Patent Application
20170156708
Field of the Invention
The present disclosure relates to a technique regarding a phantom and a packaged phantom for evaluating a photoacoustic measurement apparatus.
Description of the Related Art
In recent years, as one of optical imaging techniques, photoacoustic tomography (PAT) is discussed in Review of Scientific Instruments vol. 77 041101 2006. PAT is a technique for emitting pulse light to a specimen, such as living tissue, detecting photoacoustic waves generated by the specimen absorbing the energy of light propagated and diffused in the specimen, processing signals of the photoacoustic waves, and visualizing information related to optical characteristic values of the inside of the specimen.
Generally, to evaluate or calibrate the performance of an apparatus, a phantom simulating a specimen is used. In the phantom, a target of which the shape is known in advance is placed, and the performance of the apparatus is evaluated based on how the target is imaged. Such a phantom is discussed in the publication of Japanese Patent Application Laid-Open No. 2011-209691. In the publication of Japanese Patent Application Laid-Open No. 2011-209691, a phantom is made using a material having an optical characteristic and an acoustic characteristic close to those of human tissue. In a case where such a phantom is used to evaluate a photoacoustic tomography apparatus (a PAT apparatus) for measuring human tissue, it is possible to reduce the reflection of an acoustic wave at the interface between the apparatus and the phantom. Thus, erroneous evaluation can be reduced, which is preferable.
In the performance evaluation of a photoacoustic measurement apparatus using a phantom, however, further improvement is needed regarding an increase in the accuracy of the evaluation of the apparatus.
According to an aspect of the present disclosure, a phantom capable of being used to evaluate a photoacoustic measurement apparatus includes a recording unit having stored therein information regarding a temporal change in an optical characteristic or an acoustic characteristic of the phantom.
According to another aspect of the present disclosure, a packaged phantom includes a phantom capable of being used to evaluate a photoacoustic measurement apparatus, and a recording unit having stored therein information regarding a temporal change in an optical characteristic or an acoustic characteristic of the phantom.
Further features will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
Suitable exemplary embodiments of the present disclosure will be described below with reference to the drawings.
As illustrated in
In a case where an apparatus is evaluated using a phantom, the performance of the apparatus is evaluated using a signal from a target 103 (described in detail below) of the phantom 108 and a measured image created based on the signal. However, in the characteristic of a phantom used for a photoacoustic measurement apparatus, a change (mainly deterioration) occurs due to hardening or alteration (discoloration) of the phantom by light emitted to the phantom, hydrolysis due to soaking the phantom in water, or the interaction of materials included in the phantom. Thus, to accurately evaluate the apparatus, it is necessary to take the deterioration of the phantom into account. Specifically, in a case the phantom which has been deteriorated is measured by the apparatus, the apparatus may be erroneously evaluated as requiring adjustment although the apparatus correctly measures information of the deteriorated phantom. That is, although the measurement result (the measured image) changes due to a change in the state of the phantom, the apparatus may be erroneously evaluated as the measurement performance of the apparatus has changed. Particularly, since an oxygen saturation measured by the photoacoustic measurement apparatus requires quantitative evaluation, if the apparatus is erroneously adjusted as described above, the quantitativity of the result of the subsequent specimen measurement is impaired. This leads to a major problem.
Thus, a recording unit having stored therein information regarding a temporal change in the optical characteristic or the acoustic characteristic of the phantom is provided in the main body of the phantom, or a package is formed of a set including the recording unit and the phantom, whereby it is possible to evaluate the photoacoustic measurement apparatus based on the information stored in the recording unit. As a result, it is possible to distinguish between a change in the measurement result according to a temporal change in the phantom and a change in the measurement result according to a change in the measurement performance of the photoacoustic measurement apparatus. This increases the accuracy of the performance evaluation of the photoacoustic measurement apparatus.
An example embodiment of the phantom or the packaged phantom is described below.
As illustrated in
The phantom and the packaged phantom is not limited to the form in which the phantom and the packaged phantom include a base material and a target as described above. Alternatively, as will be described below with reference to
Examples of temporal deterioration information of the thus configured phantom include temporal deterioration information of an optical characteristic and temporal deterioration information of an acoustic characteristic, specifically, temporal deterioration information of the light attenuation characteristic of the base material 102 and temporal deterioration information of the light absorption characteristic of the target 103. These pieces of information have particularly great influence on a change in a measured image of the phantom and therefore are information particularly useful as temporal deterioration information. The acoustic characteristic is the speed of sound in the phantom or the sound pressure of an acoustic wave generated by the phantom.
As a recording unit for storing these pieces of information, an IC tag, a DVD, a USB medium, or the like can be used. The IC tag can be included in the main body of the phantom. The medium, such as the DVD and the USB medium, can be provided as an item included in a packaged formed of a set including the medium and the main body of the phantom.
Further, in an exemplary embodiment, an environment recording unit for recording information regarding the environment around the phantom may be further included. This is because the degree of progress of the deterioration of the phantom is greatly influenced by the environment state around the phantom. Thus, such information is recorded, whereby it is possible to understand the temporal change state of the phantom more accurately. Specifically, based on the information regarding the environment around the phantom recorded in the environment recording unit, the information regarding the temporal change stored in the recording unit is corrected, whereby it is possible to obtain more accurate temporal change information. For example, the recording unit for recording the temporal change in the phantom may have stored therein conversion information for correcting the information regarding the temporal change in the phantom based on the information regarding the environment around the phantom. Thus, it is possible to correct the temporal change information more easily.
In the phantom, as described above, deterioration, such as alteration, occurs due to the influence of heat or moisture. Thus, the information regarding the environment around the phantom may be temperature information and humidity information of the environment around the phantom.
Example embodiments will be described below.
An example phantom is described below with reference to the drawings.
In a phantom 108 according to a first exemplary embodiment, a target 103, which has a columnar shape having a diameter of 1 mm, is placed in a base material 102 at a depth of 20 mm from a measurement surface 105. The measurement surface 105 is a surface with which a probe comes into contact. The other surfaces are covered with an outer frame 101 to maintain the shape of the phantom 108. Further, to the phantom 108, a recorder 104 is attached, which is an environment recording unit for monitoring a change in the environment around the phantom 108.
The target 103 is used to evaluate the initial sound pressure distribution of a photoacoustic apparatus. The target 103 has an absorption coefficient of 0.2 mm−1, which is equivalent to blood. Meanwhile, the base material 102 of the phantom 108 has as optical constants an absorption coefficient of 0.005 mm−1 and an equivalent scattering coefficient of 1 mm−1, which are in the range of values of a living body. Further, in the base material 102 of the phantom 108, the speed of sound is 1450 m/s and an attenuation rate is 0.5 dB/cm MHz. The target 103 and the base material 102 are made of a polyol and fillers capable of being dispersed in the polyol. In the present exemplary embodiment, an isocyanate compound is added as a curing agent to a polyether polyol. As a filler having a light scattering property, titanium oxide is used. As a filler having a light absorption property, a carbon black pigment is used. Then, the compounding ratio between the fillers is adjusted to obtain the above characteristics.
The type of the target 103 is not limited to a target for evaluating initial sound pressure distribution. Alternatively, the target 103 may be intended to evaluate an oxygen saturation, evaluate a viewing angle, evaluate a resolution, or evaluate an optical resolution. In a case where an oxygen saturation is evaluated, a phantom including targets having oxygen saturations of 75% and 95% and each having a diameter of 1 mm can be used. These oxygen saturations can be obtained by preparing a pigment so that the ratio between the absorption coefficients of the targets is equivalent to the ratio between the oxygen saturations of the targets at wavelengths of 760 nm and 800 nm. To evaluate a viewing angle, a phantom including a ring-shaped target having an inner diameter of 10 mm and an outer diameter of 11 mm, for example, and having a light absorption coefficient can be used. Further, to evaluate a resolution, a nylon wire having a diameter of 0.1 mm and having an absorption coefficient may be used. To evaluate an optical resolution, two nylon wires having diameters of 0.3 and 0.5 mm may be placed parallel with each other with a space therebetween such that the space has a width equal to the thickness of each wire.
Further, the shape of the target 103 is not limited to a columnar shape as illustrated in
If a phantom is appropriately manufactured, it is possible to produce phantoms having equivalent parameters with excellent reproducibility of design values (reproduce the same states (characteristics) in all the phantoms). In this case, data obtained by measuring one of the phantoms can be applied as it is as data of all the other phantoms. Thus, data of a temporal change to be attached to a phantom can be obtained in advance from an acceleration test for a phantom. As a method for the acceleration test, a phantom is placed in a thermostatic bath having constant temperature and humidity, and an image of the phantom is evaluated every 24 hours, for example. The thermostatic bath may be in an environment where, for example, the temperature is 40 to 70° C., and the humidity is 50% (an environment where deterioration is more likely to occur than in a normal use environment).
The reference environment has a temperature of 25° C. and a humidity of 40%, for example. The data can be converted into changes in shorter elapsed times by interpolating the data. As a method for the interpolation, the data may be represented by a curve, using linear interpolation or polynomial regression.
Further, the information (data) of the temporal change based on the above acceleration test to be attached to the phantom is recorded in a medium 107, such as a DVD or a USB medium. As a matter of course, the information (data) is recorded not only in this format. Alternatively, the data may be printed out. In the present application, a set including a phantom and a member in which information regarding the phantom is thus recorded (written) is referred to as a “packaged phantom” or a “phantom set.” The present invention is not limited to the form of a package (the form of a set). Alternatively, the data may be stored in an IC tag 106, which is provided in the phantom. That is, a unit for recording data may be provided in the main body of the phantom itself.
The data of the phantom may be data with respect to each material. In this case, each of the base material and the target is subjected to an acceleration test, and data of the acceleration test is attached to the phantom. In this case, the value of each member according to an elapsed time and apparatus parameters are further input, and simulation is performed, whereby it is possible to obtain an image in a case where the apparatus is evaluated based on the apparatus parameters at a certain time.
An example case where an apparatus is evaluated using an image of the initial sound pressure distribution as illustrated in
As a method for displaying a screen for evaluation, for example, a reference image and an actual measured image may be arranged and displayed next to each other. Alternatively, a cursor line may be provided on an acquired image according to an evaluation item, and the contrast of the acquired image and the contrast of a reference image on the line may be displayed as line profiles (the form in the lower parts of
The deterioration of the phantom depends not only on the elapsed time but also largely on the use environment. Thus, the elapsed time may be calculated by, according to the state of use of the phantom, converting an actual elapsed time into an elapsed time in a case where the phantom is used in the reference environment. In this case, the recorder 104, which is the environment recording unit, measures the temperature of the use environment (the environment around the phantom). The temperature is recorded every 30 minutes, for example. As a matter of course, not only the temperature but also the humidity may be measured. Further, the recorded data may be able to be transferred wirelessly to the apparatus. Based on the information of the environment around the phantom, the temporal change information is corrected using, for example, the conversion information stored in the IC tag 106, which is the recording unit for recording the temporal change information of the phantom. An example of the correction of the temporal change information using the conversion information is described below.
In a case where the elapsed time of the phantom in the actual use environment is converted into an elapsed time in a case where the phantom is used in the reference environment, for example, the Arrhenius model can be used. The Arrhenius model is represented by the following formula 1 using a natural logarithm In:
where τ is a life, A is a coefficient, Ed is the activation energy of a target a, R is a gas constant, and T is an absolute temperature.
The life is measured by changing the temperature, whereby it is possible to determine A and Ea. Here, T may not be the life, and may be the time in which a certain change occurs. That is, τ may be the time in which an absorption coefficient becomes smaller than a desired value. Thus, it is possible to obtain a coefficient in this case. As a result, if a time τ2 elapses at a temperature T2, then in the case of a temperature T1 (e.g., 25° C.), the elapsed time of the phantom can be converted into an elapsed time as the following formula 2:
Consequently, based on the environment such as the temperature measured by the recorder 104, it is possible to convert the elapsed time of the phantom into an elapsed time in the reference environment.
Generally, in a case where an apparatus is used in a hospital, the apparatus is often managed for 24 hours in a constant temperature and humidity environment. Even in such a case, the phantom may be stored in a case designed for the phantom after using the phantom. The case designed for the phantom is covered with a material, such as duralumin, whereby it is possible to shield the phantom from light. The case for storing the phantom includes a mechanism for fixing the phantom. A buffer material is further placed in the case, whereby it is possible to protect the phantom from the impact of a fall. Further, a desiccant is placed in the case, whereby it is possible to maintain a constant humidity inside the case and obtain a storage state in the reference environment. Thus, the accuracy of the estimation of temporal deterioration is increased. As a matter of course, in a case where the use environment or the storage environment is different from the reference environment, the elapsed time may be converted into an elapsed time in the reference environment as described above.
In the above manner, even in a case where the state of a phantom changes, it is possible to accurately evaluate an apparatus.
Next, a description is given of a series of operations for calibrating a photoacoustic measurement apparatus for a breast using the phantom, together with the configuration of the photoacoustic measurement apparatus for a breast. A description is given below of, as the measurement of a breast by the photoacoustic measurement apparatus, the operation of calibrating the apparatus based on the measurement of an oxygen saturation using the phantom for the purpose of increasing the accuracy of the measurement of an oxygen saturation.
Between the hemispherical container 401 and the holding member 404, an acoustic matching layer, such as water, is interposed. Further, although the phantom 406 is placed in the holding member 404, a matching layer, such as water, is also placed as needed in the holding member 404 to prevent air from entering the path of an acoustic wave. The holding member 404 is formed of polyethylene terephthalate. Further, in the hemispherical container 401, a space is provided through which measurement light from a light emission unit 403 passes. Then, it is possible to emit measurement light to the specimen or the phantom 406 from a negative z-axis direction. Further, it is possible to change the position of the hemispherical container 401 using an XY stage (not illustrated). The light emission unit 403 emits pulse light to the specimen or the phantom 406 while scanning the XY stage, and the reception elements 402 detects a generated acoustic wave. Data of the detection result is reconfigured, whereby it is possible to obtain a three-dimensional photoacoustic image.
The light emission unit 403 generates pulse light to be emitted to the specimen or the phantom 406 and emits the generated pulse light. A light source for generating pulse light may be a laser light source to obtain large output. The present invention, however, is not limited to this. Alternatively, a light-emitting diode or a flash lamp may be used instead of the laser. In a case where the laser is used, various lasers, such as a solid-state laser, a gas laser, a pigment laser, and a semiconductor laser can be used.
To effectively generate a photoacoustic wave, it is necessary to emit light in a sufficiently short time according to the heat characteristic of the specimen or the phantom 406. In a case where the specimen is a living body, a suitable pulse width of pulse light generated by the light source is about 10 to 50 nanoseconds. Further, the wavelength of the pulse light may be a wavelength at which light is propagated into the specimen. Specifically, in the case of a living body, the wavelength of the pulse light is between 700 nm or more and 1100 nm or less. In the present exemplary embodiment, a titanium-sapphire laser, which is a solid-state laser, is used, and wavelengths of 760 and 800 nm are used to measure oxygen saturations.
The reception elements 402 receive a photoacoustic wave. In the present exemplary embodiment, capacitive micromachined ultrasonic transducers (CMUTs) are used. Each of the reception elements 402 has an opening having a diameter of 3 mm and has a band of 0.5 to 5 MHz as a reception characteristic. Sampling is performed 2048 times at a sampling frequency of 50 MHz, and each sample is obtained in 12-bits.
In the targets 503 and 504, a pigment is adjusted so that the ratio between the absorption coefficients at two wavelengths of 760 nm and 800 nm is the same as the ratio between the oxygen saturations of 75% and 95%.
Further, in the phantom 501, an image of initial sound pressure distribution and images of oxygen saturations that change with the lapse of time are prepared in advance. These images are obtained by performing an acceleration test on a phantom having the same configuration as that of the phantom 501. As described above, the images may be recorded in advance in a medium provided together with the phantom 501, or this information may be stored in an IC tag provided in the phantom 501.
A photoacoustic measurement apparatus is evaluated by comparing an actual measured image of a phantom acquired by the photoacoustic measurement apparatus, with an image (e.g. obtained through simulation) of the phantom according to the elapsed time prepared in advance. First, based on the state of use of the phantom (the time elapsed since the purchase of the phantom), images corresponding to an elapsed time in the reference environment (e.g., images corresponding to the phantom after a year of use) are selected as reference images. In this case, an image of an oxygen saturation and an image of initial sound pressure distribution at each wavelength are selected together. Then, an actual measured image of the initial sound pressure distribution at each wavelength and an actual measured image of the oxygen saturation are compared with the respective reference images selected first. If the proportion of the difference between the contrasts or the luminance values of compared images is within ±1%, it is possible to determine that there is no abnormality in the photoacoustic measurement apparatus. If, on the other hand, for example, the initial sound pressure distribution at each wavelength changes in the same proportion, it is determined that there may be abnormality in the transmittance of the optical system.
In a case where selected reference images (images obtained by simulation) are compared with measured images, the comparison can be made for the ratio between the absorption coefficients and the oxygen saturations of a plurality of targets (the ratio between the absorption coefficients and the oxygen saturations of the targets 503 and 504). The use of the ratio between a plurality of targets as described above has the advantage that it is not necessary to measure all the parameters of the targets. That is, if, as in the targets 503 and 504 provided in a phantom for calibrating a photoacoustic measurement apparatus using the measurement results of oxygen saturations, only the absorption coefficients at respective wavelengths are different from each other, and other members are similar in the phantom, the difference in influence of deterioration between other elements is common and therefore can be ignored. Thus, it is not necessary to obtain (compare) the absolute values of elements with high accuracy. Thus, it is possible to evaluate the apparatus by simple comparison.
As described above, even in a case where the state of a phantom changes, it is possible to accurately evaluate an apparatus.
Similarly to the first exemplary embodiment, a phantom according to a second exemplary embodiment is used to evaluate a photoacoustic measurement apparatus for a breast. The second exemplary embodiment, however, is different from the first exemplary embodiment in that the phantom includes an outer frame and a target (does not include a base material). In the present exemplary embodiment, since a base material is not included, it is possible to estimate the influence of a temporal change more easily and more accurately.
In measurement, a liquid is put into the holding member 404, and the photoacoustic measurement target 702 is located in the liquid, whereby the target 702 is measured in the state where a gap (an acoustic impedance mismatch surface) is not formed on the propagation path of a photoacoustic wave from the photoacoustic measurement target 702 to the reception elements 402. As the liquid, water or a dilution of a fat emulsion (a liquid containing soybean oil) for use in the nutritional management of veins can be used.
In this case, only the photoacoustic measurement target 702 needs to be subjected to an acceleration test to obtain a desired image (temporal change information). Thus, it is possible to estimate a temporal change in the phantom more accurately. As a result, it is possible to evaluate a photoacoustic measurement apparatus more easily and more accurately.
In the above manner, even in a case where the state of a phantom changes, it is possible to accurately evaluate an apparatus.
In the above exemplary embodiment, a description has been given of a technique for calibrating an apparatus using temporal change information attached to a phantom. Alternatively, the apparatus may be calibrated using not only the information attached to the phantom, but also temporal change information obtained (acquired) via a network. That is, it is also possible to accurately calibrate the apparatus by the following method. Temporal change information is acquired, regardless of the method for obtaining the temporal change information, and a reference image calculated based on the acquired temporal change information is compared with an image obtained by actually measuring the phantom, thereby determining a change in the apparatus and calibrating the apparatus.
According to the present disclosure, it is possible to evaluate a photoacoustic measurement apparatus with high accuracy.
While the present disclosure 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. 2015-236744, filed Dec. 3, 2015, which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2015-236744 | Dec 2015 | JP | national |
