal layer is τ (seconds), if τ is smaller than 1 / f, one data is f / N (Hz). Can be obtained by frequency.

  If the array-type optical sensor can acquire images at a sufficiently high speed and can acquire data following the vibration of sound waves, images can be acquired continuously at a frame frequency of f / N (Hz).

On the other hand, when the image input of the array type optical sensor is low speed and does not follow the sound wave vibration, the measurement light may be pulsed light and data may be acquired based on the principle of strobe photography.
FIG. 8 shows a timing chart for flash photography. The photoacoustic signal generating laser is irradiated at f (Hz). Assuming that the subject does not move during imaging, photoacoustic signals are repeatedly output with the same intensity and phase.

  Therefore, using the output of the laser for generating a photoacoustic signal as a trigger signal, the sound pressure is measured at different timings by acquiring data by irradiating the FP probe with the measurement light pulse while shifting the irradiation timing little by little. be able to.

  FIG. 8A shows the output of the photoacoustic generation laser. From the absorber irradiated with the photoacoustic generation laser, the photoacoustic signal reaches the probe with a delay of a predetermined time. FIG. 8B shows this with a waveform.

  FIG. 8C shows the waveform of a pulse laser for measurement light. That is, since the light is irradiated onto the FP probe only at this moment, the reflected light reaches the array type optical sensor. FIG. 8 (d) shows the light intensity reaching the array type photosensor. Here, for simplicity, it is assumed that the reflected light intensity is proportional to the photoacoustic wave signal. The two-dimensional distribution of the reflected light received in this way is stored in the image memory.

  In the next measurement, the reflected light intensity is measured by delaying the irradiation timing of the measurement pulse laser slightly from the trigger compared to the previous measurement. Since the same waveform is repeatedly generated in the photoacoustic wave signal, the two-dimensional distribution of the reflected light at different timings can be obtained by shifting the timing of the pulse laser irradiation for the measurement light.

  Thus, by repeating the process of irradiating the measurement light at different timings and accumulating each data in the memory, a photoacoustic wave signal for one period can be acquired. If this is arranged, it is possible to obtain a temporal change in reflected light intensity from the FP probe in each pixel. If the number of divisions when observing a photoacoustic signal with a strobe is D, data acquisition is obtained at f / (N × D) (Hz).

  When the signal is averaged m times per location, the data acquisition is f / (N × D × m) (Hz). For this reason, it is possible to acquire data at a practical speed by using a photoacoustic signal generating laser capable of repeatedly irradiating at high speed. When τ is larger than 1 / f, data acquisition is 1 / {(D × m) / f + τ × (N−1)} (Hz).

[Embodiment 2]
<Production of element>
In this embodiment, the electrode is manufactured by the same process as that of the first embodiment except that an electrode patterned in a stripe shape is used. The transparent electrode (603) in FIG. 6 is patterned after film formation. In addition, stripe electrodes are formed by employing an ink jet method or a mask deposition method. These stripe electrodes are formed so as to be orthogonal to each other to have a simple matrix configuration. Here, as shown in FIG. 9, the electrode of the substrate 1 is a common (COM) electrode 901, and the electrode of the substrate 2 is a segment (SEG) electrode 902.

  In the present embodiment, driving can be performed using a driver for a simple matrix liquid crystal generally used in a super twisted nematic (STN) liquid crystal or the like. A COM driver is mounted on the substrate 1 and an SEG driver is mounted on the substrate 2.

  When a region where the matrix electrodes intersect is defined as a liquid crystal pixel, it is preferable that the liquid crystal pixel and the pixel of the 2D image sensor have a one-to-one correspondence. However, since the purpose of the liquid crystal layer is to compensate for the film thickness distribution, when the film thickness distribution changes gradually, the liquid crystal layer may be coarser than the pixels of the image sensor.

<Adjusting the refractive index>
The voltage-reflectance profile is measured in the same manner as in the first embodiment. Using this, a lookup table (LUT) is created and stored in a storage medium.

  When the characteristics of the liquid crystal are likely to change with temperature, a similar LUT is created by changing the temperature.

  At this time, a driving condition for performing simple matrix driving is obtained. The voltage application method to each pixel may be a simple matrix liquid crystal driving method used in a general liquid crystal display. In this case, it is known that the voltage on / off ratio is determined according to the equation (7).

  For this reason, when the number of COM lines increases, it becomes impossible to ensure a sufficient on / off ratio for compensating the parylene film thickness, that is, a difference in optical path length.

Therefore, when the device is manufactured, in consideration of the number of COM lines and the parylene film thickness distribution, the liquid crystal layer thickness necessary for securing an optimal refractive index modulation amount is set. That is, in the case of horizontal alignment liquid crystal of the present embodiment, the maximum value of the optical path length is _{n r (V OFF) · d} r, the minimum value of _{n r (V ON) · d} r. Therefore, the range of compensation possible optical path length is _{{n r (V ON) -n} r (V OFF)} · d r. Here, n _r (V _OFF ) is an average extraordinary refractive index of the liquid crystal layer in the OFF state in matrix driving, and contributes to the optical path length when the polarization direction of the measurement light and the alignment processing direction are matched. It is an ingredient to do. Similarly, n _r (V _ON ) represents the extraordinary refractive index in the on state. For this reason, the compensation range can be secured by setting _dr large.

  As described above, the number of scanning lines and the on / off ratio are in a trade-off relationship. Therefore, when a sufficient on / off ratio cannot be secured over the entire surface of the FP probe region, it is effective to reduce the effective number of COM lines and increase the drive duty ratio. One is a method of simultaneously driving lines having a substantially constant film thickness. That is, a predetermined COM line group may be simultaneously selected and compensated with the same voltage for an area where the film thickness distribution is small by prior measurement. As a result, the on / off ratio can be increased because the number of duties during driving can be reduced.

Alternatively, there is a method of forming the entire image by dividing the entire element into N blocks and using N fields to form one image. That is, at the time of image acquisition, the COM line is divided into N blocks and driven. Each field is driven with a line number that is 1 / N of the entire COM line. The amount of compensation is determined by measuring the amount of light with an image sensor for each field and acquiring images N times at different locations.
Further, the number of wavelengths of the measurement light may be increased to a plurality in the same manner as described above.
The driving method and the compensation amount are determined as described above, and this is recorded on the storage medium as an LUT.

<< Use of refractive index modulation type FP probe >>
An acoustic wave can be detected while driving the compensation layer by the above simple matrix driving method and keeping the optical path length uniform within the FP probe plane. When divided into N blocks, simple matrix driving is performed for each divided area, and an acoustic wave signal in that area is received and stored in a memory. In other blocks, the acoustic signal is received, and an image of one entire element is formed using data for N fields.

  As in the first embodiment, the image can be observed with a strobe when the image acquisition of the array photosensor is slow.

[Embodiment 3]
<Production of element>
In this embodiment, an active matrix substrate for a liquid crystal display provided with a thin film transistor (TFT) element is used, and a substrate in which opposing gold electrodes are formed on the entire surface is used. Otherwise, the cell is created by the same process as in the first embodiment. Here, a gate electrode is formed in the row direction, and a source electrode is formed in the column direction.

The configuration of the liquid crystal layer of this embodiment is the same as that of a general active matrix driving liquid crystal device. As in the case of driving a twisted nematic (TN) liquid crystal, a substrate 1 on which a TFT element is disposed is used to apply a voltage in the cell thickness direction between the two substrates on which electrodes are disposed. A gate driver is mounted in the row direction and a source driver is mounted in the column direction. The substrate 2 is maintained at a potential that is an optimum condition for driving the TFT.
The alignment treatment direction and the liquid crystal material used are the same as those in the first and second embodiments.

  It is preferable that the pixels of the liquid crystal layer and the pixels of the array type photosensor have a one-to-one correspondence. However, since the purpose of the liquid crystal layer is to compensate for the film thickness distribution, It may be coarser than the pixels of the image sensor.

<Adjusting the refractive index modulation amount>
In the same manner as in the first embodiment, the optimum voltage application amount is obtained and recorded on the storage medium as an LUT for each liquid crystal pixel.

<< Use of refractive index modulation type FP probe >>
The cell used in the present embodiment can be driven in an active matrix, and an acoustic wave can be detected while keeping the optical path length uniform within the FP probe plane. As in the first embodiment, the image can be observed with a strobe when the image acquisition of the array photosensor is slow.

[Embodiment 4]
In the second and third embodiments, a configuration in which one type of compensation voltage is applied to each pixel of the liquid crystal layer has been described. However, if the liquid crystal pixel is large, that is, the optimal compensation amount varies within the pixel because the pixel is coarse relative to the amount of change in the parylene film thickness distribution, one liquid crystal pixel is divided into multiple fields. Data may be acquired. This is to spatially divide the entire device using the cell configuration of Embodiment 2 or Embodiment 3, introduce the concept of Embodiment 1, and acquire data in each pixel in a time division manner. It becomes possible to perform fine compensation.
This requires that the pixel pitch of the array photosensor is smaller than that of the liquid crystal pixel.

[Embodiment 5]
In the embodiments so far, like a nematic liquid crystal for a liquid crystal display that is generally used, it has been used while applying a voltage to the side chain polymer liquid crystal layer in actual use. In the present embodiment, a method of creating a condition of a liquid crystal layer in a manufacturing process and fixing the state will be described.

<Production of element>
The matrix electrode described in Embodiment 2 or 3 is used. As the liquid crystal material to be used, a liquid crystal material having a phase sequence of isotropic phase-nematic phase-solid phase from the high temperature side is used. At that time, when a material having the property of crystallizing is used in the phase transition from the nematic phase to the solid phase, the orientation state collapses. It is preferable to employ a material that undergoes a phase transition to a phase.

  The device is manufactured, the temperature is raised, the liquid crystal is made into a nematic phase, driving is performed under the optimum conditions for uniforming the refractive index, and a phase transition is made to a solid phase while applying a driving voltage.

<< Use of refractive index modulation type FP probe >>
When the phase transition to the solid phase is performed as described above, the orientation is stabilized, so that the state is maintained even when the voltage is turned off. Therefore, it can be used as an FP probe without applying a voltage for refractive index modulation.

  In this embodiment, the solid phase is used. However, any liquid crystal material having a higher-order liquid crystal phase may be used as long as the alignment state is fixed after the alignment is determined. For example, a liquid crystal material such as a smectic phase, a discotic liquid crystal, or a main chain type polymer liquid crystal can be used.

  As described above, even a material that does not have liquid crystallinity can be provided with a refractive index distribution and compensated. For example, a concentration gradient of an organic substance such as sucrose is given according to the film thickness distribution of the sound wave receiving layer, or a charged substance having a refractive index different depending on the concentration is used, and a concentration gradient is given from the outside using electrophoresis. Thus, a refractive index distribution can be given and compensated. When such materials are used, the concentration distribution may disappear due to convection or diffusion in the liquid state. Therefore, a partition wall should be provided at predetermined intervals to prevent diffusion, or as soon as the concentration distribution is applied, agar It is preferable to use after the refractive index distribution state is preserved by hardening.

  As mentioned above, although five embodiment was described, it is not restricted to the said embodiment, It can derive from this and can use various materials. For example, when liquid crystal is used, in Embodiments 1 to 4, parallel alignment ECB (Electrically Controlled Birefringence) type liquid crystal is used. However, various liquid crystal modes such as a VA (Vertical Alignment) method, a Bend alignment method, and a HAN (Hybrid Aligned Nematic) method can be used.

  In addition, it is considered that the adjustment of the compensation amount may be affected by changes over time. Therefore, it is preferable to periodically review the LUT not only at the time of factory shipment but also at the time of use.

  By using the biological information imaging apparatus having such a configuration, a high-resolution photoacoustic image can be obtained at high speed.

  In the above embodiment, the gold electrode is used. However, it is also possible to give the FP element light transmittance at a desired wavelength by using a transparent electrode as the electrode and a dielectric multilayer mirror as the reflective layer. is there.

  Further, when used for medical purposes, the water tank is not used as shown in FIG. 5, and an acoustic impedance matching gel is applied to the subject, that is, the affected area, and the FP probe 505 is placed on the object to perform imaging. . At this time, not only the matching gel but also any one that can achieve acoustic matching between the affected part and the FP probe 505 can be used.

  In the present embodiment, the reception of the photoacoustic wave signal has been mainly described, but an elastic wave can be detected. Therefore, the present invention is applicable to a medical ultrasonic echo probe, a nondestructive inspection ultrasonic flaw detector, a probe, or the like. Further, since this element has a wide band, it can be applied to a microphone or a stethoscope for detecting sound wave vibrations in the audible range.

This example has the configuration described in the first embodiment.
In this embodiment, the present invention is used to image a sample in which an intralipid 1% aqueous solution is hardened with agar as a subject and a rubber wire having a diameter of 300 μm for absorbing light is placed therein. Samples are placed in water.

  A dielectric multilayer film is used for the first mirror of the FP probe, and gold is used for the second mirror. This dielectric multilayer film is designed to have a reflectance of 95% or more at 900 to 1200 nm. The substrate of the FP type probe is BK7, and the AR coating is applied to the surface opposite to the surface on which the dielectric multilayer film is formed so that the reflectance is 1% or less at 900 to 1200 nm. Processing is performed. A side chain polymer liquid crystal having the molecular shape shown in FIG. 10 is used as a material for the receiving film between the mirrors. 1001 is a mesogenic group and 1002 is a main chain. A structure having a positive dielectric anisotropy is used as a mesogen group, and a horizontal alignment film is used. The liquid crystal layer is formed to have a thickness of 30 μm. Further, Parylene C is used as a protective film for the probe.

A laser diode capable of continuous oscillation at a wavelength of 915 nm is used as a light source for measurement light that emits measurement light for measuring the amount of reflected light of the FP probe.
A high-speed CCD camera is used as the array type optical sensor. This number of pixels is 100 × 100 pixels.

  At this time, the measurement light is irradiated and the amount of light detected by the CCD is monitored while appropriately changing the voltage. The voltage-reflectance characteristics are recorded, the voltage value in an optimum state is obtained, and an LUT is created for each CCD pixel.

  Thereafter, the subject is irradiated with light, and photoacoustic wave measurement is started. A light source for irradiating the subject is a titanium sapphire laser. The repetition frequency of the emitted pulsed light is 10 Hz, the pulse width is 10 ns, and the wavelength is 797 nm.

  A distribution of about 100 nm occurs in the layer thickness of the side chain type polymer liquid crystal layer produced in this example. Therefore, data is acquired by dividing the area into 10 blocks on the element.

  Image reconstruction is performed by the universal back projection algorithm using the distribution of the photoacoustic signal obtained after the measurement. At the time of reconstruction, the voxel pitch is 0.5 mm. Thereby, a rubber wire in Intralipid 1% agar which is a light diffusion medium is imaged in an imaging region having a diameter of 2 cm.

  In this example, since driving can be performed under optimum conditions according to the method described in the above embodiment, it is possible to acquire acoustic signal data with good sensitivity.

  Further, the imaging method of the present embodiment is faster than the conventionally reported raster scan method.

This example has the FP probe configuration described in the second embodiment.
The apparatus configuration and subject used in this example are the same as those described in Example 1. The liquid crystal layer is divided into 100 × 100 pixels and is driven in a simple matrix.

  A distribution of about 100 nm occurs in the film thickness of parylene produced in this example. Therefore, the data is taken by dividing the area into 10 blocks on the element.

  Image reconstruction is performed by the universal back projection algorithm using the distribution of the photoacoustic signal obtained after the measurement. At the time of reconstruction, the voxel pitch is 0.5 mm. Thereby, a rubber wire in Intralipid 1% agar which is a light diffusion medium is imaged in an imaging region having a diameter of 2 cm.

In this example, since driving can be performed under optimum conditions according to the method described in the above embodiment, it is possible to acquire acoustic signal data with good sensitivity.
Further, the imaging method of the present embodiment is faster than the conventionally reported raster scan method.

This example has the FP probe configuration described in the third embodiment.
The apparatus configuration and subject used in this example are the same as those described in Example 1. The liquid crystal layer is divided into 100 × 100 pixels and driven in an active matrix.

  A distribution of about 100 nm occurs in the film thickness of parylene produced in this example. Therefore, data is acquired by dividing the area into 10 blocks on the element.

  Image reconstruction is performed by the universal back projection algorithm using the distribution of the photoacoustic signal obtained after the measurement. At the time of reconstruction, the voxel pitch is 0.5 mm. Thereby, a rubber wire in Intralipid 1% agar which is a light diffusion medium is imaged in an imaging region having a diameter of 2 cm.

In this example, since driving can be performed under optimum conditions according to the method described in the above embodiment, it is possible to acquire acoustic signal data with good sensitivity.
Further, the imaging method of the present embodiment is faster than the conventionally reported raster scan method.

  As described in each of the above embodiments, according to the configuration of the present invention, the optical length necessary for resonance can be obtained on a two-dimensional surface even when a film thickness distribution exists due to variations in the film forming process of the receiving film. Can be kept substantially constant. Therefore, since it is possible to measure under a condition where the slope of the reflectance is steep, high sensitivity characteristics can be realized.

  In addition, if the optical path length is uniform, a single wavelength can be used for the measurement light. Alternatively, even if the correction amount is insufficient and a plurality of measurement wavelengths are used, the number of wavelengths can be significantly reduced as compared with the case where no compensation layer is used. This contributes to cost reduction of the device. Alternatively, if the price is the same, a high output light source can be used, which contributes to higher sensitivity.

  In addition, the present invention not only compensates for film formation variations, but also causes various fluctuation factors such as characteristic variations when the environmental temperature changes, characteristic changes due to aging of the elements, and assembly errors when the element is built as a device. Since it can absorb, a stable apparatus can be provided.

401 mirror 402 mirror 403 acoustic wave receiving layer

Claims (5)

An acoustic signal receiving device, a first mirror on which measurement light is incident;
A second mirror disposed opposite to the first mirror and receiving an acoustic wave from the subject;
An acoustic wave receiving layer provided between the first mirror and the second mirror and capable of modulating a refractive index;
A detector for detecting a change in optical path length between the first mirror and the second mirror according to deformation of the acoustic wave receiving layer due to the incidence of acoustic waves;
The refractive index distribution of the acoustic wave receiving layer is configured to compensate for variations in the optical path length due to the film thickness distribution of the acoustic wave receiving layer ,
Furthermore, it has a control part which controls the said acoustic wave receiving layer, The said control part controls the refractive index distribution of the said acoustic wave receiving layer, The said control part divides | segments according to the contour line of film thickness distribution An acoustic signal receiving apparatus for controlling a refractive index of the acoustic wave receiving layer for each of the plurality of regions . The acoustic signal receiving apparatus according to claim 1 , further comprising a signal processing unit that obtains an intensity of an acoustic wave from the subject based on a change in an optical path length detected by the detector. It said acoustic wave receiving layer, the acoustic signal receiving apparatus according to claim 1 or 2 characterized by having a simple matrix driving or active matrix driven liquid crystal materials. The acoustic signal receiving device according to claim 3 , wherein the liquid crystal material is disposed between two electrodes in the acoustic wave receiving layer. It said acoustic wave receiving layer, the acoustic signal receiving apparatus according to claim 1 or 2, characterized in that it has an organic matter or charged material refractive index varies depending on the concentration gradient.

Images