US20180188155A1 - Object information acquiring apparatus - Google Patents

Abstract

An object information acquiring apparatus is used, including: an optical transmission system for transmitting light from a light source; a photoacoustic probe including an irradiating end for irradiating an object with light and a receiver for receiving acoustic waves generated from the object that has been irradiated with light; a processor for acquiring information on the object based on the acoustic waves; a light quantity meter for measuring the quantity of light emitted from the irradiating end; a memory for storing a measurement value; and a presentation unit. The processor compares the measurement value with a reference value of light quantity or a history of measurement value stored in the memory, and provides a result regarding whether or not the measurement value is within a reference range to the presentation unit.

Description

BACKGROUND OF THE INVENTIONField of the Invention
• The present invention relates to an object information acquiring apparatus.
Description of the Related Art
• Photoacoustic tomography (hereinafter, called “PAT”) is attracting attentions as a technique for specifically imaging angiogenesis associated with cancer. In PAT, light (near infrared light) is irradiated on an object, which in turn generates from a depth therein photoacoustic waves to be received by an ultrasonic probe for imaging.
• A schematic view of a handheld photoacoustic apparatus described innon-patent literature 1 is shown inFIG. 7. Aphotoacoustic probe104 has a configuration in which areceiver106 for receiving photoacoustic waves is sandwiched and fixed byirradiating ends103boffiber bundles103 of an illuminating optical system. At an incident end103aof thefiber bundle103, light from alight source101 enters. The light passes through thefiber bundles103 and irradiates an object from theirradiating ends103b.This induces generation of photoacoustic waves from the interior of the object due to the photoacoustic effect, which photoacoustic waves are received by thereceiver106.
• The received signal is converted into an electrical signal, which then undergoes amplification, digitization, and image reconstruction by aprocessor107 of an ultrasonic apparatus (US). The configured image information (IMG) is transmitted to amonitor108, which serves as a display unit, and displayed as a photoacoustic image.
• Non Patent Literature 1: S. A. Ermilov et al., “Development of laser optoacoustic and ultrasonic imaging system for breast cancer utilizing handheld array probes”, Photons Plus Ultrasound: Imaging and Sensing 2009, Proc. of SPIE vol. 7177, 2009.
SUMMARY OF THE INVENTION
• However, the following problems have been associated with the prior art.
• Over repeated photoacoustic measurements, the total quantity of light emitted from an irradiating end of aphotoacoustic probe104 may decrease due to the wear of a light source or a failure of an optical transmission system, but the decrease or the failure may be left unnoticed.
• If the decreased quantity of light is ascribable to the wear of alight source101, it can be spotted by providing a photometer (not shown) between thelight source101 and theincident end103a.
• However, the above-mentioned configuration does not make it possible to notice the occurrence of decrease in a total light quantity due to a malfunction of an optical transmission system as represented by disconnection in thefiber bundle103, and displacement of an optical element (not shown). Consequently, photoacoustic signals are regarded to exhibit a sufficient quantity of light even though their actual quantity of light is lower. As a result, images and data, such as an absorption coefficient of an absorber as a signal source of photoacoustic waves obtained by correcting photoacoustic signals using light quantities, will become smaller than they actually are. Thus, the reliability of data and images will suffer.
• The above-mentioned problems are not only for photoacoustic techniques but also have been common among other optical imaging techniques using a relatively high energy density, such as diffuse optical imaging (DOI).
• The present invention addresses the above problems with an objective of enabling acquisition of reliable photoacoustic data by means of keeping track of the quantity of light emitted from an irradiating end of a probe.
• The present invention provides an object information acquiring apparatus comprising:
• an optical transmission system for transmitting light from a light source;
• a photoacoustic probe including an irradiating end for irradiating an object with the light and a receiver for receiving acoustic waves generated by the object that has been irradiated with the light;
• a processor for acquiring information on the object based on the acoustic waves;
• a light quantity meter for measuring a quantity of light emitted from the irradiating end;
• a memory for storing a measurement value measured by the light quantity meter; and
• a presentation unit, wherein
• the processor compares the measurement value with a reference value of light quantity or a history of measurement value stored in the memory to determine whether or not the measurement value is within a reference range, and has the presentation unit present a result of the determination thereon.
• In accordance with the invention, the quantity of light emitted from an irradiating end of a probe is kept track of so that reliable photoacoustic data is provided.
• Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1A is a view illustrating a configuration of a photoacoustic apparatus according to an embodiment of the invention;
• FIG. 1B is a flow chart illustrating operation of the photoacoustic apparatus according to the embodiment of the invention;
• FIG. 2A is a view illustrating a photoacoustic probe of Example 1 of the invention;
• FIG. 2B is a view illustrating the photoacoustic probe of Example 1 of the invention;
• FIG. 2C is a view illustrating the photoacoustic probe of Example 1 of the invention;
• FIG. 3A is a flow chart illustrating operation in Example 2 of the invention;
• FIG. 3B is a flow chart illustrating the operation in Example 2 of the invention;
• FIG. 4A is a view illustrating a photoacoustic probe of Example 3 of the invention;
• FIG. 4B is a view illustrating the photoacoustic probe of Example 3 of the invention;
• FIG. 4C is a view illustrating a presentation unit of Example 3 of the invention;
• FIG. 4D is a view illustrating the presentation unit of Example 3 of the invention;
• FIG. 5 is a flow chart illustrating operation in Example 4 of the invention;
• FIG. 6 is a view illustrating a photoacoustic probe of Example 5 of the invention; and
• FIG. 7 is a view illustrating a configuration of a photoacoustic apparatus of the background art.
DESCRIPTION OF THE EMBODIMENTS
• Preferred embodiment of the invention will now be described with reference to the drawings. It is noted that, the dimensions, materials, shapes, and relative positions of the components described herein should be suitably modified depending on the configuration and conditions of an apparatus to which the invention is applied, and are not intended to limit the scope of the invention to the description herein.
• The term “acoustic waves” as used herein includes sound waves, ultrasonic waves, photoacoustic waves, elastic waves called photo-ultrasound waves, and compressional waves. An object information acquiring apparatus of the invention is a photoacoustic tomography apparatus for acquiring information about properties inside an object by irradiating (electromagnetic) light to the object to thereby induce generation of acoustic waves in the object due to the photoacoustic effect, and receiving the generated acoustic waves.
• The information about the properties of an object obtained with PAT is object information that reflects an initial acoustic pressure of acoustic waves generated due to irradiation, an absorption density and absorption coefficient of light energy derived from the initial sound pressure, levels of substances constituting the object's tissue, and the like.
• Examples of substance levels include an oxygen saturation level, oxyhemoglobin level, or deoxyhemoglobin level. The property information obtained can be stored and used as numerical data, information about distribution of different locations within the object, and image data for displaying image.
• The invention will be described in detail with reference to the drawings. In some cases, the same components may be provided with the same reference numbers so that detailed explanation is omitted. The invention also directs to an object information acquiring apparatus and methods for operating and controlling the same. Further, the invention also directs to programming for an information processor or the like to execute control.
• Referring toFIGS. 1A and 1B, an embodiment of the invention will now be described.
• FIG. 1A is a schematic view of aphotoacoustic apparatus100. Alight source1 emits light (L). A first illuminatingoptical system2 forms light that enters at anincident end3aof afiber bundle3. Thefiber bundle3 transmits the light to aphotoacoustic probe4, and irradiates the light from irradiating ends3b.
• Thephotoacoustic probe4 includes the irradiating ends3bof thefiber bundle3, second illuminatingoptical systems5 for shaping the light emitted from the irradiating ends3b,and areceiver6 for receiving photoacoustic waves. Upon irradiation onto an object (OBJ) via the second illuminatingoptical systems5, light scatters within the object and absorbed in an absorber (ABS), which in turn generates photoacoustic waves (PA).
• The photoacoustic waves are converted into an electric signal (SIG) by an element contained in thereceiver6, such as a piezoelectric element, CMUT, or the like, which electrical signal is then transmitted to aprocessor7. Theprocessor7 amplifies the electrical signal, creates image information (IMG) through digital conversion and filtering, and has the image displayed on adisplay unit8. Theprocessor7 includes a CPU, memory, processing circuit, etc. , and can be an information processor that is capable of different types of processing.
• InFIG. 1A, thefiber bundle3 is branched in the middle for providing two irradiating ends3band two second illuminatingoptical systems5. However, the number of branch is not limited to two. Alternatively, no branch is made such that an irradiatingend3bmay be adjacent to one side of thereceiver6.
• Preferably, thephotoacoustic probe4 is covered with ahousing4a,as shown inFIG. 2A.
• Preferably, alight source1 emits near infrared light of a wavelength between about 600 nm to about 1,100 nm. For example, a pulse laser such as an Nd:YAG laser, alexandrite laser, or the like, is used. Also, a Ti:sa laser or OPO laser using Nd:YAG laser beams as excitation light may be used.
• InFIG. 1A, light from thelight source1 was transmitted via a first illuminatingoptical system2 and thefiber bundle3. However, an optical transmission system is not limited thereto. For example, a mirror and a prism may be combined to yield reflection and refraction, which are used for transmission. Thelight source1 may be a semiconductor laser to be placed at the irradiatingend3b.
• Light emission and the receiving of photoacoustic waves by thereceiver6 must be synchronized. This can be achieved by branching any one of the paths between thelight source1 and the second illuminating optical system(s)5 and providing a sensor(not shown), for example, a photodiode, for detection. With a detection signal as a trigger, thereceiver6 can initiate receiving. Otherwise, a pulse generator (not shown) may be used for synchronizing the illumination timing of thelight source1 and the receiving timing of theprocessor7.
• Aphotoacoustic apparatus100 includes alight quantity meter10 for measuring the quantity of light emitted from the irradiating end of thephotoacoustic probe4. Thelight quantity meter10 can be photovoltaics such as a photodiode or a heat exchange power meter such as a thermopile.
• Alternatively, light emitted from the irradiating end of thephotoacoustic probe4 may be diffused by a diffuser plate, and the diffused light may be captured by an infrared camera. In this case, the light quantity can be calculated based on the brightness values of the pixels of the infrared camera.
• Because thephotoacoustic probe4 is tethered by a cable or thefiber bundle3, its movable range is limited. Therefore, it is preferable that thelight quantity meter10 is located within a movable range of thephotoacoustic probe4. Alternatively, if thelight quantity meter10 is located outside of the movable range of thephotoacoustic probe4, thelight quantity meter10 is placed on a wagon or case (not shown) to be transferred into a movable range during measurement of light quantity.
• Anirradiation switch19 is provided so that light can be emitted when the irradiating end of thephotoacoustic probe4 is opposed to thelight quantity meter10. When theirradiation switch19 is pressed, acontroller17 executes control to set irradiating conditions. In a state that the irradiating conditions are set, an inner shutter of thelight source1 and ashutter2cin the firstoptical system2 are open. In a case thelight source1 is a Q-switched laser, a Q switch is switched on. Thus, light is emitted from the irradiating end of thephotoacoustic probe4.
• Theirradiation switch19 may be a manual switch or foot switch that can be pressed directly by an operator. Alternatively, as shown in the drawing by a dotted line, theirradiation switch19 may be provided near thelight quantity meter10 so that when the irradiating end of thephotoacoustic probe4 is opposed to thelight quantity meter10, the switch can be pressed.
• A light quantity (Q) measured by thelight quantity meter10 is stored in amemory13. Theprocessor7 determines whether the light quantity stored in thememory13 is within a reference range or not; when the light quantity is above the reference range, “anomalous” is presented on apresentation unit14.FIG. 1B shows a flow for the above situation. First, an operator directs the irradiating end of thephotoacoustic probe4 opposite to thelight quantity meter10. Subsequently, the operator presses anirradiation switch19.
• S11: Thephotoacoustic apparatus100 sets irradiating conditions. That is, the inner shutter in thelight source1 and theshutter2cin the firstoptical system2 are open, or a Q switch is switched on if thelight source1 is a Q-switched laser, and light is emitted. The quantity of light emitted from the irradiating end of thephotoacoustic probe4 is measured by thelight quantity meter10, and the light quantity measured is stored in thememory13.
• S12: A difference from a reference light quantity is determined.
• S13: When the difference in S12 is within a reference range, the procedure is terminated as “normal completion”.
• S14: When the difference in S12 is outside of the reference range, the procedure is terminated as “anomalous completion” (“abnormal completion”).
• In this flow, provided a reference value is 50 mJ for the total quantity of light emitted from the irradiating end of thephotoacoustic probe4, for example, a measurement value that falls within a range of 50 mJ±5 mJ is considered a normal completion. It should be noted that the reference value of total light quantity (50 mJ) and range (±5 mJ) are non-limiting examples. In S12, thememory13 can store not only a specified reference value but also a history of previous light quantity data for comparison. The history of measurement value can include an immediate previous measurement value, an average of a plurality of previous measurement values, and a measurement value that has undergone statistic processing such as clearing outliers.
• Thepresentation unit14 may be an LED for indicating a status by lighting or flashing and/or a unit for voice notification. Alternatively, adisplay unit8 may be used as apresentation unit14 for indicating a status by letters and/or images.
• FIG. 1A shows two processors7: one for generating images, and the other for determining whether or not a light quantity is normal. However, only oneprocessor7 can perform both operations.
• According to the above configuration, quantity changes in light emitted from thephotoacoustic probe4 can be detected immediately. Thus, faulty photoacoustic measurements due to unnoticed failure of an optical transmission system can be reduced. Consequently, the accuracy of measured light quantities and values of optical properties calculated using the light quantities will improve, whereby reliable photoacoustic data becomes available.
• In the above description, photoacoustic measurement was used as an example. However, applications of the invention are not limited thereto. For example, the invention can be used for optical imaging techniques where a relatively high energy density is used, such as diffuse optical imaging (DOI). The same holds true for the following Examples.
Example 1
• Alight quantity meter10 will now be described in detail with reference toFIGS. 2A to 2C.
• InFIG. 2A, aholder9 is provided for holding aphotoacoustic probe4 therein. Alight quantity meter10 is disposed in a position so as to oppose an irradiating end of thephotoacoustic probe4 in theholder9. The purpose of this is to reduce an adverse effect when thephotoacoustic probe4 is held by an operator during measurement of light quantity, that is, the accuracy of the measurement may be lowered affected by movement of the operator. Further, the adverse effect may be completely eliminated if the operator puts thephotoacoustic probe4 in theholder9 instead of holding it oneself and uses a switch positioned away from the photoacoustic probe to irradiate light.
• InFIG. 2A, alight quantity meter10 is shown in the form of apower meter10a.Thepower meter10acan be a photovoltaic or heat exchanging type, as described above. Thepower meter10ameasures the total quantity of light emitted from an irradiating end of aphotoacoustic probe4. Aprocessor7 compares the measured total light quantity with a reference value (or a previous measurement value) to determine whether the difference is within a predetermined range or not, and has apresentation unit14 present the result presented thereon. Thus, the total quantity of light emitted from the irradiating end of thephotoacoustic probe4 can be measured comprehensively by thepower meter10aopposite to the irradiating end of thephotoacoustic probe4.
• Turning now toFIG. 2B, alight quantity meter10 is shown in the form of aninfrared camera10b.However, it is difficult for theinfrared camera10bto directly capture light emitted from an irradiating end of aphotoacoustic probe4. To address this problem, adiffuser plate11 is provided in a position opposite to the irradiating end of thephotoacoustic probe4, on which a focal point of theinfrared camera10bis adjusted.
• Incidentally, intense light from thephotoacoustic probe4 can saturate the brightness values of pixels of an image captured by theinfrared camera10b,or can damage an image-receiving element of theinfrared camera10b.To avoid this, it is preferable that anND filter12 is provided between thediffuser plate11 and theinfrared camera10b.
• A sum of the brightness values of the pixels of an image captured by theinfrared camera10bis regarded the total quantity of light emitted from the irradiating end of thephotoacoustic probe4. Aprocessor7 compares this total light quantity with a reference value or the like to determine whether the difference is within a predetermined range or not, and has apresentation unit14 present the result presented thereon.
• The distribution of the brightness values of the pixels of an image captured by theinfrared camera10brepresents the distribution of the quantity of light emitted from the irradiating end of thephotoacoustic probe4, that is, the distribution of the quantity of light irradiating the surface of an object. Thus, theprocessor7 can use the light quantity distribution to determine the status of the light in terms of whether or not it is within a predetermined range. Here, theprocessor7 can perform determination based on the brightness values of the pixels, an extract of any pixels, or an average of the brightness values in any pixel zone.
• The use of light quantity distribution appreciates the benefit of keeping track of the status of light emission using the total light quantity as described above, and further improves the precision of photoacoustic measurement. This effect will now be described.
• A level of intensity (or an initial sound pressure p) of photoacoustic waves of light, which is irradiated on the surface of an object and which enters the object while scattering, is expressed by the following expression (1):
• 
  p=Γμ_aϕ  (1)
• where ϕ denotes a light quantity, μ_adenotes an absorption coefficient of living tissue, and Γ denotes the Gruneisen parameter.
• Thus, to determine an absorption coefficient μ_aof living tissue, data received at thereceiver6 corresponding the sound pressure p, the Gruneisen parameter of about 0.5, and a light quantity ϕ within the organism are required. The light quantity ϕ within the organism is calculated, with the light quantity distribution on the object's surface being a boundary condition, using a diffusion equation (transport equation) or the Monte Carlo method using known or estimated equivalent damping coefficient μ_effwithin the organism. If the configuration includes theinfrared camera10b,the light quantity ϕ within the organism is calculated on a high precision basis, because the light quantity distribution of the light emitted from the irradiating end of thephotoacoustic probe4 or the light quantity distribution on the object's surface as the boundary condition is available.
• Preferably, theprocessor7 is capable of calibrating the brightness values of theinfrared camera10b.In calibration, the total quantity of light emitted from the irradiating end of thephotoacoustic probe4 is pre-measured by, for example, a power meter, and the value is compared with a sum of brightness values measured by theinfrared camera10b.Thus, a light quantity per brightness gradation can be calculated. For example, provided that a pixel's brightness gradation has 256 levels, brightness values of 1280×960 pixels are added up.
• Next, still another example of alight quantity meter10 will be described with reference toFIG. 2C. Thepower meter10ainFIG. 2A has a large area so that thepower meter10acan face the entire irradiating end of thephotoacoustic probe4. In contrast, inFIG. 2C, apower meter10ahas a smaller area, and is scanned. For scanning, thepower meter10awith a smaller area is mounted on anXY stage15. In this way, without a need for using aninfrared camera10b,the relativelyinexpensive power meter10awith a smaller area is capable of measuring the light quantity distribution within an irradiated plane shone by light from an irradiating end of aphotoacoustic probe4.
• In accordance with ANSI Standard Z136.1-2000, a method for determining whether or not the irradiance energy per unit area exceeds the maximum permissible exposure (MPE) level for skin should use a spot size of a 3.5 mm beam diameter. Thus, the measurement area for thepower meter10ahas a diameter of 3.5 mm, or anaperture10cwith a diameter of 3.5 mm is disposed on thepower meter10a,such that measurement is performed as per ANSI Standard Z136.1-2000 in terms of the irradiance energy per unit area.
• Aprocessor7 determines whether or not an energy density measured by thepower meter10aexceeds a predetermined value to ensure safety of skin. The predetermined value employed was about 0.8 times the MPE level for skin taking a safety factor into consideration. When theprocessor7 determines that an irradiance energy density has exceeded the predetermined value, the illuminating intensity of alight source1 is adjusted to be lower (adjusting instruction ADJ). In this way, the light energy density is kept at or below the predetermined value so that safety is ensured.
• Other ways to keep the light energy density at or below the MPE level include inserting a filter at some point between thelight source1 and the irradiating end of thephotoacoustic probe4, or inserting a diffuser plate with a large diffusing angle into a second illuminatingoptical system5.
• TheXY stage15 shown inFIG. 2C was provided for the scanning of thepower meter10a.TheXY stage15 is operated by a drive instruction (DRV) from theprocessor7. However, a scanning mechanism is not limited to this configuration. For example, to the contrary to the above description, thephotoacoustic probe4 held in aholder9 may be scanned. That is, it is essential only that a measuring plane of thepower meter10ais scanned relative to the in-plane direction of the irradiating end of thephotoacoustic probe4.
• InFIG. 2C, thecompact power meter10awas scanned to measure the light quantity distribution within a plane shone by light from the irradiating end of thephotoacoustic probe4. On the basis of the light quantity distribution, theprocessor7 determines whether or not the light energy density is within a predetermined range, and has the result presented on apresentation unit14. Theprocessor7 may determine changes in light quantity distribution based on the brightness values of the pixels, or by averaging only arbitrary pixels or arbitrary pixel zone.
• This configuration enables not only safe measurement of total light quantity but also the computing of light quantity distribution within the object. In addition, the use of a relatively inexpensive power meter can lead to cost reduction.
Example 2
• In Example 2, contents presented by apresentation unit14 will be described.
• Reduction of the quantity of light emitted from an irradiating end of aphotoacoustic probe4 can be ascribed to reduction of the quantity of light emission from alight source1, and foreign body adherence to an optical element between a first illuminatingoptical system2 and a second illuminatingoptical system5, to an end face of afiber bundle3, and to the irradiating end of thephotoacoustic probe4.
• As shown inFIG. 1A, areflection element2band aphotometer2afor measuring its reflected light are provided in the first illuminating optical system2 (or in the light source1). Thereflection element2bis parallel-plate glass or a mirror that reflects a few percent of emitted light. Thephotometer2ais a photodiode or photomultiplier tube for measuring reflected light. Thus, a reflected light quantity is monitored by thephotometer2a,such that decrease in emitted light quantity at thelight source1 is detected. In this case, thepresentation unit14 indicates a light quantity error and also indicates that maintenance is required for thelight source1.
• FIG. 3A shows a flow of light emission.
• S31: During light emission, thephotometer2ais monitored.
• S32: The value indicated by thephotometer2ais compared with a reference value; if the resulting light quantity is within a reference range, the procedure returns to S31 during light emission. If the light quantity is outside the reference range, the procedure will be terminated as abnormal completion (S33). Thephotometer2ameasures reflected light from thereflection element2b,which is only about a few percent of the original light; therefore, when an emitted light quantity at thelight source1 is 100 mJ, 5 mJ that is 5 percent of 100 mJ will be measured. Thus, a reference range for determination is 5±0.5 mJ. Note that the light quantities and percentage above are examples only, and do not intend to limit actual values thereto.
• S33: In a case of abnormal completion, thepresentation unit14 indicates that maintenance is required for thelight source1. For safety reasons, maintenance of thelight source1 should not be performed by an operator, for example, a physician. Therefore, preferably, the indication in the step urges the operator to contact service personnel.
• As for foreign body adherence between the first illuminatingoptical system2 and the second illuminatingoptical system5, such incident should occur less frequently because the path is usually covered. In case of a failure in the cover, it will be readily and visually checked from the exterior.
• Foreign body adherence to the irradiating end of thephotoacoustic probe4 occurs more frequently. A major cause for this is sonar gel (acoustic matching gel) that is applied between thereceiver6 and the object during photoacoustic measurement. Foreign objects such as contaminants in the sonar gel and opaque matters that are dried leftovers of the sonar gel remaining on the irradiating end of thephotoacoustic probe4 become adherent.
• Thus, in a case of anomalous completion in the flow shown inFIG. 1B, preferably an operator is urged to clean. A flow inFIG. 3B depicts a procedure upon anomalous completion, which will be described as follows.
• S34: Apresentation unit14 presents a message urging an operator to clean an irradiating end of aphotoacoustic probe4.
• Checking the message, the operator cleans the irradiating end of thephotoacoustic probe4, directs thephotoacoustic probe4 opposite to alight quantity meter10 again, and presses anirradiation switch19.
• S35: Aphotoacoustic apparatus100 sets irradiating conditions. That is, an inner shutter (not shown) in alight source1 and ashutter2cin a firstoptical system2 are open, or a Q switch is switched on if thelight source1 is a Q-switched laser, and light is emitted. The quantity of light emitted from the irradiating end of thephotoacoustic probe4 is measured by thelight quantity meter10, and the data is stored in amemory13.
• S36: A difference between a reference light quantity and a measurement value is determined.
• S37: When the difference in S36 is within a reference range, the procedure is terminated as “normal completion”.
• S38: When the difference in S36 is outside of the reference range, “abnormal completion” is presented. This means that the light quantity change failed to recover to the reference range despite cleaning; therefore, thepresentation unit14 presents that maintenance is required. In this situation, foreign body adherence somewhere between the first illuminatingoptical system2 and the second illuminatingoptical system5, or displacement or damage in optical system(s) are highly likely, in which cases cause investigation and/or repairs can be difficult. Hence, rather than letting the operator, such as a physician, deal with trouble oneself, it is preferable to advise the operator through presentation to contact service personnel.
• In S38, before presenting the abnormal completion message right away, the message urging cleaning may be presented once again by thepresentation unit14.
• As described above, even though a total light quantity once turned out outside a normal range, a procedure as simple as cleaning the irradiating end of thephotoacoustic probe4 may work and enable good measurement. According to this Example, in the preceding situation, the operator is informed that a normal status may be restored by cleaning oneself without maintenance by professional; therefore, the operator has a chance to readily restore the light quantity within a reference range. As a result, the degree of capacity utilization of thephotoacoustic apparatus100 is improved.
Example 3
• At an irradiating end of aphotoacoustic probe4, light with a relatively high energy in the range of several tens to a hundred and several tens mJ is emitted from a relatively small area. Even if the irradiance energy density does not exceed the MPE level for skin, as described in Example 1, it may exceed that for retina, which has a smaller reference value. Thus, for safety of the object and the operator, it is preferable to provide a mechanism near the irradiating end of thephotoacoustic probe4 to prohibit emission when not in contact with the object.
• In this Example, as shown inFIG. 4A, acontact sensor16 for determining a contact status is provided outside the irradiating end of thephotoacoustic probe4. An optical, electrostatic or mechanical sensor, or a strain gauge may be used as thecontact sensor16. Alternatively, a receiver6 (FIG. 1A) may determine contact by transmitting and receiving ultrasonic waves. Thecontact sensor16 outputs contact information (CONT) when the irradiating end is in contact with an object and no-contact information (NCNT) when the irradiating end is not in contact with the object.
• Acontroller17 outputs a shutter open/close instruction (OP/CL) in accordance with the contact/no-contact information. That is, when the no-contact information is output, thecontroller17 closes ashutter2cin a first illuminatingoptical system2 and an inner shutter (not shown) in alight source1. In a case thelight source1 is a Q-switched laser, a Q switch is switched off by thecontroller17. Thus, thecontroller17 prevents light (L) from being emitted from the irradiating end of thephotoacoustic probe4.
• In contrast, when the contact information is output, thecontroller17 permits emission of light from the irradiating end of thephotoacoustic probe4. That is, thecontroller17 opens theshutter2cand the inner shutter in thelight source1, or switches on the Q switch if thelight source1 is a Q-switched laser.
• The above configuration ensures safety when the probe is not in contact with an object. However, measurement of total light quantity according to the invention with aphotoacoustic probe4 being held in aholder9 can have problems depending on the shape of theholder9.
• That is, the presence of a space (gap) in theholder9 at a position opposing acontact sensor16 causes an output of the no-contact information from thecontact sensor16. Consequently, when an operator presses theirradiation switch19, thecontroller17 does not permit light emission from the irradiating end of thephotoacoustic probe4. Hence, light is not emitted. As a result, alight quantity meter10 provided in theholder9 cannot perform measurement. Thus, when theholder9 houses thephotoacoustic probe4 in a normal manner, it is necessary that thecontact sensor16 output the contact information. To this end, some examples of configuration and method will be described as follows.
• In the first example, regardless of a gap in aholder9 at a position opposing acontact sensor16, light is forced to irradiate when anirradiation switch19 is pressed. That is, when theirradiation switch19 is pressed, an irradiation instruction (IRD) is output to acontroller17.
• With the above configuration, however, the pressing of theirradiation switch19 will cause light emission anyway, even when aphotoacoustic probe4 is not held in theholder9 in a normal manner. Hence, it is preferable that theirradiation switch19 is disposed adjacent to theholder9 so as to draw attentions of an operator to make sure thephotoacoustic probe4 is in place in theholder9.
• Alternatively and more preferably, as shown inFIG. 4B, acover20 is provided, which enables operation of theirradiation switch19 only when thephotoacoustic probe4 is in theholder9, but disables operation when thephotoacoustic probe4 is not in theholder9. When thephotoacoustic probe4 is in theholder9, light emission is allowed only when theirradiation switch19 is pressed.
• In the next example, the gap between acontact sensor16 and a position within aholder9 opposing thecontact sensor16 is narrowed so that thecontact sensor16 can sense a contact status . For this method, the gap between aphotoacoustic probe4 and theholder9 is filled with anelastic body9a,and the shapes of the interior of the holder, ahousing4aof the probe, and theelastic body9aare individually adjusted to shield light.
• In this way, when thephotoacoustic probe4 is not held in place, the contact information is not output and light is not emitted. When thephotoacoustic probe4 is held in place, and when light is emitted, the leakage of light to the outside of theholder9 is restricted because theelastic body9aas a light shield fills the gap between thephotoacoustic probe4 and theholder9.
• In another example, as shown inFIG. 4A, amovable part18 is provided in aholder9 at a position opposing acontact sensor16. Themovable part18 is moved to a position where thecontact sensor16 is capable of sensing a contact status when anirradiation switch19 is pressed (movable part operation instruction MV) . In this way, when thephotoacoustic probe4 is held in theholder9, light emission is allowed only when theirradiation switch19 is pressed.
• With the above configurations and methods, light emission is enabled in a state in which thephotoacoustic probe4 is held in theholder9 in a normal manner. As described above, the use of thelight quantity meter10 enables measurement of the total quantity of light emitted from the irradiating end of thephotoacoustic probe4.
• In addition, ahold sensor9bmay be provided in theholder9 or adjacent to thelight quantity meter10, which sensor outputs hold information to thecontroller17 upon sensing that thephotoacoustic probe4 is held (accommodated) in theholder9. Receiving the hold information, thecontroller17 enables emission of light (for example, opens ashutter2c). A mechanical, optical, or electrostatic switch may be used preferably for thehold sensor9b.
• In this way, thelight quantity meter10 can measure total light quantity only when thephotoacoustic probe4 is held in place in theholder9 at a predetermined position. Additionally, by providing a plurality ofhold sensors9b,measurement of total light quantity is performed only when the irradiating end of thephotoacoustic probe4 and the light quantity meter is parallel, which contributes to reproduction of measurement conditions for thelight quantity meter10 and to improvement of the measurement accuracy.
• The configurations and controlling methods described above maybe used alone or in combination. Consequently, the irradiating end of thephotoacoustic probe4 is not allowed to emit light when not in contact with an object; thus, safety of the object and the operator is ensured. Conversely, when thephotoacoustic probe4 is held in place in theholder9 at the predetermined position, light is emitted so as to enable light quantity measurement.
• InFIG. 4A, anelastic body9awas provided in an inner periphery of theholder9 at a position that comes in contact with thephotoacoustic probe4. Deformable resin such as different kinds of rubber and urethane is preferably used for theelastic body9a.The body of theholder9 was formed from relatively rigid materials such as metals, resin such as plastic, or ceramics. By providing theelastic body9a,the gap between thephotoacoustic probe4 and theholder9 is filled when thephotoacoustic probe4 is held in theholder9. In this way, the leakage of light to the exterior of theholder9 can be decreased while the quantity of light emitted from the irradiating end of thephotoacoustic probe4 is being measured by thelight quantity meter10. As a result, object and operator safety is improved.
• In this Example, thepresentation unit14 is provided on a side of theholder9.FIG. 4C exemplifies apresentation unit14 including an LED for indicating contents to be presented for an operator. The operator is notified by lighting or flashing of the LED.
• InFIG. 4D, apresentation unit14 is provided with a liquid crystal monitor, on which contents to be presented for an operator are expressed in letters. These methods facilitate an operator to understand the status of the apparatus and instructions for the operator.
Example 4
• With reference to a flow chart inFIG. 5, directions for use of aphotoacoustic apparatus100 including aphotoacoustic probe4 and aholder9 will be described. In this Example, aninfrared camera10b,such as the one shown inFIG. 2B, was used as alight quantity meter10.
• At start-up of the apparatus or during standby, thephotoacoustic probe4 is held in theholder9.
• S51: At start-up of the apparatus, acontroller17 performs a sequence of auto-emission. Alternatively, during standby, when an operator places thephotoacoustic probe4 in theholder9 and presses anirradiation switch19, thecontroller17 performs a sequence for emission (S52).
• S52: If thephotoacoustic probe4 is provided with acontact sensor16, thecontact sensor16 is made ready to sense a contact status. For example, amovable part18 is moved as shown inFIG. 4A. Thecontroller17 sets irradiating conditions. If theholder9 has ahold sensor9btherein, thecontroller17 sets irradiating conditions after thehold sensor9bsensed thephotoacoustic probe4. If neither of thecontact sensor16 and thehold sensor9bis present, thecontroller17 sets irradiating conditions after S51.
• Here, the irradiating conditions set by thecontroller17 includes opening an inner shutter in alight source1 and a shutter in a firstoptical system2, or switching on a Q switch if thelight source1 is a Q-switched laser. Thus, light is emitted from the irradiating end of thephotoacoustic probe4. The duration and the number of times of irradiation are programmed in thecontroller17; in this Example, irradiation was executed 100 times (10 sec×10 Hz).
• S53: Light emitted from the irradiating end of thephotoacoustic probe4 is diffused by adiffuser plate11 to be captured by theinfrared camera10b.The brightness values of pixels of theinfrared camera10bare stored in amemory13.
• S54: Concurrent with S53, aphotometer2aprovided in a second optical system2 (FIG. 1A) measures the emitted light. It is intended that thephotometer2ahas already undergone calibration for converting total light quantity.
• The calibration takes advantage of the direct proportional relation between a light quantity measured by thephotometer2aand a total light quantity from the irradiating end of thephotoacoustic probe4 . That is, using a total light quantity measured by thelight quantity meter10 inFIG. 1A and the then light quantity detected by thephotometer2a,calibration is performed in advance for creating a conversion expression.
• S55: Aprocessor7 calculates a sum of the brightness values of the pixels of the image captured in S53, and executes calibration of the brightness values using the calculated sum and the measurement value provided by thephotometer2ain S54. This enables the brightness for imaging by theinfrared camera10bto be calibrated, whereby a light quantity can be derived from the brightness values. Using the brightness values of thephotometer2aorinfrared camera10a,the total quantity of light emitted from the irradiating end of thephotoacoustic probe4 is obtained.
• S56: Theprocessor7 computes a boundary condition based on the total light quantity obtained in S55 and an irradiated area of the object, or based on the light quantity distribution on the object's surface established using the calibrated brightness values. Using this boundary condition, the distribution of the quantity of light entering the object as scattering and being absorbed is calculated, and light quantity distribution correction data is created.
• S57: Any of the total light quantity data, brightness data, and light quantity distribution data within organism is compared with corresponding previous data or a preset reference value. In this Example, a preset value for a total light quantity was 50 mJ and a predetermined reference range was 50±5 mJ. When a difference exceeds the predetermined value, in other words, when the total light quantity is 45 mJ or less or 55 mJ or more, “anomalous completion” (abnormal completion) is determined and indicated on either apresentation unit14 or adisplay unit8.
• In a case where a decrease in total light quantity is exhibited, such a change in the total light quantity may be ascribed to contamination of the irradiating end of thephotoacoustic probe4 or diffuser plate11 (light quantity meter10); therefore, the presented message at this point will contain the wording “clean and resume” to call attentions. To resume measurement, it is instructed to return to S51. It is also possible that optical transmission, such as thelight source1 and/or afiber bundle3 has trouble; therefore, if there is no improvement after measurement is resumed, “abnormal completion” is determined.
• If the difference is less than predetermined, the operator holds thephotoacoustic probe4 and performs photoacoustic measurement of the object. Thus, variations in total light quantity or brightness data that is derived from the total light quantity, or in light quantity distribution data within organism can be minimized; therefore, stable photoacoustic measurement data becomes available.
• S58: A photoacoustic image is created based on obtained photoacoustic signals, which image is displayed on thedisplay unit8. The total light quantity when the photoacoustic signals were obtained is converted based on the measurement value provided by thephotometer2a.Using the converted total light quantity and the light quantity distribution correction data from S56, the light quantity distribution within the object when the photoacoustic signals were obtained is corrected and established.
• As described above, photoacoustic is expressed by the following expression (1):
• 
  p=Γμ_aϕ  (1)
• where p denotes a photoacoustic initial sound pressure, Γ denotes the Gruneisen parameter, μ_adenotes an absorption coefficient, and ϕ denotes a light quantity. The absorption coefficient μ_acan be determined from the photoacoustic signal (p), the corrected light quantity distribution within object (ϕ), and the Gruneisen parameter Γ of about 0.5.
• Further, by varying the wavelength of light from thelight source1, spectral characteristics of an absorber as a photoacoustic sound source can be figured out. For example, when blood (hemoglobin) is an absorber, an oxygen saturation level of hemoglobin is measurable. Consequently, it becomes possible to measure precisely the light quantity distribution on the object's surface, which is a boundary condition used for precise measurement of light quantity distribution within the object. This leads to improved measuring performance of absorption coefficient μ_aand oxygen saturation level.
• The above-mentioned flow is also usable when apower meter10ais used as alight quantity meter10. Here, when thepower meter10aitself has been calibrated, the calibration procedure for thelight quantity meter10 described in S55 is unnecessary; thephotometer2acan be calibrated as is described in Example 1, as with S54.
Example 5
• In Example 5, alight quantity meter10 is provided for aphotoacoustic probe4. InFIG. 6, in a state1 (photoacoustic measurement standby mode), aprobe cover4bcovers an irradiating end of thephotoacoustic probe4, while in a state2 (photoacoustic measuring mode), theprobe cover4bsplit opens. Thus, measurement can be performed while the cover is open. Alight quantity meter10 is provided inside theprobe cover4b,or at the irradiating end side of thephotoacoustic probe4.
• With such configuration, thephotoacoustic probe4 and thelight quantity meter10 can be combined into a single unit.
• 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. 2013-204513, filed on Sep. 30, 2013, which is hereby incorporated by reference herein in its entirety.

Claims (19)

What is claimed is:

1. An object information acquiring apparatus comprising:

an optical transmission system for transmitting light from a light source;

a photoacoustic probe including an irradiating end for irradiating an object with the light and a receiver for receiving acoustic waves generated by the object that has been irradiated with the light;

a processor for acquiring information on the object based on the acoustic waves;

a light quantity meter for measuring a quantity of light emitted from the irradiating end;

a memory for storing a measurement value measured by the light quantity meter; and

a presentation unit, wherein

the processor compares the measurement value with a reference value of light quantity or a history of measurement value stored in the memory to determine whether or not the measurement value is within a reference range, and has the presentation unit present a result of the determination thereon.

2. The object information acquiring apparatus according toclaim 1, further comprising:

an irradiation switch enabling an operator to direct light emission; and

a controller for setting irradiation conditions such that light is emitted from the irradiating end when the irradiation switch is pressed.

3. The object information acquiring apparatus according toclaim 1, wherein

when a difference between the measurement value and the reference value of light quantity or the history of measurement value stored in the memory is larger than a predetermined value, the processor has the presentation unit present thereon an indication to the effect that the measurement value is anomalous.

4. The object information acquiring apparatus according toclaim 3, wherein

the processor provides an indication urging to perform maintenance of the object information acquiring apparatus to the presentation unit when providing the indication that the measurement value is anomalous.

5. The object information acquiring apparatus according toclaim 4, wherein

the maintenance is cleaning of the irradiating end.

6. The object information acquiring apparatus according toclaim 1, wherein

the light quantity meter is a power meter.

7. The object information acquiring apparatus according to claim1, further comprising:

a diffuser plate for diffusing light emitted from the irradiating end, wherein

the light quantity meter is an infrared camera for capturing light diffused by the diffuser plate;

the processor computes at least one of a total quantity of light emitted from the irradiating end and a light quantity distribution within a plane shone by the light, on the basis of brightness values of pixels obtained from the infrared camera imaging.

8. The object information acquiring apparatus according toclaim 7, wherein

the processor performs calibration to convert the brightness values of the pixels of the infrared camera into a total quantity of light emitted from the irradiating end.

9. The object information acquiring apparatus according toclaim 1, further comprising a stage on which the light quantity meter and the irradiating end are scanned relative to each other.

10. The object information acquiring apparatus according toclaim 1, wherein the processor lowers an energy density of light emitted from the irradiating end when the energy density of light emitted from the irradiating end exceeds a predetermined value.

11. The object information acquiring apparatus according to claim1, further comprising a holder for holding the photoacoustic probe.

12. The object information acquiring apparatus according toclaim 11, wherein the holder has an elastic body for shielding light while holding the photoacoustic probe.

13. The object information acquiring apparatus according toclaim 11, further comprising a cover for disabling operation of the irradiation switch operated by an operator directing light emission in a state where the holder is not holding the photoacoustic probe and for enabling the operation of the irradiation switch in a state where the holder is holding the photoacoustic probe.

14. The object information acquiring apparatus according toclaim 13, wherein the photoacoustic probe has a contact sensor for sensing whether or not the photoacoustic probe is in contact with the object,

the object information acquiring apparatus further comprising:

a controller for having the contact sensor determine a contact status when the irradiation switch is pressed.

15. The object information acquiring apparatus according toclaim 14, further comprising a movable part disposed inside the holder at a position opposing the contact sensor when the photoacoustic probe is held in the holder, wherein

when the irradiation switch is pressed, the controller moves the movable part to a position at which the contact sensor can detect a contact status.

16. The object information acquiring apparatus according toclaim 14, wherein

the holder has a hold sensor for sensing whether or not the photoacoustic probe is held in the holder, wherein

the controller sets light irradiating conditions to emit light from the irradiating end when the hold sensor has sensed that the photoacoustic probe is held in the holder.

17. The object information acquiring apparatus according toclaim 1, further comprising a display unit for displaying information on the object.

18. The object information acquiring apparatus according toclaim 17, wherein the presentation unit is the display unit.

19. The object information acquiring apparatus according toclaim 1, wherein the presentation unit uses an LED for presentation.

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