WO2015118880A1

WO2015118880A1 - Object information acquiring apparatus and signal processing method

Info

Publication number: WO2015118880A1
Application number: PCT/JP2015/000546
Authority: WO
Inventors: Takuji Oishi; Yoshiko Nakamura
Original assignee: Canon Kabushiki Kaisha
Priority date: 2014-02-10
Filing date: 2015-02-06
Publication date: 2015-08-13
Also published as: JP2015149994A; JP6391249B2

Abstract

The present invention employs an object information acquiring apparatus including: an acoustic detector that receives acoustic waves generated from an object at a plurality of measurement positions and converts the acoustic waves to a plurality of signals; a shape information acquirer that acquires surface shape information of the object; a signal estimator that estimates a target signal derived from an acoustic wave generated at a specific position from the plurality of signals based on the surface shape information; a reducer that reduces the target signal estimated by the signal estimator; and a characteristic information acquirer that acquires characteristic information on the inside of the object using the plurality of signals in which the target signal is reduced by the reducer.

Description

OBJECT INFORMATION ACQUIRING APPARATUS AND SIGNAL PROCESSING METHOD

The present invention relates to an object information acquiring apparatus and a signal processing method.

In recent years, in a medical field, photoacoustic tomography (PAT) which obtains biological functional information using light and ultrasonic waves has been proposed and developed as one of apparatuses that image the inside of a living body in a non-invasive manner.

Photoacoustic tomography is a technique of imaging tissues inside a living body, serving as an acoustic wave generation source using a photoacoustic effect. Photoacoustic effect is a phenomenon that, when an object is irradiated with a pulsed light generated from a light source, light having propagated and diffused inside the object is absorbed, whereby acoustic waves (typically ultrasonic waves) are generated. A change over time in the received acoustic waves is detected at a plurality of positions to obtain signals, the obtained signals are mathematically analyzed (that is, reconstructed), and information related to optical characteristics such as an absorption coefficient inside the object is visualized three-dimensionally.

When near-infrared rays are used as the pulsed light, since the near-infrared rays easily pass through water which constitutes a major part of a living body and are easily absorbed by hemoglobin in the blood, it is possible to image blood vessels. Further, by comparing blood vessel images associated with pulsed lights of different wavelengths, it is expected that an oxygen saturation in the blood which is functional information can be measured. That is, since it is thought that the blood around a malignant tumor has a lower oxygen saturation than the blood around a benign tumor, it is possible to distinguish a benign tumor from a malignant tumor based on the oxygen saturation.

Moreover, an ultrasonic examination apparatus is an example of an apparatus that receives acoustic waves to image biological functional information similarly to photoacoustic tomography. The ultrasonic examination apparatus transmits acoustic waves to a living body, receives acoustic waves reflected inside the living body, and images the reflected acoustic waves. Acoustic waves have such properties that the acoustic waves reflect from an interface where the acoustic impedance which is the product of a propagation velocity and the density of acoustic waves changes. Thus, the ultrasonic examination apparatus can visualize a distribution of acoustic impedances in a living body.

The use of acoustic waves has a problem in that multiple reflection of acoustic waves may decrease the image quality. Multiple reflection means that, when a layer having a different acoustic impedance is present between the object and an acoustic detector, acoustic waves reflect from both surfaces of the layer multiple times. A layer where multiple reflection occurs is referred to as a multiple-reflection layer. When a strong acoustic wave having propagated from an acoustic wave source (an object surface or a light absorber or a reflector inside the object) reaches an acoustic detector with a delay due to the influence of multiple reflection and the acoustic wave is imaged, an artifact is generated at a position where the acoustic wave source is not actually present.

If the delay associated with multiple reflection is small, since the acoustic wave having undergone multiple reflection does not overlap an acoustic wave having propagated from another acoustic wave source, artifacts have a small influence on diagnosis. However, if the delay associated with multiple reflection is large, since the acoustic wave having undergone multiple reflection overlaps another acoustic wave, artifacts have a large influence on diagnosis.

The delay associated with multiple reflection is large if the multiple-reflection layer is thick, and the delay is small if the multiple-reflection layer is thin. However, if the object is a living body, since an object surface is a curved surface which changes for each measurement, it is necessary to match the shape of an acoustic detector with the object in order to make the multiple-reflection layer thin, which is very difficult. Thus, the multiple-reflection layer becomes thick and the influence of artifacts associated with multiple reflection increases.

Japanese Patent Application Laid-Open No. H7-178081

Due to limitations on measurement such as holding of an object or scanning of an acoustic detector, when the layer (multiple-reflection layer) between the acoustic detector and the object is made thick, conventionally, the shape of the multiple-reflection layer is defined as disclosed in PTL 1. Here, defining the shape of the multiple-reflection layer can be understood as defining the surface shape of an object. By defining the shape of the multiple-reflection layer, it is possible to guarantee that acoustic waves associated with multiple reflection appear always at the same positions. Conventionally, acoustic waves having undergone multiple reflection are identified in this manner and are reduced by signal processing.

On the other hand, when measurement is performed without defining the shape of the object, since the shape of the object changes for each measurement, it is difficult to estimate, identify, and reduce multiple-reflection signals.

Moreover, when the object is a living body such as a human body, the surface shape of the object differs from person to person. Due to this, it is difficult to define a multiple-reflection layer as a fixed shape and apply the shape to every person.

The present invention has been made based on recognition of such problems. An object of the present invention is to reduce the influence on diagnosis, of artifacts generated by multiple reflection without defining the shape of an object.

The present invention provides an object information acquiring apparatus comprising:
an acoustic detector configured to receive acoustic waves generated from an object at a plurality of measurement positions and convert the acoustic waves to a plurality of signals;
a shape information acquirer configured to acquire surface shape information of the object;
a signal estimator configured to estimate a target signal derived from an acoustic wave generated at a specific position from the plurality of signals based on the surface shape information;
a reducer configured to reduce the target signal estimated by the signal estimator; and
a characteristic information acquirer configured to acquire characteristic information on an inside of the object using the plurality of signals in which the target signal is reduced by the reducer.

The present invention also provides a signal processing method comprising:
a signal estimating step of estimating a target signal derived from an acoustic wave generated at a specific position, from a plurality of signals obtained by receiving acoustic waves generated from an object at a plurality of measurement positions, based on surface shape information of the object;
a reducing step of reducing the target signal estimated by the signal estimator; and
a characteristic information acquiring step of acquiring characteristic information on an inside of the object using the plurality of signals in which the target signal is reduced.

According to the present invention, it is possible to reduce the influence on diagnosis, of artifacts generated by multiple reflection without defining the shape of an object.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

Fig. 1 is a schematic diagram illustrating the arrangement of an apparatus according to an embodiment of the present invention. Figs. 2A to 2D are schematic diagrams for describing the principle of the apparatus according to the embodiment of the present invention. Fig. 3 is a schematic diagram for describing the principle of the apparatus according to the embodiment of the present invention. Fig. 4 is a schematic diagram illustrating the configuration of the apparatus according to the embodiment of the present invention. Figs. 5A to 5G are schematic diagrams for describing the process of the apparatus according to the embodiment of the present invention. Fig. 6 is a schematic diagram illustrating an implementation method of an apparatus according to an embodiment of the present invention. Fig. 7 is a schematic diagram illustrating the arrangement of the apparatus according to the embodiment of the present invention. Fig. 8 is a schematic diagram illustrating the configuration of the apparatus according to the embodiment of the present invention. Fig. 9 is a schematic diagram illustrating an implementation method of an apparatus according to an embodiment of the present invention. Fig. 10 is a schematic diagram illustrating the configuration of the apparatus according to the embodiment of the present invention. Fig. 11 is a schematic diagram illustrating an implementation method of the apparatus according to the embodiment of the present invention. Fig. 12 is a schematic diagram illustrating the configuration of the apparatus according to the embodiment of the present invention. Fig. 13 is a schematic diagram illustrating the configuration of the apparatus according to the embodiment of the present invention. Fig. 14 is a schematic diagram illustrating the configuration of the apparatus according to the embodiment of the present invention. Fig. 15 is a schematic diagram illustrating the arrangement of the apparatus according to the embodiment of the present invention. Figs. 16A and 16B are diagrams illustrating the processing results of the apparatus according to the embodiment of the present invention. Figs. 17A to 17D are diagrams illustrating the processing results of the apparatus according to the embodiment of the present invention.

Hereinafter, a preferred embodiment of the present invention will be described with reference to the drawings. The dimensions, materials, shapes and relative positions, and the like of the constituent components described below should be changed appropriately depending on the configuration and various conditions of the apparatus to which the invention is applied, and it is not intended to limit the scope of the invention to the description given below.

The present invention relates to a technique of detecting acoustic waves propagating from an object to generate and acquire characteristic information on the inside of the object. Thus, the present invention can be understood as an acoustic wave measurement apparatus or a control method thereof, or an acoustic wave measurement method and a signal processing method and can be understood as an object information acquiring apparatus or a control method thereof, or an object information acquisition method. Further, the present invention can be understood as a program for causing an information processing apparatus having hardware resources such as a CPU to execute these methods and a storage medium storing the program.

An object information acquiring apparatus of the present invention includes an apparatus which uses a photoacoustic tomography technique of irradiating an object with light (electromagnetic waves) and receiving (detecting) acoustic waves generated and propagated at specific positions inside the object or on the object surface. Such an object information acquiring apparatus can be also referred to as a photoacoustic apparatus because the apparatus obtains characteristic information on the inside of the object based on photoacoustic measurement in the form of image data or the like.

The characteristic information in the photoacoustic apparatus represents a generation source distribution of acoustic waves generated by light irradiation, an initial acoustic pressure distribution inside an object, or a light energy absorption density distribution and an absorption coefficient distribution derived from the initial acoustic pressure distribution, and a density distribution of materials that constitute tissues. Examples of the materials that constitute tissues includes blood components such as an oxygen saturation distribution or a redox hemoglobin density distribution, or fat, collagen, and water.

The object information acquiring apparatus of the present invention includes an ultrasonic apparatus that transmits acoustic waves to an object, receives reflection waves (echo waves) reflected from specific positions inside the object, and obtains characteristic information in the form of image data or the like. The characteristic information in the ultrasonic apparatus is information that reflects surface shape based on reflection waves at positions where the acoustic impedances of a tissue inside the object are different.

The acoustic waves referred to in the present invention are typically ultrasonic waves and include elastic waves called sound waves and acoustic waves. The acoustic waves generated by a photoacoustic effect are referred to as photoacoustic waves or photoultrasonic waves. Electrical signals converted from acoustic waves by a probe are also referred to as acoustic signals.

<First Embodiment>
The present invention acquires a surface shape of an object and predicts reflection signals using the surface shape. In photoacoustic tomography according to the present embodiment, reflection signals are reduced using a delay profile described later. However, the following technique can be also applied to an ultrasonic examination apparatus.
In the present specification, reflection signals means signals of acoustic waves having undergone multiple reflection which cause artifacts, unless otherwise stated. First, the principle of the present invention and the present embodiment will be described, and then, constituent components, an implementation method, and a specific processing method will be described, followed by the effects lastly.

(Principle)
The principle of the present invention will be described. Fig. 1 illustrates the arrangement of an object and an acoustic wave detector and a propagation path of acoustic waves generated from the surface of a living body. Hereinafter, as in Fig. 1, a case where a multiple-reflection layer (acoustic matching member), an object, and an acoustic detector have different acoustic impedances will be considered.

In Fig. 1, a plurality of acoustic detection elements 103 included in an acoustic detector 102 receives photoacoustic waves generated and propagated from an object 101 irradiated with a pulsed light 104 with a multiple-reflection layer (acoustic matching member) 105 interposed.
Only one acoustic detection element may be provided. In this case, a scanning mechanism that moves a measurement position on the object, of the acoustic detection element may be provided so that photoacoustic waves can be detected at a plurality of measurement positions. When the technique of a reduction process according to the present invention is applied to acoustic signals obtained at respective measurement positions, the same effects as respective embodiments are obtained.

In photoacoustic tomography, at positions where pulsed lights emitted from a light source are absorbed, acoustic waves corresponding to the amount of absorption are generated. As illustrated in Fig. 1, since the object surface, the acoustic detector surface, and the like are irradiated with a strong pulsed light that is not decayed, strong acoustic waves are generated. The propagation direction of the generated acoustic waves is normal to the object surface and the acoustic detector surface.

The acoustic waves generated from the object surface and the acoustic detector surface propagate through a multiple-reflection layer (acoustic matching member) and reach the acoustic detector surface and the object surface, respectively. Some components pass and propagate as they are and the remaining components are reflected. The proportion of transmitted and reflected components depends on acoustic impedances of respective materials. Acoustic waves are reflected so that an incidence angle is equal to a reflection angle similarly to light.

Since the velocity of a pulsed light is sufficiently faster than acoustic waves, it can be considered that the acoustic waves occur at the same time regardless of the occurrence position. Thus, a signal obtained when a signal generated from the object surface reaches first the acoustic detector is delayed by the amount corresponding to the thickness of the multiple-reflection layer.
Moreover, since a signal obtained when an acoustic wave generated from the acoustic detector surface returns after being reflected from the object surface has passed through the multiple-reflection layer twice, the signal is also delayed by the amount corresponding to the thickness of the multiple-reflection layer. When reflection is repeated further, a delay corresponding to the thickness of the multiple-reflection layer occurs.

Since the thickness of the multiple-reflection layer is determined by the shape of the object surface and the arrangement (or the measurement position) of a probe device (that is, the shape of the acoustic detector), the delay of acoustic waves associated with multiple reflection can be estimated from the shape of the object surface and the shape of the probe. The same can be said to be true when the shape of the acoustic detector is not planar.

In the present embodiment, a case where the shape of the acoustic detector is planar will be described. In this case, the delay of acoustic waves associated with multiple reflection can be easily estimated by approximation.

First, as illustrated in Fig. 2A, a case where the acoustic detector surface is parallel to the object surface will be considered. In Fig. 2A, an object 201 and an acoustic detector 202 including acoustic detection elements 203 (A to E) are in contact with each other with an acoustic matching member 205 which is a multiple-reflection layer interposed.
In the case of Fig. 2A, an incident wave and a reflection wave follow the same path. Thus, the delay associated with multiple reflection is an integer multiple of the thickness of the multiple-reflection layer.

Fig. 2B illustrates signals obtained by the acoustic detection elements in Fig. 2A, in which the element positions are identical in Figs. 2A and 2B. Moreover, the vertical axis of respective signals represents a voltage and shows the intensity of a photoacoustic wave. Moreover, the horizontal axis represents time and the time when light is emitted is the origin 0. When only one acoustic detection element is used, each element position is interpreted as each measurement position.

In Fig. 2B, a signal indicated by N1 is a signal generated from the acoustic detector surface. A signal N2 is a signal detected when the signal generated from the object surface reaches the acoustic detector. A signal N3 is a signal detected when an acoustic wave propagated toward the object among the acoustic waves generated from the acoustic detector surface is reflected from the object surface and returns to the acoustic detector. A signal N4 is a signal detected by the acoustic detector when an acoustic wave generated from the object surface propagates up to the acoustic detector and is then reflected from the acoustic detector and is further reflected from the object surface. Similarly, acoustic waves generated from the acoustic detector surface and the object surface undergo multiple reflection and are detected as signals N5, N6, ..., and so on.

Here, the period from time 0 to respective signals will be referred to as a delay amount. When the object surface and the acoustic detector surface are parallel as in Fig. 2A, since incident and reflection waves follow the same path, the obtained acoustic signals are detected with the same delay amount regardless of the element position. In other words, in the case of Fig. 2A, the signals obtained with the respective elements are at the same phase.
Thus, when a relation between the acoustic element position and a relative delay amount of signals obtained by respective acoustic detection elements is referred to as a delay profile, the delay profile is a straight line or a flat surface since signals are detected at the same point in time at all element positions.

Next, as illustrated in Fig. 2C, a case where the acoustic detector surface is slightly inclined with respect to the object surface will be considered. Fig. 3 illustrates how reflections occur in a multiple-reflection layer in this case. As can be understood from Fig. 3, reflection waves follow different paths from an incident wave.

In this case, if the inclination between the acoustic detector surface and the object surface is small, the length (A, B) of a propagation path of an incident wave is approximately equal to the length (A', B') of a propagation path of a reflection wave, it can be approximated that A=A' and B=B'. Thus, as illustrated in Fig. 2D, the length from N1 and N2 of the signals obtained in each element can be assumed to be the same as the lengths from N2 to N3, N3 to N4, N4 to N5, and N5 to N6.
The length from N1 to N2 is proportional to the thickness of the multiple-reflection layer in each element. If the shape of the acoustic detector is planar, the thickness of the multiple-reflection layer can be obtained by measuring the surface shape of the object. Thus, by multiplying the length from N1 to N2 by an integer, it is possible to calculate the position of signals N3 and so on.

Moreover, the signals N1 are detected at the same point in time by the respective acoustic detection elements, and the signals N2 are detected with a delay corresponding to the length of a perpendicular line extended from the corresponding acoustic detection element to the object. Multiple-reflection signals are delayed by an integer multiple of the delay amount obtained for N2 in such a manner that delay amounts of N3, N4, N5, and N6 are twice, three times, four times, and five times the delay amount of N2, respectively. Thus, the delay profiles of signals N3 and so on are obtained by delaying the delay profile of N2 by an integer multiple in a time direction.

On the other hand, when an inclination between the acoustic detector surface and the object surface is large, the length from N1 to N2 cannot be approximated to be the same as the length from N2 to N3. Thus, it is not easy to know the positions and the delay profiles of the signals N3 and so on from the position and the delay profile of the signal N2. However, since an acoustic detection element generally has such a directivity that the sensitivity is high in the direction toward the front surface and is low in an oblique direction, an acoustic wave reflected when the inclination is large is obliquely incident on the acoustic detection element. Thus, when the acoustic detector surface and the object surface are further inclined with respect to each other, the reflection waves do not cause any problem in most cases.

(Constituent Components)
Next, constituent components of the present invention will be described with reference to Fig. 4. The object information acquiring apparatus of the present invention includes a light source 1, a light irradiation unit 2, an acoustic matching member 4 which is a multiple-reflection layer, an acoustic detector 5, an electrical signal processing unit 6, a delay acquiring unit 7, a data processing unit 10, an imaging processing unit 14, and a display unit 15. Moreover, the delay acquiring unit 7 includes a shape information acquisition unit 8, and a reflection signal estimator 9. The data processing unit 10 includes a delay adjustment unit 11, a spatial frequency filter 12, and a delay restoring unit 13. Moreover, a measurement target of the present invention is an object 3.

(Light Source)
The light source 1 is a device that generates a pulsed light. In order to obtain a large output, a laser is preferred as the light source, and a light-emitting diode or the like may also be used. In order to generate photoacoustic waves efficiently, it is necessary to emit light in a sufficiently short period according to thermal properties of an object. When the object is a living body, the pulse width of a pulsed light generated by the light source is preferably several tens of nanoseconds or shorter.
Moreover, the wavelength of the pulsed light is in a near-infrared region called a biological window and is preferably in the range of approximately 700 nm to 1200 nm. Light in this region is preferable to obtain information on the deep part since it reaches relatively a deep part of a living body. Further, the wavelength of the pulsed light preferably has a high absorption coefficient with respect to an observation target.

(Light irradiation unit)
The light irradiation unit 2 is a device that guides the pulsed light generated by the light source 1 to the object 3. Specifically, the light irradiation unit 2 is an optical device such as an optical fiber, a lens, a mirror, and a diffuser. These optical devices are used for changing irradiation conditions such as an irradiation shape of a pulsed light, an optical density, or an irradiation direction in which the object is irradiated with light. These conditions may be adjusted by the light source 1. Moreover, in order to acquire a wide range of data, the light irradiation unit 2 may be moved for scanning so that the irradiation position of the pulsed light is scanned. In this case, it is preferable to perform scanning in synchronization with the acoustic detector 5. Optical devices other than the optical devices mentioned above can be used as long as the devices have the above-described functions.

(Object)
The object 3 is a measurement target. Examples of the object 3 include a living body or a phantom that simulates the acoustic and optical properties of the living body. A photoacoustic diagnosis apparatus can image a light absorber having a large absorption coefficient present inside the object 3.
In the case of living bodies, examples of an imaging target include hemoglobin, water, melanin, collagen, and fat. In the case of phantoms, a material that simulates the optical properties of such an imaging target is enclosed in a phantom as a light absorber. Moreover, the shape and properties of a living body changes from person to person and from sample to sample. Further, a living body or a phantom in which a contrast agent, a molecule probe, or the like is injected may be used as the object.

(Acoustic Matching Member)
The acoustic matching member 4 is provided between the object 3 and the acoustic detector 5 so as to couple the two acoustically so that acoustic waves can easily propagate from the object 3 to the acoustic detector 5. The acoustic impedance of the acoustic matching member 4 is preferably set based on the acoustic impedances of the object 3 and the acoustic detector 5 so that acoustic waves undergo a small number of reflections. However, it is practically difficult to eliminate reflection completely, and the acoustic matching member 4 serves as a multiple-reflection layer.

The acoustic matching member 4 is preferably formed from a material in which a pulsed light is less likely to be absorbed. By doing so, it is possible to reduce the occurrence of photoacoustic waves from the acoustic matching member and thus to suppress artifacts on an image. Further, it is possible to irradiate the object with a large amount of light. Moreover, the acoustic matching member is preferably uniform. An acoustic matching GEL, water, oil, and the like are used as the acoustic matching member.

(Acoustic Detector)
The acoustic detector 5 includes at least one acoustic detection element that converts acoustic waves into electrical signals. In photoacoustic tomography, acoustic waves are received from a plurality of positions to perform three-dimensional imaging. Due to this, one acoustic detection element is moved to a plurality of positions for scanning, or a plurality of acoustic detection elements is provided at different positions to receive acoustic waves from a plurality of positions.
The acoustic detector 5 preferably has a high sensitivity and a broad frequency range. Specifically, acoustic detectors which use PZT, PVDF, cMUT, and a Fabry-Perot interferometer can be used. Acoustic detectors other than the detectors mentioned above can be used as long as the detectors have the above-described functions.

(Electrical Signal Processing Unit)
The electrical signal processing unit 6 amplifies electrical signals obtained by the acoustic detector 5 and converts the same into digital signals. A specific example of the electrical signal processing unit 6 includes an amplifier, an analog-digital converter (ADC), and the like formed of electric circuits. In order to acquire data efficiently, preferably, the same number of amplifiers and ADCs as the number of detection elements of the acoustic detector 5 are provided. However, one amplifier and one ADC may be sequentially connected and used.

(Delay Acquiring Unit)
The delay acquiring unit 7 is a device that obtains a delay profile of acoustic waves having undergone multiple reflection in the acoustic matching member 4 and performs the main process of the present invention. In the present embodiment, how a delay profile is obtained will be described. The delay acquiring unit 7 includes the shape information acquisition unit 8 and the reflection signal estimator 9.

(Shape Information Acquisition Unit)
The shape information acquisition unit 8 acquires surface shape information of the object 3 included in a reception region of the acoustic detector 5. The surface shape information means the shape of a surface close to the acoustic detector among the object surfaces. When the acoustic detector 5 scans two-dimensionally to acquire three-dimensional data including time, the acquired surface shape of the object 3 needs to be a three-dimensional shape. When the acoustic detector 5 acquires two-dimensional data, although it is sufficient that the surface shape of the object 3 is two-dimensional so as to conform with the acoustic detector 5, it is preferable to acquire a three-dimensional surface shape in order to improve accuracy. The shape information acquisition unit corresponds to a shape information acquirer of the present invention.

The surface shape of the object 3 may be obtained from photoacoustic signals, and alternatively, the same can be obtained using a camera capable of measuring stereoscopic information or a laser range finder or by irradiation of ultrasonic waves. In the present embodiment, a method of obtaining the surface shape from photoacoustic signals (electrical signals originating from photoacoustic waves) will be described. Other methods will be described in a fifth embodiment. In this case, a specific example of the shape information acquisition unit 8 includes a computer, a digital signal processor such as FPGA, and the like. When the surface shape of the object 3 is obtained from photoacoustic signals, it is possible to acquire the surface shape of the object 3 without introducing a new device.

Moreover, the shape information acquisition unit 8 may acquire surface shape by reading surface shape corresponding to the shape of an object during measurement from a plurality of pieces of surface shape stored in advance in the shape information acquisition unit 8. In this case, a user may input the shape of an object during measurement and the type or the like of a member that holds the object with the aid of an input unit and the shape information acquisition unit 8 may read the surface shape of the object corresponding to the input data. Alternatively, the shape information acquisition unit 8 may detect the type of a member that holds an object and read the surface shape of the object corresponding to the detected member type.

A specific processing method of this technique will be described. Although it is possible to obtain a strong acoustic wave from the surface shape of the object 3, it is not possible to obtain a strong acoustic wave from the acoustic matching member located closer to the acoustic detector. Further, since the signals obtained from the surface of the acoustic detector appear at the same time regardless of the object, it is possible to easily specify the signals based on the points in time when intensity peaks appear in advance. Thus, an appropriate threshold may be provided for the obtained signals, and the earliest signal other than the surface signal of the acoustic detector among the signals equal to or higher than the threshold may be determined to be the surface signal of the object. When the time at which the surface signal appears is obtained, it is possible to acquire the time corresponding to the distance from the acoustic detector to the object surface.

Since this time is the time taken for an acoustic wave to propagate from the object surface to the acoustic detector, it is possible to calculate the distance to the object surface by using the propagation velocity of the acoustic wave in the acoustic matching member. As a result, it is possible to acquire the surface shape.

The time acquisition process is performed on the signals obtained at a plurality of measurement positions and the acquired points in time are arranged so as to correspond to the measurement positions, whereby a time distribution (that is, the delay profile) corresponding to the surface shape of the object is obtained. In this case, preferably, processes such as noise reduction or template matching may be applied to the signals to enhance the signals from the object surface. In this way, robustness of the process is improved. Moreover, although it is preferable to automatically acquire the surface shape of the object 3 from signals, a user may manually designate the surface shape by judgment based on the signals.

(Reflection Signal Estimator)
The reflection signal estimator 9 is a device that estimates reflection signals from the surface shape of the object obtained by the shape information acquisition unit 8 using the above-described principle. The reflection signal estimator corresponds to a signal estimator according to the present invention.

The reflection signal may be estimated by a method of estimating a delay profile which is a relative delay amount at a plurality of measurement positions to identify a signal identical to the delay profile as the reflection signal, and alternatively, a method of estimating an absolute delay amount of the reflection signal. The former method will be described in the present embodiment, and the latter method will be described in a third embodiment. The reflection signal estimator 9 estimates the delay profile only, and a subsequent device determines whether a signal is identical to the delay profile.

As described above in connection the principle, when the delay profile of a signal indicating the object surface is delayed by an integer multiple in the time direction, the delay profile of the reflection signal is obtained. Specifically, such a delaying process is performed by multiplying the delay times of portions of the signal indicating the object surface, forming the delay profile by integers. A relative relation of respective delay times obtained as a result is the delay profile of the reflection signal which is delayed by an integer multiple.

Moreover, theoretically, although multiple reflections continue endlessly, since reflection waves are decayed every reflection, if the reflection waves are sufficiently decayed as compared to a signal to be measured, the subsequent multiple-reflections may be ignored. Thus, it is preferable to determine the number of delay profiles of a reflection signal to be estimated according to the number of reflections when the reflection signal is sufficiently decayed.
The predetermined number of delay profiles is preferably stored in the reflection signal estimator 9 or a storage unit. In this way, it is possible to reduce the user's operations. Moreover, the user may designate the number of delay profiles for each measurement. In this way, even when decay of reflections is different from object to object, it is possible to execute an appropriate amount of processing.

The number of delay profiles may be determined based on the size of an object and the propagation period of a reflection wave and may be determined based on the number of reflections when the reflection wave becomes sufficiently small. When the determined number of delay profiles is M, and the delay profile of a signal indicating the object surface is extended twice, three times, ..., and M times in the time direction, the delay profiles of (M-1) reflection signals are obtained.

(Data Processing Unit)
The data processing unit 10 reduces a multiple-reflection signal based on the delay profile of the estimated multiple-reflection signal. In the present embodiment, the data processing unit 10 reduces signals having the same shape as the delay profile of the multiple-reflection signal estimated by the delay acquiring unit 7. When a plurality of delay profiles is obtained, a plurality of processes is performed in such a way that the process is performed using one delay profile to obtain an output, and the same process is performed on the output using another delay profile.

In the present embodiment, the data processing unit 10 includes the delay adjustment unit 11, the spatial frequency filter 12, and the delay restoring unit 13. As the process performed by the data processing unit 10, in addition to the above technique, a method of reducing the reflection signal using optimization may be used also. However, methods other than the above methods may be used as long as the methods reduce the signal of the delay profile obtained by the delay acquiring unit 7. The data processing unit corresponds to a reducer according to the present invention.

(Delay Adjustment Unit)
The delay adjustment unit 11 adjusts the delays of the digital signals at respective measurement positions obtained by the electrical signal processing unit 6 based on the delay profile of the reflection signal estimated by the delay acquiring unit 7 so that the reflection signals at all measurement positions are delayed at the same time.

As a specific processing method, a time offset (delay) is provided to the signals of respective measurement positions so that the reflection signal is delayed at the same time. In this case, such a time offset that the reflection signal is delayed at the same time is obtained by inverting the delay profile in the time direction. In this way, the signals having the same delay profile as the delay profile of the reflection signal have the same delay (the same phase). This signal will be referred to as a delay adjustment signal.

Fig. 5A illustrates digital signals at measurement positions of the elements A to E obtained by the electrical signal processing unit 6. N1 to N6 are the same as those of Fig. 2D. When the digital signals are adjusted based on the delay profile of the object surface signal N2 so that the living body surface signals N2 have the same delay, such a delay adjustment signal as illustrated in Fig. 5B is obtained.

(Spatial Frequency Filter)
The spatial frequency filter 12 reduces components having a low spatial frequency in the arrangement direction of the temporal origins of the delay adjustment signals output from the delay adjustment unit 11 when the delay adjustment signals are arranged as in Figs. 5A to 5G in all periods of each time period.
In each time period, when signals are seen in the arrangement direction, in-phase signals are DC components (that is, low-frequency components) having the same signal intensity at all measurement positions. On the other hand, signals of which the phases are out of alignment have different signal intensities depending on the measurement position and include high-frequency components. Thus, by reducing components having a low spatial frequency in the arrangement direction, it is possible to reduce in-phase signals mainly.

Moreover, in a time region or a spatial region, it may be difficult to reduce the in-phase signals to be reduced smoothly because other necessary signals are superimposed on the signals. However, in such a case, as described above, by executing a reduction process in the frequency region using a frequency filter, it is possible to reduce in-phase components mainly and to obtain necessary signals.
Theoretically, the delay profile of a reflection signal is an integer multiple of the delay profile of a signal indicating the object surface. However, practically, the degree of adhesion of respective layers and the acoustic wave propagation velocity are not uniform, and both delay profiles are not completely in an integer-multiple relation. Thus, when the low-frequency components are reduced by the spatial frequency filter 12, it is preferable to reduce components close to the high-frequency side as well as the DC components having the lowest frequency.

The low-frequency components up to which frequency will be reduced is determined according to the uniformity of the degree of adhesion of respective layers and the acoustic wave propagation velocity. Since the variation in respective measurements and devices is not large but depends on a material and a configuration, it is preferable to determine a spatial frequency to be reduced by test measurement performed in advance. Moreover, the spatial frequency to be reduced may be determined by the user based on measured data and may be determined for respective devices based on test measurement performed in advance.
When in-phase signals of the delay adjustment signal of Fig. 5B are reduced by the spatial frequency filter 12, a signal as illustrated in Fig. 5C is obtained. As illustrated in this drawing, signals of which the phases are in alignment are reduced mainly, and signals of which the phases are out of alignment are rarely reduced.

(Delay Restoring Unit)
The delay restoring unit 13 performs a reverse process of restoring the time offset provided by the delay adjustment unit 11 in a reverse direction on the signals in which the in-phase signals are reduced and which are output by the spatial frequency filter 12. In this way, the positions of the portions corresponding to the delay profile returns to the positions of the original signal. As a result, it is possible to reduce the signal mainly having the same shape as the delay profile obtained by the delay acquiring unit 7 from the original signal.
In the drawings, this process corresponds to a process in which the delay restoring unit 13 restores the delay of the signal in Fig. 5C in which the in-phase signals are reduced by the spatial frequency filter 12 to obtain the signal illustrated in Fig. 5D.

When a plurality of delay profiles is obtained due to multiple reflection, as described above, signals having the same shape as a certain delay profile are reduced based on the delay profile to obtain resultant signals, and signals having the same shape as another delay profile among the resultant signals are reduced based on the other delay profile.

A case of reducing the object surface signal N3 further from signals obtained by reducing the object surface signal N2 illustrated in Fig. 5D will be described as an example. First, when the delay adjustment unit 11 adjusts the delay using the delay profile of the object surface signal N3 obtained by delaying the delay profile of the object surface signal N2 twice so that the object surface signals N3 have the same phase, the signal illustrated in Fig. 5E is obtained. When the spatial frequency filter 12 reduces the in-phase signals from this signal, the signal illustrated in Fig. 5F is obtained. When this signal is processed by the delay restoring unit 13, a signal in which the object surface signals N2 and N3 are reduced is obtained as illustrated in Fig. 5G.

(Imaging Processing Unit)
The imaging processing unit 14 reconstructs the signals at a plurality of measurement positions obtained by the data processing unit 10 to acquire image data indicating a spatial distribution of signal generation sources. The image obtained herein is an initial acoustic pressure distribution indicating a spatial distribution of an acoustic pressure generated from the light absorber that absorbs light, for example.

As a method of the reconstructing process, a universal back-projection method of projecting differentiated signals in a backward direction from the acquisition positions so that the signals overlap each other is preferred. However, other methods can be used as long as the methods can image a spatial distribution of signal generation sources. The imaging processing unit corresponds to a characteristic information acquirer according to the present invention.

The shape information acquisition unit 8, the reflection signal estimator 9, the data processing unit 10, the delay adjustment unit 11, the spatial frequency filter 12, the delay restoring unit 13, and the imaging processing unit 14 are formed of a computer having devices such as a CPU or a GPU or circuits such as FPGA or ASIC. Moreover, the respective units may be formed of one device or circuit and may be formed of a plurality of devices or circuits. Moreover, the respective processes performed by the respective units may be executed by any device or circuit. Further, the respective units may share the device or circuit.

(Display Unit)
The display unit 15 displays images obtained by the imaging processing unit 14. Specifically, the display unit 15 is a display or the like. Due to this, it is possible to visually perceive the information on the inside of the object.

(Flow of Implementation Method)
Next, the implementation method of the present embodiment will be described with reference to the flowchart of Fig. 6.
First, an object is irradiated with a pulsed light (S1), and an acoustic wave generated inside the object is received at a plurality of positions (S2). The surface shape of the object is acquired from the received signal using the processing method described in connection with the shape information acquisition unit (S3), and the delay profile of the multiple-reflection signal is estimated based on the surface shape (S4).

Here, since at least one delay profiles are obtained, the processes of S5 to S7 are sequentially performed on the respective delay profiles. The delay of a signal obtained using a certain delay profile so that the delay profiles of reflection signals are synchronized is adjusted (S5), and in-phase signals are reduced using the spatial frequency filter (S6) to restore the delay to the original delay (S7). It is determined whether the processes of S5 to S7 have been performed on all delay profiles corresponding to a desired number of reflections (S8), and the flow returns to S5 when the processes have not been completed for all delay profiles. When the processes have been completed for all delay profiles, imaging is performed using the processed signals (S9) and images are displayed (S10).

According to the apparatus of the present invention, it is possible to easily reduce multiple-reflection signals associated with the multiple-reflection layer and to acquire images in which artifacts associated with multiple reflection are reduced. In this way, it is possible to reduce the influence on diagnosis, of artifacts associated with multiple reflection. With the respective embodiments of the present invention, it is possible to detect and reduce surface wave components of the object as well as reducing reflection wave components. That is, a reduction target signal includes signals having undergone multiple reflection and signals having propagated from a light absorber. Further, with the respective embodiments of the present invention, even when it is not possible to reduce reflection wave components and surface wave components completely, it is possible to obtain the effect of reducing the influence of artifacts by reducing these components.

<Second Embodiment>
In the present embodiment, a case where parallel reflection layers are included in the multiple-reflection layer of the first embodiment and multiple reflection occurs in a complex manner will be described. The difference from the first embodiment is the number of reflection profiles used for the reflection signal estimator 9 to estimate the delay profile. Thus, the principle and the reflection signal estimator 9 will be described mainly.

A case where two acoustic matching layers are present between an object and a probe as illustrated in Fig. 7 will be considered. In Fig. 7, a plurality of acoustic detection elements 703 included in an acoustic detector 702 receives photoacoustic waves from an object 701 irradiated with a pulsed light with an acoustic matching member 705 interposed. The acoustic matching member 705 includes two

acoustic matching layers

705A and 705B, and the boundary between both layers is denoted by 705C.

The reason why a two-layer structure is provided is to guarantee that the acoustic detector scans smoothly and the object is held reliably. That is, when the acoustic matching layer 705B is formed of a hard flat plate that can guarantee a flat surface, it is possible to maintain the contact between the acoustic matching layer and the scanning acoustic detector. On the other hand, the acoustic matching layer 705A is formed of a material such as soft gel in order to conform with the shape of the object. In such a case, a situation where a plurality of acoustic matching layers is present and at least one of the layers is parallel may occur. Since the properties (hardness) of respective acoustic matching layers are different, the acoustic impedances at the boundary 705C may mismatch.

In this case, multiple reflection occurs in the respective acoustic matching layers, and a complex multiple-reflection signal appears in the observed signal. In this case, when propagation of acoustic waves is considered, since the acoustic matching layer B is a flat plate, the reflection on the acoustic matching layer B does not cause any difference in wave-front. That is, the delay profile does not change. Thus, even when complex reflections occur, it is possible to multiply the delay profile of an object surface in the time direction by an integer to acquire a delay profile and to determine that a signal identical to the delay profile is a reflection signal similarly to the first embodiment.
This is satisfied when a plurality of acoustic matching layers is present, one acoustic matching layer is a curved surface, and the other acoustic matching layers are parallel to each other. Thus, when the acoustic matching layer is made up of three layers as well as two layers, it is possible to estimate the reflection signal according to the same method.

In the present embodiment, as compared to the first embodiment, it is necessary to take the delay profiles of two reflection signals into consideration. When an acoustic wave generated from the acoustic detector surface is reflected from the acoustic matching layer B and is detected, the reflection signals are received at the same point in time (at the same phase) by the respective elements.

Moreover, when an acoustic wave generated from the object surface is detected after making one-round trip within the acoustic matching layer B, the delay profile of the reflection signal is the same as the delay profile of a signal indicating the object surface. Further, when an acoustic wave generated from the acoustic detector surface propagates up to the object surface and is reflected from the object surface and is then detected after making one-round trip within the acoustic matching layer B, the delay profile of the reflection signal is the same as the delay profile of a signal indicating the object surface.
In these cases, when the reflection signal estimator 9 extends the delay profile of a signal indicating the object surface twice, three times, ..., and M times in the time direction, the delay profiles of (M+1) reflection signals are obtained. Here, M is a predetermined number of delay profiles of the reflection signal.

According to the apparatus of the present embodiment, even when a plurality of acoustic matching layers is present, it is possible to easily estimate and reduce the delay profile of the reflection signal.

<Third Embodiment>
In the present embodiment, a case where the delay amount is acquired instead of the delay profile of the first embodiment will be described. The difference from the first embodiment is the data processing unit 10 and the processing of the reflection signal estimator 9. Thus, the reflection signal estimator 9, the data processing unit 10, and the implementation method will be described mainly.

Hereinafter, description is provided with reference to Figs. 2B and 2D.
As described in connection with the principle in the first embodiment, it can be assumed that the lengths from N2 to N3, N3 to N4, N4 to N5, and N5 to N6 are the same as the length from N1 to N2. Thus, if it is possible to acquire the occurrence time of the signal N1 generated from the surface of the acoustic detector and the occurrence time of the signal N2 indicating the surface shape of the object, it is possible to determine the delay amounts of the subsequent signals N3 and so on.

Since the signals N1 appear at the same time regardless of the object, the signals N1 can be identified in advance, and the signals N2 are obtained from the shape information acquisition unit 8. Thus, if the delay amounts of the signals N1 to N6 are TN1 to TN6, respectively, the reflection signal estimator 9 can acquire the delay amounts TN3=TN2+TN2-TN1, TN4=TN3+TN2-TN1, TN5=TN4+TN2-TN1, and TN6=TN5+TN2-TN1. In this way, the position of the reflection signal (that is, the delay amount of the reflection signal) is obtained. Moreover, similarly to the first embodiment, it is preferable to determine the number of reflection signals to be estimated.

Although the data processing unit 10 uses the delay profile only in the first embodiment, reflection signals are reduced using the delay amount only in the present embodiment. Unlike the first embodiment, the data processing unit 10 includes a reflection signal reducer 16 as illustrated in Fig. 8.

The reflection signal reducer 16 reduces the reflection signal by reducing the signal intensity of the reflection signal to zero using the delay amount of the reflection signal obtained from the reflection signal estimator 9.
With this process, the intensity of a signal superimposed the reflection waves is also reduced to zero. However, since the apparatus measures the signals at a plurality of measurement positions, the signals of different measurement positions are not superimposed on the reflection signal unless the source of the signal is identical to the source of the reflection waves. Thus, the influence of reducing the intensity of the signal superimposed on the reflection waves to zero is limited.

The implementation method of the present embodiment is illustrated in the flowchart of Fig. 9.
S1 to S3 are the same as those of the first embodiment. In the present embodiment, the delay amount is estimated from the surface shape of the object according to the above-described method (S11), and the reflection signal is reduced (S12). In this case, it is determined whether the process has been completed for all estimated reflection signals (S8). When the process has not been completed, the flow returns to S12, and the reduction process is repeated. When the reduction process has been completed for all reflection signals, imaging is performed using the processed signals (S9), and images are displayed (S10).

By using the apparatus of the present embodiment, it is possible to reduce reflection signals even when the number of measurement positions is small and the delay profile is not reliable.
Moreover, the first embodiment and the present embodiment may be combined so that the reflection signals are reduced using both the delay profile and the delay amount. In this case, it is possible to selectively reduce low-spatial frequency components of a delay time corresponding to the calculated delay amount. In this way, it is possible to prevent signals from the light absorber in the object from being reduced and to reduce reflection signals with high accuracy.

<Fourth Embodiment>
In the present embodiment, an apparatus that only displays reflection signals without reducing the reflection signals will be described. The difference from the first embodiment is that the data processing unit 10 and the imaging processing unit 14 on the subsequent stage of the delay acquiring unit 7 are not present. Thus, the subsequent process of the delay acquiring unit 7 will be described. In the present embodiment, the reflection signal estimator 9 obtains the delay profile or the delay amount of a reflection signal.

First, a case where the delay profile is obtained will be described. The constituent components of the present embodiment will be described with reference to Fig. 10. Constituent components 1 to 9 are the same as those of the first embodiment. A reflection signal extractor 17 compares the delay profile obtained from the reflection signal estimator 9 with the signal obtained from the electrical signal processing unit 6 and retrieves and extracts components having the same shape as the delay profile within the signal. As a retrieval and extraction method, template matching which uses delay profiles as templates is preferable because this method can increase a calculation speed. Other methods such as a neighborhood method or statistical pattern recognition may be used as long as the methods can retrieve shapes.

The imaging processing unit 14 performs reconstruction similarly to the first embodiment. In the present embodiment, since the reflection signal extracted by the reflection signal extractor 17 is used for reconstruction, image data made up of the reflection signal is obtained. The display unit 15 displays a reconstructed image.

Further, preferably, the imaging processing unit 14 reconstructs the digital signals which have not been processed and obtained from the electrical signal processing unit 6 to obtain an image and the display unit 15 displays the image so that the difference from an image made up of the reflection signal only is understood. In this way, it is possible to detect artifacts associated with a multiple-reflection signal.
In this case, both images may be displayed side by side, may be superimposed on each other in a semi-transparent manner, may be displayed in an alternating manner, and may be switched at an optional point in time by the user.

The implementation method will be described with reference to the flowchart of Fig. 11. S1 to S4 are the same as those of the first embodiment. Signals identical to the obtained delay profile are identified and extracted (S13), and it is determined whether all reflection signals corresponding to a desired number of reflections have been extracted (S8). When the extraction has not been completed, the flow returns to S8. When the extraction has been completed, imaging is performed (S9), and images are displayed (S10).

Next, a case where the delay amount is obtained will be described. The constituent components of the present embodiment will be described with reference to Fig. 12. Constituent components 1 to 9 are the same as those of the third embodiment. Since the reflection signal can be identified from the delay amount obtained from the reflection signal estimator 9, the reflection signal extractor 17 is not provided. After that, images are reconstructed and displayed similarly to the case where the delay profile is used.

By using the apparatus of the present embodiment, it is possible to display artifacts associated with a multiple-reflection signal in an image and to reduce the influence on diagnosis, of the artifacts by comparing the artifacts with a normal image.
In the present embodiment, although the influence of artifacts on diagnosis is reduced by imaging the artifacts, the influence of the artifacts on diagnosis may be reduced just by displaying the reflection signal without imaging the artifacts.

<Fifth Embodiment>
In the present embodiment, a case where the surface shape of the object is acquired from a camera or a laser range finder capable of measuring stereoscopic information rather than from signals will be described. The difference from the first embodiment is the delay acquiring unit. Thus, the delay acquiring unit will be described.

Constituent components of the present embodiment will be described with reference to Fig. 13. In the present embodiment, in order to obtain the surface shape of an object, an arrow extends directly from the object 3 to the shape information acquisition unit 8. In other words, the shape information acquisition unit 8 of the present embodiment directly measures the stereoscopic shape of the object. Thus, spatial distance is obtained from the shape information acquisition unit 8.
Specifically, the shape information acquisition unit 8 is a camera or a laser range finder capable of measuring stereoscopic information. The shape information acquisition unit 8 is not limited to these devices but other devices capable of measuring the stereoscopic shape of an object may be used.

A surface shape converter 18 converts the distance in the forward direction of the acoustic detector among the distances of the object surface shapes obtained by the shape information acquisition unit 8 to time by taking the propagation velocity of acoustic waves in the acoustic matching layer into consideration. The propagation velocity of used acoustic waves may be a predetermined fixed value, may be determined from a table prepared in advance based on a measured temperature, and may be designated by the user each time as necessary.

The surface shape converter 18 obtains the delay profile or the delay amount of a signal indicating the object surface. Thus, the reflection signal estimator 9 estimates the reflection signal using the delay profile or the delay amount of the signal indicating the object surface as described in the first to third embodiments.
The implementation method of the present embodiment is the same as that of the first embodiment.

By using the apparatus of the present embodiment, since the surface shape is obtained with a higher signal-to-noise (SN) ratio than a method of obtaining the object surface shape from signals, it is possible to acquire the object surface shape stably.

<Sixth Embodiment>
In the first embodiment, the reflection signal is estimated easily by assuming that the acoustic detector is planar. In the present embodiment, a method of estimating the reflection signal using simulations which can be used when the acoustic detector is not planar will be described. For example, as illustrated in Fig. 14, even when the acoustic wave detector is disposed on a curved surface, it is possible to estimate reflection signals using the technique of the present invention. In Fig. 14, a plurality of acoustic detection elements 1703 included in an acoustic detector 1702 receives photoacoustic waves from an object 1701 irradiated with a pulsed light with an acoustic matching member 1705 interposed. In this case, reflection waves reflected within the acoustic matching member, of the photoacoustic waves generated from the object surface are extraction and reduction target signals.

The constituent components of the present embodiment are the same as those of the first embodiment. However, the processing of the reflection signal estimator is different. The reflection signal estimator simulates propagation of acoustic waves generated from the object surface or the acoustic detector surface using the shape of the object surface shape, the shape of the acoustic matching layer, the shape of the acoustic detector, and the like and predicts reflection waves obtained as signals.
In this prediction process, the shape of the object surface shape is acquired by the shape information acquisition unit for each measurement. Moreover, since the shape of the acoustic detector is always the same, it is preferable to store the shape in advance. The shape of the acoustic matching layer can be acquired by assuming a layer between the acoustic detector and the object as the acoustic matching layer as long as the surface shapes of the acoustic detector and the object can be obtained.

Moreover, fixed values stored in advance are preferably used for the propagation velocity of acoustic waves in the acoustic matching layer and the acoustic impedances of the object, the acoustic matching layer, and the acoustic detector, which are the parameters required for simulation of propagation of acoustic waves. Since the propagation velocity of acoustic waves in the acoustic matching layer and the acoustic impedances of the object, the acoustic matching layer, and the acoustic detector change depending on temperature, the parameters may be obtained from a table prepared in advance based on a measured temperature.

Moreover, when the object is a living body, the acoustic impedance changes from person to person, the acoustic impedance of the object may be measured in advance. Here, the higher the accuracy of the prepared parameters, the higher the accuracy of reflection waves obtained through the simulation. When the reflection signal estimator can acquire reflection signals, the reflection signals are sent to the data processing unit, whereby the reflection signals can be reduced.
The implementation method of the present embodiment is the same as that of the first embodiment.

By using the apparatus of the present embodiment, even when the acoustic detector is disposed on a curved surface, it is possible to estimate reflection waves with high accuracy. Moreover, even when the acoustic detector is planar, the accuracy of estimation of reflection waves increases. Due to this, it is possible to reduce artifacts associated with multiple reflection with high accuracy. When only one acoustic detection element or a small number of acoustic detection elements is present, the same effects are obtained by scanning the device according to the target measurement positions.

<Seventh Embodiment>
In the first embodiment, when the acoustic detector surface and the object surface are approximately parallel to each other, the delay profile of a signal indicating the object surface is delayed in the time direction by an integer multiple, the delay profile of the reflection signal is obtained. In the present embodiment, a case where the acoustic detector surface and the object surface are not parallel to each other and the signal indicating the object surface and the delay profile of the reflection signal are not in an integer multiple relation will be described.

The constituent components of the present embodiment are the same as those of the first embodiment, but the processing of the reflection signal estimator 9 is different. In the present embodiment, the reflection signal estimator 9 performs a process of delaying the delay profile of a signal indicating the object surface in the time direction with a small change rate to obtain the delay profile in each step. Specifically, the process of delaying the delay profile with a small change rate is a process of repeatedly delaying the delay profile of the signal indicating the object surface in the time direction using a predetermined small change rate.

For example, the small change rate is R. In this case, the delay profile of the signal indicating the object surface is delayed in the time direction by a multiple of (1+R), (1+2R), (1+3R), ..., and so on to acquire respective delay profiles. This process is performed until the number of acquired delay profiles reaches a predetermined upper limit number of delay profiles of the reflection signal to be estimated. A shape identical to the delay profile acquired in this manner can be estimated to be the reflection signal.

In this example, since signals identical to the delay profile are reduced, the subsequent data processing unit 10 reduces a signal based on the respective obtained delay profiles. Although the small change rate and the upper limit of delay profiles of the reflection signal to be estimated are preferably determined in advance, the data may be input by the user each time as necessary.

By using the apparatus of the present embodiment, even when it is not possible to estimate reflection signals completely by the process of delaying the delay profile of a signal indicating the object surface in the time direction by an integer multiple (for example, when the acoustic detector surface and the object surface are not parallel to each other), it is possible to estimate and reduce the reflection signals.

<Test Example>
The effects of the present invention were verified using a test system illustrated in Fig. 15.
In Fig. 15, an acoustic detector 1402 receives photoacoustic waves generated from an object 1401 irradiated with a pulsed light 1404 with an acoustic matching member 1405, an object holding plate 1406, and an acoustic matching liquid 1407 covering the object holding plate 1406 interposed.

An object was the calf of a living body, and a gel-shaped acoustic matching member was provided in contact with the object. The acoustic matching member was a flexible member and was fit to the shape of the living body. Moreover, a 7 mm-thick object holding plate formed from polymethylpentene was provided. An acoustic matching liquid which is castor oil was filled in a 3 mm-thick space between the acoustic detector and the object holding plate. Both surfaces of the object holding plate were parallel to the acoustic matching liquid.

The acoustic detector and the pulsed light were moved in synchronization for scanning so that all regions being in contact with the object were measured. A PZT of which the diameter of a receiving unit was 2 mm and of which a bandwidth was 80% at a central frequency of 1 MHz was used as the element of the acoustic detector. 15 * 23 elements were arranged in a planar direction to form one acoustic detector. A TiS laser that generates a pulsed light having a wavelength of 797 nm and a pulse width of several nanoseconds was used as the light source of the pulsed light.

In this test system, irradiation of pulsed lights, collection of acoustic signals, and scanning were performed repeatedly to obtain all pieces of signal data. In this case, an analog-digital converter having a sampling frequency of 20 MHz and a resolution of 12 bit was used.

Fig. 16A illustrates the obtained signals arranged in conformity with the measurement positions. The object surface was observed at the position of 200 samples and this shape is the delay profile of the object surface. After that, a group of multiple-reflection signals appeared at the positions of 400 to 600 samples. The reason why a plurality of reflection signals rather than one reflection signal appears is because there is a plurality of multiple-reflection layers and reflections occur at different intervals. Moreover, a group of multiple-reflection signals also appeared at the positions of 800 to 100 samples. In this region, reflections repeat and the signal intensity decreases.

Fig. 16B illustrates multiple-reflection signals which are reduced using the apparatus described in the second embodiment. In this example, the reflection signal was reduced under conditions that the number of delay profiles of the reflection signal to be estimated was 4, and the shapes obtained by delaying the delay profile of the object surface in the time direction by 0, 1, 2, and 3 times were used as the delay profiles of the reflection signal. When Figs. 16A and 16B are compared, it can be understood that the multiple-reflection signal is reduced.

Subsequently, the signal was imaged and processed to obtain a 3-dimensional image. Universal back-projection was used for the imaging. Fig. 17A illustrates an image obtained by imaging the non-processed signal illustrated in Fig. 16A and displaying the slice of the reflection signal. Moreover, Fig. 17B illustrates an image obtained by imaging the signal processed using the apparatus described in the second embodiment, illustrated in Fig. 16B and displaying the same slice as Fig. 17A.
According to the comparison between both images, when the signal is not processed, the reflection signal reflecting the surface shape of the object is imaged to appear as artifacts. However, when the reflection signal is reduced using the apparatus of the present invention, artifacts are reduced.

Moreover, Figs. 17C and 17D illustrate 3-dimensional images created from non-processed signals and processed signals in the slice in which a structure derived from a living body appears remarkably. When both images are compared, it can be understood that the structure derived from the living body is rarely influenced.
From the above, it was confirmed that by using the apparatus of the present invention, it is possible to reduce artifacts mainly without having a significant influence on the structure derived from the living body.

Other Embodiments
Embodiments of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions recorded on a storage medium (e.g., non-transitory computer-readable storage medium) to perform the functions of one or more of the above-described embodiment(s) of the present invention, and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more of a central processing unit (CPU), micro processing unit (MPU), or other circuitry, and may include a network of separate computers or separate computer processors. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)^TM), a flash memory device, a memory card, and the like.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

This application claims the benefit of Japanese Patent Application No. 2014-023284, filed on February 10, 2014, which is hereby incorporated by reference herein in its entirety.

Claims

An object information acquiring apparatus comprising:
an acoustic detector configured to receive acoustic waves generated from an object at a plurality of measurement positions and convert the acoustic waves to a plurality of signals;
a shape information acquirer configured to acquire surface shape information of the object;
a signal estimator configured to estimate a target signal derived from an acoustic wave generated at a specific position from the plurality of signals based on the surface shape information;
a reducer configured to reduce the target signal estimated by the signal estimator; and
a characteristic information acquirer configured to acquire characteristic information on an inside of the object using the plurality of signals in which the target signal is reduced by the reducer.
The object information acquiring apparatus according to claim 1, wherein
the specific position includes at least one of a surface of the object which is in contact with an acoustic matching member disposed between the acoustic detector and the object, and a surface of the acoustic detector which is in contact with the acoustic matching member.
The object information acquiring apparatus according to claim 1 or 2, wherein
the signal estimator calculates a distance between each of the plurality of measurement positions and the object surface based on the surface shape information of the object, and estimates a position of the target signal in each of the plurality of signals based on the distance.
The object information acquiring apparatus according to any one of claims 1 to 3, wherein
the signal estimator estimates, as the target signal, a signal derived from an acoustic wave which has been generated from at least one of a surface of the object and a surface of the acoustic detector and then undergone multiple reflection between the surface of the object and the surface of the acoustic detector.
The object information acquiring apparatus according to claim 4, wherein
the signal estimator estimates the signal derived from the acoustic wave having undergone multiple reflection, by multiplying the time for the acoustic wave to propagate between the surface of the object and the plurality of measurement positions, by an integer.
The object information acquiring apparatus according to claim 4 or 5, wherein
the signal estimator sets the number of multiple reflections to be estimated.
The object information acquiring apparatus according to any one of claims 1 to 5, wherein
the signal estimator estimates the target signal by simulating propagation of the acoustic wave using the surface shape information of the object.
The object information acquiring apparatus according to any one of claims 1 to 7, wherein
the shape information acquirer acquires the surface shape information using the acoustic wave generated from the object.
The object information acquiring apparatus according to any one of claims 1 to 7, wherein
the shape information acquirer acquires the surface shape information using a camera capable of measuring stereoscopic information.
The object information acquiring apparatus according to any one of claims 1 to 7, wherein
the shape information acquirer acquires the surface shape information using a laser range finder.
The object information acquiring apparatus according to any one of claims 1 to 10, wherein
the acoustic wave is generated when the object is irradiated with light.
The object information acquiring apparatus according to any one of claims 1 to 11, wherein
the acoustic wave is reflected after being transmitted to the object.
The object information acquiring apparatus according to any one of claims 1 to 12, wherein
the characteristic information acquirer acquires image data on the inside of the object based on the characteristic information.
The object information acquiring apparatus according to claim 13, further comprising:
a display unit configured to display the image data.
A signal processing method comprising:
a signal estimating step of estimating a target signal derived from an acoustic wave generated at a specific position, from a plurality of signals obtained by receiving acoustic waves generated from an object at a plurality of measurement positions, based on surface shape information of the object;
a reducing step of reducing the target signal estimated by the signal estimator; and
a characteristic information acquiring step of acquiring characteristic information on an inside of the object using the plurality of signals in which the target signal is reduced.