JP2011229556A - Photoacoustic tomography apparatus and control method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique for reducing power consumption by appropriately controlling the stop period of a signal processing circuit in a photoacoustic tomography apparatus.SOLUTION: The photoacoustic tomography apparatus includes: an acoustic wave acquisition unit for converting acoustic waves emitted from a subject to electric signals; a signal processor for performing amplification and AD conversion to the electric signals, which is operated in a sleep state of not performing the processing of at least one of the amplification and the AD conversion while sleep signals are input and shifts to the state of performing the processing of both of the amplification and the AD conversion through recovery time when the sleep signals are stopped; a sleep signal generator for generating the sleep signals; and a photodetector for detecting light and generating trigger signals. The sleep signal generator does not output the sleep signals after the trigger signals are input until photoacoustic acquisition time elapses, and outputs the sleep signals after it elapses.

Description

  The present invention relates to a photoacoustic tomography apparatus and a control method thereof.

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.

  In photoacoustic tomography, first, a subject is irradiated with pulsed light from a light source, and an acoustic wave (hereinafter also referred to as a photoacoustic wave) emitted from a living tissue that absorbs the energy of light propagated and diffused in the subject is absorbed. Detect at multiple locations. Then, 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.

In photoacoustic tomography, the initial sound pressure (P ₀ ) of the photoacoustic wave emitted from the light absorber in the subject by light absorption can be expressed by the following equation (1).
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 light absorber. Φ is the amount of light in a local region (the amount of light irradiated to the light absorber, also referred to as light fluence). According to the prior art, the initial sound pressure distribution (P ₀ ) of the subject can be imaged by measuring and analyzing the change in the sound pressure P, which is the magnitude of the photoacoustic wave, at a plurality of locations. it can. 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). .

In photoacoustic tomography, the photoacoustic wave is detected by an acoustic wave detecting element and converted into an electrical signal. Subsequently, in the signal processing circuit, the electric signal is amplified by an amplifier, and the AD converter is digitized. By performing image reconstruction based on this digital signal, it is possible to image biological information in the specimen.
In an apparatus using such photoacoustic tomography, it is a condition for obtaining a good image to acquire photoacoustic waves emitted from a subject with as many acoustic wave detection elements as possible. However, in an apparatus that amplifies electric signals from a large number of acoustic wave detection elements and forms an image through AD conversion or the like, the circuit scale of an amplifier or an AD converter increases. For this reason, the power consumed by these signal processing circuits also increases, and the heat generated increases.

  The above problem of increased power consumption also exists in an ultrasonic diagnostic apparatus that uses ultrasonic echoes. Patent Document 1 discloses a method for reducing power consumption by stopping a signal processing circuit while an ultrasonic echo signal is not acquired in an ultrasonic diagnostic apparatus. In the method disclosed in Patent Document 1, the stop of the signal processing circuit is canceled and the circuit is operated based on a clock signal for starting periodic imaging. This clock signal becomes a trigger signal for operating the signal processing circuit. However, this method employs a very short recovery time from the stop state of the signal processing circuit, so that the amount of reduction in power consumption is small.

JP-A-61-213042

  On the other hand, in an apparatus based on photoacoustic tomography, when it is time to start imaging, a control signal based on a clock is transmitted to the light source to irradiate the subject with pulsed light such as laser light. However, even when a control signal is transmitted to the light source, the timing until light is actually emitted may be shifted depending on the apparatus. This shift is caused by aging of light sources or individual differences. When the timing of light emission is shifted, the timing at which the photoacoustic wave is emitted from the subject is also shifted.

Therefore, when the control signal for the light source is used as a trigger for controlling the stop period (sleep period) of the signal processing circuit, there is a difference between the period during which the photoacoustic wave can be received and the period during which the signal processing circuit is operated. There was a thing. That is, the signal processing circuit may be stopped even when the photoacoustic wave is emitted, or the signal processing circuit may operate even when the photoacoustic wave is not emitted.
Further, when the signal processing circuit is returned from the stop at the same time as the control signal for causing the light source to emit light, the photoacoustic wave immediately after light emission may be missed, and signal processing may not be performed. This is because a certain amount of time is required for returning from the stop of the signal processing circuit.

  Therefore, when the signal processing circuit is to be stopped outside the period of receiving and processing the photoacoustic wave, it is detected that light is actually emitted, and the signal processing circuit is stopped based on the detected timing. It is necessary to generate a control signal.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a technique for reducing power consumption by appropriately controlling a stop period of a signal processing circuit in a photoacoustic tomography apparatus. There is.

  In order to achieve the above object, the present invention adopts the following configuration. That is, an acoustic wave acquisition unit that acquires an acoustic wave emitted from a subject that has absorbed light emitted from a light source and converts the acoustic wave into an electrical signal; And while the sleep signal is input, it operates in a sleep state in which at least one of the amplification and the AD conversion is not performed, and when the input of the sleep signal is stopped, after a predetermined return time, Signal processing means for transitioning to a state capable of performing both amplification and AD conversion processing, sleep signal generation means for generating the sleep signal and outputting it to the signal processing means, and light irradiated on the subject A photoacoustic tomography apparatus comprising: a light detection unit that detects a trigger signal to generate a trigger signal and outputs the trigger signal to the sleep signal generation unit; Means for receiving the sleep signal from the time when the trigger signal is input until the time when a photoacoustic acquisition time, which is a predetermined time for the acoustic wave acquisition means to acquire the acoustic wave emitted from the subject, elapses; Is output, and the sleep signal is output after the photoacoustic acquisition time has elapsed.

The present invention also employs the following configuration. That is, an acoustic wave acquisition unit that acquires an acoustic wave emitted from a subject that has absorbed light emitted from a light source and converts the acoustic wave into an electrical signal, and a signal processing unit that performs amplification and AD conversion on the electrical signal And while the sleep signal is input, it operates in a sleep state in which at least one of the amplification and the AD conversion is not performed, and when the input of the sleep signal is stopped, after a predetermined return time, A signal processing means for transitioning to a state capable of performing both the amplification and the AD conversion, wherein the trigger signal is detected by detecting the light irradiated on the subject. Generating the sleep signal based on the trigger signal and outputting the sleep signal to the signal processing means, wherein the trigger signal is generated The output of the sleep signal is stopped until the photoacoustic acquisition time, which is a predetermined time for acquiring the acoustic wave emitted from the subject, by the acoustic wave acquisition means, and the light A method for controlling a photoacoustic tomography apparatus comprising: a step of outputting the sleep signal after an acoustic acquisition time has elapsed.

  According to the present invention, in the photoacoustic tomography apparatus, it is possible to reduce power consumption by appropriately controlling the stop period of the signal processing circuit.

The figure which shows the structure of a photoacoustic tomography apparatus. The timing chart which shows the timing of the process of an Example. The block diagram which shows the internal structure of a sleep signal production | generation means.

  Exemplary embodiments of the present invention will be described in detail below with reference to the drawings. The apparatus according to the present invention images information inside a subject such as a living body using photoacoustic tomography (PAT). In the present invention, an acoustic wave generated by light irradiation is also referred to as a “photoacoustic wave”. A photoacoustic wave is an elastic wave generated in a subject by light irradiation, and is typically an ultrasonic wave.

<Example>
The configuration of the apparatus will be described with reference to FIG. The photoacoustic tomography apparatus described here is intended for the diagnosis of malignant tumors, vascular diseases, etc., and the follow-up of chemotherapy. To that end, the optical characteristic value distribution (light absorption coefficient, etc.) in the living body is obtained based on the photoacoustic wave measurement data, and the concentration distribution of the substance constituting the subject such as living tissue is obtained from the information. Make an image.

The photoacoustic tomography apparatus includes a photoacoustic wave acquisition unit 101, a signal processing unit 102, a trigger signal generation unit 103, a photoacoustic acquisition signal generation unit 104, a sleep signal generation unit 105, and an image reconstruction device 106.
The photoacoustic wave acquisition unit 101 is a device that detects a photoacoustic wave generated from a light absorber and converts it into an electric signal (analog signal). Hereinafter, this analog signal is also referred to as a photoacoustic signal. The photoacoustic wave is generated when the subject irradiated with light from a light source (not shown) expands and contracts. Normally, an acoustic wave detection element such as a piezoelectric element is used as the photoacoustic wave acquisition means 101, but any other acoustic wave detector may be used.

The signal processing means 102 receives the analog electrical signal (photoacoustic signal) converted from the photoacoustic wave by the photoacoustic wave acquisition means 101, amplifies it, performs AD conversion, and converts it into a digital signal. Then, the signal processing unit 102 records the digital signal and transfers it to the image reconstruction device 106.
The trigger signal generation unit 103 detects the laser light that has generated the photoacoustic light with an optical sensor such as a PD (photodiode), and generates a signal indicating the timing at which the laser light is irradiated. Acquisition of an actual photoacoustic signal is controlled using the signal output from the trigger signal generation unit 103 as a trigger signal. The trigger signal generation means corresponds to the light detection means of the present invention.

  The photoacoustic acquisition signal generation unit 104 generates a photoacoustic acquisition signal indicating a period for acquiring the photoacoustic signal from the trigger signal, and outputs the photoacoustic acquisition signal to the signal processing unit. The photoacoustic acquisition signal is turned on in synchronization with the rising edge of the trigger signal, and is turned off after a certain time has elapsed. Acquisition of a photoacoustic signal is performed for this fixed time. At that time, the predetermined time is determined in consideration of the thickness of the subject to be measured and the propagation speed of the photoacoustic wave in the subject. Such a photoacoustic acquisition signal generation means is realized by, for example, a down counter. The photoacoustic signal acquisition time changes by changing the initial value of the down counter.

  The sleep signal generation unit 105 generates a sleep signal that is a signal for causing the signal processing unit 102 to transition to the sleep mode, which is a power saving mode, from the trigger signal. When the sleep signal is turned on, the signal processing unit 102 transitions to the sleep mode and stops its operation. When the sleep signal is turned off, the operation is restarted. Here, the sleep mode is a state in which at least one of photoacoustic signal amplification and AD conversion is stopped and power consumption is reduced. It is preferable to stop both amplification of the photoacoustic signal and AD conversion because power consumption is further reduced. Recovery from the sleep mode takes a certain amount of recovery time (a predetermined value depending on the device, for example, several milliseconds). Therefore, the return from the sleep mode is slow after the next trigger signal comes, and the photoacoustic signal may be missed. Therefore, the return from the sleep mode must be started earlier by the return time than the trigger signal rises. In other words, the sleep signal needs to be turned off before the next trigger signal rises.

  The image reconstruction device 106 performs reconstruction processing on the digital signal processed by the signal processing means 102, and generates the internal structure of the subject as image data based on the optical characteristic value distribution. The image data is sent to a display device (not shown) and displayed as an image.

  FIG. 2 is a detailed timing chart of the trigger signal, photoacoustic acquisition signal, and sleep signal of the present invention. The output timing of the signal of the present invention will be described with reference to FIG. The trigger signal 201 and the trigger signal 202 are trigger signals for taking a timing to acquire the photoacoustic signal, and are generated by the above-described trigger signal generation unit 103. The generation period of the trigger signal is, for example, 100 milliseconds according to the light emission period of the pulse laser beam that emits light at a predetermined interval. Pulsed laser light such as that used in photoacoustic tomography devices is very high energy. A high-energy pulsed laser device can emit light only with a period of several milliseconds to several hundred milliseconds. As a high energy pulse laser device, for example, an Nd: YAG laser device can be used.

  The photoacoustic acquisition signal 203 is a signal indicating a period during which the signal processing unit 102 acquires a photoacoustic signal, and is generated by the photoacoustic acquisition signal generation unit 104. The photoacoustic acquisition signal 203 is turned on in synchronization with the rising edge of the trigger signal 201, and is turned off after a predetermined photoacoustic acquisition time has elapsed. For example, when the thickness of the subject that acquires the photoacoustic wave is several centimeters, the photoacoustic wave is emitted between several tens of microseconds to several hundred microseconds. Hereinafter, it is set as 100 microseconds as an example.

The sleep signal 204 is a signal that causes the signal processing unit 102 to transition to the sleep mode, and is generated by the sleep signal generation unit 105. Since the sleep signal is a signal that causes the signal processing unit 102 to transition to the sleep mode, it should not be turned on during the photoacoustic acquisition time. That is, the signal is turned on after the photoacoustic acquisition time has elapsed from the rising edge of the trigger signal 201. In addition, it is necessary to turn off the sleep signal and restart the operation of the signal processing unit 102 before the next trigger signal 202 rises, by the time required for returning from sleep. When the return time of the signal processing means 102 is, for example, 5 milliseconds,
The sleep release time with reference to the rising edge of the trigger is 95 milliseconds, which is the difference between the trigger period of 100 milliseconds and the return time of 5 milliseconds.

  FIG. 3 shows an internal detailed block of the sleep signal generation means 105. The sleep signal generation means 105 includes down counters 301 and 302, comparators 303 and 304, and an AND circuit 305. The input of the down counter is a system clock, a trigger signal as an initialization signal, and an initial value.

The system clock is a clock for the down counter to measure time, and the same clock as the sampling frequency of the signal processing means 105 is usually used. However, other clocks may be used, and a separate clock may be used for each down counter.
The rising edge of the trigger signal is used as the down counter initialization signal. That is, the down counter starts operation in synchronization with the rising edge of the trigger signal.

  As an initial value of the down counter 301, photoacoustic acquisition time / clock cycle is input. By doing so, the down counter 301 can measure the photoacoustic acquisition time. On the other hand, the sleep release time / clock cycle is input as the initial value of the down counter 302. Thereby, the down counter 302 can measure the sleep release time.

  The counter 303 receives the counter output value of the down counter 301. The input counter value is compared with zero, and when the counter value is zero, the output of the comparator 303 is turned on. That is, the output is turned off from the rising edge of the trigger signal, and then turned on after the photoacoustic acquisition time has elapsed. Furthermore, the output is turned off again in response to the rising edge of the next trigger signal.

  The counter 304 receives the counter output value of the down counter 302. The input counter value is compared with zero, and if it is greater than zero, the output of the comparator 304 is turned on. That is, the output is on from the rising edge of the trigger signal to the sleep release time, and the output is turned off after the sleep release time has elapsed. Furthermore, the output is turned on again in response to the rising edge of the next trigger signal.

Output signals of the comparator 303 and the comparator 304 are input to the AND circuit 305. Then, the result of ANDing the output signals of both comparators is output as a sleep signal from the sleep signal generating means 105.
Then, since the output of the comparator 303 is off and the output of the comparator 304 is on until the photoacoustic acquisition time elapses from the rising edge of the trigger signal, the sleep signal is turned off as a result of the AND operation. .
Subsequently, since the output of the comparator 303 is on and the output of the comparator 304 is also on from when the photoacoustic acquisition time elapses until the sleep release time elapses, the sleep signal is turned on as a result of the AND operation. Become.
Since the output of the comparator 303 is on and the output of the comparator 304 is off from the elapse of the sleep release time to the next trigger signal, the sleep signal is turned off as a result of the AND operation.

When the sleep signal generation unit 105 performs the operation as described above, it is detected that light is actually emitted from the light source, and a photoacoustic signal is acquired. As a result, the period during which the signal processing unit normally operates can be matched with the period in which the photoacoustic wave emitted from the subject is detected, so that the sleep period can be extended and power consumption can be reduced. be able to. Further, there is no difference between the period in which the photoacoustic wave is emitted and the period in which the signal processing means operates. Also, before the next trigger signal comes, it is possible to take a sufficient time for the signal processing means to return from the sleep state, and the photoacoustic signal can be prevented from being missed.

  101: Photoacoustic wave acquisition means, 102: Signal processing means, 103: Trigger signal generation means, 105: Sleep signal generation means

Claims (3)

An acoustic wave acquisition means for acquiring an acoustic wave emitted from a subject that has absorbed light emitted from a light source and converting the acoustic wave into an electrical signal;
Signal processing means for performing amplification and AD conversion on the electrical signal, and operates in a sleep state in which at least one of the amplification and AD conversion is not performed while a sleep signal is input, When the input of the sleep signal is stopped, a signal processing means for transitioning to a state where both the amplification and the AD conversion can be performed after a predetermined return time;
Sleep signal generation means for generating the sleep signal and outputting the signal to the signal processing means;
A light detection means for detecting the light applied to the subject to generate a trigger signal and outputting the trigger signal to the sleep signal generation means;
A photoacoustic tomography device comprising:
The sleep signal generation means includes
Until the photoacoustic acquisition time elapses from when the trigger signal is input until the acoustic wave acquisition unit acquires the acoustic wave emitted from the subject, the sleep signal is not output,
The photoacoustic tomography apparatus outputs the sleep signal after the photoacoustic acquisition time has elapsed. The object is irradiated with light from the light source at a predetermined interval,
The sleep signal generation means includes
After the photoacoustic acquisition time elapses after the trigger signal is input, until the sleep release time that is shorter than the interval at which the light source emits light is shorter than the return time, Output sleep signal,
The photoacoustic tomography apparatus according to claim 1, wherein the sleep signal is not output after the sleep release time has elapsed. An acoustic wave acquisition unit that acquires an acoustic wave emitted from a subject that has absorbed light emitted from a light source and converts the acoustic wave into an electrical signal; and a signal processing unit that performs amplification and AD conversion on the electrical signal. When the sleep signal is input, the amplifier operates in a sleep state in which at least one of the amplification and the AD conversion is not performed, and when the input of the sleep signal is stopped, the amplification is performed after a predetermined recovery time. And a signal processing means for transitioning to a state capable of performing both of the processes of AD conversion, and a control method for a photoacoustic tomography apparatus comprising:
Detecting the light applied to the subject to generate a trigger signal;
Generating the sleep signal based on the trigger signal and outputting the sleep signal to the signal processing means;
The output of the sleep signal is stopped until the photoacoustic acquisition time, which is the time for the acoustic wave acquisition means to acquire the acoustic wave emitted from the subject, elapses after the trigger signal is generated. Steps,
Outputting the sleep signal after the photoacoustic acquisition time has elapsed, and
A control method for a photoacoustic tomography apparatus, comprising:

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