JP2013150745A - Photoacoustic imaging method and device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To secure desired image quality and to suppress a data amount to an appropriate amount even when a pulse width of pulsed light with which a subject is irradiated is changed, in a photoacoustic imaging method.SOLUTION: A photoacoustic imaging device 10 includes means 13, 21, 22 irradiating a subject with pulsed light, detecting acoustic waves emitted from the subject at the time, and generating photoacoustic data. In the photoacoustic imaging device, a sampling frequency adjusting means 50 adjusts the frequency of sampling so as to be higher when the pulse width of the pulsed light is smaller. More preferably, the frequency of sampling is adjusted so as to have a roughly fixed magnification with respect to the frequency of a waveform obtained by differentiating a time waveform of the light intensity of the pulsed light.

Description

  The present invention relates to a photoacoustic imaging method, that is, a method of irradiating a subject such as a living tissue with light and imaging the subject based on an acoustic wave generated by the light irradiation.

  The present invention also relates to an apparatus for performing a photoacoustic imaging method.

  Conventionally, as shown in Patent Documents 1 and 2 and Non-Patent Document 1, for example, photoacoustic imaging apparatuses that image the inside of a living body using a photoacoustic effect are known. In this photoacoustic imaging apparatus, a living body is irradiated with pulsed light such as pulsed laser light. Inside the living body that has been irradiated with the pulsed light, the living tissue that has absorbed the energy of the pulsed light undergoes volume expansion due to heat and generates acoustic waves. Therefore, this acoustic wave can be detected by a detection means such as an ultrasonic probe, and the inside of the living body can be visualized based on the electrical signal (photoacoustic signal) obtained thereby.

  Since the photoacoustic imaging apparatus constructs an image based only on the acoustic wave radiated from a specific absorber, it is suitable for imaging a specific tissue in a living body, such as a blood vessel. It has become.

JP 2005-21380 A Special table 2010-512929

A High-Speed Photoacoustic Tomography System based on a Commercial Ultrasound and a Custom Transducer Array, Xueding Wang, Jonathan Cannata, Derek DeBusschere, Changhong Hu, J. Brian Fowlkes, and Paul Carson, Proc.SPIE Vol. 7564, 756424 (Feb. 23, 2010)

  According to the photoacoustic imaging method, it is possible to image a tissue that has entered from the surface of the subject, such as a blood vessel of a living body, as described above. However, in the field of clinical and medical research, Accordingly, the depth from the subject surface of the tissue to be imaged particularly clearly and the resolution required for the photoacoustic image are often different. There is also a demand for imaging a tissue existing relatively close to the surface of the subject with particularly high definition.

  The depth at which the acoustic wave generated by the irradiation of the pulsed light can reach the surface of the subject becomes deeper as the photoacoustic pulse width increases, and the acoustic pulse width is the pulse width of the irradiated pulsed light. Therefore, in order to meet the above-described requirements, a photoacoustic imaging apparatus that can change the pulse width of the pulsed light has been conventionally considered. From the viewpoint of optimizing the diagnostic characteristics required for the displayed image and the band of the acoustic wave detection means, a photoacoustic imaging apparatus is also considered that can change the pulse width of the pulsed light. However, in the conventional photoacoustic imaging apparatus configured as described above, there is a problem that the image quality is excessive and the amount of data is meaninglessly increased, or the desired image quality cannot be obtained. It is allowed to get.

  The present invention has been made in view of the above circumstances. Light that can obtain a desired image quality even when the pulse width of the pulsed light is changed, and can suppress the amount of data carrying the photoacoustic image to an appropriate amount. An object is to provide an acoustic imaging method.

  It is another object of the present invention to provide a photoacoustic imaging apparatus that can implement such a photoacoustic imaging method.

The photoacoustic imaging method according to the present invention comprises:
As described above, after irradiating the subject with pulsed light having a wavelength absorbed therein, the acoustic wave emitted from the subject is detected by the acoustic wave detection means to obtain the acoustic wave detection signal. A photoacoustic imaging method of sampling the acoustic wave detection signal to obtain photoacoustic data, imaging the subject based on the photoacoustic data, and displaying the image on an image display means, wherein the pulse width of the pulsed light is In a photoacoustic imaging method that can be changed,
The sampling frequency is adjusted to be higher as the pulse width is smaller.

  As described above, as described above, in order to optimize the diagnostic characteristics required for the displayed image and the bandwidth of the acoustic wave detection means as an example of changing the pulse width of the pulsed light as described above. Examples include changing the pulse width.

  More specifically, the sampling frequency is preferably adjusted to be in a range of 4 to 10 times the frequency of the waveform obtained by differentiating the time waveform of the light intensity of the pulsed light.

  Furthermore, the sampling frequency may be adjusted so as to have a substantially constant magnification with respect to the frequency of the waveform obtained by differentiating the time waveform of the light intensity of the pulsed light.

In the photoacoustic imaging method of the present invention,
The correspondence between the sampling frequency and the pulse width is stored in the storage means,
When the pulse width is determined, the frequency stored in the storage means corresponding to the pulse width is read out,
It is preferable to set the sampling frequency according to the read frequency.

Further, in the photoacoustic imaging method of the present invention, the reflected ultrasonic wave irradiated to the subject in a pulsed manner and reflected by the subject is detected by the reflected ultrasonic detection means,
When the reflected ultrasonic detection signal obtained by this detection is sampled, it is desirable to adjust the sampling frequency so as to increase as the ultrasonic pulse width decreases.

  However, the present invention is not limited to this, and the sampling frequency of the reflected ultrasonic wave detection signal may be set constant regardless of the ultrasonic pulse width.

On the other hand, the photoacoustic imaging apparatus according to the present invention, as described above, irradiates the subject with pulsed light having a wavelength that is absorbed inside the subject, thereby causing the acoustic wave emitted from the subject to be emitted by the acoustic wave detecting means. A photoacoustic imaging apparatus that detects and obtains photoacoustic data, images the subject based on the photoacoustic data, and displays the image on the image display means, and is configured to change the pulse width of the pulsed light. In the photoacoustic imaging device,
A sampling frequency adjusting means for adjusting the sampling frequency so as to be higher as the pulse width of the pulsed light is smaller is provided.

  Note that, as described above, the pulse width can be changed freely. There is only one pulse width of the pulsed light applied to the subject when acquiring one image, and the pulse width is appropriately set according to the subject or the like. For example, a configuration for changing, or a configuration for irradiating a subject by alternately switching pulsed light having a plurality of different pulse widths at short time intervals when acquiring one image. More specifically, as the former configuration, a configuration using one pulse light source with a variable pulse width, or a plurality of light sources that emit pulse lights having different pulse widths are provided, and one of them is Configurations that are appropriately selected and used are included.

  More specifically, the sampling frequency adjusting means adjusts the sampling frequency so as to have a substantially constant magnification with respect to the frequency of the waveform obtained by differentiating the time waveform of the light intensity of the pulsed light, for example. The

  When the sampling frequency adjusting means is configured as described above, the substantially constant magnification is preferably set within a range of 4 to 10 times.

In the photoacoustic imaging apparatus of the present invention,
Storage means for storing the correspondence between the sampling frequency and the pulse width of the pulsed light is provided,
When the pulse width of the pulsed light is determined, the sampling frequency adjusting means reads the frequency stored in the storage means corresponding to the pulse width, and sets the sampling frequency according to the read frequency. It is desirable that

  In addition, the photoacoustic imaging apparatus of the present invention generates a signal obtained by deconvolution of a photodifferential waveform, which is a differential waveform of a time waveform of the light intensity of pulsed light irradiated on a subject, from photoacoustic data. Convolution means may be provided.

  In photoacoustic imaging, a waveform obtained by differentiating a time waveform of the light intensity of pulsed light is a basic element of an acoustic waveform. Therefore, this frequency is hereinafter referred to as a basic waveform frequency. The pulse width of the pulsed light is generally defined by the full width at half maximum (ΔFWHM) of the time waveform of the light intensity as shown in FIG. When ½ of ΔFWHM is Δdiff, the inverse of Δdiff is the basic waveform frequency.

  In photoacoustic imaging, in general, if the sampling frequency of the acoustic wave detection signal is set in the range of 4 to 10 times the basic waveform frequency, the diagnostic performance can be improved without increasing the amount of data. It is believed that a high photoacoustic image can be obtained. Therefore, if a certain magnification within the range of 4 to 10 times is applied, the higher the fundamental waveform frequency, that is, the differentiated waveform (in this specification, this may be referred to as “differential waveform”). The smaller the width Δdiff is, the higher the sampling frequency at which the desired image quality can be ensured. Since Δdiff = 0.5 · ΔFWHM, the photoacoustic image can be maintained at a desired image quality if the sampling frequency is set higher as the pulse width ΔFWHM of the pulsed light is smaller.

  On the other hand, the amount of data bearing the photoacoustic image increases as the sampling frequency increases. In other words, the amount of data carrying the photoacoustic image becomes smaller as the sampling frequency is lower.

  As described above, according to the photoacoustic imaging method of the present invention in which the sampling frequency is adjusted to be higher as the pulse width is smaller, a photoacoustic image having a desired image quality can be obtained. If the sampling frequency is relatively large, it is possible to avoid increasing the sampling frequency meaninglessly and to suppress the data amount to an appropriate amount.

  Further, in the photoacoustic imaging method of the present invention, as described above, the correspondence between the sampling frequency and the pulse width is stored in the storage means, and when the pulse width is determined, it corresponds to the pulse width. Thus, when the frequency stored in the storage means is read out and the sampling frequency is set according to the read frequency, the photoacoustic imaging method of the present invention can be implemented more easily.

  The photoacoustic imaging apparatus according to the present invention is provided with the sampling frequency adjusting means for adjusting the sampling frequency so as to be higher as the pulse width of the pulsed light is smaller. A photoacoustic imaging method can be implemented.

The block diagram which shows schematic structure of the photoacoustic imaging device by one Embodiment of this invention. The block diagram which shows the partial structure of the photoacoustic imaging device by another embodiment of this invention. Graph showing the relationship between the resolution of photoacoustic images and the amount of data with respect to the pulse width of pulsed light Schematic diagram showing the time waveform of the light intensity of pulsed light and the optical pulse differential waveform obtained by differentiating it. 1 is a block diagram showing a configuration of a laser light source unit that can be applied to the present invention. Schematic which shows the structural example of the wavelength selection means in the laser light source unit of FIG. The graph which shows the relationship between the wavelength and the transmittance | permeability in the transmission area | region of the wavelength selection means of FIG. Block diagram showing a specific configuration example of a part of the laser light source unit Timing chart showing flash lamp light emission timing and pulsed laser light timing in the laser light source unit Timing chart showing pulsed laser beam emission

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram showing a basic configuration of a photoacoustic imaging apparatus 10 according to an embodiment of the present invention. This photoacoustic imaging apparatus 10 can acquire both a photoacoustic image and an ultrasonic image as an example. An ultrasonic probe (probe) 11, an ultrasonic unit 12, a laser light source unit 13, an image A display means 14, a sampling frequency adjusting means 50, and a storage means 51 are provided.

  The laser light source unit 13 emits laser light having a center wavelength of 756 ns, for example. The subject is irradiated with the laser beam emitted from the laser light source unit 13. The laser light is preferably guided to the probe 11 using light guide means such as a plurality of optical fibers, and irradiated from the probe 11 portion toward the subject.

  The probe 11 performs output (transmission) of ultrasonic waves to the subject and detection (reception) of reflected ultrasonic waves reflected back from the subject. For this purpose, the probe 11 has, for example, a plurality of ultrasonic transducers arranged one-dimensionally. The probe 11 detects ultrasonic waves (acoustic waves) generated by the observation object in the subject absorbing the laser light from the laser light source unit 13 by using the plurality of ultrasonic transducers. The probe 11 detects the acoustic wave and outputs an acoustic wave detection signal, and also detects the reflected ultrasonic wave and outputs an ultrasonic detection signal.

  When the above-described light guide means is coupled to the probe 11, the end portion of the light guide means, that is, the tip portions of the plurality of optical fibers, are arranged along the arrangement direction of the plurality of ultrasonic transducers. From there, laser light is irradiated toward the subject. Hereinafter, the case where the light guide means is coupled to the probe 11 as described above will be described as an example.

  When acquiring a volume image (3D image) as a photoacoustic image or ultrasonic image of a subject, the probe 11 is moved in a direction substantially perpendicular to the one-dimensional direction in which the plurality of ultrasonic transducers are arranged. A subject is two-dimensionally scanned with laser light and ultrasonic waves. This scanning may be performed by an inspector moving the probe 11 manually, or a more precise two-dimensional scanning may be realized using a scanning mechanism.

  The ultrasonic unit 12 includes a reception circuit 21, an AD conversion unit 22, a reception memory 23, a data separation unit 24, an image reconstruction unit 25, a detection / logarithmic conversion unit 26, and an image construction unit 27. The output of the image construction unit 27 is input to the image display unit 14 including, for example, a CRT or a liquid crystal display device. Further, the ultrasonic unit 12 includes a transmission control circuit 30 and a control unit 31 that controls the operation of each unit in the ultrasonic unit 12.

  The receiving circuit 21 receives the acoustic wave detection signal and the ultrasonic wave detection signal output from the probe 11. The AD conversion means 22 is a sampling means, which samples the acoustic wave detection signal and the ultrasonic detection signal received by the receiving circuit 21 and converts them into photoacoustic data and ultrasonic data, which are digital signals, respectively. This sampling is performed at a predetermined sampling period in synchronization with, for example, an AD clock (sampling clock) signal input from the outside. The sampling frequency of the reflected ultrasonic detection signal may be changed according to the pulse width of the ultrasonic wave applied to the subject, or may be constant regardless of the pulse width.

  The laser light source unit 13 includes a Q-switched pulse laser 32 made of a Ti: Sapphire laser, an alexandrite laser, or the like, and a flash lamp 33 that is an excitation light source thereof. The laser light source unit 13 is supplied with a light trigger signal for instructing light emission from the control means 31. When the light trigger signal is received, the flash lamp 33 is turned on and the Q switch pulse laser is turned on. 32 is excited. For example, when the flash lamp 33 sufficiently excites the Q switch pulse laser 32, the control means 31 outputs a Q switch trigger signal. When receiving the Q switch trigger signal, the Q switch pulse laser 32 turns on the Q switch and emits a pulse laser beam having a wavelength of 756 ns.

  Here, the time required from when the flash lamp 33 is turned on until the Q-switch pulse laser 33 is sufficiently excited can be estimated from the characteristics of the Q-switch pulse laser 33 and the like. In place of controlling the Q switch from the control means 31 as described above, the Q switch may be turned on after the Q switch pulse laser 32 is sufficiently excited in the laser light source unit 13. In that case, a signal indicating that the Q switch is turned on may be notified to the ultrasonic unit 12 side.

  As described above, the pulse width of the pulse laser beam is defined by the full width at half maximum (ΔFWHM) of the time waveform of the light intensity of the pulse laser beam. In the present embodiment, this pulse width is taken as an example for 5 nsec (nanosecond). ), The Q-switch pulse laser 32 is configured so that it can be changed in four ways of 15 nsec, 50 nsec, and 100 nsec.

  The control unit 31 inputs an ultrasonic trigger signal for instructing ultrasonic transmission to the transmission control circuit 30. When receiving the ultrasonic trigger signal, the transmission control circuit 30 transmits an ultrasonic wave from the probe 11. The control means 31 outputs the optical trigger signal first, and then outputs an ultrasonic trigger signal. The light trigger signal is output to irradiate the subject with laser light and the acoustic wave is detected, and then the ultrasonic trigger signal is output to transmit the ultrasonic wave to the subject and the reflected ultrasonic wave. Is detected.

  The control means 31 further outputs a sampling trigger signal that instructs the AD conversion means 22 to start sampling. The sampling trigger signal is output after the optical trigger signal is output and before the ultrasonic trigger signal is output, more preferably at the timing when the subject is actually irradiated with the laser light. Therefore, the sampling trigger signal is output in synchronization with the timing at which the control means 31 outputs the Q switch trigger signal, for example. When receiving the sampling trigger signal, the AD conversion means 22 starts sampling the acoustic wave detection signal output from the probe 11 and received by the receiving circuit 21.

  After outputting the optical trigger signal, the control means 31 outputs the ultrasonic trigger signal at the timing when the detection of the acoustic wave is finished. At this time, the AD conversion means 22 continues the sampling without interrupting the sampling of the acoustic wave detection signal. In other words, the control unit 31 outputs the ultrasonic trigger signal in a state where the AD conversion unit 22 continues sampling the acoustic wave detection signal. When the probe 11 transmits ultrasonic waves in response to the ultrasonic trigger signal, the detection target of the probe 11 changes from acoustic waves to reflected ultrasonic waves. The AD conversion unit 22 continuously samples the acoustic wave detection signal and the ultrasonic wave detection signal by continuously sampling the detected ultrasonic wave detection signal.

  The AD conversion unit 22 stores photoacoustic data and ultrasonic data obtained by sampling in a common reception memory 23. The sampling data stored in the reception memory 23 is photoacoustic data up to a certain point, and becomes ultrasonic data from a certain point. The data separation unit 24 separates the photoacoustic data and the ultrasonic data stored in the reception memory 23.

  The sampling frequency adjusting means 50 inputs a sampling frequency control signal to the control means 31 and changes the frequency of the AD clock (sampling clock) signal, thereby changing the sampling frequency by the AD converting means 22. The storage means 51 stores sampling frequencies corresponding to the pulse widths 5 nsec, 15 nsec, 50 nsec, and 100 nsec of the four types of pulse laser light described above.

  Hereinafter, generation and display of a photoacoustic image will be described. The ultrasonic data read from the reception memory 23 and the photoacoustic data obtained by irradiating the subject with pulsed laser light having a wavelength of 756 ns are input to the data separation unit 24 in FIG. The data separation unit 24 inputs only the photoacoustic data to the subsequent image reconstruction unit 25 when generating the photoacoustic image. The image reconstruction means 25 reconstructs data indicating a photoacoustic image based on the photoacoustic data.

  The detection / logarithm conversion means 26 generates an envelope of data indicating the photoacoustic image, and then logarithmically converts the envelope to widen the dynamic range. The detection / logarithm conversion means 26 inputs these processed data to the image construction means 27. Based on the input data, the image construction unit 27 constructs a photoacoustic image related to the cross section scanned by the pulse laser beam, and inputs data indicating the photoacoustic image to the image display unit 14. Thereby, the photoacoustic image regarding the said cross section is displayed on the image display means 14. FIG.

  As described above, the probe 11 is moved to scan the subject two-dimensionally with laser light, and a desired part of the subject, such as a blood vessel, is detected based on the image data regarding a plurality of cross sections obtained by the scanning. It is also possible to generate and display a photoacoustic image for three-dimensional display.

  It is also possible to generate and display an ultrasonic image of the subject based on the ultrasonic data separated by the data separation means 24. The generation and display of the ultrasonic image may be performed by a conventionally known method, and since it is not directly related to the present invention, a detailed description is omitted, but such an ultrasonic image and a photoacoustic image are superimposed. It can also be displayed.

Next, the point that the sampling frequency of the AD conversion means 22 is changed in accordance with the pulse width of the pulsed laser light applied to the subject will be described in detail. In the present embodiment, the pulse width (ΔFWHM) of the pulsed laser light applied to the subject is changed to four types of 5 nsec, 15 nsec, 50 nsec, and 100 nsec as described above. The storage means 51 stores sampling frequencies 1600 MHz, 520 MHz, 160 MHz, and 80 MHz corresponding to these pulse widths 5 nsec, 15 nsec, 50 nsec, and 100 nsec, respectively. The combinations of pulse width and sampling frequency are shown in Table 1 below. Table 1 also shows Δdiff corresponding to the above four pulse widths, the basic waveform frequency, and the value of the data amount by sampling. This data amount is a data amount per channel (channel) when data up to a depth of 40 mm (27 μsec) is acquired and the number of quantization bits is 8 bits.

  Here, the reason why the pulse width of the pulse laser beam is set in the above four ways will be described with reference to FIG. In general, the relationship between the pulse width of the pulsed laser beam and the resolution of the photoacoustic image obtained from the acoustic wave generated by the optical pulse (right vertical axis in the figure) is plotted as white dots in the figure. ing. This relationship is a relationship when it is defined that about 1/100 of the depth determined by the attenuation of the ultrasonic wave reported in various papers is a resolution that can be realized empirically. For example, in the case of blood vessel observation from the body surface, it is desirable that the resolution is 0.5 mm or less in consideration of diagnostic performance, so the left side of the dashed line drawn in the figure (the side where the pulse width is small) The four pulse widths in the region are set. In FIG. 3, the relationship between the pulse width and the data amount (the left vertical axis in the figure) is also plotted with black dots.

  In the configuration of FIG. 1, a signal indicating the set pulse width or switching the pulse width is input from the Q switch pulse laser 32 to the sampling frequency adjusting means 50. The sampling frequency adjusting means 50 reads the sampling frequency stored in the storage means 51 corresponding to the pulse width indicated by the signal, and causes the AD converting means 22 to set the sampling frequency as described above. For example, if the set pulse width is 100 nsec, the sampling frequency is set to 80 MHz as shown in Table 1.

  As described above, in the present embodiment, the sampling frequency is set to be higher as the pulse width is smaller. More specifically, the sampling frequency is set so as to always be four times the basic waveform frequency. By setting the sampling frequency in this way, it is possible to keep the desired image quality and to suppress the data amount to an appropriate amount. The detailed reason is as described above.

  The sampling frequency is set to 4 times the basic waveform frequency, and may be set to other constant magnification within a range of 4 times to 10 times. The magnification may be changed to 0 times, 5.0 times, 4.5 times, or the like. Normally, it is preferable to increase this magnification in terms of image quality. However, the data amount may increase unnecessarily, and therefore an appropriate magnification may be set in consideration of both the image quality and the data amount. Note that the minimum value is four times because the phase can be determined if at least four points can be sampled during one wavelength. On the other hand, the reason why the maximum value is 10 times is that the data amount becomes too large if the ratio is exceeded.

  Further, the present invention is not limited to the case where a subject is irradiated with light of a plurality of types of pulse widths from a single light source, as in the above-described embodiment, and light having a different pulse width from each of the plurality of light sources. The present invention can also be applied to the case of irradiating pulsed light of a plurality of types of light from one or a plurality of light sources alternately at short time intervals, and the pulse width of each pulsed light is different. Any of such cases corresponds to the configuration in which the pulse width of the pulsed light can be changed as defined in the present invention.

  Here, an example of a light source that emits light having different wavelengths and pulse widths as described above will be described with reference to FIGS.

  The laser light source unit 113 shown in FIG. 5 can be applied in place of the laser light source unit 13 shown in FIG. 1, and a case where such replacement is made will be described below as an example. The laser light source unit 113 switches and emits a plurality of pulsed laser beams having different pulse widths and wavelengths from each other. Thereby, it becomes possible to separate and detect photoacoustic signals from a plurality of tissues having different absorption characteristics for each frequency band. “The wavelengths are different from each other” means that the peak wavelengths having the highest light intensity in the wavelength distribution are different from each other. The wavelength of the pulse laser beam is appropriately determined depending on the light absorption characteristics of the substance in the subject to be measured. For example, using a pulse laser beam having a pulse width of 4.2 nsec and a wavelength of 800 nm and a pulse laser beam having a pulse width of 45 nsec and a wavelength of 750 nm, and based on the intensity ratio of the acquired photoacoustic signal of each wavelength It is possible to separate and image the artery and vein inside the living body.

  The laser light source unit 113 can be configured as follows, for example. As shown in FIG. 5, the laser light source unit 113 includes a laser rod 151, a flash lamp 152, mirrors 153 and 154, a condensing lens 155, a wavelength selection unit 156, a drive unit 157, a drive state detection unit 158, and a control unit. 159.

  A condensing lens 155 and a wavelength selection unit 156 are arranged in the resonator composed of the mirrors 153 and 154. The wavelength selection unit 156 controls the wavelength of light resonating in the resonator to any one of a plurality of wavelengths to be emitted. The condensing lens 155 is disposed between the laser rod 151 and the wavelength selection unit 156, converges the light incident from the laser rod 151 side, and emits the light to the wavelength selection unit 156 side. That is, the condensing lens 155 reduces the beam diameter of the light traveling in the resonator toward the wavelength selection unit 156.

  The wavelength selection unit 156 includes, for example, a plurality of transmission regions and non-transmission regions that are alternately arranged along the circumferential direction. The plurality of transmission regions selectively transmit light having predetermined wavelengths corresponding to the plurality of wavelengths. The wavelength selection unit 156 has, for example, two transmission regions and two non-transmission regions. For example, a first band pass filter (BPF) that transmits light having a wavelength of 750 nm (center wavelength) is provided in one of the transmission regions, and a wavelength of 800 nm (center wavelength) is provided in the other. A second band-pass filter that transmits the light is provided.

  The wavelength selecting unit 156 having the above-described configuration selectively inserts any of a plurality of bandpass filters on the optical path of the resonator as it rotates. For example, the wavelength selecting unit 156 sequentially inserts an opaque region, a first band pass filter, an opaque region, and a second band pass filter in this order on the optical path of the resonator. By inserting the first bandpass filter on the optical path of the resonator, the oscillation wavelength by the resonator can be 750 nm, and by inserting the second bandpass filter on the optical path of the resonator, The oscillation wavelength by the device can be 800 nm.

  The wavelength selection unit 156 is configured to change the insertion loss in the resonator from a large loss (first loss) to a small loss (second loss) in accordance with the rotational drive. When the first or second bandpass filter is inserted on the optical path of the resonator, the insertion loss of the resonator is small (high Q), and when an opaque region is inserted on the optical path, The insertion loss is large (low Q). The wavelength selection unit 156 also serves as Qsw, and the wavelength selection unit 156 rapidly changes the insertion loss in the resonator from a large loss (low Q) to a small loss (high Q) with the rotational drive. A pulsed laser beam can be obtained.

  The driving unit 157 drives the wavelength selecting unit 156 so that the resonator performs Qsw pulse oscillation. That is, the drive unit 157 drives the wavelength selection unit 156 so that the wavelength selection unit 156 rapidly changes the insertion loss in the resonator from a large loss (low Q) to a small loss (high Q). For example, when the wavelength selection unit 156 is configured by a filter rotating body in which transmission regions (bandpass filters) and non-transmission regions are alternately arranged along the circumferential direction, the driving unit 157 is placed on the optical path of the resonator. The filter rotator is continuously rotated so that the non-transmissive areas and the transparent areas are alternately inserted. It is preferable that the switching time when the insertion loss in the resonator is switched from the high loss to the low loss accompanying the driving of the wavelength selection unit 156 is shorter than the Qsw pulse generation delay time. By switching the region inserted on the optical path of the resonator from the non-transmissive region to the transmissive region (first or second bandpass filter), the transmissive region (bandpass filter) inserted on the optical path transmits. The resonator can be subjected to Qsw pulse oscillation at a wavelength corresponding to the wavelength of light.

  The drive state detection unit 158 detects the drive state of the wavelength selection unit 156. The drive state detection means 158 detects the rotational displacement of the wavelength selection means 156 that is a filter rotator, for example. The drive state detection means 158 outputs BPF state information indicating the rotational displacement of the filter rotating body to the control unit 159 as a BPF state signal.

  The control unit 159 includes a rotation control unit 160 and a light emission control unit 161. The rotation control unit 160 controls the driving unit 158 so that the wavelength selection unit 156 rotates at a predetermined rotation speed. The rotation speed of the wavelength selection unit 156 is based on, for example, the number of wavelengths of pulsed laser light to be emitted from the laser light source unit 113 (number of bandpass filters in the filter rotator) and the number of pulsed laser light per unit time. Can be determined. The rotation control unit 159 controls the drive unit 157 so that the amount of change per predetermined time of the rotation position detected by the drive state detection unit 158 is constant. For example, the rotation control unit 159 controls the driving unit 157 so that the amount of change in the BPF state information during a predetermined time becomes a change amount according to a predetermined bandpass filter switching speed (rotational speed of the filter rotating body). Control.

  The light emission control unit 161 controls the flash lamp 152. That is, the light emission control unit 161 outputs a flash lamp (FL) control signal to the flash lamp 152, and irradiates the laser rod 151 with excitation light from the flash lamp 152. The light emission control unit 161 outputs the FL control signal to the flash lamp 152 at a time before the time when the wavelength selection unit 156 switches the insertion loss of the resonator from the large loss to the small loss, and irradiates the excitation light. Let That is, the light emission control unit 161 has the rotational position detected by the drive state detection unit 158 at a position that is a predetermined amount before the rotational position at which the wavelength selection unit 156 switches the insertion loss of the resonator from large loss to small loss. Then, an FL control signal is sent to the flash lamp 152 to irradiate excitation light.

  For example, the light emission control unit 161 indicates that the information represented by the BPF state signal indicates that the laser rod from the drive position of the wavelength selection unit 156 in which a bandpass filter corresponding to the wavelength of the pulsed laser light to be emitted is inserted on the optical path of the resonator. When the information indicating the position obtained by subtracting the amount of displacement of the wavelength selection unit 156 during the time required for the excitation 151 is output, an FL control signal is output, and the laser lamp 151 is irradiated with the excitation light from the flash lamp 152. After the output of the FL control signal, the light emission control unit 161 detects that the rotation position detected by the driving state detection unit 158 is a rotation position where the wavelength selection unit 156 switches the insertion loss of the resonator from large loss to small loss. A Qsw synchronization signal indicating the timing when is turned on is generated and output to the ultrasonic unit 12 (see FIG. 1).

  The control means 30 shown in FIG. 1 controls each part in the ultrasonic unit 12. The trigger control circuit 29 outputs a BPF control signal for controlling the rotation speed of the wavelength selection unit 156 to the laser light source unit 113. The trigger control circuit 29 outputs an FL standby signal for controlling the light emission of the flash lamp 152 to the laser light source unit 113. For example, the trigger control circuit 29 receives the current rotational displacement position of the filter rotator from the rotation control unit 160 of the laser light source unit 113 and outputs an FL standby signal at a timing based on the received rotational displacement position.

  The trigger control circuit 29 receives from the laser light source unit 113 a Qsw synchronization signal indicating the timing when Qsw is turned on, that is, the laser emission timing. When receiving the Qsw synchronization signal, the trigger control circuit 29 outputs a sampling trigger signal to the AD conversion means 22. The AD conversion unit 22 starts sampling of the photoacoustic signal based on the sampling trigger signal.

  FIG. 6 shows a detailed configuration example of the wavelength selection unit 156. As shown here, the wavelength selection unit 156 is configured as a filter rotating body 170 including a plurality of transmission regions (bandpass filters) having different transmission wavelengths. The filter rotator 170 includes a first transmission region 171 that selectively transmits light having a wavelength of 750 nm, a second transmission region 172 that selectively transmits light having a wavelength of 800 nm, and a non-transmission region 173 that does not transmit light. 174. The non-transparent area is not required to have the ability to completely block light. The light may be slightly transmitted to such an extent that unnecessary laser oscillation does not occur.

  The first transmission region 171 and the second transmission region 172 are formed, for example, in fan shapes with central angles θ1 and θ2, respectively. The magnitudes of the central angles θ1 and θ2 are appropriately set according to the length of the pulse width of the pulse laser beam having each wavelength. The light condensed by the condensing lens 155 is applied to the vicinity of the periphery of the filter rotating body 170 shown in FIG. When the filter rotating body 170 is rotated clockwise, the first transmission region 171, the non-transmission region 173, the second transmission region 172, and the non-transmission region 174 are inserted in this order on the optical path of the resonator. As described above, the first transmission region 171 and the second transmission region 172 can change the wavelength of light transmitted through each region, that is, by changing the transmission wavelength of the bandpass filter provided in each transmission region. Pulsed laser light having a different wavelength for each pulse can be obtained.

  FIG. 7 shows the relationship between the wavelength and the transmittance in the transmission region. It is assumed that the transmittance for light having a center wavelength of 750 nm in the first transmission region (first bandpass filter) 171 is 90% or more. Its bandwidth is approximately 10 nm. In the second transmission region (second bandpass filter) 72, the transmittance with respect to light having a central wavelength of 800 nm is 90% or more. Its bandwidth is approximately 10 nm.

  Here, it is assumed that the rotation frequency of the filter rotating body 170 is 100 Hz (rotational speed 6000 rpm). In that case, since light passes through two transmission regions per rotation, the number of pulse laser beams emitted from the laser unit 113 is 200 per second (200 Hz operation). For example, a filter rotating body having a radius of 2 inches (50.4 mm) is considered as the filter rotating body 170. The beam diameter is 100 μm. The angular velocity is ω = 2πf = 628.3 [rad / sec], and the linear velocity is v = rω = 628.8 [rad / sec] × 50.4 [mm] = 31.7 [m / s]. The time for crossing the beam (switching time) is 3.15 μsec.

  As a characteristic of Qsw, the switching time (for example, the switching time from the non-transmissive region to the first or second transmissive region) is approximately several microseconds or less (smaller than the generation delay time of the Qsw pulse). This is a condition for obtaining a pulse. The central angle θ of the transmission region is selected on the condition that the beam is not disturbed during the time of traversing the beam + Qsw delay time. In the above numerical example, the transmissive region may be continued for 3.15 μsec + several μsec = approximately 10 μsec. Therefore, 31.7 [m / s] × 10 μsec = 317 μm is the width of the width, and 0.35 ° in angle. Considering the production, the central angle θ should be 1 ° to several degrees.

  FIG. 8 shows a part of the laser unit 113. The wavelength selection means 156 is configured as a filter rotating body 170 as shown in FIG. 6, for example. The beam diameter on the filter rotating body 170 is preferably small. Therefore, in this example, the converging lens 155 is used to converge the beam. The beam diameter on the filter rotating body 170 is desirably 100 μm or less. The lower limit is determined by the diffraction limit and is several μm. The filter rotator 170 is preferably inclined by about 0.5 ° to 1 °, for example, with respect to the optical axis so as to rotate in a plane inclined at a predetermined angle with respect to the optical axis of the resonator. In this way, by arranging it slightly oblique to the optical axis of the resonator, it is possible to prevent unnecessary reflection components from causing parasitic oscillation.

  The driving means 157 is a servo motor, for example, and rotates the filter rotating body 170. The rotation frequency of the filter rotator 170 is preferably high. Mechanically, it can be up to about 1 kHz. The drive state detection means 158 is composed of, for example, a rotary encoder. The rotary encoder detects the rotational displacement of the filter rotator with a slit-type rotating plate attached to the output shaft of the servo motor and a transmission type photo interrupter, and the rotation of the filter rotator 170 is converted into an electrical signal (BPF state signal). Convert. This BPF state signal is used as a master clock and is sent to the light emission control unit 161 as a synchronization signal. The light emission control unit 161 determines the flash lamp light emission timing in accordance with the rotation of the filter rotating body 170 rotating with high accuracy.

  FIG. 9 shows the timing of flash lamp light emission and the timing of pulsed laser light. Time t2 is assumed to be a time corresponding to a rotational position at which the rotating filter rotator 170 switches from the non-transmissive region to the transmissive region. Time t1 is the time obtained by subtracting the time required for excitation of the laser rod 151 from time t2. The light emission controller 161 causes the flash lamp 152 to emit light when the rotational position of the filter rotator 170 reaches a position corresponding to time t1 ((a) of FIG. 9). As the flash lamp 152 emits light, the laser rod 151 is excited.

  After the flash lamp emission, at time t2, the filter rotator 170 switches from the non-transmissive area to the transmissive area (the first transmissive area 171 or the second transmissive area 172) at approximately the same time that the flash lamp is extinguished. ((B) of FIG. 9). The time required for switching from the non-transmissive region to the transmissive region (switching time) is preferably as short as possible, and is several μsec or less, more desirably 0.5 μsec or less. When a transmission region that transmits light of 750 nm is inserted on the optical path of the resonator, light having a wavelength of 750 nm oscillates at time t3, and pulse laser light having a wavelength of 750 nm is obtained ((c) in FIG. 9). ). On the other hand, when the transmission region inserted on the optical path of the resonator is a transmission region that transmits light having a wavelength of 800 nm, light having a wavelength of 800 nm oscillates in a Qsw pulse, and pulse laser light having a wavelength of 800 nm is obtained. After the transmissive region portion of the filter rotating body 170 continues for about 10 μsec, the filter rotator 170 switches to the non-transmissive region again at time t4.

  FIG. 10 shows the emission waveform of the pulse laser beam. If the filter rotator 170 as described above is used, as shown in FIG. 10, one pulse of a pulse laser beam having a relatively wide pulse width of 750 nm and a pulse laser beam having a relatively narrow pulse width of 800 nm. It becomes possible to switch and emit each. For example, if the rotation frequency of the filter rotator 170 is 100 Hz, 200 pulsed laser beams can be obtained per second while alternately switching the pulse width and wavelength.

  The photoacoustic imaging apparatus and method of the present invention are not limited to the above embodiment, and various modifications and changes from the configuration of the above embodiment are also included in the scope of the present invention.

  For example, the present invention can also be applied to a photoacoustic imaging apparatus and method in which a deconvolution process is performed. FIG. 2 is a block diagram showing a part of the photoacoustic imaging apparatus configured to perform the deconvolution processing. 2 is inserted, for example, between the image reconstruction means 25 and the detection / logarithmic conversion means 26 shown in FIG. 1, and is connected to the optical differential waveform deconvolution means 40 and its subsequent stage. Correction means 46. The split waveform deconvolution means 40 includes Fourier transform means 41 and 42, an inverse filter calculation means 43, a filter application means 44, and a Fourier inverse transform means 45.

  The partial waveform deconvolution means 40 deconstructs an optical pulse differential waveform obtained by differentiating the time waveform of the light intensity of the pulsed laser light irradiated to the subject from the data indicating the photoacoustic image output from the image reconstruction means 25. Volute. By this deconvolution, photoacoustic image data showing an absorption distribution is obtained.

  Hereinafter, this deconvolution will be described in detail. The Fourier transform means (first Fourier transform means) 41 of the optical differential waveform deconvolution means 40 converts the reconstructed photoacoustic image data from a time domain signal to a frequency domain signal by discrete Fourier transform. . The Fourier transform means (second Fourier transform means) 42 converts a signal obtained by sampling the optical pulse differential waveform at a predetermined sampling rate from a time domain signal to a frequency domain signal by discrete Fourier transform. For example, FFT can be used as the Fourier transform algorithm.

  In the following description, it is assumed that the sampling rate of the acoustic wave detection signal in the AD conversion means 22 is equal to the sampling rate of the optical pulse differential waveform. For example, the acoustic wave detection signal is sampled in synchronization with a sampling clock of Fs = 40 MHz, and the optical differential pulse is also sampled at a sampling rate of Fs_h = 40 MHz. The Fourier transform unit 41 Fourier transforms the photoacoustic image data output from the image reconstruction unit 25 obtained as a result of sampling at 40 MHz, for example, by 1024 points of Fourier transform. Further, the Fourier transform means 42 performs Fourier transform on the optical pulse differential waveform sampled at 40 MHz by 1024 points of Fourier transform.

The inverse filter calculation unit 43 obtains the inverse of the Fourier transformed optical pulse differential waveform as an inverse filter. For example, the inverse filter calculation means 43 obtains conj (fft_h) / abs (fft_h) ² as an inverse filter, where fft_h is a signal obtained by Fourier transforming the optical pulse differential waveform h. The filter applying unit 44 applies the inverse filter obtained by the inverse filter calculating unit 43 to the photoacoustic image data Fourier-transformed by the Fourier transform unit 41. For example, the filter application unit 44 multiplies the Fourier coefficient of the photoacoustic image data by the Fourier coefficient of the inverse filter for each element. By applying the inverse filter, the optical pulse differential waveform is deconvolved in the frequency domain signal. The Fourier inverse transform unit 45 transforms the photoacoustic image data to which the inverse filter is applied from a frequency domain signal to a time domain signal by Fourier inverse transform. An absorption distribution signal in the time domain is obtained by inverse Fourier transform.

  By performing the processing described above, the optical differential term can be removed from the acoustic wave detection signal in which the optical differential term is convoluted, and the absorption distribution can be obtained from the acoustic wave detection signal. When such an absorption distribution is imaged, a photoacoustic image showing the absorption distribution image is obtained.

  The correction means 46 corrects the data obtained by deconvolution of the optical pulse differential waveform, and removes the influence of the reception angle dependent characteristic of the ultrasonic transducer in the probe 11 from the data obtained by deconvoluting the optical pulse differential waveform. Further, the correction means 46 removes the influence of the incident light distribution of the light on the subject from the data obtained by deconvolution of the optical pulse differential waveform in addition to or instead of the reception angle dependent characteristics. Note that a photoacoustic image may be generated without performing such correction.

DESCRIPTION OF SYMBOLS 10 Photoacoustic imaging apparatus 11 Probe 12 Ultrasonic unit 13, 113 Laser light source unit 14 Image display means 21 Reception circuit 22 AD conversion means 23 Reception memory 24 Data separation means 25 Image reconstruction means 26 Detection / logarithm conversion means 27 Image construction Means 30 Transmission control circuit 31 Control means 32 Q switch laser 33 Flash lamp 41, 42 Fourier transform means 43 Inverse filter operation means 44 Filter application means 45 Fourier inverse transform means 50 Sampling frequency adjustment means 51 Storage means

Claims (11)

An acoustic wave detection signal is obtained by irradiating a subject with pulsed light having a wavelength that is absorbed within the subject, thereby detecting an acoustic wave emitted from the subject by an acoustic wave detection means, and then detecting the acoustic wave detection signal. Is a photoacoustic imaging method in which photoacoustic data is obtained by sampling, and the subject is imaged based on the photoacoustic data and displayed on the image display means, and the pulse width of the pulsed light can be changed. In the photoacoustic imaging method,
The photoacoustic imaging method, wherein the sampling frequency is adjusted to be higher as the pulse width is smaller.   2. The light according to claim 1, wherein the sampling frequency is adjusted to be in a range of 4 to 10 times a frequency of a waveform obtained by differentiating a time waveform of the light intensity of the pulsed light. Acoustic imaging method.   2. The photoacoustic imaging method according to claim 1, wherein the sampling frequency is adjusted so as to have a substantially constant magnification with respect to a frequency of a waveform obtained by differentiating a time waveform of the light intensity of the pulsed light. The correspondence between the sampling frequency and the pulse width is stored in a storage means,
When the pulse width is determined, the frequency stored in the storage means corresponding to the pulse width is read out,
4. The photoacoustic imaging method according to claim 1, wherein a sampling frequency is set according to the read frequency. The reflected ultrasound that is irradiated on the subject in a pulsed manner and reflected by the subject is detected by reflected ultrasound detection means,
5. When sampling a reflected ultrasonic detection signal obtained by this detection, the sampling frequency is adjusted to be higher as the ultrasonic pulse width is smaller. The photoacoustic imaging method as described. The reflected ultrasound that is irradiated on the subject in a pulsed manner and reflected by the subject is detected by reflected ultrasound detection means,
5. The photoacoustic according to claim 1, wherein when the reflected ultrasonic detection signal obtained by this detection is sampled, the sampling frequency is constant irrespective of the pulse width of the ultrasonic wave. Imaging method. The object is irradiated with pulsed light having a wavelength that is absorbed inside the object, thereby detecting an acoustic wave emitted from the object by an acoustic wave detecting means to obtain photoacoustic data, based on the photoacoustic data, In a photoacoustic imaging apparatus that images a subject and displays the image on an image display unit, the photoacoustic imaging apparatus is configured to be capable of changing the pulse width of pulsed light.
A photoacoustic imaging apparatus comprising sampling frequency adjusting means for adjusting a sampling frequency to be higher as the pulse width of pulsed light is smaller.   The sampling frequency adjusting means adjusts the sampling frequency so as to be within a range of 4 to 10 times the frequency of the waveform obtained by differentiating the time waveform of the light intensity of the pulsed light. The photoacoustic imaging apparatus according to claim 7.   The sampling frequency adjusting means adjusts the sampling frequency so as to have a substantially constant magnification with respect to the frequency of the waveform obtained by differentiating the time waveform of the light intensity of the pulsed light. 8. The photoacoustic imaging apparatus according to 7. Storage means for storing the correspondence between the sampling frequency and the pulse width is provided,
When the pulse width is determined, the sampling frequency adjusting means reads the frequency stored in the storage means corresponding to the pulse width, and sets the sampling frequency according to the read frequency. 10. The photoacoustic imaging apparatus according to claim 7, wherein the photoacoustic imaging apparatus is configured as described above.   A photodifferential waveform deconvolution means for generating a signal obtained by deconvolution of a photodifferential waveform that is a waveform obtained by differentiating a time waveform of the light intensity of the pulsed light irradiated to the subject from the photoacoustic data is provided. The photoacoustic imager according to any one of claims 7 to 10.

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