JP6218908B2 - Method - Google Patents

Description

The present invention relates to a square method.

  As an apparatus for acquiring information inside an object to be inspected (subject) using ultrasonic waves (acoustic waves), for example, a photoacoustic apparatus used for medical diagnosis has been proposed. The photoacoustic apparatus irradiates a subject with laser pulse light, and receives photoacoustic waves generated as a result of the tissue in the subject absorbing the energy of the irradiation light at a plurality of locations surrounding the subject. Subsequently, the time change of the received acoustic wave is mathematically analyzed, that is, an image is reconstructed. Then, information related to the optical characteristic value inside the subject can be visualized in two dimensions or three dimensions. Such a technique is also called photoacoustic tomography (PAT).

  The intensity of the acoustic wave generated from the tissue in the subject varies depending on the intensity of the irradiation light reaching the tissue and the absorption coefficient with respect to the wavelength of the irradiation light. In the subject, there are tissues that absorb a lot of light such as melanin, fat, moisture, hemoglobin, cholesterol, collagen, and the like, and each has a different absorption coefficient with respect to the wavelength. Therefore, even for the same tissue of the same subject, different absorption coefficient distribution images can be obtained by measuring at different wavelengths and reconstructing the image.

  A method for obtaining the blood oxygen saturation level of a subject using an absorption coefficient distribution image by this wavelength is known. Since the absorption spectra of two types of hemoglobin (oxyhemoglobin and deoxyhemoglobin) existing in the blood vessel in the subject are different, different absorption coefficient distribution images are obtained when measured at different wavelengths. From these absorption coefficient distribution images, the distribution ratio (oxygen saturation in blood) of oxyhemoglobin and deoxyhemoglobin can be calculated. For example, in Non-Patent Document 1, oxygen saturation in a blood vessel is acquired from a reconstructed image using four wavelengths (578-596 nm).

FUNCTIONAL PHOTOACOUSTIC MICROSCOPY, HAO ZHANG, Texas A & M University, August 2006

However, the four wavelengths (578nm, 584nm, 590nm, 596nm) used in Non-Patent Document 1
) Is the measurement target hemoglobin, epidermis or dermis as shown in FIG.
) In the wavelength region where the absorption coefficient μ _a is high. For this reason, there has been a problem that the irradiation light can only reach a few millimeters from the skin surface, and the deep blood vessels in the subject cannot be measured.

  In addition, since the calculation formula for the oxygen saturation includes the calculation of the results obtained from different absorption coefficient distribution images, the noise in the absorption distribution image and the displacement of the absorber due to the body movement of the subject are the oxygen saturation. Significantly affects the resulting noise and error.

As a method for making it difficult to see the oxygen saturation noise due to the noise in the absorption coefficient distribution image, it is conceivable that the oxygen saturation result is trimmed and displayed at the blood vessel position. Here, the blood vessel position can be considered as the place where the absorption coefficient distribution of hemoglobin exists. However, as can be seen from the absorption coefficient spectra of the two types of hemoglobin shown in FIG. 7, the absorption coefficient of the arteries is low and the absorption coefficient of the veins is high at a wavelength where both absorption coefficients are different (for example, 756 nm).

If a position having an absorption coefficient greater than a certain value in this absorption coefficient distribution (more than 30% when the vein strength is 100%) is a blood vessel, even if the blood vessel has the same thickness, it is shown in FIG. Since different intensity values are calculated as shown, it looks like blood vessels of different thickness. Therefore, when the oxygen saturation is trimmed with this absorption coefficient distribution, as shown in FIGS. 5B and 5C, even in the blood vessel having the same thickness, in the oxygen saturation distribution image, There is a problem that blood vessels of different thickness are displayed (P is a vein and A is an artery).

  On the other hand, when the subject moves, the position of the absorption coefficient distribution changes as shown in FIG. Since the oxygen saturation is calculated by the absorption coefficient at the same position, there has been a problem that a change in the absorption coefficient distribution due to body movement affects the calculation result of the oxygen saturation. In particular, when the body movement is larger than the thickness of the blood vessel, the calculation result of the oxygen saturation does not make sense.

  The present invention has been made in view of the above problems, and can generate image data that makes it easy to understand the relationship between the biological characteristic distribution of a subject and the position of a blood vessel, and can reduce the influence of measurement position deviation. The object is to provide a possible photoacoustic apparatus.

The present invention employs the following configuration. That is, light having a first wavelength of 700 nm or more with the same or substantially the same absorption coefficient of oxyhemoglobin and deoxyhemoglobin is irradiated on the subject.
Using the first reconstructed image data obtained based on the acoustic wave generated by being emitted, a specifying step for specifying a blood vessel position, and light having a plurality of wavelengths are irradiated to the subject A display step of highlighting on the display means the object characteristic distribution information corresponding to the blood vessel position obtained based on the generated acoustic wave .

  According to the present invention, photoacoustics capable of measuring the biological characteristic distribution in the deep part of the subject, generating image data that can easily understand the relationship with the position of the blood vessel, and reducing the influence of the displacement of the measurement position. An apparatus can be provided.

The block diagram of the photoacoustic apparatus of this invention. FIG. 3 is a flowchart showing processing of the first embodiment. FIG. 6 is a flowchart showing processing of the second embodiment. FIG. 9 is a flowchart showing processing of Example 3; The figure which shows the measurement result by Example 1. FIG. The figure which shows the absorption spectrum of the substance which comprises a test object. The figure which shows the absorption spectrum of oxyhemoglobin and deoxyhemoglobin. The figure which shows the difference in the appearance of the artery and the vein by wavelength. The figure which shows the shift | offset | difference of the absorption coefficient distribution by a body motion. The figure which shows the shift | offset | difference at the time of the synthesis | combination of the reconstruction image by a body movement. FIG. 10 is a diagram illustrating composition of a reconstructed image according to a fourth embodiment.

  Hereinafter, embodiments of the photoacoustic apparatus of the present invention will be described with reference to the drawings. In the following description, a photoacoustic diagnostic apparatus that images the inside of a subject using the photoacoustic tomography technique will be described as an example.

  The photoacoustic apparatus of the present invention is an apparatus using a photoacoustic effect that receives acoustic waves generated in a subject by irradiating the subject with light (electromagnetic waves) and acquires subject information as image data. is there. The acoustic wave is typically an ultrasonic wave, and includes an elastic wave called a sound wave, an ultrasonic wave, an acoustic wave, a photoacoustic wave, and an optical ultrasonic wave.

FIG. 1 is a block diagram of the photoacoustic diagnostic apparatus according to the present embodiment.
The photoacoustic diagnostic apparatus includes a light source 1, an ultrasonic probe 3, an image reconstruction unit 4, an image memory 5, a wavelength selection unit 6, an absorber specifying condition memory 7, a blood vessel position specifying unit 8, a biological characteristic distribution calculating unit 9, an image A trimming unit 10, a display unit 11, and an information processing device 12 are provided. The operation and function of each block will be described in detail later. The subject 2 is a measurement object such as a living body.

(Operation of the photoacoustic diagnostic apparatus)
FIG. 2 is a flowchart of the first embodiment.

In step S201, the photoacoustic diagnostic apparatus irradiates the subject 2 as a target with pulsed light from the light source 1. This pulsed light has a wavelength range of 700 nm or more, and has a wavelength A in which the absorption coefficients of two types of hemoglobin (oxyhemoglobin and deoxyhemoglobin) are equal. This pulsed light is absorbed by the light absorber in the subject 2, and a photoacoustic wave that is an ultrasonic wave (acoustic wave) is generated. In the present invention, “the absorption coefficient is equal” includes not only the case where the absorption coefficient is completely equal, but also the case where the absorption coefficient is substantially equal. If the absorption coefficients of the two types of hemoglobin are within 10%, the two absorption coefficients are assumed to be substantially equal.

In step S202, the generated photoacoustic wave is acquired by a plurality of elements included in the ultrasonic probe 3, and converted into an electrical signal (element signal). Thereafter, the electrical signal is subjected to signal processing such as amplification and digital conversion as necessary.
In step S203, the amplified and digitally converted electrical signal is subjected to image reconstruction processing by the image reconstruction unit 4, and an absorption coefficient distribution image A is created as a three-dimensional image indicating the distribution of absorption coefficients. The absorption coefficient distribution image A is temporarily stored in the image memory 5.

In steps S204, S205, and S206, the subject is irradiated with pulsed light having a wavelength B different from the wavelength A, and the image reconstruction unit 4 generates an absorption coefficient distribution image B from an electrical signal based on the generated photoacoustic wave. Created.
In step S207, the biological characteristic distribution calculation unit 9 generates a biological characteristic distribution using the absorption coefficient distribution image A corresponding to the wavelength A and the absorption coefficient distribution image B corresponding to the wavelength B stored in the image memory 5. . The biological characteristic distribution is an oxygen saturation distribution or a glucose distribution.

In step S208, the blood vessel position specifying unit 8 determines the position of the blood vessel included in the absorption coefficient distribution image A obtained at the wavelength A where the absorption coefficients of the two types of hemoglobin are substantially equal. The absorber specifying condition memory 7 stores conditions necessary for specifying the blood vessel position of the image and can be rewritten.

In step S209, the image trimming unit 10 creates an image in which the biological characteristic distribution such as the oxygen saturation distribution is emphasized at the blood vessel position diagnosed by the blood vessel position specifying unit 8.
In step S210, the absorption coefficient distribution image and the oxygen saturation image created by the image trimming unit 10 are displayed by the display unit 11. Further, the enhanced oxygen saturation image and the absorption coefficient distribution image A may be superimposed and displayed.

Here, the order of the measurement at the wavelength A indicated by S201 to S203 and the measurement at the wavelength B indicated by S204 to S206 may be reversed. Further, S204 and S205 may be performed after S201 and S202, and then steps S203 and S206 for reconfiguration may be performed. Further, the order of the biological characteristic calculation S207 and the blood vessel position specification S208 may be reversed.

  Hereinafter, the operation and function of each block will be described in detail with reference to FIG. 1 again.

(light source)
The light source 1 only needs to generate nanosecond order pulsed light (PLS) having a wavelength of 700 nm or more. With a light source having a wavelength of 700 nm or less, it is desirable to use a wavelength of 700 nm or more because light cannot sufficiently reach the deep part of the subject because of the large absorption of hemoglobin, collagen, and the like as shown in FIG. A laser is preferable for obtaining a large output, but a light emitting diode or the like can be used instead of the laser. As the laser, various lasers such as a solid laser, a gas laser, a dye laser, and a semiconductor laser can be used. The timing, waveform, intensity, etc. of irradiation are controlled by a light source control unit (not shown).

In the present invention, light having a plurality of wavelengths is used. In order to generate light having a plurality of wavelengths, for example, one wavelength variable light source may be used, or a plurality of light sources corresponding to each wavelength may be prepared. A plurality of wavelengths of light can be individually irradiated at different timings.
Also, in order to guide light from the light source to the subject, a mirror that reflects the light, a lens that condenses or enlarges the light, changes its shape, a prism that disperses, refracts, or reflects light, and light that propagates the light. Optical members such as fibers and diffusion plates may be used.
The light may be irradiated to the subject from the same side as the ultrasonic probe or from the opposite side. Further, irradiation may be performed from both sides of the test pair.

(Wavelength selector)
The wavelength selection unit 6 selects the wavelength of the pulsed light emitted from the light source 1, and passes selection wavelength information (SEL) to the image reconstruction unit.

(Ultrasonic probe)
The ultrasonic probe 3 is an acoustic detector having one or more elements that receive acoustic waves (ultrasonic waves). If a plurality of elements are arranged in the plane of the ultrasonic probe, signals at a plurality of positions can be acquired at a time. As a result, the reception time can be shortened and the influence of vibration of the subject can be reduced.
This ultrasonic probe receives and amplifies an ultrasonic wave (USW) that is an acoustic wave, converts it into an electric signal, and outputs it. Examples of the element used for the ultrasonic probe include a conversion element using a piezoelectric phenomenon, a conversion element using optical resonance, and a conversion element using a change in capacitance. Any device that can receive an acoustic wave and convert it into an electrical signal may be used.

(Image reconstruction unit)
An electrical signal is input from the ultrasonic probe 3 to the image reconstruction unit 4. Image reconstruction is performed using the input electrical signal, and absorption coefficient distribution image information (ABS), which is image data representing the distribution of the absorption coefficient in the subject, is generated. At that time, processing can be performed for each wavelength in accordance with the selected wavelength information (SEL) received from the wavelength selection unit 6. It is preferable to perform processing such as amplification and digital conversion before or after inputting the electric signal to the image reconstruction unit 4. The image reconstruction unit corresponds to the absorption coefficient distribution generation unit of the present invention.

  As an image reconstruction algorithm for generating this image data, for example, back projection in the time domain or Fourier domain normally used in tomography technology can be used. In addition, when much time is required for reconstruction, an image reconstruction technique such as an iterative method using an iterative process can also be used. Typical image reconstruction methods for PAT include a Fourier transform method, a universal back projection method, and a filtered back projection method.

(Image memory)
The image memory 5 is a place where the absorption coefficient distribution image information (ABS) generated by the image reconstruction unit 4 is recorded. The recorded information is output to the blood vessel position specifying unit 8, the biological characteristic distribution calculating unit 9, and the display unit 11 as necessary.

(Biological characteristic distribution calculation unit)
The biological characteristic distribution calculation unit 9 is a biological characteristic distribution generation unit that generates biological characteristic distribution information (DST). Examples of biological characteristic distribution information (DST) include oxygen saturation distribution, glucose distribution, and collagen distribution.
The calculation method of biological characteristic distribution information will be described using an example of oxygen saturation. Equation (1) shows an equation for obtaining the blood vessel absorption coefficient μ _a (λ _i ) measured at a certain wavelength λ _i . The absorption coefficient μ _a (λ _i) is the product of _HbO2 absorption coefficient of oxyhemoglobin epsilon and (lambda _i) and the product of the concentration [HbO _2], the absorption coefficient of deoxy-hemoglobin ε _HbR (λ _i) and the concentration [HbR] It is represented by the sum of

From this, the concentration [HbO ₂ ] of oxyhemoglobin and the concentration [HbR] of deoxyhemoglobin at a certain position (x, y, z) are obtained as shown in Equation (2).

μ_a（λ_i,x,y,z）は波長λiに対する(x,y,z)位置の吸収係数である。 Here, E and M _a (x, y, z) are expressed by the following equation (3).

μ _a (λ _i , x, y, z) is an absorption coefficient at the (x, y, z) position with respect to the wavelength λi.

Therefore, the oxygen saturation SO _{2 at} a certain position (x, y, z) is calculated as shown in Equation (4).

According to the formula (2), [HbO ₂ ] and [HbR] can be obtained from the measurement results of the two wavelengths.
Using measurement results with three or more wavelengths, [HbO ₂ ] and [HbR] are equivalent to those obtained by the least squares method.
The most probable value is obtained.
Therefore, when measurement is performed using three or more wavelengths, first, a plurality of combinations of wavelengths excluding at least one result of the measured wavelengths is prepared. And after calculating several hemoglobin density | concentration using Formula (2) for every group of those wavelengths, the method of calculating oxygen saturation using the least squares method or an average can be considered.

この場合は１波長に対する吸収係数によってグルコースの濃度分布が求められる。 In addition to oxygen saturation, it is also possible to know blood glucose distribution as biological characteristic distribution information (DST). The glucose distribution [Glc] is calculated by the following equation (5).

In this case, the glucose concentration distribution is determined by the absorption coefficient for one wavelength.

Oxygen saturation and glucose distribution calculation results are obtained from the absorption coefficient distribution μ _a (λ _i , x, y, z at each wavelength λ _i.
) Greatly depends on the position accuracy. Therefore, when the subject to be measured moves during the measurement, a large error may occur in the oxygen saturation result.
Therefore, _a method of minimizing the oxygen saturation error can be used by intentionally blurring the absorption coefficient distribution μ _a (λ _i , x, y, z). As a method of blurring the absorption coefficient, there are a method of taking an average of surrounding absorption coefficients and a method of using a filter.

(Vessel position specifying part)
The blood vessel position specifying unit 8 determines and determines the position of the blood vessel from the absorption coefficient distribution image information created by the image reconstruction unit 4. Conditions for determination are stored in the absorber specifying condition memory 7. The blood vessel position specifying unit 8 accesses the absorber specifying condition memory 7 as necessary to read out the conditions. As a determination method, there is a method in which a predetermined threshold is set in advance and a position where the absorption coefficient is equal to or larger than the predetermined threshold is set as the blood vessel position. In addition, the arrangement and shape of positions with strong absorption coefficients, and the shape of artifacts that appear around the absorber are correlated with the blood vessel conditions stored in the absorber specifying condition memory 7, and the position with high correlation is set as the blood vessel position. There is a way.

The absorption coefficient strength and the correlation threshold for determining the blood vessel position may be stored in the absorber specifying condition memory 7, or the noise level may be evaluated and automatically set to an appropriate value. For example, a histogram of the absorption coefficient in the image is created and the concentration value of the absorption coefficient at a position lower than the absorption coefficient of the blood vessel is used as the noise level, or the range considered as noise is specified and the absorption coefficient within that range is specified. There is a method of using the average as the noise level. Further, a knob for adjusting the predetermined threshold value may be provided on the display unit, and the surgeon may adjust the predetermined threshold value while referring to the display screen. This blood vessel position determination information (POS) is sent to the absorber specifying condition memory 7. If a rewritable memory is used as the absorber specifying condition memory 7, the changed condition can be recorded.

(Image trimming part)
The image trimming unit 10 receives the oxygen saturation information from the biological characteristic distribution calculation unit 9 and the absorption coefficient information distribution image from which the blood vessel position has been determined from the blood vessel position specifying unit 8. An oxygen saturation image (IMG) is created so as to emphasize the oxygen saturation information of the blood vessel position in the absorption coefficient information image, and is output to the display unit 11. The image trimming unit 10 may output an image in which the enhanced oxygen saturation is combined with the absorption coefficient distribution image.

(Display section)
The image output from the image trimming unit 10 and the reconstructed absorption coefficient distribution image are displayed. As a display method, an MIP (Maximum Intensity Projection) image or a slice image can be considered, but other display methods can be applied. For example, there is a method of displaying a 3D image from a plurality of different directions. There is also a method in which the user changes the display image tilt, display area, window level, and window width while checking the display. There are also a method of displaying the oxygen saturation image and the absorption coefficient distribution image side by side, and a method of displaying the display image in alignment with each other.
It is also conceivable that an input means such as a knob capable of adjusting the blood vessel specifying conditions used for determining the blood vessel position in the blood vessel position specifying unit 8 while installing the displayed image on the display unit.

(Information processing device)
The processing of each block described above can be realized as a control method in which the information processing apparatus 12 such as a PC executes a predetermined program. The information processing apparatus transmits control contents to each block by a control line (not shown), wireless, or other methods. If necessary, there may be an instruction from the user to the information processing apparatus or each block.

<Example 1>
An example of the photoacoustic diagnostic apparatus according to the present invention will be described.
Ti: S (titanium sapphire laser) is used as the light source, and two wavelengths (756 nm, 797 nm) in the same optical path
The light was irradiated. The light density at these wavelengths was 15 mJ / cm ² . Nd: YAG laser light (pulsed light of nanosecond order with a wavelength of 1064 nm) was used for excitation of Ti: S. Measurements were performed using a wavelength of 700 nm or more reaching the deep part of the subject.

First, light having a wavelength of 797 nm was used. This wavelength is a wavelength around the wavelength where the absorption coefficients of the two types of hemoglobin in the region of 700 nm or more are equal. Light having a wavelength of 797 nm corresponds to light having the first wavelength of the present invention. According to our knowledge, the wavelength region where the absorption coefficient is equal is the thickness of the blood vessel when trimmed using a value that is moderately larger than the noise value of the absorption coefficient distribution image measured at that wavelength as a threshold value. It is desirable that the wavelength range does not change by 10% or more. In this embodiment, when the trimming is performed with 30% of the maximum absorption coefficient of the blood vessel position as a threshold, the blood vessel thickness is ± 10%.
The wavelength in the wavelength range of 778 nm to 950 nm that does not change is used.

Next, light having a wavelength of 756 nm, which has different absorption coefficients of the two hemoglobins, was used. At a wavelength of 756 nm, there is a peak in the absorption coefficient of deoxyhemoglobin, and the difference in absorption coefficient between the two types of hemoglobin is large. This corresponds to the second wavelength light of the present invention. When calculating the oxygen saturation, the difference between the absorption coefficients of the two types of hemoglobin, such as the light with a wavelength of 756 nm, is large, and a wavelength having an absorption coefficient similar to the absorption coefficient for the light with the previous wavelength of 797 nm is used. When selected, oxygen saturation can be obtained with high accuracy.

Using the light as described above, an object having a thickness of 50 mm or more in the direction perpendicular to the receiving surface of the ultrasonic probe was measured.
First, the subject was irradiated with light having a wavelength of 797 nm (corresponding to S201 in FIG. 2), and the photoacoustic wave from the subject was received by the ultrasonic probe (S202). Since the wavelength used was 700 nm or more, which has a low absorption coefficient for melanin and hemoglobin, it was possible to measure photoacoustic waves from deeper than 25 mm with sufficient intensity. An absorption coefficient distribution image A was created using the photoacoustic signal obtained (S203)
. The reconstruction used time projection in the time domain.
After that, the same measurement was performed at a wavelength of 756 nm to create an absorption coefficient distribution image B (S204-S206).
).

Next, the oxygen saturation was calculated using the created absorption coefficient distributions A and B (S207). At this time, by averaging the 1.25 mm absorption coefficient distribution in the thickness direction, an error in oxygen saturation due to the displacement of each image was suppressed.

Next, a blood vessel position was determined using an absorption coefficient equal to or greater than the threshold value of the absorption coefficient distribution image A as a blood vessel (S208). The threshold value was 30% of the maximum absorption coefficient. A value that can sufficiently trim noise in the absorption coefficient distribution image was selected as the threshold value.

  Display was performed with the oxygen saturation information other than the blood vessel position set to zero so that the calculated oxygen saturation information is emphasized at the blood vessel position (S209-210).

Next, the effect of implementing the present invention will be described. FIG. 5A shows an oxygen saturation diagram in the simulated living body in the region of 30 mm in the x direction, 46 mm in the y direction, and 50 mm in the z direction. The simulated living body has the same sound speed, absorption coefficient and scattering coefficient as the living body. One simulated arterial blood vessel corresponding to an arterial blood vessel and one corresponding to a venous blood vessel were arranged at a central 25 mm position in the depth direction z in the simulated living body. The right side of FIG. 5A shows that there is a venous simulated blood vessel (indicated by “P” in the figure), and the left side has an arterial simulated blood vessel (indicated by “A” in the figure). Light was incident on each simulated living body, and the generated photoacoustic waves were measured with a 3 cm × 4.6 cm probe arranged in parallel to the xy plane, and an absorption coefficient distribution image and oxygen saturation were obtained.

FIG. 5D shows the absorption coefficient distribution measured at a wavelength of 797 nm. FIG. 5E shows an oxygen saturation image obtained by trimming with an absorption coefficient distribution having a wavelength of 797 nm. FIG. 5B shows an absorption coefficient distribution measured at a wavelength of 756 nm. FIG. 5C shows an oxygen saturation image obtained by trimming with an absorption coefficient distribution having a wavelength of 756 nm.

  In FIG. 5B to FIG. 5E, since the simulated blood vessel arranged at the position of 25 mm depth in the z direction can be recognized, the absorption coefficient distribution image is obtained up to the depth of 25 mm of the subject at any wavelength. It was confirmed that measurement was possible. When the blood vessel position is determined using the absorption coefficient distribution image B corresponding to light having a wavelength of 756 nm, in which the absorption coefficient of oxyhemoglobin is smaller than that of deoxyhemoglobin, the venous blood vessel in the image of oxidative saturation in FIG. On the contrary, the artery became thinner. In addition, disappearance was observed in part of the artery.

However, when the blood vessel position is determined using the absorption coefficient distribution image A corresponding to light with a wavelength of 797 nm where the absorption coefficients of the two types of hemoglobin are equal, the full width at half maximum of the vein and artery is within ± 5%. And displayed almost the same. Therefore, by using the absorption coefficient distribution for the wavelengths with the same absorption coefficient of two types of hemoglobin of 700 nm or more for the identification of the blood vessel position, it was confirmed that the arteries and veins in the deep part of the subject look the same. .

  In the present embodiment, the oxygen saturation is calculated using two types of wavelengths, but may be calculated using three or more types of wavelengths. In that case, the wavelength A (797nm) where the absorption coefficients of oxyhemoglobin and deoxyhemoglobin are the same, the wavelength B (756nm) where each absorption coefficient is different, and the wavelength (797nm) which is not restricted by the magnitude of the absorption coefficient (797nm) , Other than 756 nm).

<Example 2>
In the first embodiment, an apparatus for producing oxygen saturation using an absorption coefficient distribution with respect to the wavelength A where the absorption coefficients of oxyhemoglobin and deoxyhemoglobin are equal has been described. In this embodiment, a photoacoustic apparatus that calculates oxygen saturation without using the absorption coefficient distribution of wavelength A will be described as an example. A flowchart of this embodiment is shown in FIG. Here, a different part from Example 1 is demonstrated in detail.

In this example, a light source having a wavelength A of 797 nm, a wavelength B of 756 nm, and a wavelength C of 825 nm was used. Wavelength B and wavelength C are wavelengths with different absorption coefficients of the two types of hemoglobin. A Ti: S laser was used as the light source, and the subject was irradiated along the same optical path.

First, in step S301, the subject was irradiated with light of wavelength A, a photoacoustic wave was acquired in step S302, and image reconstruction was performed using the acoustic signal acquired in step S303.
Next, the subject was irradiated with light of wavelength B in step S304, photoacoustic waves were acquired in step S305, and image reconstruction was performed in step S306.

Next, in this example, the subject was irradiated with light of wavelength C in step S311, a photoacoustic wave was acquired in step S312, and image reconstruction was performed in step S313.

In step S307, the oxygen saturation was calculated using the absorption coefficient distribution images B and C for the wavelengths B and C having different absorption coefficients of the two hemoglobins. At the wavelength B of 756 nm, the absorption coefficient of deoxyhemoglobin is higher than that of oxyhemoglobin, and at the wavelength of 825 nm, the absorption coefficient of oxyhemoglobin is higher than the absorption coefficient of deoxyhemoglobin. The magnitudes of the absorption coefficients of the two types of hemoglobin are reversed such as the wavelengths 756 nm and 825 nm. By selecting a wavelength that is the same as the absorption coefficient for the wavelength A, the oxygen saturation can be calculated more accurately.

In step S308, the position of the blood vessel was determined from the absorption coefficient distribution image for wavelength A. A threshold was provided, and a position having an absorption coefficient equal to or greater than the threshold was defined as a blood vessel. The threshold value was 30% of the maximum absorption coefficient.

In step S309, an image in which the oxygen saturation at the blood vessel position is emphasized based on the oxygen saturation information and the blood vessel position information calculated in S307 and S308 is created. Oxygen saturation other than the blood vessel position was eliminated, and the oxygen saturation at the blood vessel position was emphasized.
In step S310, the image obtained in S309 is displayed.

The order of S301-S303, S304-S306, and S311-S313 corresponding to the procedure from irradiation to reconstruction of wavelengths A, B, and C may be reversed. Further, reconstruction may be performed after signals are acquired at all wavelengths. Further, the procedures of S307 and S308 may be reversed. Or S307
After finishing, the steps from S301 to S304 and S308 may be performed and vice versa.

The implemented effect will be described next. For any wavelength of wavelengths A, B, and C, 25 mm of the subject
The absorption coefficient distribution image could be measured up to the above depth. When the blood vessel position was determined at the wavelength B, an image having a larger full width at half maximum of the vein was obtained. When the blood vessel position was determined at the wavelength C, an image having a larger full width at half maximum was obtained. However, when the blood vessel position was determined at the wavelength A, the veins and the arteries were obtained in the same way, with the full width at half maximum being only about ± 5%. Therefore, by using the absorption coefficient distribution for the wavelengths with the same absorption coefficient of two types of hemoglobin of 700 nm or more for the identification of the blood vessel position, it was confirmed that the arteries and veins deep in the subject look the same. .

<Example 3>
In the first and second embodiments, the measurement of the oxygen saturation at the blood vessel position has been described. In this embodiment, a method for measuring the glucose distribution at the blood vessel position will be described. The description will focus on the differences between the present embodiment and the first and second embodiments.

In this example, a total of three wavelengths were used: a wavelength A having the same absorption coefficient of two types of hemoglobin, and two types of wavelengths B and C for measuring glucose distribution. A flowchart of this embodiment is shown in FIG. Wavelength A is mainly used to specify the blood vessel position. The wavelengths B and C are selected in the range of 700 nm to 1100 nm. This is a wavelength region where the absorption coefficient of glucose or water is relatively low. In principle, the glucose distribution can be calculated by measuring the absorption coefficient distribution for one wavelength. In this example, the glucose distribution was measured from three wavelengths.

  First, light was irradiated for each wavelength, photoacoustic waves were acquired, and image reconstruction was performed. The processing of S401-S403, S404-S406, and S411-S413 was performed while changing the light source wavelength. This part was measured in the same manner as in Example 2.

In step S407, the glucose distribution was calculated using the absorption coefficient distribution obtained from the wavelengths A, B, and C and the mathematical formula shown in Equation (5). The average of the glucose distribution obtained for each wavelength was calculated, and a glucose distribution image was created.

  In step S408, the absorption coefficient distribution image for wavelength A was multiplied by a threshold value of 30% of the maximum absorption coefficient, and an absorber having an absorption coefficient higher than that was determined as a blood vessel, and the blood vessel position was determined.

  Then, in step S409, based on the blood vessel position information obtained from S408, the glucose distribution image obtained in S407 is set so that the distribution other than the blood vessel position is zero and the blood vessel position is emphasized. Adjusted.

  Next, effects of the embodiment will be described. It was confirmed that the absorption coefficient distribution image could be measured up to a depth of 25 mm or more of the subject, and that the glucose distribution display appeared as a blood vessel of the same thickness regardless of the artery or vein.

<Example 4>
In the above embodiment, measurement is performed using light of a plurality of wavelengths, so that a time difference occurs between the measurements at each wavelength. If the measurement position of the subject is shifted due to body movement during the time difference, the positions of the plurality of reconstructed images may be shifted. As a result, for example, the accuracy of alignment and trimming may be reduced, the positions of the arteries and veins may not be aligned, and the biological characteristic distribution image may be blurred.

This is shown in FIG. FIG. 10A shows the absorption coefficient distribution when the measurement region of the subject is irradiated with light having a wavelength of 797 nm, which is approximately equal to the absorption coefficients of the two types of hemoglobin. An image P derived from a vein and an image A derived from an artery can be confirmed in the measurement region. FIG. 10B is an absorption coefficient distribution when two types of hemoglobin are irradiated with light having a wavelength of 756 nm having different absorption coefficients. At this time, if there is a body movement during measurement while changing the wavelength, the position of the blood vessel is shifted as shown in FIG.

  As one countermeasure against the influence of such body movement, it is conceivable to adjust conditions such as a threshold used for determining the blood vessel position in the blood vessel position specifying unit. It is also conceivable that the corresponding measurement positions among a plurality of reconstructed images are identified and imaged by comparing feature amounts, tracking motion vectors, alignment by markers, and the like.

  In this embodiment, when the position of the light absorber is shifted in a plurality of images due to body movement, the light absorber in the other image in the composite image is increased by enlarging the light absorber in at least one image. A method of superimposing on the body will be described. In order to enlarge the light absorber, a method of reducing the resolution in the image space is used. Thereby, the light absorber can be apparently blurred and enlarged. Such a method for reducing the resolution is preferable in that the amount of calculation can be suppressed because it is not necessary to grasp the positional deviation for each target region (voxel, pixel, etc.) of the subject. In the present invention, the method of reducing the resolution in the image space is to reduce the fluctuation of the absorption coefficient value between adjacent positions (between voxels and between pixels) in the absorption coefficient distribution, thereby reducing the image of the light absorber. Shows how to blur the appearance.

  A flow of a series of processes in the present embodiment will be described. First, the image reconstruction unit creates an absorption coefficient distribution image from the photoacoustic wave measurement results at the respective wavelengths as in the above-described embodiment. The blood vessel position specifying unit specifies the blood vessel position from the absorption coefficient distribution image corresponding to each wavelength, and grasps a rough position shift amount between the two images. Here, a detailed level (for example, voxel, pixel, etc.) is not necessary, and a rough shift amount that can determine the degree of reduction in resolution is sufficient. Examples of the method for grasping the amount of deviation include analysis of a reconstructed image, analysis performed by mechanically measuring the state of body movement or optical imaging, and manual input by a user. You can use this method. In the case of optical imaging, the position of a mark that can be a feature on the subject may be compared, or a marker may be installed. In addition, a misalignment determination unit that determines the presence or absence of misalignment and grasps the misalignment amount may be provided.

  Then, the degree of resolution reduction is determined based on the grasped deviation amount. This determination may be performed by the blood vessel region specifying unit, or may be performed by the biological characteristic distribution calculating unit, the image trimming unit, or another added block receiving the shift amount. When reducing the resolution of only one of the images, at least perform processing to the extent that the image of the blood vessel whose appearance has increased due to the resolution reduction includes the blood vessel of the image whose resolution has not been reduced. .

  When reducing the resolution, various techniques for filtering an image can be used. For example, an image of a blood vessel or the like can be blurred by applying a moving average filter or a Gaussian filter to the reconstructed image. The degree of resolution reduction can be changed by adjusting the filter range and filter coefficient. For example, the relationship of how much resolution should be reduced in accordance with the amount of positional deviation may be determined in advance and stored in the form of a table or a relational expression. In the same reconstructed image, the degree of reduction may be changed according to the size of the absorber (the thickness of the blood vessel image).

The result of applying this example is shown in FIG. FIG. 11A shows an absorption coefficient distribution when light having a wavelength of 797 nm is irradiated as in FIG. FIG. 11B shows an image obtained by performing resolution reduction processing on the image of FIG. 10B in which the position shift has occurred due to body movement. As shown in the drawing, the blood vessel image is blurred and apparently enlarged. The degree of the resolution reduction process is determined so that the light absorbers of the two images overlap each other when the synthesis process is performed as shown in FIG. After or before the synthesis, the image trimming unit may perform trimming within the range of the blood vessel image of FIG.

  According to the processing of this embodiment, even when the measurement position is shifted due to body movement, it is possible to synthesize a biological characteristic distribution image typified by oxygen saturation with a certain degree of accuracy. Furthermore, when displaying on the display unit, if the blood vessel position used for trimming is specified based on a reconstructed image corresponding to light having wavelengths with substantially the same absorption coefficient of the two types of hemoglobin, the actual blood vessel thickness is reflected. It is possible to provide easy-to-see biological characteristic distribution information.

  1: Light source, 4: Image reconstruction unit, 8: Blood vessel position specifying unit, 9: Biological characteristic distribution calculation unit, 10: Image trimming unit, 11: Display unit

Claims (20)

  Based on acoustic waves generated by irradiating a subject with light having a first wavelength of 700 nm or more having the same or substantially the same absorption coefficient of oxyhemoglobin and deoxyhemoglobin
Using the first reconstructed image data obtained in this manner, a specifying step for specifying a blood vessel position;
  A display step of highlighting on the display means object characteristic distribution information corresponding to the blood vessel position obtained based on an acoustic wave generated by irradiating the object with light of a plurality of wavelengths;
Having a method.   A specifying step of specifying a blood vessel position using first reconstructed image data obtained based on an acoustic wave generated by irradiating a subject with light having a first wavelength between 778 nm and 950 nm; ,
  A display step of highlighting on the display means object characteristic distribution information corresponding to the blood vessel position obtained based on an acoustic wave generated by irradiating the object with light of a plurality of wavelengths;
Having a method.   The object characteristic distribution information is oxygen saturation distribution information.
The method according to claim 1 or 2.   The analyte characteristic distribution information is glucose distribution information or collagen distribution information.
The method according to claim 1 or 2.   The plurality of wavelengths includes wavelengths having different absorption coefficients of oxyhemoglobin and deoxyhemoglobin.
5. A method according to any one of claims 1 to 4.   The first wavelength is a wavelength between 778 nm and 950 nm
The method of claim 1.   Absorption coefficient distribution information corresponding to light of the first wavelength as the first reconstructed image data
To get
The method according to any one of claims 1 to 6.   The plurality of wavelengths includes the first wavelength and a second wavelength different from the first wavelength;
  Based on the first reconstructed image data and the second reconstructed image data obtained based on the acoustic wave generated by irradiating the subject with the light of the second wavelength, Obtain the object characteristic distribution information
8. A method according to any one of claims 1 to 7.   Obtaining first absorption coefficient distribution information corresponding to light of the first wavelength as the first reconstructed image data;
  Obtaining second absorption coefficient distribution information corresponding to the light of the second wavelength as the second reconstructed image data;
  The method according to claim 8, wherein the object characteristic distribution information is acquired based on the first and second absorption coefficient distribution information.   The object characteristic distribution information is acquired after reducing the resolution of at least one of the first and second reconstructed image data.
The method of claim 8.   The blood vessel position is specified using the first reconstructed image data before reducing the resolution.
The method of claim 10.   Obtain the object characteristic distribution information at a position 25 mm or more away from the receiving means for receiving the acoustic wave
12. A method according to any one of claims 1 to 11.   A position where the value of the first reconstructed image data is within a predetermined range is determined as the blood vessel position.
The method according to any one of claims 1 to 12.   The plurality of wavelengths are all wavelengths of 700 nm or more.
14. A method according to any one of claims 1 to 13.   The light of the plurality of wavelengths includes the light of the first wavelength
15. A method according to any one of claims 1 to 14.   The display means displays the subject characteristic distribution information corresponding to a position other than the blood vessel position as a fixed value.
The method according to any one of claims 1 to 15.   The object characteristic distribution information corresponding to the blood vessel position is highlighted on the display means by setting the object characteristic distribution information other than the blood vessel position to zero.
The method according to any one of claims 1 to 16.   The object characteristic distribution information corresponding to a position other than the blood vessel position is displayed darker than the object characteristic distribution information corresponding to the blood vessel position.
The method according to any one of claims 1 to 17.   The object characteristic distribution information corresponding to the blood vessel position obtained based on an acoustic wave generated by irradiating the subject with light of at least two wavelengths among the plurality of wavelengths is the display unit. Highlight
The method according to any one of claims 1 to 18. A program causing an information processing apparatus to execute the method according to any one of claims 1 to 19.

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