Patent Application
20150366459
1. Field of the Invention
The present invention relates to an object information acquiring apparatus and a laser apparatus used therein.
2. Description of the Related Art
Photoacoustic tomography systems (photoacoustic measurement systems) are being developed for medical use that use short-pulse oscillation lasers (S. Manohar, et al., “Region-of-interest breast studies using the Twente Photoacoustic Mammoscope (PAM)” Proc. of SPIE, Vol. 6437, 643702). Photoacoustic tomography (PAT) refers to a technique for forming images by irradiating a measurement segment with a pulsed laser for about several tens to several hundred nanoseconds, receiving the photoacoustic waves generated therein with a probe, and processing the resulting received signals. The use of PAT makes it possible to analyze body functions from spectrum measurements based on the absorption coefficients of body tissue.
In addition, lasers using Q switching are used to generate short-pulsed light used to measure acoustic waves. Q-switched oscillation refers to a technology for generating high-output, short-pulse laser light by controlling an indicator of resonator performance in the form of a Q value that is a function of the half-width of an oscillation pulse. Laser oscillation at that time is referred to as giant pulse oscillation. An apparatus has been proposed that acquires object information by emitting laser light onto an object by laser oscillation using Q switching in this manner (Japanese Patent Application Laid-open No. 2013-89680).
However, abnormal emission referred to as prelasing occurs when the properties of an apparatus become unstable in a laser apparatus using Q switching. In addition, this prelasing occurs at a timing that is earlier than the inherently required oscillation timing of short-pulsed giant pulses. Thus, this prelasing has to be detected and reduced. If prelasing occurs in an object information acquiring apparatus, light propagates to body tissue prior to the timing of giant pulse oscillation. As a result, acoustic wave signals are generated from the body tissue. Consequently, these signals become noise when analyzing the acoustic wave signals and impair the obtaining of accurate biological information (object information). In addition, since variation occurs in the width per se of a single giant pulse, desired acoustic wave signals are unable to be obtained. Moreover, there is a strong correlation between the occurrence of prelasing and the ambient temperature of the laser apparatus. However, suppressing the occurrence of prelasing by controlling the temperature of the laser apparatus has the effect of, for example, increasing the size of the object information acquiring apparatus or increasing production cost.
With the foregoing in view, an object of the present invention is to provide an object information acquiring apparatus that reduces the effects of prelasing.
In order to achieve the above-mentioned object, the present invention provides an object information acquiring apparatus, comprising: a laser light source; a detector configured to detect a portion of laser light emitted from the laser light source; a determination unit configured to determine whether or not abnormal emission is contained in the laser light, based on a detection result of the detector; a irradiator configured to radiate the laser light onto an object, a receiver configured to receive acoustic waves that propagate from the object, based on radiation of the laser light; an acquisition unit configured to acquire information relating to the object, based on a reception result of the receiver, and a controller configured to control output of the laser light, based on a determination result of the determination unit.
The present invention also provides an apparatus, comprising: a laser light source; a detector configured to detect a portion of laser light emitted from the laser light source; a determination unit configured to determine whether or not abnormal emission is contained in the laser light, based on a detection result of the detector; and a controller configured to control the laser light source, based on a determination result of the determination unit.
The present invention also provides an apparatus, comprising: a laser light source; a detector that detects a portion of laser light emitted from the laser light source; a determination unit that determines whether or not abnormal emission is contained in the laser light, based on a detection result of the detector; and a controller that controls a temperature of the laser light source, based on a determination result of the determination unit.
As has been described above, according to the present invention, an object information acquiring apparatus can be provided that reduces the effects of prelasing.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
The following provides a detailed explanation of embodiments of the present invention while referring to the drawings. Furthermore, the same reference numbers are used to indicate the same constituents as a general rule, and explanations thereof are omitted. However, detailed calculation formulas or calculation procedures and the like described to follow are to be suitably altered according to the configuration and various conditions of the apparatus to which the present invention is applied, and are not intended to limit the scope of the present invention to the following descriptions.
The object information acquiring apparatus of the present invention includes an apparatus that receives acoustic waves generated within an object as a result of irradiating the object with light (electromagnetic waves) such as near infrared rays, and uses a photoacoustic effect whereby object information is acquired in the form of image data. In the case of an apparatus that uses photoacoustic effects, the object information that is acquired refers to the generation source distribution of acoustic waves generated by light radiation, initial acoustic pressure distribution in an object, optical energy absorption density distribution or absorption coefficient distribution derived from initial acoustic pressure distribution, or the concentration distribution of a substance that composes a tissue. The concentration distribution of a substance refers to, for example, oxygen saturation distribution, total hemoglobin concentration distribution or oxidized/reduced hemoglobin concentration distribution.
In addition, object information of a plurality of locations in the form of characteristic information may also be acquired as a two-dimensional or three-dimensional characteristics distribution. Characteristics distribution can be generated in the form of image data indicating characteristic information within an object. Acoustic waves as referred to in the present invention are typically ultrasonic waves, and include those referred to as sound waves or photoultrasonic waves. Acoustic waves generated by photoacoustic effects are referred to as photoacoustic waves or photoultrasonic waves. An acoustic detector (such as a probe) receives acoustic waves generated or reflected within an object.
First, total output is smaller in the case of actual prelasing emission in comparison with giant pulse emission. Consequently, time-based resolution and detection of prelasing alone requires the use of a sensor having extremely high sensitivity and time resolution, thereby resulting in the problem of the sensor becoming expensive. Moreover, the amount of time from prelasing oscillation to the generation of a Q switching off signal for giant pulse oscillation is extremely short at several nanoseconds to several tens of nanoseconds. Consequently, this amount of time is excessively short for realistically controlling giant pulse oscillation not to be carried out during that time, thereby making the realization thereof difficult.
The object information acquiring apparatus 101 is an apparatus that acquires information on the interior of the object 111 from a photoacoustic signal. A portion of the light energy that propagates through the interior of the object 111 is absorbed by an absorbing body (sound source) such as blood hemoglobin. Whereupon, the acoustic wave signal 117 is generated due to thermal expansion of the light-absorbing body, and that acoustic wave signal 117 propagates through the object interior. The propagating acoustic wave signal 117 is then converted to the electrical signal 118 with a probe in the acoustic wave receiver 105, and is transmitted to an acquisition unit (an acquisition device) in the form of the acoustic wave signal processing unit 106. The electrical signal 118 is converted to optical characteristic value distribution information and the like within an object by the acoustic wave signal processing unit 106 where it becomes object information. In addition to optical characteristic value distribution and absorption coefficient distribution, the generated object information can also include initial acoustic pressure distribution, substance concentration and oxygen saturation based thereon. Moreover, image data can also be included for forming and displaying an image (image reconstruction) based on this information.
The laser light source 102 supplies light for preferably passing through the object 111 in the form of a body and transmitting a photoacoustic signal attributable to a measurement target in the form of hemoglobin present in a blood vessel and the like. It is necessary for high-output light to propagate to the object 111 in order to enhance the signal accuracy of the photoacoustic signal, namely the acoustic wave signal 117. Laser light is used for this purpose. In addition, since it is necessary for the light to reach the measurement target in the form of hemoglobin in a blood vessel and the like with little absorption in the object 111, light for which there are limitations on the wavelength thereof and having wavelength characteristics of about 500 nm to 1200 nm is used in particular as light that easily propagates through the object 111. Consequently, an alexandrite laser or titanium-sapphire laser, which emits light of a wavelength within that range, is used preferably. In addition, pulsed light having a short pulse width, in which the pulse width is several tens to several hundred nanometers, is used for the laser light 115 in order to improve signal accuracy of the acoustic wave signal 117. A laser capable of giant pulse oscillation by Q switching is preferably used to generate such laser light having a high output and short pulse width. The laser light source 102 may be integrally incorporated in the object information acquiring apparatus 101 or may be installed outside thereof.
The light-transmitting optical system 103 has the function of propagating light from the laser light source 102 to the light-radiating optical system 104. Since the laser light source 102 and the light-radiating optical system 104 are at a distance from each other in terms of their arrangement and the laser light 115 ends up spreading, a lens, for example, is present in the optical path of the laser to suppress this spreading. In addition, in the case of not arranging the laser light source 102 and the light-radiating optical system 104 on a straight line in terms of their arrangement, a reflecting mirror and the like is arranged there between to adjust the direction of travel of the laser light 115 and guide the laser light to a desired location. Laser light may also be guided to a measurement apparatus, such as a timing trigger that measures the timing of light transmission required by the acoustic wave signal processing unit 106 or the laser light sensor 107 in the present invention, as necessary. Therefore, the branch mirror 108 is arranged in the optical path and that branched light is led to these measurement apparatuses. In addition, there are cases in which an optic fiber may be partially used for transmitting light within the light-transmitting optical system 103.
The light-radiating optical system 104 forms the radiated light 116 from the laser light 115 propagated by the light-transmitting optical system 103, and radiates the radiated light 116 onto a target measurement segment of the object 111. Consequently, the light-radiating optical system 104 fulfills the role of deforming the distribution of the amount of the laser light 115 to a preferable distribution of the amount of light for the object 111 primarily by spreading the laser light 115, for example. A lens or diffuser is arranged to form the radiated light 116 by preferably expanding and diffusing the laser light 115 so as to preferably obtain the acoustic wave signal 117 along with preventing the radiation dose to a body in the form of the object 111 from exceeding a specified value.
The acoustic wave receiver 105 has a probe that receives the acoustic wave signal 117. This probe that receives the acoustic wave signal 117 generated, for example, on the surface or in the interior of a body by pulsed light in the form of the radiated light 116, converts the acoustic wave signal 117 into the analog electrical signal 118. Any type of probe may be used provided it is able to receive acoustic wave signals, examples of which include probes using piezoelectric phenomena, probes using optical resonance and probes utilizing a change in electrostatic capacitance. The probe of the present embodiment has a plurality of receiving elements (such as piezo elements) arranged one-dimensionally or two-dimensionally, and the receiving elements are arranged in the shape of a spiral on the bottom of a bowl-shaped stationary component. The use of this multidimensional arrangement of receiving elements makes it possible to simultaneously receive the acoustic wave signal 117 at a plurality of locations, thereby making it possible to shorten measuring time. In the case of desiring to increase the number of locations where measurements are made with the probe, the probe may be made to receive the acoustic wave signal 117 by scanning a plurality of locations. After being converted to the electrical signal 118, the acoustic wave signal 117 received with the probe is used to generate characteristics information with the acoustic wave signal processing unit 106.
The acoustic wave signal processing unit 106 is composed of a computer or other information processing apparatus and circuitry, and carries out processing and calculations on the electrical signal 118. The acoustic wave signal processing unit 106 has a conversion unit such as an A/D converter that converts electrical signals obtained from the probe from analog signals to digital signals. The conversion unit is preferably able to process a plurality of signals simultaneously. This enables the amount of time until an image is formed (image reconstruction) to be shortened. The converted digital signals are stored in memory. The acoustic wave signal processing unit 106 generates object information such as optical characteristic value distribution by back projection in a time domain, for example, using the data and the like stored in memory.
An excitation unit (an excitation device) 204, a laser medium 205 and a Q switch 206 are arranged within the resonator. Voltage applied to the excitation unit 204 and the Q switch 206 is controlled by the laser controller 211. In the case of using a flash lamp or semiconductor laser and using a rod-shaped laser medium 205, the excitation unit 204 carries out optical excitation from a side of the laser medium 205. A Pockels cell, which is an optical crystal of potassium dihydrogen phosphate (KDP) or dipotassium deuterium phosphate (DKDP) and the like, is used for the Q switch 206. A Pockels cell is an element that rotates the direction of polarization of light that passes through the element by using anisotropy to change refractive index in proportion to the strength of an electric field, and is widely used to obtain giant-pulsed light having a narrow oscillating pulse width and high output intensity. Although pulse width varies according to the type of laser medium, resonator length and resonator status, a pulse width of 100 ns or less is obtained. The configuration is as shown in
In the case of solid-state lasers using a rod-shaped laser medium in particular, if prelasing oscillation occurs in the center of the rod where excitation efficiency is high, that oscillation takes on the form of seed light causing the subsequently occurring giant pulse oscillation to concentrate in the center. As a result, intense oscillation occurs having a characteristic intensity distribution to be subsequently described. Furthermore, the direction of polarization of reciprocal light can be changed using a device such as a Pockels cell for the Q switch that induces refractive index anisotropy in an electric field. Resonance is suppressed by this characteristic of Q switching. In the case of using such an optical shutter that suppresses resonance, the polarized state of giant pulse light differs from the polarized state of prelasing. This characteristic of Q switching is used in the examples to be subsequently described.
The laser light sensor 107 described in the present invention does not detect feint emission when the Q switch is on (resonance suppression period), but rather acquires oscillation intensity so as to include both feint light resulting from prelasing and giant pulse oscillation after Q switching, which emits feint light resulting from prelasing as seed light, has been switched off on a time axis. A signal corresponding to the detection result in the form of that acquired intensity is output to a determination unit 123 shown in
The following provides an explanation of the method used to control an object information acquiring apparatus or laser apparatus contained therein in the case prelasing has occurred. Although there are cases in which a control method is introduced so as to reduce a suspected cause of prelasing provided the cause of prelasing can be presumed, there are also cases in which the object information acquiring apparatus or laser apparatus per se contained therein is shut down. In the case the cause is able to be presumed and is reversible, the member that composes the Q switch may be a member provided so as to change refractive index anisotropy by applying a voltage thereto in the manner of a Pockels cell. At this time, prelasing is presumed to occur as a result of the voltage deviating from the optimal applied voltage of the Pockels cell due to temperature effects and the like of the laser apparatus. In such cases, controlling the laser apparatus by changing the voltage applied to the Pockels cell makes it possible to suppress the occurrence of prelasing. A stable object information acquiring apparatus can be provided by providing such a control mechanism. In addition, in the case prelasing occurs randomly due to unstable operation of the Q switch, for example, identifying information as to whether or not emissions contain prelasing is output in combination with a photoacoustic signal. Use of only the acoustic wave signal obtained with emissions not containing prelasing, for example, for image reconstruction can then be used to remove noise from reconstructed images. Alternatively, information indicating that an image has been reconstructed based on laser light in which prelasing has occurred can also be output along with the reconstructed image.
Here, reference is again made to
Here, there is variation in the range of emission energy when the combined emission energy of prelasing and giant pulse oscillation is acquired. However, that energy is measured after concentrating in a range equivalent to a diameter of about 2 mm, which is narrower than the diameter of 5 mm of the range over which giant pulse oscillation occurs. The total amount of emission energy measured by each element remains at 300 mJ. Namely, this is the same as the total amount of emission energy of each element for the typical giant pulse oscillation shown in
Determination criteria were set in the determination unit 123 as to whether or not prelasing is occurring in a laser demonstrating such emission energy distribution characteristics. Namely, a determination threshold was set so that cases in which the average value of energy entering a range having a diameter of 2 mm is 0.25 mJ or more are determined to indicate the presence of prelasing. As a result, prelasing was able to be effectively detected. Namely, the determination unit 123 compares a prescribed value in the form of the above-mentioned determination threshold with the above-mentioned average value based on the results of detection by the laser light sensor 107a. As a result, when that comparison result is a result such that the above-mentioned average value exceeds the above-mentioned determination threshold, prelasing is determined to be occurring, and that determination result 119 is output. On the other hand, when that comparison result is a result such that the above-mentioned average value does not exceed the above-mentioned determination threshold, prelasing is determined not to be occurring, and that determination result 119 is output. Possible output destinations consist of the acoustic wave signal processing unit 106 and the laser light source 102.
Prelasing was able to be effectively detected by judging whether or not prelasing is occurring by monitoring the energy entering a diameter of 2 mm in a laser demonstrating such emission energy distribution characteristics. In addition, in the case prelasing has been determined to have occurred as a result of being detected, information was obtained indicating that the cause of the occurrence thereof is a rise in temperature of the laser system. Consequently, a control function was provided that suppresses the occurrence of prelasing by having the laser controller lower the set temperature of the laser system temperature control mechanism (such as an air-conditioner) by 0.1° C. As a result, an object information acquiring apparatus was able to be produced in which instability of giant pulse oscillation was suppressed. In addition, as a result of adopting a sensor configuration like that described above, the occurrence of prelasing was able to be detected easily even with the laser light sensor 107 in which time resolution is not that high.
As a result of using the above-mentioned configuration, data containing abnormal emissions can be used while thinning out the data during image formation, thereby allowing the production of an object information acquiring apparatus that enables favorable data acquisition.
As a result, data based on abnormal oscillation can be used after being thinned out when forming an image from data that contains emissions attributable to abnormal oscillation, namely prelasing. Thus, an object information acquiring apparatus can be produced that uses an inexpensive laser light sensor 107b and is provided with the laser light sensor unit 126 that enables favorable data acquisition.
Practical Example 5 was produced using the same constituent members as the object information acquiring apparatus used in Practical Example 1, and feedback control was carried out for control following determination of prelasing. More specifically, the occurrence of prelasing in this laser is characterized by a rise in temperature of the Q switch due to continuous use of the laser, and the voltage applied to the Q switch was determined to have a tendency to be below the lower limit voltage of the threshold value at which prelasing occurs. A set value of 2 kV was used for the voltage applied to the Q switch. Consequently, a feedback circuit was provided as a control function in the laser controller so that the voltage applied to the Q switch is raised by 100 V when prelasing is detected. As a result of providing this control function, an object information acquiring apparatus can be produced that easily detects prelasing and enables favorable data acquisition.
Practical Example 6 was produced using the same constituent members as the object information acquiring apparatus used in Practical Example 1, and feed-forward control was carried out for control following determination of prelasing. More specifically, the occurrence of prelasing is detected based on the above-mentioned determination criteria by the determination unit 123 when prelasing has occurred. A mechanism, whereby information indicating that prelasing has occurred is output in combination with photoacoustic signal data for each pulse at that time, is provided as feed-forward control.
As a result, data containing abnormal emissions can be used while thinning out the data during image formation, thereby allowing the production of an object information acquiring apparatus that enables favorable data acquisition. Furthermore, image reconstruction may be carried out as is without the above-mentioned thinning processing by combining information indicating that prelasing has not occurred when prelasing has actually not occurred, and image reconstruction based on laser light in which prelasing has occurred may be notified to an operator.
Explanations of each of the examples are intended to be exemplary in terms of explaining the present invention, and the present invention can be carried out by suitably modifying or combining these examples within a range that does not deviate from the gist of the present invention. The above-mentioned processing and means of the present invention can be freely combined as desired provided they do not give rise to technical contradiction. Furthermore, the various characteristics of the present invention are not limited to the above-mentioned examples, and can be applied over a wide range. In addition, the object information acquiring apparatuses of the above-mentioned Practical Examples 1 to 6 can also be carried out using an information processing apparatus provided with a CPU or memory and the like that operates according to a program (software). Alternatively, each constituent of this object information acquiring apparatus may be composed with hardware such as circuity that enables input/output and arithmetic processing of information.
Consequently, whether or not prelasing has occurred cannot be determined with the single element sensor in the form of the laser light sensor 124 according to this comparative art. Consequently, an object information acquiring apparatus using this sensor of the comparative art was unable to acquire favorable data relating to object information. On the other hand, according to each of the examples of the present invention, an object information acquiring apparatus can be provided that is capable of detecting prelasing light, reducing the effects of prelasing and enabling the acquisition of favorable images as was previously described.
Embodiment(s) of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be referred to more fully as a ‘non-transitory computer-readable storage medium’) to perform the functions of one or more of the above-described embodiment(s) and/or that includes one or more circuits (e.g., application specific integrated circuit (ASIC)) for performing the functions of one or more of the above-described embodiment(s), and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s) and/or controlling the one or more circuits to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more processors (e.g., central processing unit (CPU), micro processing unit (MPU)) and may include a network of separate computers or separate processors to read out and execute the computer executable instructions. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™) a flash memory device, a memory card, and the like.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2014-125527, filed on Jun. 18, 2014, which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014-125527 | Jun 2014 | JP | national |
