WO2014115042A1

WO2014115042A1 - Safety mechanism for photoacoustic imaging system

Info

Publication number: WO2014115042A1
Application number: PCT/IB2014/058033
Authority: WO
Inventors: Ramon Quido ERKAMP; Ladislav Jankovic; Todd Nicholas ERPELDING
Original assignee: Koninklijke Philips N.V.
Priority date: 2013-01-22
Filing date: 2014-01-03
Publication date: 2014-07-31

Abstract

A photoacoustic imaging apparatus includes: a light generator configured to produce a light beam; an imaging probe configured to deliver the light beam to a region of interest, the imaging probe including one or more acoustic transducer elements configured to receive an acoustic signal from the region of interest; and a processor configured to determine whether the imaging probe is uncovered, and to disable the light beam from being transmitted by the imaging probe when the processor determines that the imaging probe is uncovered. In some embodiments, the processor determines whether the imaging probe is uncovered based on data generated from the acoustic transducer elements. In some embodiments, the data is generated from the acoustic signal received by the acoustic transducer elements.

Description

SAFETY MECHANISM FOR PHOTO ACOUSTIC FMAGING SYSTEM

TECHNICAL FIELD This invention pertains to photoacoustic imaging systems.

BACKGROUND AND SUMMARY

The imaging contrast in a conventional acoustic imaging system (e.g., an ultrasound imaging system) is generated by the spatially varying acoustic properties of the object. An acoustic wave or beam is transmitted into a region of interest to be imaged, and the resulting acoustic signal that is received back from the region of interest is processed to generate an image. For biological tissue, acoustic images provide a great deal of structural information, but photoacoustic imaging can provide an additional contrast mechanism based upon optical absorption to provide further information about tissue function..

Photoacoustic imaging operates based on the photoacoustic effect, which is the generation of acoustic waves due to thermal expansion that follows the absorption of pulsed light. Instead of transmitting an acoustic wave to tissue within a region of interest, the tissue is illuminated with a short, high energy light pulse which diffusely scatters throughout the tissue. Light is absorbed by the tissue and tissue chromophores, but the amount of light absorption depends on the tissue type and the wavelength of the light that is used for illumination, i.e. the absorption spectrum. The absorbed light energy is immediately converted into heat, resulting in a very fast temperature rise that causes a quick thermal expansion. The brief tissue motion caused by this thermal expansion creates an acoustic wave, and an acoustic transducer including an array of acoustic transducers may be employed in a beamforming arrangement to receive the acoustic waves from the tissue from which acoustic data may be produced. The acoustic data is employed to create an image of the tissue in the region of interest.

Photoacoustic imaging is able to perform functional imaging of physiological parameters, for example the concentration and oxygen saturation of hemoglobin. The optical absorption of hemoglobin varies with laser wavelength, and this function also differs for oxygenated and deoxygenated hemoglobin. Spectroscopic imaging using several wavelengths allows measurement of the relative concentrations of these two forms of hemoglobin.

The light pulse energy used for photoacoustic imaging needs to be high enough to create via thermal expansion a sound wave of sufficient magnitude to be detectable by an acoustic transducer, which may comprise an array of acoustic transducer elements. The pulse needs to be short enough to create sound waves with high enough frequencies to use acoustic imaging transducers for their detection. Optical pulses of up to about 100ns in length may be considered to be well suited to these requirements, but the practical pulse lengths readily available are typically around 5- 10ns and pulsed lasers (e.g. Q-switched solid state lasers) are used to create such short pulses. These light pulses may be delivered to the tissue in close proximity to the aperture of the acoustic probe through a light delivery system. The light delivery system could be, for example, a fiber optic bundle (Fig. 1) or a direct free-space application, and serves to spread the light over the region of interest and keep the light intensities within safe limits for human exposure.

The light wavelengths used in photoacoustic imaging can extend into the invisible infrared range where humans have no blinking reflex to protect their eyes before damage may be done to them. Thus, unless protective measures are taken, light transmitted by a photoacoustic imaging system can create an exposure risk for the physician, patient, sonographer, equipment service personnel, cleaning personnel, and other hospital/clinic staff. In addition, allowing the photoacoustic imaging system to transmit light during time periods when probe is not in position to generate images of interest may reduce the operational lifetime of the light generation components in the system.

Accordingly, it would be desirable to prevent accidental exposure of the eye to damaging light levels from a photoacoustic imaging system. It would further be desirable to disable transmission of light by a photoacoustic imaging system when the probe is not coupled to biological tissue to be imaged.

In one aspect of the invention, a photoacoustic imaging apparatus comprises: a light generator configured to produce a pulsed light beam; a photoacoustic imaging probe configured to deliver the pulsed light beam to a region of interest, the photoacoustic probe including one or more acoustic transducer elements configured to receive an acoustic signal from the region of interest; and a processor configured to process data generated from the one or more acoustic transducer elements, to determine from the data whether the photoacoustic imaging probe is coupled to biological tissue, and to selectively disable the transmission of the light beam based on the determination. In some embodiments, the data is acoustic data generated from the received acoustic signal.

In some versions of these embodiments, the processor processes the acoustic data to determine one or more statistical properties of the acoustic data, and determines whether the photoacoustic imaging probe is coupled to biological tissue based on a comparison of the one or more statistical properties of the acoustic data and one or more reference statistical values.

In some versions of these embodiments, the processor processes the acoustic data to produce a brightness mode (B-mode) image of the region of interest, and determines whether the photoacoustic imaging probe is coupled to biological tissue based on whether most of the B-Mode image exhibits fully developed speckle

In some versions of these embodiments, the processor processes the acoustic data to produce a brightness mode (B-mode) image of the region of interest, and determines whether the photoacoustic imaging probe is coupled to biological tissue based on whether time-varying changes in the B-mode image exceed a defined threshold.

In some embodiments, the data comprises electrical impedance data for the one or more acoustic transducer elements, and the processor determines whether the

photoacoustic imaging probe is coupled to biological tissue based on whether a resonance frequency for at least one of the one or more acoustic transducer elements experiences a resonance shift which is greater than a defined threshold.

In some embodiments, the light generator comprises one or more light sources for generating the pulsed light beam, and wherein the processor selectively disables the transmission of the light beam by disconnecting a supply of power to the one or more light sources.

In some embodiments, the light generator comprises one or more light sources for generating the pulsed light beam, and one or more shutters for preventing the light beam from being emitted from the photoacoustic imaging probe, and wherein the processor selectively disables the transmission of the pulsed light beam by closing the one or more shutters.

In another aspect of the invention, a method is provided for photoacoustic imaging with an imaging probe having one or more acoustic transducer elements and a light generator configured to produce pulsed light. The method comprises: generating data from the one or more acoustic transducer elements; determining from the data generated from the one or more acoustic transducer elements whether the imaging probe is coupled to biological tissue; and when it is determined from the data that the imaging probe is not coupled to biological tissue, disabling the pulsed light from being transmitted by the imaging probe.

In some embodiments, the data is acoustic data that is generated from an acoustic signal received by the one or more acoustic transducer elements.

In some versions of these embodiments, the method further comprises: processing the acoustic data to determine one or more statistical properties of the acoustic data; and determining whether the imaging probe is coupled to biological tissue based on a comparison of the one of more statistical properties of the acoustic data and one or more reference statistical values.

In some versions of these embodiments, the method further comprises: processing the acoustic data to produce a brightness mode (B-mode) image of the region of interest; and determining whether the imaging probe is coupled to biological tissue based on whether most of the B-Mode image exhibits fully developed speckle.

In some versions of these embodiments, the method further comprises: processing the acoustic data to produce a brightness mode (B-mode) image of the region of interest; and determining whether the imaging probe is coupled to biological tissue based on whether time-varying changes in the B-mode image exceed a defined threshold.

In some embodiments, the data comprises electrical impedance data for the one or more acoustic transducer elements, and wherein the method further comprises determining whether the imaging probe is coupled to biological tissue based on whether a resonance frequency for at least one of the one or more acoustic transducer elements experiences a resonance shift which is greater than a defined threshold.

In some embodiments, the light generator comprises one or more light sources for generating the pulsed light, and wherein disabling the pulsed light from being transmitted by the imaging probe comprises disconnecting a supply of power to the one or more light sources.

In some embodiments, the light generator comprises one of more light sources for generating the pulsed light, and one of more shutters for preventing the pulsed light from being emitted from the imaging probe, and wherein disabling the light from being transmitted by the imaging probe comprises closing the one or more shutters.

In yet another aspect of the invention, an apparatus comprises: a light generator configured to produce a light beam; an imaging probe configured to deliver the light beam to a region of interest, the imaging probe including one or more acoustic transducer elements configured to receive an acoustic signal from the region of interest; and a processor configured to determine whether a light output region of the imaging probe is uncovered, and to disable the light beam from being transmitted by the imaging probe when the processor determines that the light output region is uncovered.

In some embodiments, the imaging probe further comprises a light sensor disposed on the imaging probe, and wherein the processor determines whether the light output region of the imaging probe is uncovered based on comparing a light sensed by the light sensor to a light threshold.

In some embodiments, the imaging probe further comprises an imaging sensor for detecting an image of a region coupled to the light output region of the imaging probe, and wherein the processor determines whether the light output region of the imaging probe is uncovered based on the image.

In some embodiments, the processor determines whether the light output region of the imaging probe is uncovered based on an absence of one or more defined features within the image.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one example embodiment of a photoacoustic imaging probe for a photoacoustic imaging system.

FIG. 2 is a functional block diagram of one example embodiment of a

photoacoustic imaging apparatus.

FIG. 3 is a functional block diagram of one example embodiment of a light generation system for a photoacoustic imaging apparatus.

FIG. 4 is a flowchart illustrating a method of controlling the light output of a photoacoustic imaging apparatus.

FIG. 5 is a flowchart illustrating an example embodiment of controlling the light output of a photoacoustic imaging apparatus.

FIG. 6 is a flowchart illustrating another example embodiment of a method of controlling the light output of a photoacoustic imaging apparatus. DETAILED DESCRIPTION

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided as teaching examples of the invention. Within the present disclosure and claims, when something is said to have approximately a certain value, then it means that it is within 10% of that value, and when something is said to have about a certain value, then it means that it is within 25% of that value.

FIG. 1 illustrates one example embodiment of a photoacoustic imaging probe 10 for a photoacoustic imaging system. Photoacoustic imaging probe 10 includes an acoustic transducer 12 and a light delivery system 14. In some embodiments, the light delivery system may comprise a bundle of optical fibers. In some embodiments, the light delivery system may comprise a light window or aperture through which light may be transmitted.

FIG. 2 is a functional block diagram of one example embodiment of a

photoacoustic imaging apparatus 20. Photoacoustic imaging apparatus 20 may be incorporated with one or more user controls, a display device, etc. to form a photoacoustic imaging system.

Photoacoustic imaging apparatus 20 includes a processor 21, an acoustic transducer comprising one or more acoustic transducer elements 22, and a light generation system 23. Photoacoustic imaging apparatus 20 may further include receive circuit(s) 24 for processing electrical signal(s) output from acoustic transducer element(s) 22 in response to an acoustic signal (e.g., one or more acoustic waves) received by acoustic transducer element(s) 22 from a region of interest.

In some embodiments, photoacoustic imaging apparatus 20 may include a

"conventional" acoustic imaging system which can transmit an acoustic probe beam (e.g., an ultrasonic probe beam, for example in a frequency range of 2-18 MHz) to the region of interest, for example by means of optional transmit circuit(s) 25, and receive back an acoustic signal (e.g., an ultrasonic signal, for example in a frequency range of 2-18 MHz) comprising acoustic echoes from the region of interest. In that case, photoacoustic imaging apparatus 20 may generate acoustic images in additional to photoacoustic images, to provide a more complete picture of a region of interest. Light generation system 23 may include a light delivery system as described above. Photoacoustic imaging apparatus 20 includes an imaging probe, for example photoacoustic imaging probe 10 of FIG, 1. The photoacoustic imaging probe includes at least acoustic transducer element(s) 22 and a light delivery system or light generation system 23. In some embodiments, the photoacoustic imaging probe may further include light generation system 23, receive circuit(s) 24, and/or transmit circuit(s) 25.

Processor 21 has associated therewith memory 26, which may include volatile memory (e.g., dynamic random access memory) and/or nonvolatile memory (e.g., read only memory, flash memory, ferroelectric memory, etc.). Processor 21 may execute one or more algorithms for operational control of photoacoustic imaging apparatus 20, including for example, algorithms described below with respect to FIGs. 4-6. In that case, programming instructions or code for processor 21 may be stored in memory 26.

Operationally, light generation system 23 is configured to produce light, for example a pulsed light beam, for illuminating a region of interest, for example tissue in a human body. The light is output by the photoacoustic imaging probe to the region of interest, where it causes an acoustic signal to be generated, as explained above. Acoustic transducer element(s) 22 receives the acoustic signal and converts it to one or more electrical signal(s). Receive circuit(s) 24 and/or processor 21 generates acoustic data from the acoustic signal received by acoustic transducer element(s) 22. Processor 21 may generate one or more images from the acoustic data and provide those images for display by a display device (not shown in FIG. 2).

Beneficially, in photoacoustic imaging apparatus 20, processor 21 may be configured to determine whether the photoacoustic imaging probe is uncovered, and may disable the light beam from being transmitted by the photoacoustic imaging probe when processor 21 determines that the photoacoustic imaging probe is uncovered. In some embodiments, processor 21 may be configured to generate a control signal for selectively enabling and disabling light from being output by light generation system 23, as explained in greater detail below. In some embodiments, processor 21 may be configured to process data generated from acoustic transducer element(s) 22, to determine from the data whether the photoacoustic imaging probe is coupled to biological tissue, and to selectively disable the transmission of the light beam based on that determination.

Optionally, the photoacoustic imaging probe of photoacoustic imaging apparatus 20 may include one or more light sensor(s) 28 and/or a camera 29, as discussed in detail below.

FIG. 3 is a functional block diagram of one example embodiment of a light generation system 23 for a photoacoustic imaging apparatus such as photoacoustic imaging apparatus 20. Light generation system 23 includes one or more light source(s) 32 and a power supply 34 for supplying power to light source(s) 32. Light generation system 23 may further comprise one or more light shutter(s) 36 and/or a controller 38 for controlling an operation of light generation system 23. In some embodiments, light source(s) 32 may comprise one or more pulsed laser(s). However, other light source(s) are contemplated. Light source(s) 32 generate light, for example pulsed light or a pulsed light beam, to be output by the photoacoustic imaging probe for illuminating a region of interest to produce one or more photoacoustic images thereof.

Beneficially, light generation system 23 may receive one or more control signal(s) for controlling an operation thereof. Such control signals may be received from a processor, such as processor 21 of photoacoustic imaging apparatus 20. The control signal(s) may control one or more operating parameters of light generation system 23, for example an intensity and/or frequency of the light output by light source(s) 32.

Beneficially the control signal(s) includes a light output control signal for selectively disabling light from being output by the photoacoustic imaging probe. In some embodiments, the light output control signal may be provided to power supply 34 for disconnecting a supply of power from being provided to light source(s) 32, thereby inhibiting light source(s) 32 from producing light. In some embodiments, the light output control signal may be provided to shutter(s) 36 to prevent light produced by light source(s) 32 from being output from light generation system 23, and thereby preventing the light from being output by the photoacoustic imaging probe. In some embodiments, controller 38 may include the capability to disable light source(s) 32, and in that case the light output control signal may be provided to controller 38.

As explained above, the light output control signal may be generated by processor 21 of photoacoustic imaging apparatus 20. In some embodiments, processor 21 may be configured to generate the light output control signal so as to disable the light beam from being transmitted by the photoacoustic imaging probe when processor 21 determines that the photoacoustic imaging probe is uncovered. In some embodiments, processor 21 may be configured to process data generated from acoustic transducer element(s) 22, to determine from the data whether the photoacoustic imaging probe is coupled to biological tissue, and to generate the light output control signal so as to disable the transmission of the light beam based on that determination.

FIG. 4 is a flowchart illustrating a method 40 of controlling the light output of a photoacoustic imaging apparatus. In some embodiments, method 40 may comprise an algorithm performed under control of processor 21 of photoacoustic imaging apparatus 20, for example based on computer instructions stored in memory 26.

Initially, in a step 410, light is prevented from being output by the photoacoustic imaging probe of the photoacoustic imaging apparatus. That is, until it is determined a light output region of the photoacoustic imaging probe is covered, step 410 is executed and light is prevented from being transmitted or output from the photoacoustic imaging probe. The light output region comprises the area or areas of the photoacoustic imaging probe from which light is output, and may comprise one or more light output windows or light output apertures.

In a step 420, it is determined whether or not the light output region of the photoacoustic imaging probe is covered. In some embodiments, this determination may be made by processor 21 of photoacoustic imaging apparatus 20. Various embodiments of techniques for making this determination will be described below, in particular with respect to FIGs. 5 and 6.

If it is determined in step 420 that the light output region of the photoacoustic imaging probe is uncovered, the process continues in step 410 wherein the light continues to be prevented from being output by the photoacoustic imaging probe of the photoacoustic imaging apparatus.

On the other hand, if it is determined in step 430 that the light output region of the photoacoustic imaging probe is covered, then the process proceeds to step 430 where light is allowed to be output by the photoacoustic imaging probe of the photoacoustic imaging apparatus. The process may then return to step 420 where it is may be repeatedly checked whether or not the light output region of the photoacoustic imaging probe is covered, and if it is determined that the light output region of the photoacoustic imaging probe is no longer covered, then the light may again be prevented from being output by the photoacoustic imaging probe of the photoacoustic imaging apparatus.

FIG. 5 is a flowchart illustrating an example embodiment of a method 50 of controlling the light output of a photoacoustic imaging apparatus.

In a step 510, light output from the photoacoustic imaging probe of the photoacoustic imaging apparatus is disabled. That is, until it is determined the

photoacoustic imaging probe is coupled to biological tissue, step 510 is executed and light is prevented from being transmitted or output from the photoacoustic imaging probe. The light output region comprises the area or areas of the photoacoustic imaging probe from which light is output, and may comprise one or more light output windows or light output apertures.

In a step 520, an acoustic probe signal (e.g., an ultrasonic probe beam, for example in a frequency range of 2-18 MHz) is generated and transmitted to a region of interest, for example a region coupled to the light output region of the photoacoustic imaging probe (either directly, or through an intermediary such as coupling gel, a gown, etc.). In some embodiments, the acoustic probe signal may be transmitted by acoustic transducer element(s) 22 of photoacoustic imaging apparatus 20. In that case, acoustic transducer element(s) 22 and transmit circuit(s) 25 may form an acoustic beam for probing and imaging the region of interest. In some embodiments, the acoustic probe signal, or acoustic beam, may be transmitted by a different acoustic probe, or some other device.

In a step 530, an acoustic signal (e.g., an ultrasonic signal, for example in a frequency range of 2-18 MHz) comprising one or more acoustic echoes is received from the region of interest. In some embodiments, the acoustic signal may be received by acoustic transducer element(s) 22 of photoacoustic imaging apparatus 20.

In a step 540, the received acoustic signal is processed to determine whether or not the photoacoustic imaging probe is coupled (directly, or through an intermediary such as coupling gel, a gown, etc.) to biological tissue, such as human skin. In some embodiments, the received acoustic signal may be processed by receive circuit(s) 24 and/or processor 21 to generate or produce acoustic data representing an image of the region of interest.

In some embodiments, advantage may be taken of the differences between an acoustic signal received from a region comprising biological tissue, and a signal received from a region (e.g. open air) which does not comprise biological tissue.

That is in some embodiments, in step 540 it is determined whether or not the photoacoustic imaging probe is coupled to biological tissue based on a comparison of the one of more statistical properties of the acoustic data against one or more reference statistical values that are expected to be observed when the acoustic data is produced from an acoustic signal received from biological tissue.

For example, the brightness mode (B-mode) image that an acoustic imaging device produces when the imaging probe is in contact with biological tissue is different from the B-mode image when it is not. One cannot however merely look for the presence or absence of an image. Even when the probe is clean and its aperture coupled to air, the near field will have reverberation bands from the sound bouncing around in the matching layers.

In practical situations the imaging probe may be covered with acoustic coupling gel, for example when a scan has just been completed. Sometimes there can be large amounts of such gel, such as when the sonographer applies gel to the imaging probe just prior to initiating a scan. Other materials can come in contact with the imaging probe during cleaning, such as paper towels or wet disinfecting wipes, or water when rinsing the photoacoustic imaging probe. All these scenarios can cause an image signal at significant depths.

The acoustic data resulting from soft biological tissue is largely due to acoustic reflections from areas where the acoustic properties of the tissue change rapidly over spatial distances much smaller than the acoustic wavelength and in a geometrically random fashion. This can be modeled as reflections from a large collection of finely spaced and randomly distributed point scatterers. This causes the acoustic data to be normally distributed, the envelope values of the acoustic signal to be Rayleigh distributed, and the brightness values to have an exponential distribution. This type of signal is referred to as "fully developed speckle." Such fully developed speckle should not be produced when the photoacoustic imaging probe is not coupled to biological tissue.

When imaging biological tissue this fully developed speckle is expected to be observed throughout most of the imaged region. Only areas where scatter distribution is not random (sharp edges of large B-mode contrast) are expected to deviate from fully developed speckle, for example due to bone/tissue interfaces and blood/tissue interfaces.

Acoustic data produced when the probe is not in contact with biological tissue will largely be created through reflections from surfaces and reverberations, and thus not exhibit the same statistics as fully developed speckle.

Thus one possible metric for detecting if biological tissue is being imaged may be to determine if fully developed speckle is present in most of the imaged region.

There are many possible algorithms to detect to what extent there is fully developed speckle. Motion tracking through speckle decorrelation assumes fully developed speckle and in this research field methods for detection of fully developed speckle have been developed. For example R. W. Prager, et al., "Speckle Detection in Ultra-sound Images Using First Order Statistics " TECHNICAL REPORT CUED/F-INFENG/TR 415,

Cambridge University Engineering Department, July 2001, disclose an algorithm based on R = (mean/standard deviation) and S=skewness of the square of the envelope data.

Alternatively, if one knows the transducer geometry and specifications, the decorrelation rate based on fully developed speckle can be calculated, and if it deviates from measured decorrelation rate, it can be determined that speckle is not fully developed.

Accordingly, in some embodiments, in step 540 it is determined whether or not the photoacoustic imaging probe is coupled to biological tissue based on whether most of the B-Mode image exhibits fully developed speckle.

In some embodiments, dynamic aspects of the B-mode image can be exploited to determine whether the photoacoustic imaging probe is coupled to biological tissue. When imaging living biological tissue the B-mode image is not going to be fully static even when the probe is held still, due to blood perfusion and breathing. Thus, if a static B-mode is detected one can assume there is no biological tissue and the light output may be disabled.

Accordingly, in some embodiments, in step 540 it is determined whether or not the photoacoustic imaging probe is coupled to biological tissue based on whether time-varying changes in the B-mode image exceed a defined threshold.

In a step 550, when it is determined that the photoacoustic imaging probe is not coupled to biological tissue, then the process continues in step 510 wherein the light continues to be prevented from being output by the photoacoustic imaging probe of the photoacoustic imaging apparatus. On the other hand, if it is determined in step 550 that the photoacoustic imaging probe is coupled to biological tissue, then the process proceeds to step 560 where light output by the photoacoustic imaging probe is enabled. The process may then return to step 520 where it is may be repeatedly checked whether or not the photoacoustic imaging probe is coupled to biological tissue and if it is determined that the photoacoustic imaging probe is no longer coupled to biological tissue, then the light output by the photoacoustic imaging probe may be disabled again.

FIG. 6 is a flowchart illustrating another example embodiment of a method 60 of controlling the light output of a photoacoustic imaging apparatus.

In a step 610, light is output from the photoacoustic imaging probe of the photoacoustic imaging apparatus is disabled. That is, until it is determined the

photoacoustic imaging probe is coupled to biological tissue, step 610 is executed and light is prevented from being transmitted or output from the photoacoustic imaging probe. The light output region comprises the area or areas of the photoacoustic imaging probe from which light is output, and may comprise one or more light output windows or light output apertures.

In a step 620, the resonance frequency(ies) of the acoustic transducer element(s) of the photoacoustic imaging probe is/are determined. For example, in some embodiments receive circuit(s) 24 may produce electrical impedance data from acoustic transducer element(s) 22 which indicates the resonant frequency(ies) thereof.

The electrical properties of an acoustic transducer element, such as a PZT crystal, depend on its mechanical loading, i.e. the mechanical/acoustic properties of the material with which the transducer element is in contact. Therefore, the resonance frequency will shift when the probe is brought in contact with the skin.

Accordingly, in some embodiments, in step 630 electrical impedance data from acoustic transducer element(s) 22 may be used to determine the resonance frequency(ies), thereof, and the resonance frequency(ies) may be compared to reference values to determine whether or not the photoacoustic imaging probe is coupled to biological tissue.

In a step 640, when it is determined that the photoacoustic imaging probe is not coupled to biological tissue, then the process continues in step 610 wherein the light continues to be prevented from being output by the photoacoustic imaging probe of the photoacoustic imaging apparatus. On the other hand, if it is determined in step 640 that the photoacoustic imaging probe is coupled to biological tissue, then the process proceeds to step 650 where light output by the photoacoustic imaging probe is enabled. The process may then return to step 620 where it is may be repeatedly checked whether or not the photoacoustic imaging probe is coupled to biological tissue and if it is determined that the photoacoustic imaging probe is no longer coupled to biological tissue, then the light output by the photoacoustic imaging probe may be disabled again.

In various embodiments, other techniques may be employed to determine whether or not a photoacoustic imaging probe is covered, or whether or not it is coupled to biological tissue.

For example, in some embodiments, one or more light sensor(s) 28 could be used to detect darkening when the photoacoustic imaging probe is covered by skin or other materials. That same light sensor(s) 28 could also be used to detect the laser flashes when in photoacoustic imaging mode. Absence of detected laser flashes while in photoacoustic imaging mode could indicate laser malfunction (such as broken or unplugged fiber light guide).

In other embodiments, a camera or direct contact charge coupled device 29 could be used to detect skin texture features and thereby determine whether or not the

photoacoustic imaging probe is coupled to biological tissue.

In still other embodiments, it can be determined whether or not the photoacoustic imaging probe is coupled to biological tissue by employing a capacitance measurement device at the photoacoustic imaging probe to measure a capacitance of any surface in contact with the photoacoustic imaging probe, and/or an impedance measurement device at the photoacoustic imaging probe to measure electrical conductivity of any surface in contact with the photoacoustic imaging probe, and/or a pressure sensor at the photoacoustic imaging probe to measure a contact pressure of the probe, Any one or combination of the values obtained by these measurements may be compared (e.g., by processor 21) to reference values to determine whether or not the photoacoustic imaging probe is coupled to biological tissue.

While preferred embodiments are disclosed herein, many variations are possible which remain within the concept and scope of the invention. Such variations would become clear to one of ordinary skill in the art after inspection of the specification, drawings and claims herein. The invention therefore is not to be restricted except within the scope of the appended claims.

Claims

CLAIMS What is claimed is:

1. A photoacoustic imaging apparatus, comprising:

a light generator configured to produce a pulsed light beam;

a photoacoustic imaging probe configured to deliver the pulsed light beam to a region of interest, the photoacoustic probe including one or more acoustic transducer elements configured to receive an acoustic signal from the region of interest; and

a processor configured to process data generated from the one or more acoustic transducer elements, to determine from the data whether the photoacoustic imaging probe is coupled to biological tissue, and to selectively disable the transmission of the light beam based on the determination.

2. The apparatus of claim 1, wherein the data is acoustic data generated from the received acoustic signal.

3. The apparatus of claim 2, wherein the processor processes the acoustic data to determine one or more statistical properties of the acoustic data, and determines whether the photoacoustic imaging probe is coupled to biological tissue based on a comparison of the one of more statistical properties of the acoustic data and one or more reference statistical values.

4. The apparatus of claim 2, wherein the processor processes the acoustic data to produce a brightness mode (B-mode) image of the region of interest, and determines whether the photoacoustic imaging probe is coupled to biological tissue based on whether most of the B-Mode image exhibits fully developed speckle.

5. The apparatus of claim 2, wherein the processor processes the acoustic data to produce a brightness mode (B-mode) image of the region of interest, and determines whether the photoacoustic imaging probe is coupled to biological tissue based on whether time-varying changes in the B-mode image exceed a defined threshold.

6. The apparatus of claim 1, wherein the data comprises electrical impedance data for the one or more acoustic transducer elements, and wherein the processor determines whether the photoacoustic imaging probe is coupled to biological tissue based on whether a resonance frequency for at least one of the one or more acoustic transducer elements experiences a resonance shift which is greater than a defined threshold.

7. The apparatus of claim 1, wherein the light generator comprises one or more light sources for generating the pulsed light beam, and wherein the processor selectively disables the transmission of the light beam by disconnecting a supply of power to the one or more light sources.

8. The apparatus of claim 1, wherein the light generator comprises one of more light sources for generating the pulsed light beam, and one of more shutters for preventing the light beam from being emitted from the photoacoustic imaging probe, and wherein the processor selectively disables the transmission of the pulsed light beam by closing the one or more shutters.

9. The apparatus of claim 1, wherein the or more acoustic transducer elements are further configured to transmit an acoustic beam, and wherein the acoustic signal comprises a reflection produced in response to the acoustic beam.

10. A method of photoacoustic imaging with an imaging probe having one or more acoustic transducer elements and a light generator configured to produce pulsed light, the method comprising:

generating data from the one or more acoustic transducer elements;

determining from the data generated from the one or more acoustic transducer elements whether the imaging probe is coupled to biological tissue; and

when it is determined from the data that the imaging probe is not coupled to biological tissue, disabling the pulsed light from being transmitted by the imaging probe.

11. The method of claim 10, wherein the data is acoustic data is generated from an acoustic signal received by the one or more acoustic transducer elements.

12. The method of claim 11, further comprising transmitting an acoustic beam from the one or more acoustic transducer elements, and wherein the acoustic signal comprises a reflection produced in response to the acoustic beam.

13. The method of claim 11, further comprising:

processing the acoustic data to determine one or more statistical properties of the acoustic data; and

determining whether the imaging probe is coupled to biological tissue based on a comparison of the one of more statistical properties of the acoustic data and one or more reference statistical values.

14. The method of claim 11, further comprising:

processing the acoustic data to produce a brightness mode (B-mode) image of the region of interest; and

determining whether the imaging probe is coupled to biological tissue based on whether most of the B-Mode image exhibits fully developed speckle.

15. The method of claim 11, further comprising:

determining whether the imaging probe is coupled to biological tissue based on whether time-varying changes in the B-mode image exceed a defined threshold.

16. The method of claim 10, wherein the data comprises electrical impedance data for the one or more acoustic transducer elements, and wherein the method further comprises determining whether the imaging probe is coupled to biological tissue based on whether a resonance frequency for at least one of the one or more acoustic transducer elements experiences a resonance shift which is greater than a defined threshold.

17. The method of claim 10, wherein the light generator comprises one or more light sources for generating the pulsed light, and wherein disabling the pulsed light from being transmitted by the imaging probe comprises disconnecting a supply of power to the one or more light sources.

18. The method of claim 10 wherein the light generator comprises one of more light sources for generating the pulsed light, and one of more shutters for preventing the pulsed light from being emitted from the imaging probe, and wherein disabling the light from being transmitted by the imaging probe comprises closing the one or more shutters.

19. An apparatus, comprising:

a light generator configured to produce a light beam;

an imaging probe configured to deliver the light beam to a region of interest, the imaging probe including one or more acoustic transducer elements configured to receive an acoustic signal from the region of interest; and

a processor configured to determine whether a light output region of the imaging probe is uncovered, and to disable the light beam from being transmitted by the imaging probe when the processor determines that the light output region is uncovered.

20. The apparatus of claim 19, wherein the imaging probe further comprises a light sensor disposed on the imaging probe, and wherein the processor determines whether the light output region of the imaging probe is uncovered based on comparing a light sensed by the light sensor to a light threshold.

21. The apparatus of claim 19, wherein the imaging probe further comprises an imaging sensor for detecting an image of a region coupled to the light output region of the imaging probe, and wherein the processor determines whether the light output region of the imaging probe is uncovered based on the image.

22. The apparatus of claim 21, wherein the processor determines whether the light output region of the imaging probe is uncovered based on an absence of one or more defined features within the image.