JP5473761B2 - Biological information imaging apparatus and biological information imaging method - Google Patents

Description

  The present invention relates to a biological information imaging apparatus and a biological information imaging method.

  In general, imaging devices using X-rays, ultrasound, and MRI (nuclear magnetic resonance imaging) are widely used in the medical field. On the other hand, research on optical imaging equipment that obtains in-vivo information by propagating light emitted from a light source such as a laser into a subject such as a living body and detecting the propagating light is actively promoted in the medical field. It has been. As one of such optical imaging techniques, Photoacoustic Tomography (PAT: Photoacoustic Tomography) has been proposed (Non-Patent Document 1).

  In an apparatus using the PAT technique, first, pulse light is irradiated from a light source to a subject. In the subject, an acoustic wave (typically an ultrasonic wave, hereinafter referred to as a photoacoustic wave) is generated from a living tissue that has absorbed the energy of the propagated and diffused light. Then, the generated photoacoustic wave is detected at a plurality of locations, the detected signal is analyzed, and information related to the optical characteristic value inside the subject is visualized. Thereby, it is possible to obtain an optical characteristic value distribution in the subject, particularly a light energy absorption density distribution.

According to Non-Patent Document 1, in photoacoustic tomography, the initial sound pressure (P ₀ ) of a photoacoustic wave generated from an absorber in a subject due to light absorption can be expressed by the following equation.
P ₀ = Γ · μ _a · Φ Equation (1)
Here, Γ is a Gruneisen coefficient, which is the product of the square of the volume expansion coefficient (β) and the speed of sound (c) divided by the constant pressure specific heat (C _P ). μ _a is the absorption coefficient of the absorber, and Φ is the amount of light in a local region (the amount of light irradiated to the absorber, also referred to as light fluence).
According to Non-Patent Document 1, an initial sound pressure distribution (P ₀ ) of a subject is imaged by measuring and analyzing changes in sound pressure P, which is the magnitude of a photoacoustic wave, at a plurality of locations. Can do. Note that Γ is known to assume a substantially constant value once the structure is determined. Therefore, the product of μ _a and Φ, that is, the optical energy absorption density distribution can be obtained from the relationship of the equation (1). .

M. Xu, L. V. Wang, “Photoacoustic imaging in biomedicine”, Review of scientific instruments, 77, 041101 (2006)

  In photoacoustic tomography, the photoacoustic wave is detected by an ultrasonic detector and converted into an electric signal, and the electric signal is amplified by an amplifier. Then, the amplified signal is digitized by an AD converter, and signal processing is performed to image biological information in the subject. In such a biological information imaging apparatus using photoacoustics, obtaining a photoacoustic wave generated from a subject with as little noise as possible is a condition for obtaining a good image. However, the strength of the photoacoustic wave generated from the subject depends on the situation inside the subject, and the strength of the signal cannot be determined until actual measurement is performed.

Therefore, if the gain of the amplifier that amplifies the signal is increased to the maximum, when the signal from the subject is strong, the amplifier and the AD converter at the subsequent stage are saturated and correct signal data cannot be obtained. On the other hand, if the gain of the amplifier is made small, when the signal from the subject is weak, the obtained signal is hidden by noise and only an image with a low signal-to-noise ratio (SN ratio) can be obtained. End up. That is, the biological information imaging apparatus using photoacoustic tomography has a problem that it is difficult to appropriately determine the gain of an amplifier that amplifies a signal in advance.

  The present invention has been made in view of the above problems, and provides a technique for increasing the signal-to-noise ratio while appropriately adjusting the gain of the amplifier according to the condition of the subject in photoacoustic tomography. With the goal.

  In order to achieve the above object, the present invention adopts the following configuration. That is, a detector that detects an acoustic wave generated from a light absorber inside the subject that has absorbed light emitted from a light source and converts it into a detection signal, and a gain of the amplifier is determined according to the signal strength of the detection signal A gain control unit that performs amplification, an amplifier that amplifies the detection signal according to the gain determined by the gain control unit, a number determination unit that determines the number of additions that is the number of times to add the amplified detection signal, and the amplified A biological information imaging apparatus including a signal processing unit that performs an average of the number of additions of the detection signal and generates image data, wherein the number determination unit is amplified by a gain determined by the gain control unit The biometric information image is characterized in that the number of additions is determined so that the SN ratio of the signal after the averaging is equal to or greater than a preset value based on the SN ratio when It is a packaging apparatus.

  The present invention also employs the following configuration. That is, a step of detecting an acoustic wave generated from a light absorber inside the subject that has absorbed light emitted from a light source and converting it to a detection signal, and a gain of the amplifier is determined according to the signal intensity of the detection signal. Amplifying the detection signal with the determined gain; determining an addition count that is the number of additions of the amplified detection signal; and adding an average of the addition counts of the amplified detection signal And generating image data, wherein in the determining step, the SN of the signal after addition averaging is based on the SN ratio when amplification is performed with the determined gain. In the biological information imaging method, the number of additions is determined so that the ratio is equal to or greater than a preset value.

  According to the present invention, in photoacoustic tomography, it is possible to increase the signal / noise ratio while appropriately adjusting the gain of the amplifier according to the condition of the subject.

1 is a diagram illustrating a configuration of a biological information imaging apparatus according to Embodiment 1. FIG. 5 is a flowchart illustrating a gain control and an addition count adjustment method according to the first embodiment. 9 is a flowchart illustrating a gain control and an addition count adjustment method according to the second embodiment.

The biological information imaging apparatus of the present invention is an imaging apparatus using photoacoustic tomography (PAT). The biological information imaging apparatus described in the following embodiments enables imaging of in-vivo information (optical characteristic value distribution) and a concentration distribution of substances constituting the biological tissue obtained from the information. . Thereby, objectives such as diagnosis of malignant tumor and vascular disease and follow-up of chemotherapy can be achieved.
Exemplary embodiments of the present invention will be described in detail below with reference to the drawings.

<Example 1>
FIG. 1 shows a configuration of a biological information imaging apparatus according to the present embodiment.
The biological information imaging apparatus includes a light source 101, a photoacoustic wave detector (also referred to as a probe) 106, an amplifier 107, a signal processing device 108, an addition number determination unit 109, a gain control unit 110, and a display device. 111.
The light source 101 is a device that emits light 102. The emitted light 102 is applied to a subject 103 such as a living body. In the figure, the object 103 can be irradiated from both sides, but irradiation from one side or irradiation from another direction may be possible. It is also possible to use a plurality of light sources in synchronization.

  When a part of the energy of the light propagated inside the subject 103 is absorbed by the light absorber 104 such as a blood vessel, a photoacoustic wave (for example, an ultrasonic wave) 105 is generated from the light absorber 104. The photoacoustic wave detector 106 detects the photoacoustic wave 105 generated from the light absorber 104, converts the photoacoustic wave signal into an electric signal, and generates a detection signal. The amplifier 107 amplifies the detection signal output from the photoacoustic wave detector 106. During this amplification process, the amplifier 107 performs amplification according to a predetermined gain table. The gain at the time of gain is changed by changing (regenerating) the gain table. The signal processing device 108 is a device that converts the detection signal amplified by the amplifier 107 into a digital signal, performs signal processing, and generates (image reconstruction) image (biological information image) data inside the subject. The signal processing device is composed of, for example, an AD converter and a personal computer (PC). The display device 111 is a device that displays an image based on the generated image data. Processing in the addition number determination unit 109 and the gain control unit 110 will be described later. The photoacoustic wave detector corresponds to the detector of the present invention, the signal processing device corresponds to the signal processing unit of the present invention, and the addition number determination unit corresponds to the number of determination unit of the present invention.

Since the photoacoustic wave 105 generated from the light absorber 104 is a weak signal, when an ordinary ultrasonic probe or the like is used for the photoacoustic wave detector 106, the output electrical signal is also a weak signal. . Therefore, the amplification factor of the amplifier 107 is set high. However, when the amplification factor of the amplifier 107 is increased, noise inside the device and noise from outside the device are mixed in the measured signal. As a result, the photoacoustic wave signal has a low signal / noise ratio (SN ratio). In the photoacoustic tomography apparatus, in order to reduce such noise, signal data obtained by performing a plurality of measurements are added and averaged. For example, when the measurement is performed with the number of detections being 100 and the addition average is taken, if the noise is white noise, the SN ratio is reduced to 1/10.
Therefore, in this embodiment, both the amplification factor of the amplifier 107 and the number of detections by the probe are automatically adjusted based on the data of the actual acquired signal. Thereby, it controls so that the S / N ratio of an acquisition signal may be maintained above a certain value.

A specific procedure for automatically adjusting the amplification factor and the number of additions will be described with reference to the flowchart of FIG. Steps S201 to S203 are processes performed by the signal processing apparatus 108.
In step S201, the signal processing apparatus extracts a signal in the subject from the signal data acquired before this step. As the previously acquired signal, signal data measured by trial for automatic adjustment may be used, or signal data actually measured for imaging may be used. However, it is necessary to use data obtained by measuring the same part as the part in the subject to be measured this time. The signal data may be data obtained by a single measurement, or may be data obtained by averaging a plurality of measurement data.

In step S201, signal extraction is performed in consideration of the arrival time of the signal on the probe. The reason is that since the data is arranged along the time axis, it is necessary to consider the time for the photoacoustic wave signal from the subject to reach the probe surface. Specifically, the arrival time can be calculated from the distance from the probe surface to the subject surface and the sound speed of the object sandwiched between the probe and the subject. Therefore, the signal for this time is removed from the measurement data. Furthermore, if the sound speed and thickness of the subject are known, it is possible to extract the signal portion from the actual subject. In this embodiment, it is assumed that the thickness of the subject can be measured. Alternatively, it is also possible to discriminate a signal from the subject by discriminating a high-intensity photoacoustic wave signal generated from the subject surface.

  In step S202, the signal processing device extracts the maximum value of the photoacoustic wave that is the target for gain optimization. First, it is determined which part of the photoacoustic wave signal obtained in step S201 is to be focused and optimized. At this time, any one of a method of simply extracting a portion having the maximum signal intensity, a method of extracting the maximum portion after detecting the envelope of the signal, and a method of extracting the N shape signal, which is a characteristic of the photoacoustic wave, by a pattern recognition method, etc. It may be used. After extracting the part of interest by any method, the maximum value of the signal intensity (absolute value) of that part is extracted.

  In step S203, the signal processing device calculates an optimum gain. Based on the relationship between the maximum value of the signal intensity extracted in step S202 and the optimum value viewed from the AD converter capability, it is determined how to change the gain value applied to the current amplifier. The optimum value viewed from the capability of the AD converter is, for example, 70 to 80 percent of the maximum value of the output value of the AD converter. The reason why the maximum value of the output value of the AD converter is not reached in this way is that if the maximum value of the output value of the AD converter is exceeded, the signal is saturated and distorted. Further, if saturation occurs in an analog circuit, it is necessary to set the value while suppressing the optimum value.

  Then, an optimum gain is determined from the ratio between the optimum value and the current maximum signal strength. For example, when the maximum value of the current signal strength is half of the optimum value viewed from the capability of the AD converter, the optimum gain can be determined to be twice the current gain. However, since the gain has an upper limit, if the upper limit is exceeded, the gain is not optimized and reaches the upper limit. However, even the gain value at the upper limit is optimal as the capability of this apparatus, and will be treated as an optimum value of gain below. The signal processing apparatus notifies the gain control unit and the addition number determination unit of the optimum value of the gain and the predicted value of the signal strength obtained when the gain is optimized.

Steps S204 and S205 are processing of the gain control unit.
In step S204, the gain control unit regenerates the gain table from the notified optimum value of the gain and the previously set gain curve. How the middle part of the gain curve curves depends on the purpose of correcting the attenuation of light in the subject and the attenuation of the photoacoustic wave, but the absolute value of the gain is Is determined by an appropriate gain value.
In step S205, the gain control unit sets a gain table for the amplifier. The gain table can be set directly in the amplifier or an auxiliary device for setting in the amplifier may be installed, but any method may be used.

Steps S206 to S208 are processes in which the addition number determination unit determines the number of additions from the predicted value of the SN ratio.
In step S206, the addition number determination unit predicts the SN ratio in one measurement. The number-of-addition determination unit stores in advance the relationship between the magnitude of the gain given to the amplifier and the level of white noise generated from the circuit, using tables and expressions. Therefore, white noise generated from the circuit can be estimated with reference to the optimum gain value determined in the signal processing apparatus. In addition, since the signal processing apparatus notifies the predicted value of the signal strength obtained at the optimum gain value, the SN ratio can be predicted at this point in time for one measurement.

A target value of the S / N ratio is determined in advance in the addition count determination unit by a user setting or the like. In step S207, the addition number determination unit automatically adjusts the addition number so that the target value of the SN ratio is obtained. At this time, based on the S / N ratio predicted in the previous step, it is calculated how many additions should be made to reach the set S / N ratio target value. For example, when the target SN ratio is 1/10 of the predicted SN ratio in one measurement, the number of additions is set to 100.
In step S208, the addition number determination unit notifies the calculated number of additions to the signal processing device, and the signal processing circuit sets the number of repetitions.

  According to the processing procedure of the present embodiment, the optimum value of the gain can be determined from the maximum value of the photoacoustic wave obtained from the signal in the subject and the performance of the amplifier. By using this optimum value, it becomes possible to appropriately set the gain of the amplifier in the gain control unit. In addition, since the SN ratio in one measurement can be predicted from the gain value and the signal strength when the gain is optimized, the number of measurement additions for achieving the target SN ratio can be determined.

In the above description, the previously measured signal data is not included in the number of additions, but the previously measured signal data can also be added. In this case, control is performed so that the total number of times of measurement becomes the number of additions calculated in the present embodiment. That is, when the previously measured signal data is one time, the remaining number of repetitions is the number obtained by subtracting one from the calculated number of additions.
However, in such a case, the set gain of the data measured previously is different from the subsequent optimized gain. Therefore, when adding the signal data measured previously and the signal data measured by changing the gain, it is preferable to correct the gain at the time of addition. Therefore, for example, the signal data measured previously can be corrected by multiplying the gain correction coefficient.

  In this embodiment, the number of additions increases when the signal strength to be optimized is weak. In particular, when there is no light absorber in the subject, the photoacoustic wave signal does not come out, so the signal is only noise. If this embodiment is applied to such a case, the number of additions becomes very large, and the measurement time is extended. In order to prevent this phenomenon, it is also preferable to set an upper limit for the number of additions. By setting the upper limit value, a fixed upper limit is set for the measurement time, so that the measurement is completed within the fixed time even when there is no light absorber in the subject.

<Example 2>
In the first embodiment, the optimum gain and the number of additions are determined only once at the beginning of a series of measurements. On the other hand, in this embodiment, the optimum gain resetting and the determination of the number of additions (determination of whether to end addition) are performed in each signal measurement.

With reference to the flowchart of FIG. 3, the optimum gain setting method and the number of additions determination method of the present embodiment will be described focusing on differences from the first embodiment.
In the present embodiment, in the process of determining the optimum gain (steps S301 to S303, which are the processes of the signal processing apparatus), an average of signals measured before the current measurement is used (step S301). Thereby, it is possible to set the optimum gain reflecting all the measurement signals so far every time.
The processing of the gain processing unit (Steps S304 and S305) is performed in the same manner as in the first embodiment.

In the method of determining the number of additions, the number of additions is not determined first as in the first embodiment, but by determining whether the SN ratio is sufficient using the signals measured so far, the number of additions is determined. To decide.
Specifically, first, in step S306, the signal-to-noise ratio of the signal is calculated from the addition average signal of the signals measured so far. Subsequently, in step S307, if the calculated value exceeds the set S / N ratio, the measurement is terminated (S307 = YES). If not, the measurement is repeated (S307 = NO). In this way, since the number of additions is determined while calculating the actual SN ratio, a signal exceeding the set SN ratio is reliably measured.

  According to the processing procedure of the present embodiment, it is possible to determine the optimum gain value by reflecting the addition average of the signals measured so far, and to appropriately set the gain of the amplifier. In addition, since the SN ratio is calculated for each measurement and it is determined whether or not to continue the measurement, it is possible to reliably achieve the target SN ratio.

  Note that the upper limit of the number of additions must be provided when there is no light absorber in the subject, similar to the first embodiment. In the case of the present embodiment, this can be dealt with by setting an upper limit on the number of times the measurement is repeated.

  101: light source, 106: photoacoustic wave detector (detector), 107: amplifier, 108: signal processing device (signal processing unit), 109: number of additions determination unit (number of times determination unit), 110: gain control unit

Claims (7)

A detector that detects an acoustic wave generated from a light absorber inside the subject that has absorbed light emitted from a light source, and converts the detected acoustic wave into a detection signal;
A gain control unit that determines the gain of the amplifier according to the signal strength of the detection signal;
An amplifier that amplifies the detection signal according to the gain determined by the gain control unit;
A number-of-times determining unit that determines the number of times of addition that is the number of times of adding the amplified detection signals;
A biological information imaging apparatus including a signal processing unit that performs addition averaging for the number of additions of the amplified detection signal and generates image data;
The number-of-times determining unit is configured to add the signal so that the signal-to-noise ratio after the averaging is equal to or greater than a preset value based on the signal-to-noise ratio when amplification is performed with the gain determined by the gain control unit. A biological information imaging apparatus characterized by determining the number of times. The number determination unit estimates an SN ratio in one detection from the gain determined by the gain control unit and a relationship between gain and noise stored in advance, and based on the estimated SN ratio The biological information imaging apparatus according to claim 1, wherein the number of additions is determined. The detector tries to detect an acoustic wave at least once and converts it into a detection signal before detecting the acoustic wave for generating image data,
The gain control unit determines a gain according to the signal strength of the detection signal obtained by the trial,
The biological information imaging apparatus according to claim 1, wherein the number determination unit determines the number of additions based on a gain determined according to a signal strength of a detection signal obtained by the trial. . The biological information imaging apparatus according to claim 1, wherein the gain control unit determines a gain according to an intensity of a signal obtained by adding and averaging detection signals already acquired. The number-of-times determining unit obtains the SN ratio from the addition average of the signal strengths of the already acquired detection signals,
The biological information imaging apparatus according to claim 1, wherein the detector ends detection of acoustic waves when the SN ratio is equal to or greater than a preset value. The biological information imaging apparatus according to claim 1, wherein the number determination unit sets an upper limit value in advance for the number of additions. Detecting an acoustic wave generated from a light absorber inside the subject that has absorbed light emitted from a light source, and converting the detected acoustic wave into a detection signal;
Determining a gain of an amplifier according to the signal strength of the detection signal, and amplifying the detection signal by the determined gain;
Determining the number of additions, which is the number of additions of the amplified detection signals;
A biological information imaging method comprising: performing an averaging of the number of times of addition of the amplified detection signal and generating image data,
In the determining step, the number of times of addition is determined based on an SN ratio when amplification is performed with the determined gain, so that an SN ratio of a signal after the averaging is equal to or greater than a preset value. A biological information imaging method characterized by the above.

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