JP2017131334A - Subject information acquisition device and subject information acquisition method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a device capable of removing artifact at low cost and with a high degree of precision in a photoacoustic device.SOLUTION: A subject information acquisition device includes light irradiation parts 110 and 120, an acoustic wave probe 130 for receiving an acoustic wave generated in a subject due to a pulse light, and information acquisition parts 140 and 150 for acquiring characteristic information in the subject based on the received acoustic wave. The light irradiation parts perform first light irradiation for irradiating the pulse light a plurality of times within a predetermined time, and second light irradiation for irradiating a single pulse light after the first light irradiation. The information acquisition parts acquire third data which is a difference between first data acquired based on the acoustic wave generated the by first light irradiation and second data acquired based on the acoustic wave generated by the second light irradiation, and acquire the characteristic information in the subject using the third data. The predetermined time is shorter than the time from when a predetermined region in the subject is brought into an excitation state due to the pulse light until the excitation state is mitigated.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a subject information acquisition apparatus that acquires information in a subject.

In recent years, research for imaging morphological information and physiological information in a subject, that is, functional information, has been advanced in the medical field. In recent years, photoacoustic tomography (PAT) has been proposed as one of such techniques.
When light such as pulsed laser light is irradiated onto a living body that is a subject, an acoustic wave (typically, an ultrasonic wave) is generated when the light is absorbed by a living tissue in the subject. This phenomenon is called a photoacoustic effect, and an acoustic wave generated by the photoacoustic effect is called a photoacoustic wave. Since tissues constituting the subject have different optical energy absorption rates, the sound pressures of the generated photoacoustic waves are also different. In PAT, the generated photoacoustic wave is received by a probe, and the received signal is mathematically analyzed, whereby characteristic information in the subject can be acquired.

  The characteristic information is a distribution of optical characteristic values such as an initial sound pressure distribution, a light absorption energy density distribution, and a light absorption coefficient distribution. In addition, by acquiring these pieces of information using light of a plurality of wavelengths, the concentration of a specific substance in the subject (hemoglobin concentration in blood, oxygen saturation of blood, etc.) can be quantitatively measured. .

  On the other hand, in the process of imaging the position of the sound source, since only the direct propagation component is considered, if an acoustic wave is reflected inside the subject, a virtual image (hereinafter referred to as artifact) appears in the reflected portion. As a technique for dealing with this problem, Non-Patent Document 1 describes a technique for solving the problem by tagging due to minute vibration of ARF (Acoustic Radiation Force) using an ultrasonic device.

Michael Jaeger, Jeffrey C. Bamber, Martin Frenz, “Clutter elimination for deep clinical optoacoustic imaging using localized vibration tagging (LOVIT)” Photoacoustics 1 19-29 (2013)

  The apparatus described in Non-Patent Document 1 has a problem in that artifacts can be accurately removed at low cost.

  The present invention has been made in view of such a problem of the prior art, and an object of the present invention is to provide a technique for removing artifacts with low cost and high accuracy in a subject information acquisition apparatus.

In order to solve the above problems, a subject information acquisition apparatus according to the present invention includes:
Based on the received acoustic wave, a light irradiation unit that irradiates the subject with pulsed light, an acoustic wave probe that receives an acoustic wave generated in the subject due to the pulsed light, and the received acoustic wave. An information acquisition unit that acquires characteristic information in the specimen, and the light irradiation unit includes a first light irradiation that irradiates the pulsed light a plurality of times within a predetermined time, and after the first light irradiation. The second light irradiation for irradiating a single pulsed light is performed, and the information acquisition unit includes first data obtained based on an acoustic wave generated by the first light irradiation, and the second light. Obtaining third data that is the difference between the second data obtained based on the acoustic wave generated by irradiation, obtaining characteristic information in the subject using the third data, For a predetermined time, a predetermined site in the subject is excited due to the pulsed light. From when, characterized in that the excited state is shorter than the time until the relaxation.

Further, the subject information acquisition method according to the present invention includes:
An object information acquisition method performed by an object information acquisition apparatus comprising: a light irradiation unit that emits pulsed light; and an acoustic wave probe that receives an acoustic wave generated in the object due to the light. A first light irradiation step of irradiating the pulsed light a plurality of times within a predetermined time, a second light irradiation step of irradiating a single pulsed light after the first light irradiation, and the first light irradiation step Third data which is a difference between the first data obtained based on the acoustic wave generated by the light irradiation and the second data obtained based on the acoustic wave generated by the second light irradiation. And acquiring the characteristic information in the subject using the third data, and the predetermined portion in the subject is in the pulsed light for the predetermined time. After the excited state is caused, the excited state is relaxed. Wherein the of a time shorter than the time.

  According to the present invention, it is possible to provide an object information acquisition apparatus that removes artifacts with low cost and high accuracy.

The schematic diagram of the subject information acquisition apparatus which concerns on 1st embodiment. The flowchart figure which shows the subject information acquisition method which concerns on 1st embodiment. The figure which shows the mode of the acoustic propagation in a subject. The figure explaining the photoacoustic signal which a probe receives. The schematic diagram of the subject information acquisition apparatus which concerns on 2nd embodiment.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In principle, the same components are denoted by the same reference numerals, and description thereof is omitted. In addition, the dimensions, materials, shapes, and relative arrangements of the parts used in the description of the embodiments should be appropriately changed according to the configuration of the apparatus to which the invention is applied and various conditions, and limit the scope of the present invention. Not intended to do.

(First embodiment)
The subject information acquisition apparatus according to the present embodiment irradiates a subject with pulsed light, and receives and analyzes a photoacoustic wave generated in the subject due to the pulsed light. It is an apparatus that visualizes, that is, visualizes characteristic information related to characteristics.
In general, the characteristic information related to the optical characteristics is the source distribution of the acoustic wave in the subject, the initial sound pressure distribution, the light absorption energy density distribution, the absorption coefficient distribution, or the substance constituting the tissue. It is a characteristic distribution related to concentration. The characteristic distribution related to the concentration includes, for example, the distribution of oxygen saturation, a value obtained by weighting the oxygen saturation with an intensity such as an absorption coefficient, a total hemoglobin concentration, an oxyhemoglobin concentration, or a deoxyhemoglobin concentration. Further, it may be a distribution of glucose concentration, collagen concentration, melanin concentration, volume fraction of fat or water, and the like.

In addition, the subject information acquisition apparatus according to the present embodiment has a function of transmitting ultrasonic waves and a function of receiving ultrasonic waves reflected within the subject (also referred to as reflected waves or ultrasonic echoes). The image information can also be obtained based on the above. In this case, the characteristic information related to the acoustic characteristic in the subject can be visualized, that is, imaged.

In addition, the acoustic wave in this embodiment is typically an ultrasonic wave, and includes an elastic wave called a sound wave and an acoustic wave. An acoustic wave generated by the photoacoustic effect is called a photoacoustic wave or an optical ultrasonic wave. Hereinafter, among acoustic waves, an acoustic wave transmitted into the subject by the transducer and an acoustic wave reflected in the subject are referred to as “ultrasonic waves” or “echo waves”.
Further, among acoustic waves, those generated in the subject by the photoacoustic effect are referred to as “photoacoustic waves”. However, this is for the convenience of distinguishing the two, and does not limit the wavelength or the like of each acoustic wave.

<Prior art issues>
The problem in the prior art will be described with reference to FIG.
Since the photoacoustic wave is an ultrasonic wave, it propagates through the medium with linearity. In addition, one of the characteristics of ultrasonic waves is that they are reflected at the boundary between media having different acoustic impedances.
In the example of FIG. 3, it is assumed that the subject is composed of a medium 311 and a medium 312 having different acoustic impedances, and a light absorber is provided inside each of the medium 311 and the medium 312.
When such an object is irradiated with the irradiation light 340, first, a photoacoustic wave is generated from the light absorber 321. Among the generated photoacoustic waves, part of the photoacoustic wave travels upward in the figure and directly enters the probe, and part of the photoacoustic wave travels downward in the figure and is reflected at the boundary between the medium 311 and the medium 312. In other words, the photoacoustic wave 301 that has directly propagated and the photoacoustic wave 302 that has propagated with a delay due to reflection are incident on the probe 330.

  Generally, in the reconstruction process for imaging the position of a sound source, only the direct propagation component is considered, so that an image caused by the photoacoustic wave 302 as a reflection component appears on the image as an artifact. In addition, in the example of FIG. 3, although the reflective surface is made into one place, in the target object which many scattering and reflection generate | occur | produce, such as a biological body, an infinite number of reflected signals generate | occur | produce.

Next, a light absorber 322 in the medium 312 having the same absorption coefficient as that of the light absorber 321 is considered. The intensity of light reaching the subject decreases exponentially with distance. Therefore, the initial sound pressure of the photoacoustic wave generated from the light absorber 322 is extremely smaller than the initial sound pressure of the photoacoustic wave generated from the light absorber 321.
At this time, the photoacoustic wave 303 reaching the probe should be observable if the amplitude is larger than the noise component of the system. However, when the photoacoustic wave 302 and the photoacoustic wave 303 arrive at the probe at a time interval such that the respective signals cannot be distinguished, the signal corresponding to the photoacoustic wave 302 becomes dominant. It becomes difficult to detect the presence of the acoustic wave 303.

In particular, in the shallow part of the living body (particularly the body surface side of the subcutaneous tissue), there are many melanins that serve as light absorbers, hemoglobin contained in capillaries, and the like. In addition, it has been confirmed that this causes a decrease in the discriminability of signals generated from blood vessels such as deep arteries and veins and the visibility in photoacoustic images.
Therefore, in order to improve the visibility of the deep part of the subject, it is desirable to reduce the artifact component caused by the strong photoacoustic signal generated from the vicinity of the shallow part of the living body.
Examples of the medium corresponding to the boundary include muscle fibers such as a muscle layer, mammary gland layer and other organs, and an infiltrated region of cancer cells.

<Outline of device>
Next, an outline of a method for reducing the artifact by the subject information acquiring apparatus according to the present embodiment will be described.
As described above with reference to FIG. 3, since the photoacoustic wave generated inside the subject diffuses in all directions, reflection occurs when there is an interface with different acoustic impedance in the subject. Is delayed and enters the probe, causing artifacts. Further, due to the artifact, it becomes difficult to discriminate the light absorber in the deep part of the subject.

The object information acquiring apparatus according to this embodiment first irradiates a single pulsed light twice at a time interval equal to or less than the absorption relaxation time of the light absorber distributed on the surface of the object. The photoacoustic wave generated due to the pulsed light is sampled. The pulsed light irradiation performed here is referred to as first light irradiation, and a signal obtained due to the first light irradiation is referred to as a first signal.
In the present embodiment, the absorption relaxation time (hereinafter simply referred to as relaxation time) means that after the light absorber inside the subject absorbs light energy and becomes saturated, the saturation state relaxes ( That is, the time until the ground state is restored).

  In order to irradiate the pulsed light a plurality of times within the relaxation time, typically the pulsed light may be irradiated a plurality of times within 100 picoseconds in the first light irradiation. In the first light irradiation, the pulsed light may be irradiated a plurality of times within a time of 17 picoseconds or less. In the first light irradiation, the pulsed light may be irradiated a plurality of times within a time of 0.5 picoseconds or less. For example, when the target light absorber is deoxyhemoglobin and light having a wavelength of 567 nm is used, the relaxation time is 22 ± 5 picoseconds. Therefore, in the first light irradiation, the light is pulsed several times within 17 picoseconds. May be irradiated. Further, for example, when the target light absorber is oxyhemoglobin and light having a wavelength of 567 nm is used, the relaxation time is 2.3 ± 1.8 picoseconds. What is necessary is just to irradiate pulsed light several times within picosecond.

  It is desirable that the pulse width of the light irradiated by the first light irradiation is short enough for the relaxation time of the target light absorber and can provide energy that causes absorption saturation with energy equal to or lower than MPE. . For example, when the target light absorber is deoxyhemoglobin and light having a wavelength of 576 nm is used, absorption saturation can be caused by energy of MPE or less by setting the pulse width to 70 picoseconds or less. In addition, when the target light absorber is oxyhemoglobin and light having a wavelength of 576 nm is used, absorption saturation can be caused with energy of MPE or less by setting the pulse width to 70 picoseconds or less.

If comprised in this way, the light absorber of object will absorb energy by irradiation of the 1st pulsed light, and the 2nd pulsed light will be irradiated before energy release by vibration occurs. Therefore, in the second pulsed light, energy absorption is suppressed in the absorber in a portion that is saturated with the first pulsed light. As a result, the generation of photoacoustic waves due to vibrations can be suppressed, while in the absorber in the deep part where saturation is not achieved by the first pulse light, the linearly absorbed energy is converted into vibrations to generate photoacoustic waves. Is done.
The light reaches the deep part while being affected by scattering. For this reason, the pulse width becomes longer due to the effect of changing the propagation distance in the deep part, and the saturation phenomenon hardly occurs in the light absorber in the deep part.

  Next, only the first pulse irradiated in the first light irradiation is irradiated toward the subject, and the generated photoacoustic wave is sampled. The pulsed light irradiation performed here is referred to as second light irradiation, and a signal obtained due to the second light irradiation is referred to as a second signal.

The first and second signals obtained in this way each include a component corresponding to the photoacoustic wave generated near the surface of the subject and an artifact corresponding to the photoacoustic wave reflected inside the subject. Contains ingredients. In the first light irradiation, since the pulsed light reaches the deep part of the subject, the first signal has a component corresponding to the photoacoustic wave generated in the deep part of the subject from the second signal. Also appears strongly. In the first light irradiation, the second pulsed light is irradiated within a period in which the light absorber is in a saturated state, and therefore the second pulsed light is not absorbed by the light absorber and is not absorbed. It reaches the deep part of the specimen. Therefore, the signal intensity of the photoacoustic wave generated from the deep part is given an effect that appears stronger in the first signal than in the second signal.

  Therefore, by calculating the difference between the first signal and the second signal, the influence caused by the signal generated from the light absorber region (hereinafter referred to as the saturation region) near the surface of the subject is canceled, and the subject It is possible to acquire a signal generated from the deep part of the area more strongly. That is, by reconstructing this signal, artifacts due to the light absorber in the saturation region can be suppressed, and the deep part of the subject can be clearly imaged. Specific examples of signal strength will be described later.

  In particular, when a living body is a subject, there are many planar light absorbers such as melanin and epidermis on the surface, and when a photoacoustic wave is generated from it, a plane wave with a large amplitude or a focused wave is generated as a synthesized wave May occur, resulting in artifacts and affecting the image. However, by performing the processing as described above, even if a synthetic wave is generated due to a light absorber near the surface, the influence of the synthetic wave can be canceled, so that improvement of the image is expected. it can.

<System configuration>
The configuration of the subject information acquisition apparatus according to the first embodiment for performing the processing described above will be described with reference to FIG. The subject information acquisition apparatus according to this embodiment includes a light source 110, an optical system 120, a probe 130, a control device 140, a signal processing device 150, and a display device 160. Hereinafter, each means which comprises the subject information acquisition apparatus which concerns on this embodiment is demonstrated.

<< Light source 110 >>
The light source 110 is a device that generates pulsed light that irradiates a subject. The light source is preferably a laser light source in order to obtain a large output, but a light emitting diode, a flash lamp, or the like may be used instead of the laser. When a laser is used as the light source, various lasers such as a solid laser, a gas laser, a dye laser, and a semiconductor laser can be used. The timing, waveform, intensity, etc. of irradiation are controlled by a light source control unit (not shown). The light source control unit may be integrated with the light source.
The wavelength of the pulsed light is preferably a specific wavelength that is absorbed by a specific component among the components constituting the subject, and is a wavelength at which light propagates to the inside of the subject. Specifically, when the subject is a living body, the thickness is desirably 500 nm or more and 1400 nm or less.
In order to effectively generate photoacoustic waves, light must be irradiated in a sufficiently short time according to the thermal characteristics of the subject. When the subject is a living body, the pulse width of the pulsed light generated from the light source is preferably on the order of several femto to several microseconds.

  In order to efficiently generate a photoacoustic wave while maintaining a saturated state in light absorption, a pulse width of about several picoseconds may be used, but the time required for the target to saturate is less than, It is more preferable to use a pulse width on the order of femtoseconds. Specifically, on the basis of MPE, 5 picoseconds in the case of deoxygenated hemoglobin and 70 picoseconds in the case of oxygenated hemoglobin are used as a guideline. Accordingly, the pulse width is preferably at least 100 picoseconds or less.

  The pulsed light generated by the light source 110 is received from an emission end 121 provided at an end of the optical system 120 via an optical system 120 (described later) having optical members such as a lens, a mirror, a diffusion plate, and an optical fiber. The specimen is irradiated.

<< Optical system 120 >>
The subject information acquiring apparatus according to the present embodiment branches light generated by a light source into two systems using a half mirror and delays one of them using a delay line. As a result, it is possible to irradiate two pulses of light at intervals of several picoseconds. However, if two light sources are provided and pulse light can be generated with a time difference, a delay line is not necessary.
Specifically, when the delay time is 10 picoseconds, the optical path is designed so that the difference between the optical path lengths is 3 mm. A shutter (not shown) is disposed on the optical path on the delay line side so that the optical path on the delay side can be blocked. Thereby, it is possible to switch between emitting pulsed light with two continuous waves or emitting only one wave.

The light emitted from the light source is typically guided to the subject while being processed into a predetermined light distribution shape by an optical component such as a lens or a mirror. It is also possible to propagate light using an optical waveguide such as an optical fiber. The optical system is, for example, a mirror that reflects light, a lens that collects or enlarges light, or changes its shape, or a diffusion plate that diffuses light. As such an optical component, any optical component may be used as long as the light emitted from the light source is irradiated on the subject in a desired shape. Note that it is preferable to spread light over a certain area, rather than condensing light with a lens, from the viewpoint that the safety to the living body and the diagnostic area can be expanded.
The light source 110 and the optical system 120 are a light irradiation unit in the present invention.

<< Subject 100 and Light Absorber 101 >>
These do not constitute a part of the subject information acquisition apparatus of the present invention, but will be described below. The object information acquiring apparatus according to the present embodiment is mainly intended for diagnosis of human or animal malignant tumors, vascular diseases, etc., follow-up of chemical treatment, and the like. Therefore, the subject is assumed to be a living body, specifically, a target site for diagnosis such as the breast, neck and abdomen of a human body or animal.
Further, the light absorber inside the subject refers to one having a relatively high absorption coefficient inside the subject. For example, if the human body is a measurement target, oxyhemoglobin or deoxyhemoglobin, a blood vessel containing many of them, or a malignant tumor containing many new blood vessels becomes a light absorber. In addition, melanin, carotid artery plaque, and the like are also light absorbers.

<< Probe 130 >>
The probe 130 is means for detecting acoustic waves (photoacoustic waves and echo waves) coming from within the subject and converting them into analog electrical signals. The probe is also called an acoustic wave probe, an acoustic wave detector, or a transducer. Any probe may be used such as a probe using a piezoelectric phenomenon, light resonance, change in capacitance, or the like.
Here, the probe preferably has a plurality of transducers that transmit and receive acoustic waves arranged in an array. Thereby, since an acoustic wave can be received at a plurality of positions and a plurality of signals can be output, a reduction in measurement time and an improvement in the SN ratio can be expected. The element arrangement is preferably one-dimensionally arranged or arranged on the spherical shell surface.

It is preferable that the position of the probe 130 can be changed with respect to the subject. For example, when the probe 130 is a handheld probe, the position is changed by the operator's technique. Further, a scanning mechanism for moving the probe 130 may be installed. Further, the subject information acquiring apparatus may give a swing guide instruction to the worker in the form of voice or screen display.
The probe 130 preferably has both a transmitter function for transmitting ultrasonic waves to the subject and a receiver function for receiving echo waves that have propagated through the subject. As a result, signals can be detected for the same region, and space saving can be expected. However, the transmitter and the receiver may be separated. Further, the photoacoustic wave and echo wave receivers may be provided separately.

  In the present embodiment, the probe 130 is formed by providing a hemispherical support member as shown in the drawing and arranging a plurality of transducers on the inner surface thereof. In addition, a pulse light emitting end 121 is provided at the bottom of the probe 130 so that the subject can be irradiated with the pulsed light.

<< Control device 140 >>
The control device 140 is a means including an amplifier, an A / D converter, a FPGA (Field Programmable Gate Array) chip, a CPU, and the like, and a means for amplifying the electric signal converted by the probe 130 and converting it into a digital signal. It is.
When the probe 130 has a plurality of transducers and outputs a plurality of received signals, it is desirable that the control device 140 is also configured to process a plurality of signals simultaneously. Thereby, processing time can be shortened.
The control device 140 controls the timing of pulsed light irradiation and subsequent photoacoustic wave reception. Specifically, control of pulsed light irradiation timing, switching of illumination (two waves and one wave) by shutter control, and transmission / reception timing of electrical signals using pulsed light as a trigger signal are performed.
Hereinafter, the term photoacoustic signal will be used as a term representing both an analog time-series electrical signal output from the probe 130 and a digital time-series signal processed by the control device 140.
The signal converted by the control device 140 is transmitted to the signal processing device 150.

<< Signal processing device 150 >>
The signal processing device 150 is means for acquiring characteristic information inside the subject by calculating a light amount distribution, image reconstruction, and the like based on the acquired digital signal (photoacoustic signal).
The signal processing device 150 has functions of performing signal processing, image reconstruction, and image processing. Note that the signal processing device 150 can apply signal processing to both 2D space and 3D space.
The signal processing device 150 is typically a workstation, and the above-described processing is performed by software stored in advance.

Signal processing algorithms executed by the signal processing apparatus include, for example, filtering using an existing bandpass filter and response correction of a known probe response.
As an image reconstruction algorithm, for example, a technique known in tomography technology such as back projection in the time domain or Fourier domain, phasing addition (delay and sum), or the like can be executed. When much reconstruction time can be used, an image reconstruction method such as an inverse problem analysis method using iterative processing may be used.
As an image processing algorithm, there are an existing speckle reduction process, an averaging of image frames in a time direction, and the like.

In photoacoustic imaging, by using the focused probe 130, it is possible to generate an image representing in-vivo characteristic information without image reconstruction. In such a case, it is not necessary to perform signal processing using an image reconstruction algorithm.
The signal processing device 150 may be composed of elements such as a CPU and a GPU, and circuits such as an FPGA and an ASIC. Further, the signal processing device 150 is not limited to a single element or circuit, but may be configured from a plurality of elements or circuits. In addition, any element or circuit may execute each process performed by the signal processing device 150.
Further, the control device 140 and the signal processing device 150 may be integrated. In this case, the characteristic information such as the acoustic impedance of the subject and the optical characteristic value distribution may be generated by hardware processing instead of software processing performed at the workstation.
The control device 140 and the signal processing device 150 are information acquisition units in the present invention.

<< Display device 160 >>
The display device 160 is means for displaying characteristic information such as an optical characteristic value distribution output from the signal processing device 150. As the display device, for example, a liquid crystal display, a plasma display, an organic EL display, a head mounted display, or the like can be used. The display device may be provided separately from the subject information acquisition device. For example, the acquired subject information may be transmitted to an external display device by wire or wireless.

<Processing flow>
Next, the procedure of the subject information acquisition method performed by the subject information acquisition apparatus according to the first embodiment will be described with reference to the flowchart of FIG. The flowchart shown in FIG. 2 is executed when the control device 140 reads a program stored in a memory (not shown) and controls the operation of each component of the device.

Step S100 is a step of generating a photoacoustic signal by performing first light irradiation, receiving a photoacoustic wave generated from the subject.
First, pulse light is emitted from the light source 110. The emitted pulsed light is branched into two paths by a half mirror. The branched pulsed light is irradiated to the subject 100 through the common optical system 120. At this time, one of the branched optical paths is set longer than the other, thereby setting a delay between the pulses.

  It is preferable that the optical path difference can be changed depending on the installation position of the mirror. For example, the position of the mirror may be adjustable by a fine adjustment mechanism using a manual micrometer. Further, the mirror position may be controlled using an automatic stepping motor or piezo. The interval between pulses to be set is preferably within the relaxation time of the target light absorber. For example, when the relaxation time is 10 picoseconds, the optical path difference is approximately 3 mm. Since the pulse width is shorter than the delay, a single pulsed light is continuously output from the emission end 121. Hereinafter, these are referred to as the first pulse light and the second pulse light, respectively.

Details of the photoacoustic wave generated by the first light irradiation will be described with reference to FIG.
When the first pulse light is irradiated as the irradiation light 340, the light absorber 321 in the medium 311 absorbs the energy of light and becomes saturated, and then the energy changes to vibration energy, and the photoacoustic wave 301 is generated. Occur.
At this time, if the acoustic impedances of the medium 311 and the medium 312 are different, the photoacoustic wave that has traveled to the opposite side of the probe is reflected and enters the probe as the photoacoustic wave 302.

  Further, when the light reaches the light absorber 322 in the medium 312, the photoacoustic 303 is generated by the photoacoustic phenomenon. The light reaching at this time has a wide pulse width because the optical path difference of the light changes randomly due to the scattering phenomenon in the living body. Here, it is assumed that the first pulsed light reaches the light absorber 322 in a state where the energy is lower than the saturation energy.

  FIG. 4A shows a signal to be observed by the probe 330 by the first pulse light irradiation. As illustrated, in this example, the photoacoustic wave 301 directly propagated from the light absorber 321 in the saturated state, the photoacoustic wave 302 that is a reflection component and causes artifacts, and the light absorber 322 that is not saturated. A signal is obtained in the order of the directly propagated photoacoustic wave 303.

Next, consider a case where the subject is irradiated with the second pulsed light emitted after the delay time.
When the second pulse light reaches the light absorber 321 that is saturated by the first pulse light, no further light absorption occurs, so that further photoacoustic phenomenon occurs in the light absorber 321. do not do. That is, the pulsed light enters the medium 312 without energy loss.
Further, since the light absorber 322 in the medium 312 is not saturated, the photoacoustic wave 303 is generated. At this time, since there is no loss due to energy absorption, the sound pressure of the generated acoustic wave increases compared to the first wave.
Here, FIG. 4B shows a signal to be observed by the probe 330 by irradiation with the second pulsed light. The photoacoustic wave 304 directly propagated from the light absorber 322 in which light is not saturated becomes the main observation signal.

  Note that since the photoacoustic wave sampling is performed in the ultrasonic frequency band (MHz band), the above-described separation of the first wave signal and the second wave signal cannot be performed on the signal. That is, the photoacoustic signal actually obtained in step S100 is the sum of the signal shown in FIG. 4A and the signal shown in FIG. 4B, as shown in FIG.

  In step S100, the control device 140 causes the probe 130 to start receiving photoacoustic waves according to the emission timing of the pulsed light. The photoacoustic signal output from the probe 130 is stored in a memory (not shown) as a first signal after being processed by the control device 140.

Step S200 is a step of performing a second light irradiation, receiving a photoacoustic wave generated from the subject, and generating a photoacoustic signal.
First, pulse light is emitted from the light source 110. The emitted pulsed light is branched into two paths by a half mirror. At this time, the optical path on the delay side closes the shutter and suppresses irradiation of the subject. Since the pulsed light emitted from the emission end 121 behaves the same as the first wave in step S100, the same signal as shown in FIG. 4A is obtained. The photoacoustic signal output from the probe 130 is stored in the memory as a second signal after being processed by the control device 140.

  Step S300 is a step in which the signal processing device 150 generates a third signal by taking the difference between the first signal acquired in Step S100 and the second signal acquired in Step S200.

The first signal and the second signal both contain a component that reaches the probe directly and a component that reaches the probe, in the photoacoustic wave generated from the light absorber that is both saturated. It is. Further, the ratio of the intensity of the photoacoustic wave 303 to the intensity of the photoacoustic wave 301 is smaller than the ratio of the intensity of the photoacoustic wave 305 to the intensity of the photoacoustic wave 301.
Therefore, by taking the difference between the first signal and the second signal, the signal shown in FIG. 4B is extracted. The extracted signal is stored in the memory as a third signal.

  At this time, when the second pulse light is irradiated in a state where the light absorber 321 is not completely saturated, a further photoacoustic wave is generated from the light absorber 321. However, even in that case, the effect is less than when saturation is not occurring.

Step S400 is a step in which the signal processing device 150 processes a signal based on the third signal calculated in Step S300 and reconstructs an image.
A known method may be used for signal processing and image reconstruction. For example, a deconvolution process by a probe response or a band pass filter process is performed on the third signal, and the time domain is reconfigured. The reconstructed photoacoustic wave image is displayed by the display device 160. At this time, only the image reconstructed from the third signal may be displayed, or in addition to this, an image reconstructed from the first signal or the second signal may be displayed. For example, the images may be displayed side by side in a comparable form, or the images may be switched and displayed.

In the first embodiment, the difference between the photoacoustic signals is processed. However, the images are reconstructed using the first signal and the second signal, and the difference between the images is obtained. The same effect can be obtained. If it is the data based on the received photoacoustic wave, the object which takes a difference will not be limited.

(Second embodiment)
The second embodiment is an embodiment in which pulsed light is irradiated using a plurality of light sources. In the second embodiment, a method for removing a reflection / scattering signal caused by a surface vascular network existing near the surface of a living body will be described.

  FIG. 5 shows the configuration of the subject information acquisition apparatus according to the second embodiment. In the second embodiment, two light sources 510 and 511 are used as laser light sources. Further, the respective light emission timings are controlled using the delay generator 500. In this embodiment, the delay time is set to 500 femtoseconds (0.5 picoseconds) shorter than the hemoglobin relaxation time. Each pulse width is also 500 femtoseconds. The pulsed light is emitted from the emission end 521 provided in the probe 530 through the optical system 520.

  The probe 530 is a 1D linear array probe. In addition, the control device 540 controls the emission timing of the pulsed light through the delay generator 500, and collects photoacoustic signals. Since the signal processing device 550 and the display device 560 are the same as those in the first embodiment, description thereof will be omitted.

Based on the processing described in FIG. 2, a subject information acquisition method in the second embodiment will be described.
In the second embodiment, first, in step S100, the subject is irradiated with pulsed light using both the light source 510 and the light source 511, and a photoacoustic signal as a first signal is acquired.
Next, in step S200, the subject is irradiated with pulsed light using only the light source 510, and a photoacoustic signal as a second signal is acquired.
In the second embodiment, when irradiating pulsed light in each step, the emission intensity (light quantity distribution of the light irradiated to the subject) is obtained with a photodiode built in the probe, and the light quantity Using the distribution, the intensities of the first signal and the second signal are corrected. As a result, the first and second signals are normalized, and the influence of fluctuations in the laser output can be removed.

Next, in step S300, the third signal is acquired by taking the difference between the corrected first and second signals.
In the second embodiment, in step S400, image reconstruction processing is performed on each of the second signal and the third signal to generate two types of images. Hereinafter, an image reconstructed from the second signal is referred to as a first image, and an image reconstructed from the third signal is referred to as a second image.
For image reconstruction processing, deconvolution based on a probe response, band pass filter processing, universal back projection processing, or the like can be used.
The reconstructed image data is transmitted to the display device 160 and displayed.

The first image (image reconstructed from the second signal using the conventional method) and the second image (image reconstructed from the third signal using the method of the present invention) were respectively compared. Describe the results.
According to the method of the present invention, it is possible to obtain an effect of reducing artifacts caused by a signal generated from a light absorber that is saturated particularly at a shallow portion. Therefore, the second image can acquire an image having a deeper contrast. On the other hand, in the second image, since the signal generated from the saturation region is canceled, the luminance is lower than that of the first image. Therefore, when deep information is necessary, the necessary information can be supplemented by using the first image together.

(Modification)
The description of each embodiment is an exemplification for explaining the present invention, and the present invention can be implemented with appropriate modifications or combinations without departing from the spirit of the invention.
For example, the present invention can be implemented as a subject information acquisition apparatus including at least a part of the above processing. Moreover, it can also be implemented as a subject information acquisition method including at least a part of the above processing. The above processes and means can be freely combined and implemented as long as no technical contradiction occurs.

In the description of the embodiment, the time from when the target light absorber is saturated by the irradiation of pulsed light until it returns to the ground state is defined as the relaxation time. However, the light absorber is not necessarily saturated. That is, the time until the light absorber that has absorbed the light energy and returned to the ground state (including the substantially ground state) in the excited state may be defined as the relaxation time.
The delay time may be any value as long as it is within the relaxation time of the target light absorber.

  The present invention is also realized by executing the following processing. That is, a program that realizes one or more functions of the above-described embodiments is supplied to a system or apparatus via a network or various storage media, and one or more processors in the computer of the system or apparatus read the program. It can also be realized by processing to be executed. It can also be realized by a circuit (for example, FPGA or ASIC) that realizes one or more functions.

  DESCRIPTION OF SYMBOLS 110 ... Light source, 120 ... Optical system, 130 ... Probe, 140 ... Signal processing apparatus, 150 ... Signal processing apparatus

Claims (16)

A light irradiation unit that irradiates the subject with pulsed light; and
An acoustic probe for receiving an acoustic wave generated in the subject due to the pulsed light; and
Based on the received acoustic wave, an information acquisition unit for acquiring characteristic information in the subject;
Have
The light irradiation unit performs first light irradiation that irradiates pulsed light a plurality of times within a predetermined time, and second light irradiation that irradiates a single pulsed light after the first light irradiation,
The information acquisition unit includes first data obtained based on the acoustic wave generated by the first light irradiation, and second data obtained based on the acoustic wave generated by the second light irradiation. And obtaining the third data that is the difference between, and using the third data to obtain the characteristic information in the subject,
The predetermined time is a time shorter than a time from when a predetermined site in the subject is in an excited state due to the pulsed light until the excited state is relaxed. A subject information acquisition apparatus. The subject information acquiring apparatus according to claim 1, wherein the subject is a human body, and the predetermined part is located closer to the body surface than a subcutaneous tissue. The first data is an electrical signal obtained by converting an acoustic wave generated by the first light irradiation, and the second data is a conversion of an acoustic wave generated by the second light irradiation. The object information acquiring apparatus according to claim 1, wherein the object information acquiring apparatus is an electric signal obtained by performing the above process. The first data is image data generated based on an electrical signal obtained by converting an acoustic wave generated by the first light irradiation, and the second data is the second light. The object information acquiring apparatus according to claim 1, wherein the object information acquiring apparatus is image data generated based on an electrical signal obtained by converting an acoustic wave generated by irradiation. The information acquisition unit acquires a difference between the first and second data after normalizing the first and second data based on a light amount distribution of light irradiated on the subject. The object information acquiring apparatus according to claim 1, wherein: In the subject, there is a first region corresponding to the predetermined region and a second region deeper than the first region, and corresponds to an acoustic wave generated from the first region. When the signal is the first signal and the signal corresponding to the acoustic wave generated from the second region is the second signal,
The ratio of the intensity of the second signal obtained due to the second light irradiation to the intensity of the first signal obtained due to the first light irradiation is:
It is smaller than the ratio of the intensity of the second signal obtained due to the first light irradiation to the intensity of the first signal obtained due to the first light irradiation. Item 6. The subject information acquiring apparatus according to any one of Items 1 to 5. The subject according to any one of claims 1 to 6, wherein the pulse width in the second light irradiation is the same as or shorter than the pulse width in the first light irradiation. Information acquisition device. 8. The information acquisition unit according to any one of claims 1 to 7, wherein the information acquisition unit acquires characteristic information in the subject using both the second data and the third data. The subject information acquisition apparatus described. The said information acquisition part outputs the characteristic information produced | generated based on said 2nd data, and the characteristic information produced | generated based on said 3rd data in the format which can be compared. The subject information acquisition apparatus described. The object information acquiring apparatus according to claim 1, wherein the predetermined time is a time of 100 picoseconds or less. The object information acquiring apparatus according to claim 1, wherein the predetermined time is a time of 17 picoseconds or less. The object information acquiring apparatus according to claim 1, wherein the predetermined time is 0.5 picosecond or less. An object information acquisition method performed by an object information acquisition apparatus comprising: a light irradiation unit that emits pulsed light; and an acoustic wave probe that receives an acoustic wave generated in the object due to the light. And
A first light irradiation step of irradiating the pulsed light multiple times within a predetermined time;
A second light irradiation step of irradiating a single pulsed light after the first light irradiation;
The difference between the first data obtained based on the acoustic wave generated by the first light irradiation and the second data obtained based on the acoustic wave generated by the second light irradiation. Obtaining third data and obtaining characteristic information in the subject using the third data, and
The predetermined time is a time shorter than a time from when a predetermined site in the subject is in an excited state due to the pulsed light until the excited state is relaxed. A method for acquiring subject information. The object information acquiring method according to claim 13, wherein the predetermined time is 100 picoseconds or less. 14. The object information acquiring method according to claim 13, wherein the predetermined time is a time of 17 picoseconds or less. The object information acquiring method according to claim 13, wherein the predetermined time is 0.5 picosecond or less.

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