JP2014100605A - Photoacoustic measurement apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a photoacoustic measurement apparatus that can evenly and efficiently illuminate a surface of a living body.SOLUTION: The photoacoustic measurement apparatus includes: a light source; movable holding means for holding a subject; light diffusion means where a distance between the light diffusion means and the holding means for diffusing light emitted from the light source is fixed; and acoustic wave acquisition means for acquiring an acoustic wave emitted from the subject by the light emitted via the holding means and the light diffusion means.

Description

  The present invention relates to a photoacoustic measurement apparatus, and more particularly to an apparatus having movable holding means for holding a subject.

  In recent years, photoacoustic tomography has been proposed in which an optical characteristic value distribution in a living body is obtained with high resolution by utilizing the characteristics of ultrasonic waves that are less scattered in the living body than light. Hereinafter, the photoacoustic tomography is referred to as PAT. In a measuring apparatus using the principle of PAT, first, when pulsed light generated from a light source is irradiated on a living body, it propagates while diffusing in the living body. The absorber contained in the living tissue absorbs the energy of the propagated pulsed light and generates an acoustic wave (this acoustic wave is also called a photoacoustic wave, and is typically an ultrasonic wave). By analyzing the acoustic wave signal obtained by detecting and processing the acoustic wave, it is possible to obtain an optical characteristic distribution in the living body, particularly a light energy absorption density distribution.

In PAT, the sound pressure (P) of an acoustic wave obtained from an absorber in a living body by light absorption can be expressed by the following equation (1).
P = Γ · μ _a · Φ Equation (1)
Here, Γ is a Gruneisen coefficient which is an elastic characteristic value, which 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 (C _p ). μ _a is an absorption coefficient of the absorber, and Φ is a light amount in a local region (amount of light irradiated to the absorber).

  In recent years, breast diagnosis has been studied as a biological application of PAT. In this specification, a PAT apparatus for breast diagnosis is described as photoacoustic mammography (PAM). The PAM device is a device that detects a tumor position in the breast by imaging a new blood vessel formed around the tumor at the time of tumor formation and a region having a high absorption coefficient including the new blood vessel. In order to diagnose the whole breast by PAM, it is necessary to measure from the living body surface to a deep region of 4-5 cm or more. The intensity of the acoustic wave signal is proportional to the amount of light Φ, but the light beam incident by the living cells is scattered and the amount of light reaching the deep portion decreases exponentially. Therefore, in order to enable deep observation, it is preferable to illuminate a wide range with irradiation irradiance (maximum irradiation tolerance MPE: Maximum Permissible Exposure) allowed for the living body. Therefore, a flash lamp pumped solid-state laser having a large output is generally used as the light source.

  The flash lamp excitation solid-state laser has a local high-energy output distribution, and the light amount distribution in the light flux is not uniform as compared with a semiconductor laser, a He—Ne laser, or the like. When a light beam having a local high energy density is used, the local area becomes the upper limit of the illumination intensity. Therefore, it is preferable to make the light quantity distribution of the light beam uniform by using a light modulation member such as a diffusion plate (Patent Document 1).

US Patent Application Publication No. 2006/0184042

In the case of a PAM equipped with a breast holding mechanism that fixes an observation site according to the breast shape, the following problems occur, unlike the X-ray mammography. In the X-ray mammography, since the X-rays travel substantially straight in the living body, the X-ray intensity distribution in the living body is not affected by the position of the parallel plate of the breast holding mechanism.

  On the other hand, in the PAM, when a light diffusing plate, which is a general optical component, is installed in the light beam propagation path in order to make the light quantity distribution in the light beam uniform, the emitted light beam spreads according to the propagation distance. Therefore, when the subject is measured at a plurality of points, if the distance from the living body surface to the diffusion plate is different, the irradiation area is different. As a result, the illumination illuminance at each point changes according to the breast fixed position, causing a problem that the photoacoustic signal is deteriorated.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a photoacoustic measurement apparatus that can uniformly and efficiently illuminate the surface of a living body.

  In order to achieve the above object, the present invention adopts the following configuration. That is, a light source, a movable holding means for holding a subject, a light diffusing means for diffusing light incident from the light source, a fixed distance from the holding means, the holding means, and the light diffusing means. And an acoustic wave acquisition unit that acquires an acoustic wave generated from the subject by the irradiated light.

  According to the photoacoustic measurement apparatus of the present invention, it is possible to uniformly and efficiently illuminate the surface of a living body.

Schematic of a front detection type PAT apparatus. Schematic of a rear detection type PAT apparatus. Schematic of a double-sided irradiation type PAT apparatus. The figure which shows the irradiation area | region in the case of having a biological body holding mechanism including a diffusion mechanism. Another figure which shows the irradiation area | region in the case of having a biological body holding mechanism including a diffusion mechanism. The figure which shows the irradiation area | region in the case of the comparative example to which the spreading | diffusion mechanism is being fixed. 1 is a schematic diagram of a front detection type PAT apparatus in Embodiment 1. FIG. FIG. 6 is a schematic diagram of a rear detection type PAT apparatus according to a second embodiment. 6 is a schematic diagram of a double-sided irradiation type PAT apparatus that performs volume diffusion in Example 3. FIG.

  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 changed as appropriate according to the configuration of the apparatus to which the invention is applied and various conditions. It is not intended to limit the following description.

  The photoacoustic measurement apparatus includes a living body holding mechanism that fixes a measurement site of a subject. Breast fixing methods for breast diagnosis include fixing using two parallel plates from the side of the breast, fixing by pressing from the front of the chest as a whole, and fixing from the periphery of the breast in an arc. Proposed. In particular, the breast holding mechanism using parallel plates has the advantage of being arbitrarily movable according to the size of the breast. Further, the breast holding mechanism using a parallel plate is also employed in X-ray mammography, and has an additional advantage that image comparison with X-ray mammography is easy.

When a breast is fixed using a parallel plate in PAM, there are several possible light irradiation directions and ultrasonic detector arrangement directions on the parallel plate, which will be described with reference to FIGS. In these explanations, it is assumed that there are two parallel flat plates (not shown) that are held across the breast.

  FIG. 1 shows a front detection type apparatus in which an ultrasonic detector 104 is arranged on the side facing the subject 101 with respect to the illumination light 103. The illumination light is irradiated as shown in the diffused light area 102 by diffusion in the living body. FIG. 2 shows a backward detection type apparatus in which an ultrasonic detector 204 is disposed on the same side as the illumination light 203 with respect to the subject 201. The illumination light is irradiated as shown in the diffused light area 202. FIG. 3 shows a double-sided illumination type apparatus in which the object 301 is irradiated with illumination light 303b from the opposite side of the ultrasonic detector 304 and illumination light 303a from the same side. The illumination light is irradiated as shown in the diffused light area 302.

  The parallel flat plate used for holding the object in between can be selected from a variety of movable forms, such as moving the parallel flat plate on the ultrasonic detector side or the opposing side independently or only one of them. Since the parallel plate is movable, the breast can be appropriately compressed and held according to individual differences.

  In illumination used for PAM, a region having a high local irradiation energy density can be removed by using a light diffusion mechanism. Furthermore, it is possible to make the illuminance uniform in the entire irradiation region with uniformity according to the diffusion angle. The light beam that has passed through the light diffusing mechanism has a wider irradiation range depending on the diffusion angle. Here, when a movable parallel plate is used to fix the breast, if the light diffusion mechanism is fixed, the distance from the light diffusion mechanism to the parallel plate, that is, the light beam propagation distance changes with the movement of the plate. The area changes.

  Incidentally, the amount of light allowed for irradiation on the surface of a living body is defined by irradiation energy or irradiation amount per unit area. For this reason, when the irradiation area is changed and the irradiation area is enlarged, the amount of light per unit area on the surface of the living body decreases. On the other hand, when the irradiation area is reduced, the amount of light per unit area on the surface of the living body is increased, and there is a possibility that the illuminance is higher than MPE (Maximum Permissible Exposure). In a PAM that requires deep observation, it is preferable that the illumination illuminance be constant with a large illumination illuminance within a range of safe illumination light quantity below MPE and enabling deep observation. For this purpose, it is necessary to make the irradiation region constant irrespective of the movement of the movable parallel plate.

  In the present invention, a method for making the irradiation region constant will be described below. In FIG. 4, light 402 is emitted from a light source (not shown). In the breast holding mechanism shown in FIG. 4, a light diffusion mechanism 404 is provided on the parallel plate 403 itself. FIG. 4A and FIG. 4B show two types of configurations in which the parallel plate intervals are different in this breast holding mechanism. As shown in this figure, the irradiation area formed by the light flux through the light diffusion mechanism does not change regardless of the interval between the parallel plates 403, and has a constant area. Alternatively, even if the movable parallel plate is moved to hold the breast, the irradiation area does not change before and after the movement.

  In order to realize the light diffusing mechanism, there is a method of making the light quantity distribution of the luminous flux uniform by providing a surface diffusing mechanism that roughens the surface shape on the incident side like a frosted glass. However, when frosted glass is used, a lot of 0th-order light is included such that a light beam perpendicularly incident on the substrate is emitted vertically. Therefore, in order to further improve the uniformity of the light quantity distribution, it is effective to use a holographic diffusion plate in which micron-level surface structures based on surface relief hologram patterns are randomly arranged on the substrate surface. The uneven shape on the surface is made of epoxy, and can be patterned by UV curing. The diffusion angle can be selected depending on the non-periodic uneven shape, has no wavelength dependence from visible to near-infrared wavelengths, and can diffuse a light beam with high transmittance.

In addition, it is also possible to use volume diffusion in which particles having different refractive indexes are contained in a parallel plate base material and diffusion control is performed inside the parallel plate. In particular, titania fine particles can be contained in a substrate such as acrylic or polycarbonate to be volume diffused. In addition to containing fine particles uniformly in the substrate, it is possible to increase the diffusion angle by increasing the diffusion coefficient by increasing the content of fine particles from the incident side to the emission side. When laser light having high coherence is used, there is a possibility that the light beam is locally condensed near the surface inside the substrate due to refraction at the substrate surface. By gradually increasing the diffusion, the possibility that the light beam collects in this way is reduced, and the light can be uniformly diffused.

  As described above, the light diffusing mechanism of the parallel flat plate refers to a structure in which the light diffusing mechanism is provided on the surface or inside the flat plate. As a result, the entire flat plate has a diffusion angle defined within a predetermined range. In the present invention, the light diffusion mechanism (light diffusion means) has a diffusion angle of 0.5 ° or more in the whole light diffusion mechanism when the diffusion angle is shown as the full width at half maximum. The diffusion angle is preferably 1 ° or more, more preferably 5 ° or more. By such a light diffusion mechanism, the subject is irradiated with uniform light.

  Further, as shown in FIG. 5, a configuration may be provided in which a jig 507 is provided as a fixing unit that keeps the distance between the light diffusion mechanism 504 and the parallel plate 503 constant. In this case, since the jig provided with the diffusion mechanism can be moved together with the movement of the holding mechanism, the irradiation region 505 can be made constant. For example, even when the parallel flat plate moves from the state shown in FIG. 5A in the direction in which the breast is compressed and held to the state shown in FIG. 5B, the irradiation area on the breast surface does not change. In the configuration of FIG. 5, unlike the case where the light diffusing mechanism is applied to the parallel plate described above, the surface diffusing mechanism is provided on the light emitting side.

  However, the light diffusion mechanism 504 is not limited to a member using a surface diffusion mechanism, but may be a member using a volume diffusion mechanism. In addition, when a diffusion mechanism is provided on the surface of the light diffusion mechanism 504 installed in a space where light propagates, the diffusion surface may be on the incident side or the emission side. However, when laser light having a high light intensity and high coherence is used, there is a possibility that the light beam is locally condensed near the surface inside the parallel plate due to refraction on the surface of the parallel plate. Therefore, in the case of a light diffusing member using a material having low damage resistance, it is preferable that the exit side be a diffusing surface.

  FIG. 6 shows a configuration of a comparative example with respect to the embodiment of FIG. In this configuration, the position of the light diffusion mechanism 604 is fixed, and the distance between the light diffusion mechanism and the flat plate increases as the parallel flat plate moves in the direction of pressing the subject. In this case, with the movement of the parallel plate, the irradiation area 605 due to the diffused light beam changes (enlarges) from FIG. 6A to FIG. 6B. As a result, the illuminance in the irradiation region is significantly lower than the MPE, and the measurement efficiency is lowered.

<Example 1>
In this embodiment, a configuration example of a forward detection type PAT apparatus to which the present invention is applied will be described.

  As shown in FIG. 7, a front detection type PAT apparatus including a plane parallel flat plates 705 a and 705 b, an ultrasonic detector 704, and a light source 707 for illuminating illumination light 703 is prepared for a living tissue 701 as a subject. To do. Then, the irradiated light is diffused to illuminate the diffused light area 702. In this apparatus, the ultrasonic detector scans on the flat plate surface 705a. In addition, the light beam on the opposite side also moves in synchronization with the movement of the ultrasonic detector and always illuminates from the front surface (left side of the paper) of the ultrasonic detector 704.

The light source uses Nd: YAG, which is a pulsed laser having an oscillation wavelength of 1064 nm. In addition, it is possible to use a visible to near-infrared wavelength band of about 500 nm to 1400 nm. It is also possible to use TI: sa (titanium sapphire) and OPO (optical parametric generation) used with Nd: YAG laser to change the wavelength or use Alexandrite laser using Alexandrite crystal that oscillates in the wavelength band around 750 nm. It is. The parallel plates 705a and 705b use polymethylpentene having a refractive index of 1.46 and have a thickness of 10 mm.

  In this embodiment, a holographic diffusion mechanism, which is a surface diffusion mechanism, is provided on the incident side of the flat plate 705b opposite to the flat plate 705a on which the ultrasonic detector 704 is disposed, that is, on the outer side of the flat plate not in contact with the subject. For this purpose, a sheet 706 having a holographic diffusion mechanism is bonded to the parallel plate 705b. Alternatively, the holographic diffusion mechanism may be formed by directly processing the flat plate surface. That is, the surface diffusion mechanism may be a part of a flat plate or may be a member different from the flat plate. The parallel plate corresponds to the holding means in the present invention. The sheet provided with the holographic diffusion mechanism corresponds to the light diffusion means in the present invention.

  The diffused light is absorbed by the living tissue 701, and ultrasonic waves (acoustic waves) are generated by the expansion and contraction of the living tissue. This is acquired by the ultrasonic detector 704. The ultrasonic detector is composed of, for example, a piezoelectric element, and can convert the acquired ultrasonic wave into an electric signal and use it for image reconstruction inside the living tissue. The ultrasonic detector corresponds to acoustic wave acquisition means in the present invention.

  Using the PAT apparatus configured as described above, the breast is held by moving one or both of the parallel plates 705a and 705b in accordance with the size of the breast, and measurement is performed from the light diffusion mechanism to the subject. The distance is kept constant. As a result, it is possible to prevent the irradiation region from fluctuating due to light diffusion, and it is possible to effectively perform illumination.

<Example 2>
In this embodiment, a configuration example of a backward detection type PAT apparatus to which the present invention is applied will be described.

  For the biological tissue 801, a rear detection type PAT device including a parallel plate 805, an ultrasonic detector 804, and illumination light 803 of a light source (not shown) as shown in FIG. 8 is prepared. The same light source as that of the first embodiment can be used. The light emitted from the light source is diffused to illuminate the diffused light area 802.

  A film having a size larger than that of the detector surface, which is acoustically matched, is fixed on the front surface of the ultrasonic detector. This film is more preferable as it has higher ultrasonic transmission and also requires light transmission. In this example, a polycarbonate film having a thickness of 200 μm was used. A unit in which the ultrasonic detector and the polycarbonate film are integrated is moved in parallel by bringing the unit into contact with the flat plate holding the breast through a solution serving as an acoustic matching layer that easily transmits ultrasonic waves such as castor oil. As the light diffusion mechanism, the polycarbonate film is provided with a surface diffusion mechanism 806 which is a holographic diffusion mechanism. The surface diffusion mechanism is applied to a portion other than the bonded portion between the polycarbonate film and the ultrasonic detector 804, and a light beam enters from this region.

  When the measurement is performed using the PAT apparatus having such a configuration, the distance from the light diffusion mechanism to the subject can be reduced even when the parallel plate 805 is moved in accordance with the size of the breast in order to hold the breast. Kept constant. As a result, it is possible to prevent the irradiation region from fluctuating due to light diffusion, and it is possible to effectively perform illumination.

  Although the rear detection type PAT apparatus has been described here, the method of the present embodiment is combined with the method of the first embodiment, and both sides are illuminated by irradiating light from the light source on the side facing the ultrasonic detector. A type PAT apparatus can be realized.

  Further, in contrast to the configuration of the present embodiment, it is possible to eliminate the flat plate on the opposite side of the ultrasonic detector and to form a single plate. In that case, the measurement is performed by pressing and fixing the breast and holding it. Even in this configuration, it is possible to prevent the irradiation region from being fluctuated due to diffusion, and illumination can be performed effectively. Moreover, it is applicable also when pressing and fixing with an arc-shaped member.

<Example 3>
In this embodiment, a configuration example for realizing a light diffusion mechanism by volume diffusion instead of the surface diffusion used in the above-described embodiment will be shown.

  FIG. 9 shows the configuration of the PAT apparatus in this embodiment. Here, a polycarbonate film for imparting surface diffusion to the front surface of the ultrasonic detector as shown in the above-described embodiments is not necessary. A living tissue 901 such as a breast is a subject, and is held between parallel plates 905. Then, illumination light 903 a and 903 b guided by an optical system from a light source (not shown) is irradiated onto the subject and diffused into the diffused light area 902. The ultrasonic detector 904 detects an acoustic wave generated from the subject.

  In this example, titania fine particles were included in order to adjust the refractive index inside the polymethylpentene resin, which is the material of the parallel plate 905. Two types of titania were prepared, in which the content of the titania fine particles was increased so that the diffusion effect was gradually increased from the incident side to the exit side of the light flux, when contained uniformly in the flat plate.

  When using any of the two types of flat plates described above, if the PAT apparatus having such a configuration is used to move the parallel flat plate 905 according to the size of the breast to hold the breast and perform measurement, The distance from the light diffusion mechanism to the subject is kept constant. As a result, it is possible to prevent the irradiation region from fluctuating due to light diffusion, and it is possible to effectively perform illumination. Of the two types of flat plates, a configuration in which the titania content is gradually increased from the incident side is preferable in that it is possible to prevent the light flux from locally gathering near the surface due to refraction.

  As another aspect of the present embodiment, a configuration example including a diffusion mechanism that moves in conjunction with the breast holding mechanism may be taken. In this case, the parallel plate that is the breast holding mechanism changes the plate interval depending on the size of the breast. A movable parallel plate and a member having a diffusion mechanism are connected to maintain a distance between the flat plate and the diffusion member. As in the first embodiment, when measuring the acoustic wave signal, the opposite side light beam is also operated in the plane in synchronization with the ultrasonic detector, so that the vertical and horizontal sizes are sufficiently large so that the light beam is always irradiated through the diffusing member. A diffusion member is used.

  By adopting such a configuration, it is possible to prevent the irradiation region from being fluctuated due to diffusion, and illumination can be effectively performed.

  703: Illumination light, 704: Ultrasonic detector, 705a and 705b: Parallel plate, 706: Light diffusion mechanism, 707: Light source

In order to achieve the above object, the present invention adopts the following configuration. That is, the light source and, a hold unit that holds the object, located at a said holding means and spacing spreads the light incident from the light source, a light diffusing said gap between said holding means is fixed A photoacoustic measuring device comprising: means; and acoustic wave acquiring means for acquiring an acoustic wave generated from the subject by light irradiated through the holding means and the light diffusing means.

Claims (8)

A light source;
Movable holding means for holding a subject;
A light diffusing means for diffusing the light incident from the light source and having a fixed distance from the holding means;
Acoustic wave acquisition means for acquiring an acoustic wave generated from the subject by light irradiated through the holding means and the light diffusion means;
A photoacoustic measurement apparatus having 2. The photoacoustic measurement apparatus according to claim 1, wherein the light diffusing unit is configured to perform surface diffusion by processing a light incident side of the holding unit. The photoacoustic measurement apparatus according to claim 1, wherein the light diffusing unit is a holographic diffusion plate attached to the holding unit. 2. The photoacoustic measurement apparatus according to claim 1, wherein the light diffusing unit is configured to perform volume diffusion by causing the holding unit to contain fine particles. 5. The photoacoustic measurement apparatus according to claim 4, wherein the light diffusion means has a diffusion coefficient that increases from the light incident side toward the subject side. The photoacoustic measurement apparatus according to claim 1, further comprising a fixing unit that keeps a distance between the light diffusing unit and the holding unit constant. The photoacoustic measurement apparatus according to claim 1, wherein the holding unit holds the subject between two flat plates. The photoacoustic measurement apparatus according to claim 1, wherein the holding unit is configured to press and hold a subject.

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