WO2016136206A1

WO2016136206A1 - Phantom

Info

Publication number: WO2016136206A1
Application number: PCT/JP2016/000853
Authority: WO
Inventors: Nobuhito Suehira; Yukio Furukawa; Takuji Oishi
Original assignee: Canon Kabushiki Kaisha
Priority date: 2015-02-26
Filing date: 2016-02-18
Publication date: 2016-09-01
Also published as: JP2016158673A; JP6548405B2

Abstract

In a phantom, a difference in acoustic property between a first target and a base material is smaller than a difference in acoustic property between a second target and the base material, a difference in optical property between the second target and the base material is smaller than a difference in optical property between the first target and the base material, and the first target is provided at a position closer to a measurement surface than the second target.

Description

PHANTOM

The present invention relates to a phantom for evaluation of an apparatus, and particularly relates to a phantom for evaluation of an apparatus capable of performing measurement using photoacoustic waves and measurement using ultrasonic echoes.

Photoacoustic tomography (PAT) is known as one of optical imaging techniques. In the photoacoustic tomography, when a test object is irradiated with pulsed light, an acoustic wave (hereinafter may also be referred to as "a photoacoustic wave") is generated from a light-energy absorbing region of the test object, and then detected. From the detected photoacoustic waves, information about an optical characteristic value of an inner structure of the test object can be visualized.

The PAT technology is known to be used with an ultrasonic echo technology. PTL 1 discusses an apparatus that obtains a distribution of new blood vessels included in a tissue of the breast of a subject by performing measurement using photoacoustic waves. This apparatus also obtains a morphologic image of the subject by using ultrasonic echoes.

In general, a phantom having a known inner structure is used to evaluate performance of a PAT apparatus. An image based on photoacoustic waves is obtained by using this phantom as a test object, so that the performance of the PAT apparatus can be evaluated based on the obtained image. PTL 2 discusses a phantom imitating a human body.

However, conventionally, no study has been made for a configuration of a phantom suitable for evaluation of a PAT apparatus, which is also capable of performing measurement using ultrasonic echoes.

Japanese Patent Application Laid-Open No. 2005-021380 Japanese Patent Application Laid-Open No. 2011-209691

According to an aspect of the present invention, a phantom includes a base material having a measurement surface, and a first target and a second target that are provided in the base material, wherein a difference in acoustic property between the first target and the base material is smaller than a difference in acoustic property between the second target and the base material, wherein a difference in optical property between the second target and the base material is smaller than a difference in optical property between the first target and the base material, and wherein the second target is provided at a position further away from the measurement surface than the first target.

According to another aspect of the present invention, a phantom includes a first target, and a second target, wherein, of the first target and the second target, one target absorbs light more easily than other target, and wherein the other target reflects an acoustic wave more easily than the one target.

Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

Fig. 1A is a diagram illustrating a configuration example of a phantom. Fig. 1B is a diagram illustrating a configuration example of a phantom Fig. 2 is a diagram illustrating a configuration example of a photoacoustic tomography (PAT) apparatus. Fig. 3A is a diagram illustrating a configuration example of a phantom. Fig. 3B is a diagram illustrating a configuration example of a phantom. Fig. 4A is a diagram illustrating a configuration example of a PAT apparatus. Fig. 4B is a diagram illustrating a configuration example of a PAT apparatus. Fig. 5A is a diagram illustrating a configuration example of a phantom. Fig. 5B is a diagram illustrating a configuration example of a phantom.

For easy understanding of an issue, a phantom for a photoacoustic tomography (PAT) apparatus (hereinafter may also be referred to as "photoacoustic apparatus") will be described first.

A phantom to be used for evaluation of a PAT apparatus has a target suitable for measurement of photoacoustic waves. This target has a light absorption rate sufficient for generation of a photoacoustic wave having intensity detectable by a transducer. However, a target material suitable for generation of photoacoustic waves is not suitable for measurement of ultrasonic echoes in many cases. Therefore, a conventional phantom for a PAT apparatus may not be suitable for evaluation of a PAT apparatus that can also perform measurement using ultrasonic echoes.

To address such an issue, a phantom having a target suitable for generation of photoacoustic waves and a phantom having a target suitable for measurement using ultrasonic echoes may be prepared. The phantoms may be switched to one another depending on which measurement is to be evaluated. However, in this case, the replacement of the phantom according to the measurement is burdensome for an operator.

Therefore, in each exemplary embodiment of the present invention, one phantom has a target suitable for photoacoustic wave measurement, and a target suitable for ultrasonic echo measurement. Specifically, the present inventors have some favorable layouts of the targets in the phantom. The phantom according to each exemplary embodiment of the present invention will be described below. The target suitable for the photoacoustic wave measurement will be hereinafter referred to as "photoacoustic measurement target", and the target suitable for the ultrasonic echo measurement will be hereinafter referred to as "ultrasonic measurement target".

First Embodiment

Fig. 1A is a diagram illustrating a phantom according to a first exemplary embodiment of the present invention. In this phantom, an area surrounded by an outer frame 101 is filled with a base material 102, and part of the base material 102 is provided with a photoacoustic measurement target 103 serving as a first target and an ultrasonic measurement target 104 serving as a second target. In the present exemplary embodiment, the photoacoustic measurement target 103 and the ultrasonic measurement target 104 are both cylindrical and provided along an x-axis. For easy understanding, Fig. 1A illustrates an example in which the outer frame 101 does not cover a z-y plane of the base material 102. However, the outer frame 101 may cover the z-y plane of the base material 102.

The outer frame 101 serves as a support member for maintaining the shape of the base material 102. In the present exemplary embodiment, the base material 102 is exposed on a surface orthogonal to a z-axis, and this surface serves as a measurement surface 105. A material having higher rigidity than that of the base material 102 may be used for the support member, so that the phantom can be easily handled, when, in particular, the base material 102 is soft. Further, the base material 102 is fixed to the support member via a surface different from the measurement surface 105. The measurement surface 105 of the base material 102 may be exposed as illustrated in Fig. 1A. Alternatively, a protection member for covering the measurement surface 105 of the base material 102 may be provided if the protection member is transparent for the wavelength of light to be used for the photoacoustic measurement. The protection member may be configured integrally with the support member. When the PAT apparatus is evaluated, a probe is disposed in proximity to the measurement surface 105.

The photoacoustic measurement target 103 is intended to be detected in the photoacoustic measurement and not to be easily observed in the ultrasonic echo measurement. This target is provided to evaluate an image contrast of an initial sound pressure distribution, an oxygen saturation, a resolution, and the like obtained by the PAT apparatus. Therefore, the photoacoustic measurement target 103 is configured not to be observed easily in the ultrasonic echo measurement. Specifically, the acoustic properties such as sound velocity and acoustic attenuation in the photoacoustic measurement target 103 are close to those of the base material 102, while the optical properties such as a light absorption coefficient are different from those of the base material 102.

The ultrasonic measurement target 104 is intended to be observed in the ultrasonic echo measurement and not to be easily observed in the photoacoustic measurement. This target is provided to evaluate an image contrast, a resolution, and the like obtained by the ultrasonic echo measurement. Therefore, the acoustic properties such as sound velocity and an attenuation rate are different from those of the base material 102, and the optical properties such as a light absorption coefficient are low and close to those of the base material 102.

Based on the above description, the difference in acoustic properties between the photoacoustic measurement target 103 and the base material 102 is smaller than the difference in acoustic properties between the ultrasonic measurement target 104 and the base material 102. Further, it may be said that the difference in optical properties between the ultrasonic measurement target 104 and the base material 102 is smaller than the difference in optical properties between the photoacoustic measurement target 103 and the base material 102. In other words, it may be said that the photoacoustic measurement target 103 absorbs light more easily than the ultrasonic measurement target 104, and the ultrasonic measurement target 104 reflects an acoustic wave more easily than the photoacoustic measurement target 103.

Materials satisfying the above-described properties will be described next.

In the present exemplary embodiment, the photoacoustic measurement target 103 may be fabricated using a material similar to that of the base material 102. When the same material as the base material 102 is used for the photoacoustic measurement target 103, the values of the acoustic properties can be close to those of the base material 102. Therefore, the photoacoustic measurement target 103 is not easily observed in the ultrasonic measurement. The base material 102 may be made of a polyol, and a filler capable of dispersing in the polyol. Examples of the polyol may include a polyether polyol, a polyester polyol, and a polycarbonate polyol. Of these, the polyether polyol is more desirable in terms of correlation concerning the sound propagation properties of human tissue. The polyol is usually in a liquid state, and may contain a curing agent as necessary so that a resin can be cured to become solid. An isocyanate compound can be used to achieve an approximation to the sound propagation properties of human tissue.

Further, to make the light propagation properties of the base material 102 and the photoacoustic measurement target 103 close to those of the human tissue, it is necessary to set each of an equivalent scattering coefficient and an absorption coefficient of light at an appropriate value by dispersing the filler. As a light scattering filler having light scattering properties, there is an inorganic oxide such as titanium oxide. As a light absorbing filler having light absorption properties, it is desirable to use a pigment. Examples of the pigment include a black pigment such as carbon black, a cyan pigment such as copper phthalocyanine, a magenta pigment such as a monoazo lake pigment and a monoazo pigment, and a yellow pigment such as diarlide yellow.

As an example of the ultrasonic measurement target 104, it is conceivable to adopt a urethane resin obtained by diluting a polybutadiene polyol serving as a main agent, and causing the resultant to react with a diphenylmethane diisocyanate serving as a curing agent. An organic powder filler is added to this urethane resin to achieve uniform reflection and scattering of ultrasonic waves. For the organic powder filler, a powder of polypropylene, polyethylene, ethylene vinyl acetate, a vinyl alcohol copolymer, or the like may be used.

Next, the shape and placement of each target in the base material 102 will be described. Fig. 1B is a cross-sectional diagram in the z-y plane of the phantom according to the present exemplary embodiment.

In the present exemplary embodiment, the base material 102 is a cuboid having a size of 60 mm, 80 mm, and 60 mm along the x-axis, a y-axis, and the z-axis, respectively. The photoacoustic measurement target 103 is shaped like a cylinder having a diameter of 1 mm, and is provided at a depth of 10 mm from the measurement surface 105, while being parallel with the x-axis. The ultrasonic measurement target 104 is also shaped like a cylinder having a diameter of 1 mm, and is provided at a depth of 20 mm from the measurement surface 105, while being parallel with the x-axis. In the present exemplary embodiment, the photoacoustic measurement target 103 and the ultrasonic measurement target 104 are placed at the respective positions that are the same in the y coordinate and different in the z-axis direction. The shape of each of these targets is not limited to the cylinder, and may be other shape such as a ball. However, each of the targets is molded by pouring a resin into a die and therefore, a pillar shape such as a cylinder is desirable since this shape allows fabrication with high reproducibility.

In Fig. 1B, a dotted line connected between each of the targets and the measurement surface 105 is an acoustic ray indicating a range of acoustic waves to be detected by an ultrasonic probe (not illustrated), when the ultrasonic probe of a linear type extending in the x-axis direction is used. In general, the ultrasonic probe of the linear type exhibits favorable properties including a resolution, when a target is placed on the centerline of the probe. Therefore, when the targets are placed as illustrated in Figs. 1A and 1B, the apparatus can be evaluated at the same position without moving the ultrasonic probe, for both of the ultrasonic echo and photoacoustic wave measurements.

In the present exemplary embodiment, the photoacoustic measurement target 103 serving as the first target has a small difference in acoustic properties from the base material 102, as compared with the ultrasonic measurement target 104 serving as the second target. Therefore, in the evaluation of the ultrasonic measurement, an acoustic wave reflected by the ultrasonic measurement target 104 and then heading for the measurement surface 105 is not easily reflected by the photoacoustic measurement target 103. In other words, even if the photoacoustic measurement target 103 is present between an acoustic-wave receiving surface of the ultrasonic probe and the ultrasonic measurement target 104, the ultrasonic measurement is not easily affected by interference from the photoacoustic measurement target 103. Assume that the photoacoustic measurement target 103 is provided at a position deeper than the ultrasonic measurement target 104. In this case, a photoacoustic wave generated from the photoacoustic measurement target 103 may be reflected by the ultrasonic measurement target 104. Therefore, it is desirable to place the photoacoustic measurement target 103 almost at the same position as that of the ultrasonic measurement target 104, or at a position closer to the measurement surface 105. In other words, it is desirable to provide the second target at a position further away from the measurement surface 105 than the first target.

When the PAT apparatus to be evaluated uses a living body as a test object, it is desirable that the optical properties and the acoustic properties of the phantom also imitate those of the living body. For example, as for the base material 102, an optical constant may be an absorption coefficient of 0.005 mm^-1 within a range of values of the living body, an equivalent scattering coefficient may be 1 mm^-1, a sound velocity within the base material 102 may be 1,450 m/s, and an attenuation factor may be 0.5 dB/cmMHz. As for the photoacoustic measurement target 103, an absorption coefficient is assumed to be 0.2 mm^-1 that is equivalent to that of blood, and an equivalent scattering coefficient is assumed to be 1 mm^-1. Further, as for the ultrasonic measurement target 104, a sound velocity is assumed to be 1,530 m/s, and an attenuation factor is assumed to be on the order of 0.5 dB/cmMHz. In the ultrasonic measurement target 104, an absorption coefficient and an equivalent scattering coefficient are assumed to be small values that are about the same as those of the base material 102.

The light propagation properties of the base material 102 and each of the targets of the phantom may be evaluated by, for example, measuring, with a spectrophotometer, transmittance and reflectance of specimens made when the base material 102 and each of the targets are fabricated. For the results obtained by the measurement, conditioning is performed by a Monte Carlo simulation to achieve a minimum difference between the obtained measurement values and the calculated values, so that the equivalent scattering coefficients and the absorption coefficients can be obtained. On the other hand, the sound propagation properties can be measured by, for example, placing a specimen made when the phantom is fabricated, between a transducer for transmission and a needle-type hydrophone for reception. The sound velocity can be measured from a difference between the arrival times of ultrasonic waves, and the attenuation properties can be measured from a difference between the ultrasonic amplitudes of the ultrasonic waves, when the thickness of the specimen is changed.

Next, the PAT apparatus the performance of which is to be evaluated using the phantom according to the present exemplary embodiment, and an evaluation method therefor will be described.

Fig. 2 illustrates a state where the phantom is measured by the PAT apparatus. The PAT apparatus according to the present exemplary embodiment includes two light irradiation units 201, and an ultrasonic probe 202 of a handheld type. The ultrasonic probe 202 receives photoacoustic waves, and also transmits and receives ultrasonic waves. The PAT apparatus further includes a light control unit 203, an ultrasonic wave control unit 204, a device control unit 205, and a display unit 206. In this PAT apparatus, the photoacoustic measurement is enabled by adjusting timing for sampling of an acoustic wave by the ultrasonic probe 202, to timing for emission of light 207 from the light irradiation unit (a light source) 201. The device control unit 205 controls the timing. In addition, the ultrasonic measurement is enabled by transmission and reception of ultrasonic waves by the ultrasonic probe 202.

Here, a part of the light 207 emitted from the light irradiation unit (the light source) 201 propagates while diffusing in the base material 102, and is then absorbed by the photoacoustic measurement target 103. Upon absorbing the light 207, the photoacoustic measurement target 103 generates photoacoustic waves 208. In contrast, in the ultrasonic measurement target 104, since the light absorption coefficient is close to that of the base material and the value is low, the photoacoustic waves 208 can be ignored. Further, ultrasonic waves transmitted from the ultrasonic probe 202 penetrate the photoacoustic measurement target 103. The ultrasonic waves are then reflected by the ultrasonic measurement target 104, and then received by the ultrasonic probe 202.

In the evaluation of the PAT apparatus, the photoacoustic measurement and the ultrasonic measurement of the phantom are sequentially performed. Subsequently, from an image based on the photoacoustic waves generated by the photoacoustic measurement target 103, it is determined whether an initial sound pressure distribution and a resolution indicate the desired performance. Next, from an image based on the ultrasonic waves reflected by the ultrasonic measurement target 104, it is determined whether the resolution of a contrast indicates the desired performance. This determination may be performed by an operator of the PAT apparatus, or may be automatically performed by the PAT apparatus.

The light irradiation unit 201 is a device that emits pulsed light for irradiating a test object. In the present exemplary embodiment, the phantom is irradiated from two directions and thus, the light for the irradiation becomes as uniform as possible. The light control unit 203 controls the emission timing, waveform, and intensity of the pulsed light. The light source is desirably a laser light source to obtain high power. However, the light source is not limited to the laser light source. A light emitting diode or flash lamp may be used in place of the laser light source. As the laser light source, any of various types including a solid-state laser, a gas laser, a dye laser, and a semiconductor laser can be used.

To generate the photoacoustic waves effectively, the light can be emitted in a sufficiently short time according to the thermal properties of the test object. In particular, when the test object is a living body, the pulse width of the pulsed light generated from the light source can be on the order of 10 to 50 nanoseconds. In addition, the wavelength of the pulsed light is desirably a wavelength of light that can propagate into the test object. When, for example, the test object is a living body, the wavelength is 700 nm or more and 1,100 nm or less. Here, a titanium sapphire laser, i.e., a solid-state laser, is assumed to be used, and the wavelength is assumed to be 800 nm.

The ultrasonic probe 202 of the linear type according to the present exemplary embodiment includes a transducer array having transducers arranged at least one-dimensionally. The ultrasonic probe 202 is used to receive photoacoustic waves, and to transmit and receive ultrasonic waves. For example, 192 transducers may be arranged in a line. When receiving an ultrasonic wave or a photoacoustic wave, the ultrasonic probe 202 outputs an analog electrical signal. An element to be used for the transducer may be an element made of lead zirconate titanate (also referred to as PZT) that is one kind of piezoelectric ceramic, a capacitive micromachined ultrasonic transducer (CMUT), or the like.

Further, the analog electrical signal output from the transducer is transmitted to the ultrasonic wave control unit 204 and then converted into a digital signal via components such as an amplifier and an analog-to-digital (A/D) converter. The digital signal is then sent to the device control unit 205. The band of the ultrasonic probe 202 is, for example, 2 to 5 MHz. Further, sampling is performed 2,048 times at a sampling frequency of 50 MHz. Data is assumed to be signed 12 bits.

According to the phantom described above, evaluating the photoacoustic measurement and the ultrasonic echo measurement can be implemented by one phantom. In particular, when the first target for the photoacoustic measurement is provided at a position closer to the measurement surface than the second target for the ultrasonic measurement, interference by one of the targets with the measurement of the other can be suppressed.

Second Embodiment

A phantom according to a second exemplary embodiment has a spherical measurement surface, and is assumed to be used for evaluation of a PAT apparatus that uses an ultrasonic probe disposed on a hemispheric surface. Such a PAT apparatus is used, for example, for measurement of a breast.

In the phantom according to the present exemplary embodiment, in addition to a photoacoustic measurement target serving as a first target and an ultrasonic measurement target serving as a second target, a shared target serving as a third target to be used for both of the photoacoustic measurement and the ultrasonic measurement is embedded in a base material. The shared target is to be observed in both of a photoacoustic apparatus and an ultrasonic apparatus. Specifically, the acoustic properties and the optical properties of the shared target are both different from those of the base material. In other words, the third target has such properties that a light absorption coefficient is high to the extent of generating photoacoustic waves, and most of incident acoustic waves are reflected off an interface with the base material. This can be used for registration of an image of the photoacoustic apparatus and an image of the ultrasonic apparatus.

Fig. 3A illustrates a phantom 301 according to the present exemplary embodiment. In the phantom 301, a photoacoustic probe is disposed in a hemispheric container, and used for evaluation of the PAT apparatus. In this example, a base material 302 has such a structure that a measurement surface 306, which is a curved surface, is provided at a column having a diameter of 120 mm. The apex of the curved surface forming the measurement surface 306 is on a central axis of the above-described column. The curved surface may be, for example, a spherical surface of a sphere the center of which is located on the central axis of the above-described column. Further, a shared target 305 serving as the third target is disposed at a depth of 10 mm from the apex of the measurement surface 306. The shared target 305 has a diameter of 0.1 mm. Furthermore, a photoacoustic measurement target 303 serving as the first target is disposed at a depth of 15 mm from the apex of the measurement surface 306. The photoacoustic measurement target 303 has a diameter of 1 mm. The two photoacoustic measurement targets 303 are used and disposed 32 mm away from each other, between which the central axis of the column is located. Further, an ultrasonic measurement target 304 is disposed at a depth of 25 mm from the apex of the measurement surface 306. The ultrasonic measurement target 304 has a diameter of 5 mm. The two ultrasonic measurement targets 304 are used and disposed 16 mm away from each other, between which the central axis of the column is located. These two types of targets are provided in parallel with each other. As illustrated in Fig. 3A, the first, second, and third targets are provided at the different depths, and therefore, when a cross section of one of the targets is measured, the other targets can be prevented from appearing.

Fig. 3B illustrates an example of an acoustic ray, which extends from each of the photoacoustic measurement targets 303, the ultrasonic measurement targets 304, and the shared target 305 to the probe for image generation using photoacoustic waves, with a dotted line. The photoacoustic waves emitted from each of the photoacoustic measurement targets 303 and the shared target 305 propagate in all directions of the target, and are received by the probe provided on the curved surface. Therefore, the spread of the acoustic ray is wide. On the other hand, ultrasonic waves reflected by the ultrasonic measurement target 304 can form a focus and therefore, the spread of the acoustic ray is narrower than the acoustic ray of the photoacoustic waves. The shared target 305 can be used as a target for registration of an ultrasonic image and a photoacoustic image. On the other hand, the photoacoustic measurement targets 303 are intended to evaluate the accuracy of oxygen saturation measurement of the PAT apparatus. One of the two photoacoustic measurement targets 303 is made to correspond to an oxygen saturation of 75%, and the other is made to correspond to an oxygen saturation of 95%. The ultrasonic measurement targets 304 are used to evaluate the contrast of an ultrasonic image.

The ultrasonic measurement target 304 has a property of easily reflecting an acoustic wave. For this reason, when the ultrasonic measurement target 304 is disposed within the range of the acoustic ray connecting the photoacoustic measurement target 303 or the shared target 305 with the measurement surface 306, the ultrasonic measurement target 304 interferes with the propagation of acoustic waves. Therefore, for example, an artifact may appear in an image based on photoacoustic waves. Accordingly, interference with the photoacoustic measurement can be prevented by providing the ultrasonic measurement target 304 at a position further away from the measurement surface 306 than, or at the same depth as those of, the photoacoustic measurement target 303 and the shared target 305.

The shared target 305 can be fabricated, by differentiating the acoustic properties from the acoustic properties of the photoacoustic measurement target 303, thereby increasing the difference from the base material 302 in terms of acoustic properties. The shared target 305 may be fabricated, by adding a coloring material to the ultrasonic measurement target 304, thereby increasing the difference from the base material 302 in terms of optical properties. Alternatively, a nylon wire to which a coloring material is added may be used for the shared target 305. In general, the nylon wire can be made thin and hard, unlike urethane resins, and therefore, the nylon wire is suitable for evaluation of the resolution of the PAT apparatus. The shared target 305 is made thinner than a thickness resolvable by each of the photoacoustic measurement and the ultrasonic echo measurement. Therefore, even in a case where the resolutions are different, such as a case where the resolution of a photoacoustic wave is 0.3 mm and the resolution of an ultrasonic wave is 0.2 mm, the shared target 305 appears to have a thickness larger than the original thickness. Accordingly, the resolution of each of the photoacoustic wave and the ultrasonic wave can be evaluated.

Further, in the present exemplary embodiment, the two photoacoustic measurement targets 303 for evaluating the oxygen saturation are designed to be made of a certain type of material. This is such a material that, when two wavelengths of 800 nm and 760 nm are used, the ratio between the absorption coefficients in the respective wavelengths can be equal to the ratio between the oxygen saturation of 75% and 95%.

In the present exemplary embodiment, the pair of targets different from each other in optical property is provided as the first target. However, in a different configuration, the phantom may have two or more of these pairs. In this configuration, when the thicknesses of the respective targets are different between the two or more pairs, it is useful in evaluating the resolution of the PAT apparatus. Further, as for each of the second and third targets as well, targets having the respective thicknesses different from each other may be provided to vary the optical properties and the acoustic properties such as a reflectance of an acoustic wave. When each of the first to third targets includes the targets having the different thicknesses, the thinner target can be provided at a position in proximity to the measurement surface.

Further, the first target may have one pair of targets having the same thicknesses, and another target having a thickness different from the thicknesses of this one pair of targets. In this case, the one pair of targets having the same thicknesses can be provided in closer proximity to each other, than to another target having the different thickness. The second and third targets may each have a pair of targets having the same thickness and another target having a different thickness.

Figs. 4A and 4B each illustrate a part of the PAT apparatus for the breast measurement according to the present exemplary embodiment. Fig. 4A is a cross-sectional diagram illustrating a configuration of a part, which holds a test object, of the PAT apparatus. Fig. 4B is a top view of a surface parallel with an x-y plane of the configuration of the part holding the test object. A plurality of ultrasonic transducers 402 is disposed in a spiral shape inside a container 401 having a hemispheric shape. The number of the ultrasonic transducers 402 is 512, for example.

The PAT apparatus has a holding member 405 provided to hold the test object. The holding member 405 is made of, for example, polyethylene terephthalate. The material of the holding member 405 is not limited to the polyethylene terephthalate, and may be any kind of material as long as the material allows acoustic waves and light emitted from a light irradiation unit 403 to pass therethrough. In Figs. 4A and 4B, a phantom 406 mounted on the holding member 405 is indicated with a dotted line. As illustrated in Fig. 3, the phantom 406 has a spherical measurement surface, and is in contact with the holding member 405 having a spherical shape. The holding member 405 may be filled with a solution to serve as an acoustic matching layer, such as water.

Further, as with the holding member 405, the container 401 having a hemispheric shape may be filled with a solution to serve as an acoustic matching layer, such as water. Furthermore, to provide a matching layer between the container 401 and the holding member 405 likewise, a sealed space may be provided therebetween, or the container 401 may be brought closer to the holding member 405 so that no gap is formed therebetween at the time of measurement. The container 401 has a space that allows the light from the light irradiation unit 403 to pass therethrough. The test object can be irradiated with the light from a negative direction of z.

The position of the container 401 relative to the phantom 406 serving as the test object can be changed by an XY stage (not illustrated). The test object is irradiated with pulsed light while being scanned by the XY stage, and acoustic waves generated by the ultrasonic transducer 402 are detected. A three-dimensional optical ultrasonic image can be obtained by reconfiguring data obtained by the detection. Further, an ultrasonic image is obtained by an ultrasonic probe 404 of a linear type. The ultrasonic probe 404 of the linear type is allowed to perform scanning by the XY stage.

To generate the photoacoustic waves effectively, the light must be emitted during a quite short time according to the thermal properties of the test object. When the test object is a living body, the pulse width of the pulsed light to be emitted from a light source can be on the order of 10 to 50 nanoseconds. For example, a titanium sapphire laser, i.e., a solid-state laser, is employed, and light having wavelengths of 760 nm and 800 nm are used to measure an oxygen saturation.

The ultrasonic transducer 402 receives a photoacoustic wave generated by the test object and outputs an electrical signal. Here, a CMUT is used. This transducer is a single element and has an opening of 3 mm in diameter, and the frequency band is 0.5 to 5 MHz. Even if a blood vessel is on the order of 3 mm, inclusion of this low frequency makes it possible to prevent appearing of a ring-shaped blood vessel with no central part. Sampling is performed 2,048 times at a sampling frequency of 50 MHz. Data is assumed to be signed 12 bits.

The ultrasonic probe 404 of the linear type transmits and receives ultrasonic waves, so that a morphologic image can be obtained. For such an element, PZT is used. The number of elements is 256, and the frequency band is 5 to 10 MHz. Sampling is performed 2,048 times at a sampling frequency of 50 MHz. Data is assumed to be signed 12 bits.

In the evaluation of the PAT apparatus, the ultrasonic measurement and the photoacoustic measurement of the phantom are sequentially performed. Subsequently, from an image of the photoacoustic measurement target 303, it is determined whether an oxygen saturation indicates the desired performance. Next, from an image of the ultrasonic measurement target 304, it is determined whether the resolution of a contrast indicates the desired performance. Further, when superimposition of a photoacoustic image and an ultrasonic image is performed, an evaluation is made to check whether there is no misregistration between these images, based on the shared target 305. The shared target 305 can also be used to evaluate the resolution of the photoacoustic apparatus and the resolution of the ultrasonic apparatus. As with the first exemplary embodiment, the determination as to whether the PAT apparatus exhibits the desired performance may be performed by an operator of the PAT apparatus, or the PAT apparatus may have a function of performing the determination.

The shared target may be a target having optical properties and acoustic properties different from those of the base material. This is effective for the evaluation of the ultrasonic image and the ultrasonic image. On the other hand, in evaluation of the photoacoustic imaging, a target having acoustic properties similar to those of the base material may be prepared, which produces an effect of allowing evaluation of a photoacoustic image unaffected by acoustic scattering.

As with the first exemplary embodiment, the evaluation of the photoacoustic measurement and the ultrasonic echo measurement can be implemented by one phantom. In particular, when the first target for the photoacoustic measurement is provided at a position closer to the measurement surface than the second target for the ultrasonic measurement, interference by one of the targets with the measurement of the other can be suppressed. Further, a misregistration between the image based on the photoacoustic measurement of the PAT apparatus and the image based on the ultrasonic echoes can be evaluated, by providing the shared target serving as the third target usable for both of the photoacoustic measurement and the ultrasonic echo measurement.

Third Embodiment

A phantom according to a third exemplary embodiment is used to evaluate the photoacoustic apparatus for breast described in the second exemplary embodiment.

Figs. 5A and 5B illustrate the phantom according to the present exemplary embodiment. The phantom according to the present exemplary embodiment has a photoacoustic measurement target 502 and an ultrasonic measurement target 503 in an outer frame 501. The outer frame 501 is a transparent body made of a material such as acrylic, and absorbs no light. Therefore, the outer frame 501 hardly generates photoacoustic waves. The photoacoustic measurement target 502 and the ultrasonic measurement target 503 are each fixed through a hole formed in the outer frame 501. For example, the photoacoustic measurement target 502 has a diameter of 5 mm, and the ultrasonic measurement target 503 has a diameter of 0.5 mm. The photoacoustic measurement target 502 is disposed at a position close to a measurement surface, and the ultrasonic measurement target 503 is disposed at a position away from the measurement surface.

The measurement is performed in a state where an area defined by each of the outer frame 501 and the holding member 405 is provided with a liquid that allows photoacoustic waves and ultrasonic waves to propagate therein. In other words, in the present exemplary embodiment, this liquid serves as a base material. For the liquid, for example, water or a diluted fat emulsion (a liquid containing soybean oil) used for nutritional management of veins can be used. In this case, the holding member 405 serves as the measurement surface. The holding member 405 may hold the liquid, and the outer frame 501 may be submerged in the liquid. In this case as well, the holding member 405 serves as the measurement surface.

Fig. 5B schematically illustrates a target-sensor relationship. By the layout illustrated in Fig. 5B, an acoustic ray 504 of photoacoustic waves can reach the probe on the container 401 having a hemispheric shape, without interference from the ultrasonic measurement target 503. On the other hand, a range of an acoustic ray 505, which extends from the ultrasonic measurement target 503 to the detection surface of the ultrasonic probe 404, includes the photoacoustic measurement target 502. However, if the acoustic properties of the photoacoustic measurement target 502 are about the same as those of the surrounding liquid, the influence of the photoacoustic measurement target 502 can be ignored.

In a case where a plurality of probes for photoacoustic measurement is provided, even if an ultrasonic measurement target is on the acoustic rays of the photoacoustic measurement target 502 and an ultrasonic measurement target 503, an influence on an image may be small, depending on the locations and the number of interfering acoustic rays. For example, starting from the probe close to an optical axis, 70% of acoustic rays may be included.

By disposing the phantom as described above, acoustic waves from the photoacoustic measurement target and the shared target can be detected without interference, even if the ultrasonic measurement target is present.

As with the first exemplary embodiment, the evaluation of the photoacoustic measurement and the ultrasonic echo measurement can be implemented by one phantom, if the phantom according to the present exemplary embodiment is used. In particular, when the first target for the photoacoustic measurement is provided at a position closer to the measurement surface than the second target for the ultrasonic measurement, interference by one of the targets with the measurement of the other can be suppressed.

The exemplary embodiments described above are each provided as an example, and may be combined in a range without deviating from the scope of the principles of the present invention.

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. 2015-037431, filed February 26, 2015, which is hereby incorporated by reference herein in its entirety.

Claims

A phantom comprising:
a base material having a measurement surface; and
a first target and a second target that are provided in the base material,
wherein a difference in acoustic property between the first target and the base material is smaller than a difference in acoustic property between the second target and the base material,
wherein a difference in optical property between the second target and the base material is smaller than a difference in optical property between the first target and the base material, and
wherein the second target is provided at a position further away from the measurement surface than the first target.
The phantom according to claim 1, wherein the first target includes a polyol in which a filler is dispersed.
The phantom according to claim 2, wherein the polyol includes any of a polyether polyol, a polyester polyol, and a polycarbonate polyol.
The phantom according to claim 2 or 3, wherein the filler includes a light scattering filler and a light absorbing filler.
The phantom according to claim 4, wherein the light absorbing filler is a pigment.
The phantom according to any one of claims 1 to 5, wherein the second target is made of urethane resin including an organic powder filler.
The phantom according to claim 6, wherein the organic powder filler includes any of polypropylene, polyethylene, ethylene, vinyl acetate, and vinyl alcohol copolymer.
The phantom according to any one of claims 1 to 7, wherein the first target and the second target each have a cylindrical shape.
The phantom according to claim 8, wherein the first target and the second target are provided in parallel with each other.
The phantom according to any one of claims 1 to 9, further comprising a support member having higher rigidity than rigidity of the base material,
wherein the base material is fixed to the support member via a surface different from the measurement surface.
The phantom according to any one of claims 1 to 10, wherein the measurement surface is covered by a transparent protection member.
The phantom according to any one of claims 1 to 11, wherein the measurement surface is a curved surface.
The phantom according to any one of claims 1 to 12, further comprising another first target, wherein the another first target has different optical property from the first target.
The phantom according to claim 13, wherein the first target and the another first target include two or more pairs of targets having respective optical properties different from each other, and the pair of targets included in each of the first target and the another first target are different from each other in terms of thickness.
The phantom according to claim 12 or 13, wherein the optical properties are light absorption coefficients in respective wavelengths different from each other.
The phantom according to any one of claims 1 to 15, further comprising another second target, wherein the another second target has an acoustic property different from the second target.
The phantom according to any one of claims 1 to 16, further comprising a third target,
wherein a difference in acoustic property between the third target and the base material is greater than a difference in acoustic property between the first target and the base material, and
wherein a difference in optical property between the third target and the base material is greater than a difference in optical property between the second target and the base material.
The phantom according to claim 17, wherein the third target includes at least two targets each having a first thickness, and a target having a second thickness larger than the first thickness.
The phantom according to claim 18, wherein the at least two targets each having the first thickness are provided closer to each other, than to the target having the second thickness.
The phantom according to any one of claims 1 to 19, wherein the first target, the second target, and the third target are provided at different depths in the base material with respect to the measurement surface.
The phantom according to claim 17, wherein the first target, the second target, and the third target include targets having thicknesses different from each other.
The phantom according to claim 21, wherein among the targets having the thicknesses different from each other, the target having a smaller thickness is provided closer to the measurement surface.
A phantom comprising:
a first target; and
a second target,
wherein, of the first target and the second target, one target has a higher light absorbance than the other target, and
wherein the other target has higher acoustic reflectance than the one target.