WO2009057021A2

WO2009057021A2 - Photoacoustic imaging contrast agent and system for converting optical energy to in-band acoustic emission

Info

Publication number: WO2009057021A2
Application number: PCT/IB2008/054392
Authority: WO
Inventors: Yao Wang; William T. Shi
Original assignee: Koninklijke Philips Electronics N. V.
Priority date: 2007-10-31
Filing date: 2008-10-24
Publication date: 2009-05-07
Also published as: WO2009057021A3

Abstract

The present disclosure provides systems and methods related to contrast agents for photoacoustic and ultrasound imaging. A contrast agent includes a nanoparticle -based contrast medium that is adapted to absorb optical energy and to undergo a phase change upon absorption of a requisite level of optical energy, such phase change being effective to generate nano-bubbles and/or micro-bubbles that resonate at a frequency or in a frequency range. Various constructions of the PA contrast agent are provided including one substance droplets with both absorbing and evaporating capabilities, and multiple-component complexes of one substance with both absorbing and evaporating materials. Highly absorbing nano-particles (such as gold nano-rods) may be coated with an evaporating material. In addition, disclosed agents (including either one substance or multi-component complex) may be encapsulated or coated with an optically transparent and mechanically elastic shell that allows the droplet to expand and form stabilized nano- and/or microbubbles during phase change.

Description

PHOTOACOUSTIC IMAGING CONTRAST AGENT AND SYSTEM FOR CONVERTING OPTICAL ENERGY TO IN-BAND ACOUSTIC EMISSION

BACKGROUND

1. Technical Field

The present disclosure relates to photoacoustic imaging contrast agents and to systems/methods for generating in-band acoustic emissions.

2. Background Art

Photoacoustic (PA) imaging is a non-invasive imaging technique that may be used in medical environments, e.g., to detect, inter alia, vascular disease, skin abnormalities and some types of cancer. PA imaging generally involves flashing a laser at low energy with a near infrared wavelength onto a target area or region. The near infrared wavelength helps the light penetrate deeply into the body. This penetration behavior creates a large radiated area for a more detailed picture. As light illuminates the body, the rapid absorption of laser energy expands the tissue (composed of microscopic absorbers) through transient thermo-elastic expansion. This expansion creates ultrasonic acoustic waves that can be detected by ultrasound detectors of appropriate sensitivity, e.g., ultrasound transducers. The transducer readings can be processed and interpreted using different mathematical equations/algorithms to create two dimensional or three dimensional images of the target area, showing the tissue structure via spatial distribution of microscopic absorbers.

PA imaging is effective in anatomical applications based on its unique contrast mechanism. Typically, each tissue or target region absorbs different amounts of the laser energy, making each different target region or tissue potentially unique from a PA imaging standpoint. For purposes of blood vessel-related imaging, hemoglobin generally exhibits high optical contrast when a near infrared wavelength is applied. This contributes to the sensitivity of blood vessel imaging with photoacoustic techniques, enabling doctors/healthcare providers to see abnormalities in the skin, vascular disease and cancer which can then be treated directly. PA images may be combined with those from other modalities (e.g., ultrasound) to create highly detailed depictions of the target area with complementing contrast. For example, the generated images may facilitate valuable diagnostics, e.g., allowing doctors/health care providers to identify small lesions that may be difficult to pick up using other techniques/technologies.

Photoacoustics is also an emerging field within molecular medicine. (See, e.g.,

X. Wang, Y. Pang, G. Ku, G. Stoica and L. -H. Wang, "Three-dimensional laser-induced photoacoustic tomography of mouse brain with the skin and skull intact " Optics Letters 28 (19), 1739-1741, 2003.) Recent progress towards the use of contrast agents to assist in generating images has further attracted considerable interest for commercialization of this technology. (See, e.g., Y. Wang, X. Xie, X. Wang, G. Ku, K. L. Gill, D. P. O'Neal, G. Stoica, and L.-H. Wang, "Photoacoustic tomography of a nanoshell contrast agent in the in vivo rat brain " Nano Letters 4 (9), 1689-1692, 2004.) Such PA contrast agents (up to a few hundred nanometers) are much smaller than micro-bubbles (up to 3-5 microns) used in ultrasound imaging. Thus, these agents can permeate the vascular wall and can be exploited for the purpose of quantifying vascular permeability.

PA contrast agents (e.g., nano-rods, nano-shells, quantum-dots) under development may significantly increase optical absorption, which can translate into enhanced PA contrast. For example, a nanoparticle ellipsoid (cylinder) with a diameter of 25 nm and a length of 100 nm has been designed and validated. (See, D. Guzatov et. al., "Plasmon Resonance in Ellipsoidal Nano-particles with Shells " Quantum Electronics, 33(9), 817-822, 2003; A.

Oraevsky, "High Contrast Opto acoustical Imaging Using Nano-particles " Patent Publication No. WO 2004/068405; J.Copland et. al. "Bioconjugated Gold Nano-particles as a Molecular Based Contrast Agent: Implications for Imaging of Deep Tumors Using Optoacoustic Tomography," Molecular Imaging and Biology, 6(5), 341-349, 2004.) Compared to a 40 nm diameter gold nano-sphere, the ellipsoid design leads to a narrowed and shifted (from 532 nm to 757 nm) absorption spectrum and an increase in absolute absorption cross-section by a factor of almost twenty-eight.

PA contrast agents currently available and under development aim at directly increasing optical absorption and consequently suffer from low sensitivity due to their inherently low acoustical emission capability. Furthermore, incident optical pulsing is generally out of the reception bandwidth of ultrasound instrumentation and, thus, the optical energy is inefficiently converted into ultrasound signal(s). Indeed, existing PA agents were primarily developed for optical purposes and rely on increased optical absorption alone to be effective and operable. To date, insufficient attention has been directed to PA agents that improve upon the conversion of absorbed optical energy to in-band ultrasound signal. This lack of effective conversion functionality is a hindrance to the overall effectiveness and applicability of PA agents and PA imaging techniques.

For example, a short laser pulse (e.g., approximately 5ns) induces a broad-band PA signal from a point absorber (10 μm in diameter). This signal can be calculated analytically according to a previously developed mathematical formula. (See, C.G.A. Hoelen et al., "A New Theoretical Approach to Photoacoustic Signal Generation " Journal of the Acoustical Society of America, 106(2), 695-706, 1999.) FIGS. l(a) - l(b) illustrate simulated results that demonstrate the broadband nature of a generated photo-acoustic transient waveform. FIG. l(a) illustrates an exemplary simulated PA signal generated from a point absorber of 10 μm in diameter while a virtual receiving hydrophone is located 25 mm away from the source. FIG. l(b) illustrates the spectrum of the generated PA signal. The simulation associated with FIGS. l(a) - l(b) assumes no dispersion or attenuation in medium.

Assuming a constant response between 0 and 24 MHz for ultrasound detection (e.g., Philips HDI-5000 scanner has an A/D sampling frequency of 24 MHz), typically only about 4% of all absorbed energy is detected due to the mismatch in the spectra of the PA signal and the pass-band of the ultrasound detector (e.g., ultrasound transducer array). Since red blood cells typically have a diameter of less than 10 μm (hemoglobin is the most prominent endogenous PA contrast agent), the noted calculation should represent realistic conversion efficiency in PA imaging without the use of contrast agents. Of note, the use of nanoparticles to boost optical absorption may prove to be more problematic with respect to energy conversion efficiency. FIGS. 2(a) - 2(b) show results related to using a nanoparticle with a diameter of 300 nm using the same laser pulse as described with respect to FIGS. l(a) - l(b). FIG. 2(a) illustrates a simulated PA signal generated from a point absorber 300 nm in diameter while the virtual receiving hydrophone is located 25 mm away from the source. FIG. 2(b) illustrates a spectrum of the generated PA signal. The simulation assumes no dispersion or attenuation in medium. As demonstrated in FIGS. 2(a) - 2(b), the bandwidth of the PA transient increases dramatically in the presence of nanoparticles of the noted diameter and the mismatch in spectra worsens (only about 1 ppm of absorbed energy is within the pass-band of the transducer). Thus, the use of nanoparticles in small animal experiments is likely to require greater concentrations of PA chromophores. This requirement may further complicate future human studies based on potential safety concerns and issues with required injection volumes.

Microbubble -based ultrasound contrast agents offer certain recognized advantages in enhancing regular backscatter signals and generating distinct backscatter signals (e.g., super- harmonics and sub-harmonics of incident ultrasound waves) within the ultrasound receive pass- band. (See, e.g., Shi WT, Forsberg F, Liu JB, Merritt CRB, Goldberg BB: "New US media boosts imaging quality " Diagnostic Imaging Global: Special Supplement, Nov. 2000, pp 8-12.) Such benefits may be attributed to the fact that the acoustic properties (especially density and compressibility) of a gas within a microbubble are significantly different relative to those of blood and tissue surrounding the microbubble. The distinct backscatter signal associated with microbubble-based contrast agents arises from both nonlinear bubble dynamics and insoniflcation using unique ultrasound pulse sequences. (See, Simpson DH, Chin CT, Burns PN, "Pulse inversion Doppler: a new method for detecting nonlinear echoes from microbubble contrast agents," IEEE Trans Ultrason Ferroelec Freq Contr 1999; 46:372-382; U.S. Patent No. 6,494,841 to Thomas et al., "Medical diagnostic ultrasound system using contrast pulse sequence imaging.") The sizes of these microbubble-based contrast agents are relatively large. For example, a microbubble-based contrast agent that is commercially available from Bristol- Myers-Squibb (Definity® contrast agent) has a documented size distribution between 1.1 μm and 3.3 μm and cannot permeate capillary walls. The relatively large size of known microbubble-based contrast agents thus supports rapid clearance from the patient's system, but prevents known microbubble-based contrast agents from measuring vascular parameters, such as permeability. The use of nano -bubbles ~ which would overcome the limitations associated with known microbubble -based contrast agents ~ in ultrasound backscatter imaging has not been realized for several reasons. For example, the life-time of nano-bubbles is too short for intravenous injection and subsequent human circulation, mainly because of the tremendous surface tension against the shell material in this size range. Additionally, the backscatter cross- section of such nano-bubbles is very small. Since backscatter cross-section is determined by 6th power on scatterer size, a factor of 10 reduction in bubble diameter may lead up to 10 times (6OdB) reduction in backscatter power, which is a diminishing return.

Thus, despite efforts to date, a need still exists for effective PA signal conversion systems and methods capable of generating ultrasound bandwidth signals for effective PA imaging. These and other needs are addressed and/or overcome by the systems and methods of the present disclosure.

SUMMARY The present disclosure provides contrast agent-based systems and methods that offer enhanced photoacoustic and ultrasound imaging results. More particularly, the disclosed PA contrast agents are optimally tuned for the ultrasonic receive pass-band of the PA transducer to provide more effective imaging systems. In addition, the disclosed contrast agents are sufficiently small to permeate capillary walls and like anatomical structures, thereby permitting an expansion of PA imaging/measurement applications, e.g., to vascular parameters such as permeability. The disclosed contrast agents are sufficiently stable for advantageous clinical use, e.g., by intravenous injection and circulatory migration to desired location(s)/region(s).

In an exemplary embodiment, the present disclosure provides nanoparticle-based contrast agent that is adapted for in situ activation to form nano-bubbles and/or micro-bubbles. Activation of the nanoparticles is achieved through the introduction of optical energy, i.e., laser irradiation at a near infrared wavelength. The resonance of the activated nano-bubbles (and/or micro-bubbles) generated through activation of the disclosed nanoparticle-based contrast agents is advantageously adapted to function as an efficient acoustic radiator, i.e., an ultrasonic source for purposes of PA imaging systems of the present disclosure. In a first exemplary embodiment of the present disclosure, a nanoparticle-based contrast agent is provided that is adapted for absorption of optical energy and thermal evaporation in response to such energy absorption. The disclosed contrast agent advantageously takes the form of droplets that are adapted to experience/undergo a phase change therewithin in response to localized energy absorption, i.e., evaporation, thereby forming nano-bubbles (and/or micro-bubbles). More particularly, the relatively rapid gaseous expansion resultant from the disclosed energy absorption (laser energy) temporarily creates a bubble with a resonant frequency within the desired ultrasound receive pass-band. In this way, the disclosed nanoparticle -based contrast agents generate a PA signal that may be captured/used for PA imaging/measurement according to the present disclosure. Indeed, the volumetric expansion of the contrast agent/contrast medium emits an acoustic signal that can be received by a detection apparatus, such as an ultrasound transducer. The emission of the acoustic signal is advantageously within the detectable in-band frequency range of the transducer.

In exemplary embodiments, the disclosed nanoparticle -based contrast agents/contrast medium may be defined by a nanoparticle, e.g., a gold nano-sphere, nano-rod, or the like, that is encapsulated and/or coated by a coating material. The coating material may be a perfiuorocarbon material/composition and is typically optically transparent, while exhibiting mechanical elasticity, thereby allowing the droplet to advantageously expand during phase change. Of note, the localized heat resulting from optical stimulation of the light absorbing nanoparticle material is generally characterized by a temperature change of only a few degrees Celsius. For delivery purposes, the nanoparticle -based contrast agent/contrast medium may be suspended in a carrier solution for injection with respect to a target tissue region. The carrier solution can be a saline -based solution, such as phosphate buffered saline (PBS). Due to the relatively small size, e.g., between about 50 nm and 500 nm in diameter, the disclosed contrast agent/contrast medium is free to permeate through capillary walls and the like. Thus, the disclosed contrast agents/contrast media significantly expand the utility of PA imaging/measuring techniques.

In a second exemplary embodiment of the present disclosure, a bi-layered or multi-layered nanoparticle contrast agent/contrast medium is provided for PA imaging/measurement applications. The core of the disclosed multi-layered nanoparticle contrast agent/contrast medium generally takes the form of an optically absorbing material that is adapted to undergo a phase change, i.e., evaporation, upon absorption of optical energy (e.g., laser energy). An outer encapsulating shell layer may be provided according to this second exemplary embodiment, such shell layer also being adapted for optical absorption and exhibiting a desired level of elasticity. Through the combined energy absorption of the core and shell layer, the disclosed multi-layered contrast agent/contrast medium absorbs sufficient energy to effect the desired phase change/evaporation of the core. As with the previously disclosed exemplary embodiment, such evaporative phenomenon translates to a nano-bubble (and/or micro-bubble) that resonates at a desired frequency for PA imaging/measurement. Of note, the shell layer enhances the stability of the multi-layered contrast agent/contrast medium, thereby supporting significant expansion thereof through internal bubble formation, e.g., to a diameter of 500 nm to 5000 nm (based on starting diameter as small as 50 nm).

In an exemplary embodiment, the acoustic signal is characterized by a frequency that can be tuned by changing parameters associated with the disclosed contrast agent/contrast medium, e.g., droplet size, materials of fabrication, coating/encapsulation materials/thickness, properties of multilayered droplets, and the like. The acoustic signals generated by the disclosed contrast agents/contrast media are typically characterized by a frequency (e.g., 10 MHz).

The present disclosure further provides an exemplary photoacoustic imaging system including: (a) a transducer adapted to receive ultrasound signals and optionally transmit the signals to an imaging system; (b) a photoacoustic transmission source adapted to transmit a laser pulse to a target tissue region; and (c) a contrast agent including a nanoparticle -based material that is adapted to absorb energy and undergo a phase change (e.g., evaporation), thereby emitting a detectable signal in a given resonance frequency region. The contrast agent is adapted to be injected into the target tissue region and may be adapted to permeate through vascular walls and the like.

The present disclosure further provides for an exemplary method for ultrasound imaging using photoacoustic signal transmission including the steps of: (a) injecting a nanoparticle -based contrast agent into a target tissue region, wherein the contrast agent is adapted for light absorption and phase change (e.g., evaporation) based on such light absorption; (b) optically stimulating the nanoparticle -based contrast agent by transmitting a laser pulse from a laser source to the target tissue region; and (c) providing a transducer to receive ultrasound signals resulting from the optical stimulation and phase change of the nanoparticle-based contrast agent. The ultrasound signals received by the transducer may be advantageously transmitted to an imaging system or other processing unit, as is known in the art.

Additional features, functions and benefits of the disclosed contrast agents, systems and methods will be apparent from the description which follows, particularly when read in conjunction with the appended figures.

BRIEF DESCRIPTION OF THE DRAWINGS

To assist those of ordinary skill in the art in making and using the disclosed systems and methods, reference is made to the appended figures, wherein: FIG. l(a) illustrates a prior art/reference simulated PA signal generated from a point absorber of lOum in diameter while a virtual receiving hydrophone is located 25mm away from the source; FIG. l(b) illustrates a prior art/reference spectrum of the PA signal generated in the system of FIG. l(a); FIG. 2(a) illustrates prior art/reference simulated PA signal generated from a point absorber 300nm in diameter while the virtual receiving hydrophone is located 25mm away from the source; FIG. 2(b) illustrates a prior art/reference spectrum of the PA signal generated in the system of FIG. 2(a); FIG. 3 illustrates an exemplary contrast agent droplet according to the present disclosure; FIG. 4 illustrates an exemplary laser pulse train for use in PA imaging systems of the present disclosure, the pulse train including 50ns laser irradiation with 10 individual laser pulses at a temporal spacing of 10ns, each laser pulse having a temporal duration of 6ns;

FIG. 5 illustrates an exemplary laser irradiation waveform for use in PA imaging of the present disclosure that is composed of 50ns of continuous laser irradiation;

FIG. 6 illustrates an exemplary embodiment of a laser irradiation waveform that advantageously matches the contrast agent response with the pass-band shape of a PA transducer; FIG. 7 illustrates an exemplary contrast agent according to the present disclosure that includes a bi-layered nanoparticle with a core of absorbing and evaporating material and an outer shell of an elastic coating material. Of note, the disclosed core may be an evaporating material only, while the disclosed elastic shell may be an optically absorbing material; FIG. 8 illustrates an exemplary contrast agent composed of two-liquid emulsion nano-droplets, wherein smaller evaporating droplets are embedded inside larger droplets; FIG. 9 illustrates an exemplary contrast agent made of optically absorbing particles covered with a coating (FIG. 9(a)) or droplets (FIG. 9(b)) of evaporating material according to the present disclosure; and FIG. 10 illustrates an exemplary contrast agent consisting of a complex of optically absorbing particles and evaporation droplet. In FIG. 10(a), each evaporation droplet is covered by absorbing particles and, in

FIG. 10(b), multiple absorbing particles are embedded inside each droplet.

DESCRIPTION OF EXEMPLARY EMBODIMENT(S) The present disclosure provides advantageous systems and methods for increasing photoacoustic (PA) imaging sensitivity and expanding the applicability of PA imaging techniques, e.g., to vascular environments (and other clinical regions requiring permeation of small diameter, e.g., 50 nm, contrast agent droplets). The advantageous clinical results that may be achieved according to the disclosed systems and methods are achieved through exemplary contrast agents/contrast media. Performance of the disclosed contrast agents/contrast media may be enhanced when used in combination with disclosed optical pulsing techniques.

The disclosed contrast agents/contrast media include nanoparticles, e.g., nano- spheres, nano-rods, and the like, that are adapted to absorb energy and to undergo a phase change, e.g., evaporation, in response to such energy absorption. Thus, the disclosed contrast agents/contrast media ~ which generally take the form of droplets ~ are generally adapted to undergo a liquid-to-gas phase shift after exposure to optical pulses, thereby forming/defining tiny gas-filled bubbles. The bubbles (e.g., nano-bubbles and/or micro-bubbles) resonate at frequencies that advantageously match (or substantially match) the pass-band of a receiving ultrasound transducer. Of note, up until the time the disclosed contrast agents are exposed to activating energy (e.g., laser energy), the contrast agents ~ which are based on nanoparticles ~ are sized for migration to and through the vasculature. Moreover, the contrast agents may be conjugated to specific targeting molecules (e.g., heat shock protein for targeting endothelium transduction, α_vβ₃ and inter-cellular adhesion molecule-1 (ICAM-I) for selectively targeting endothelium inflammation associated with cardiovascular diseases and rheumatoid arthritis, HerII for targeting breast cancer, and the like) for molecular imaging and therapy. In an exemplary embodiment, a PA imaging system according to the present disclosure is effective for converting absorbed optical energy through a spectrum match of generated PA signal(s) and the pass-band of a receiving transducer. This advantageous match can be achieved according to the present disclosure by a judicious choice of contrast agent material, design and size as well as potentially altered designs in the excitation optical waveforms.

Exemplary systems according to the present disclosure enable in situ activation of the contrast agents via optical energy, typically laser energy to form/define nano-bubbles and/or micro-bubbles. The resonant frequency of the nano-bubbles/micro-bubbles advantageously matches (or substantially matches) the pass-band of an associated receiving ultrasound transducer. Appropriately selected contrast agents according to the present disclosure do not overlap with known ultrasound micro-bubble based systems, but rather complement ultrasound micro-bubbles in absorbing and generating readable signals. Thus, the disclosed nanoparticle- based contrast agents may be used in combination with conventional micro-bubble generating media so as to realize the benefits of both contrast agents. With reference to FIG. 3, a first exemplary embodiment of the disclosed nanoparticle -based contrast agent is schematically depicted. The contrast agent takes the form of a nanoparticle -based droplet of optically absorbing material. The nanoparticle -based droplet is further adapted to undergo a phase change, i.e., evaporation, upon absorption of the requisite optical energy, e.g., laser energy associated with a PA imaging system. The disclosed contrast agents may vary in diameter. In an exemplary implementation of the present disclosure, the contrast agent droplets exhibit diameters of from about 50 nm to 500 nm. The droplets may also exhibit a diameter distribution that includes diameters of between about 50 nm to 500 nm.

The nanoparticles that define (in whole or in part) the exemplary droplet of FIG. 3 may take the form of nano -spheres, nano-rods and the like. The nanoparticles are generally optically stimulated by laser energy, thereby generating localized heat that in turn induces a phase change to the droplet, i.e., converts the contrast medium from liquid to gas. Such phase change conversion creates nano-bubble(s) and/or micro-bubble(s). Thus, with further reference to FIG. 3, evaporation occurs inside the spherical droplet once sufficient light is absorbed thereby. The rapid gaseous expansion associated with such evaporation temporarily creates a bubble with a resonant frequency that advantageously matches (or substantially matches) the ultrasound receive pass-band of the associated transducer. More particularly, nano-bubble and/or micro-bubble formation leads to generation of signature PA signals that correspond to the droplet size of the contrast medium.

In an exemplary embodiment, a system according to the present disclosure includes gold nano-particles (e.g. nano-sphere, nano-rods, etc.) encapsulated or coated with a coating material (FIG.8), such as perfluorocarbon chemical(s) with low boiling points. Once a coated particle is irradiated by laser light, optical energy is absorbed and local temperature of the absorbing nano-particle rises. If this sudden temperature increase is sufficiently large, local phase-shift (evaporation) of the coating and/or internal nanoparticle material occurs. Thus, this process creates a bubble, typically a tiny bubble, whose resonant frequency is determined by its size.

Of note, local temperature increase due to laser light absorption can be calculated according to the following equation:

AT = ^F°^NP x^x(l - Λ ) (1)

' NP P NP^ NP ^ L

With respect to Equation (1):

• F is the fiuence of input laser light;

• O_NP, V_NP, P_NP, and C_NP are absorption cross-section, volume, density and heat capacity of the nanoparticle, respectively; and • T_HD and T_L are the heat diffusion time (time required for the transfer of two thirds of the thermal energy absorbed by a nanoparticle) and the full temporal width of laser pulse, respectively.

The absorption cross-section of a nanoparticle (e.g., a gold nano-sphere that is 40nm in diameter or a nano-rod that is 25 nm in diameter and 100 nm in length) is in the range 10^"11 to 10⁹ cm², where V_NP is in the range of 10 ¹⁶ to 10 ¹⁷CC, P_NP and C_NP are 19.3g/cc and 0.128J/g°C, respectively, for gold nanoparticles. Thus, in order to generate a temperature increase on the order of tens of degrees Celsius (up to the boiling point of water), an estimated fluence level up to a few mJ/cm is required, where 20 mJ/cm is the ANSI current limit. Of note, a low threshold of only 0.22 mJ/cm is required for water evaporation with a gold nano-rod (25 nm in diameter and 100 nm in length) immersed in water using a 10 ns Q-switched laser pulse.

In an exemplary embodiment, liquid vaporization (i.e., liquid-to-gas phase conversion) can be utilized for the creation of ultrasound contrast agents. An exemplary contrast agent for use according to the present disclosure is commercially available as QW7437 (Sonus Pharmaceuticals, Bothell, Washington). The QW7437 material is a premixed liquid-in-liquid emulsion of dodecafiuoropentane (DDFP) droplets with a mean diameter of approximately 300nm. On intravenous administration at body temperature (37°C), nano-droplets become a dispersion of micro-bubbles with an average diameter of 2 to 5 μm and a mean concentration of 10¹² micro-bubbles per ml. The liquid-to-gas conversion for such emulsions can also be achieved by heating droplets through radiation from intense light pulses, e.g., laser energy.

After nano-droplets (e.g., droplets as small as 50 nm) permeate outside vessels, phase conversion can be activated by a series of heating laser pulses (e.g., FIG 4). The process may be controlled so that the resultant gas-filled bubbles may resonate at a desired frequency (e.g., 10 MHz). An optically activated bubble will oscillate around the resonant frequency as an efficient acoustic radiator (ultrasonic source). The ringing (oscillation) of the bubble will readily be received by a regular medical ultrasound system. FIG. 4 illustrates an exemplary laser pulse train, composed of 50 ns laser irradiation with 10 individual laser pulses at a temporal spacing of 10ns, each laser pulse has a temporal duration of 6ns (e.g., Philips Nd: YAG laser source defines this specification). Depending on the optical and thermal diffusivity of a selected material, the actual thermal expansion drive on the contrast agent may resemble a prolonged (approximately 50 ns) impact, as shown in FIG. 5, which can also serve as an exemplary laser irradiation waveform according to the present disclosure. In an exemplary embodiment, the conversion efficiency can be increased to about 50%. FIG. 5 illustrates a possible laser irradiation waveform composed of 50 ns of continuous laser irradiation. This form of excitation may be more easily achieved with laser-diode type laser sources of the type commonly used in telecommunications applications.

FIG. 6 shows a further laser irradiation waveform according to the present disclosure that further matches the contrast agent response with the pass-band shape of the receiving transducer. The laser irradiation waveform of FIG. 6 includes two 50 ns laser waveforms separated at 100 ns which achieves an upward modulation of the spectrum into the transducer pass-band. The pass-band of an ultrasound detector generally resembles a Gaussian or Lorentian shape more than a rectangular window. The waveform excitation associated with FIG. 6 shifts the spectral centroid upward and away from the direct current (DC), thereby achieving an improved coupling in shape.

In the first exemplary PA contrast agents shown in FIG. 3, a possibility exists that the nano-bubble and/or micro-bubble created in situ may potentially burst after one irradiation. FIG. 7 illustrates an exemplary multi-layered/bi-layered nanoparticle with a nanoparticle core and an outer shell to provide enhanced mechanical stability to the nano-bubble/micro -bubble and associated droplet structure during optical stimulation. The coating material is typically optically transparent and mechanically flexible to allow for the internal nanoparticle medium to expand appropriately during the light-induced phase change, i.e., evaporation. Thus, the exemplary embodiment that is schematically depicted in FIG. 7 includes a nanoparticle core that is adapted to absorb optical energy and to undergo a phase change, e.g., evaporation, in response to a requisite level of energy absorption. Of note, the outer shell advantageously encapsulates the nano-bubbles/micro-bubbles generated by reason of the phase change/evaporation disclosed herein. The size and resonant properties/frequency of the nano-bubbles/micro-bubbles formed by the multi-layer/bi-layer contrast agent of FIG. 7 is akin to that described with reference to the exemplary droplet of FIG. 3.

Another exemplary embodiment of the present disclosure is schematically depicted in FIG. 8. Instead of one liquid with both optical absorption and evaporation capabilities (e.g., as depicted in FIG.3), the exemplary agent of FIG. 8 is an emulsion with two liquids. The emulsion includes at least one evaporating smaller nano-drop inside a larger absorbing nano-drop. Two-liquid emulsion droplets can be produced using various techniques, e.g., by mechanical stirring, ultrasound soniflcation, etc. Once an emulsion nano-drop is irradiated by laser light, optical energy is absorbed and local temperature of the absorbing nano- drop rises. If this sudden temperature increase is sufficiently large, local phase-change (evaporation) of the enclosed smaller nano-drops occurs. Thus, this process creates a bubble, typically a relatively small bubble whose resonant frequency is determined by its size. As schematically depicted in FIGS. 9(a) and 9(b), a further exemplary system/implementation according to the present disclosure includes optically absorbing nano- particles (e.g. nano-sphere, nano-rods, etc.) encapsulated or coated with a coating material, such as perfiuorocarbon chemical(s). The liquid coating may shrink into tiny droplets on certain surface pockets (e.g., dents, concave defects and the like) of optically absorbing nano-particles, as shown in FIG.9(b). Once a coated particle is exposed to laser light, optical energy is absorbed and local temperature of the absorbing nano-particle rises. If this sudden temperature increase is sufficiently large, local phase-change (evaporation) of the coating material (e.g., surface layer shown in FIG. 9(a)), surface droplets (e.g., droplets shown in FIG. 9(b)) and/or internal nanoparticle (e.g., as shown in FIG. 8) occurs. Again, this process creates a bubble, typically a tiny bubble, whose resonant frequency is determined by its size.

In a further exemplary embodiment of the present disclosure, a system as schematically depicted in FIGS. 10(a) and 10(b) includes optically absorbing nano-particles (e.g. nano-spheres, nano-rods, etc.) that cover an evaporating material, such as perfiuorocarbon chemical(s) (FIG. 10(a)) or are embedded inside the evaporating material (FIG. 10(b)). Once the droplet-particle complex is heated with laser light to effect a sufficient sudden temperature increase, local phase-change (evaporation) of the evaporating material occurs. As with the other embodiments/implementations disclosed herein, this process creates a bubble, typically a tiny bubble, whose resonant frequency is determined by its size.

It is noted that attachment of absorbing particles to evaporating droplets can be achieved based on various chemical and/or physical interactions, e.g., based on chemical affinity, molecular or biological conjugation, etc. For example, particles can be bound by ligands that are embedded on the surface of droplets. With respect to this type of conjugation, the present disclosure may benefit from a specific technique which is based on an avidin-biotin adhesion, as this conjugation provides an extremely strong non-covalent interaction between a protein and a ligand, with an affinity of 10¹⁵ IVT¹ at pH 5. (See Journal of Controlled Release, 2007).

With reference to multi-layer/bi-layer contrast agent embodiments of the present disclosure, after phase change/evaporation is effected, a gas-filled nano-bubble or micro-bubble with a shell of optically transparent material is generally created. The size of the bubble- containing droplets may be on the order of 500 nm to 5000 nm, although the present disclosure is not limited by or to such exemplary dimensional values. In an exemplary embodiment, the nano- bubbles/micro-bubbles generated by a multi-layer/bi-layer contrast agent according to the present disclosure may have a resonant frequency of about 5 to 15 MHz, although the present disclosure is not limited by or to such exemplary frequencies. Of note, the disclosed frequency range is well matched to the pass-band of existing diagnostic transducers intended for PA applications. The disclosed nanoparticle -based contrast agents offer substantial benefits. The small size of the nanoparticles enables capillary permeability to desired clinical locations/regions, e.g., within the vasculature, before such materials are optically activated. Thereafter, delivery of laser energy to the desired clinical location/region is effective to effect a phase change (evaporation) for such nanoparticle -based contrast agents. The nano-bubbles and/or micro-bubbles formed by such phase change/evaporation resonates at an identifiable frequency that advantageously matches (or substantially matches) the pass-band of existing PA transducers. Bi-layer and multi-layer contrast agents offer enhanced structural stability/strength, thereby increasing the ability of the disclosed nanoparticle -based contrast agents to cycle through multiple optical activations. The disclosed nanoparticle -based contrast agents may be suitable for use in untargeted photoacoustic imaging applications at clinically acceptable dosage levels. In addition, targeted photoacoustic imaging for molecular imaging and/or diagnostics may also be effected using the disclosed nanoparticle -based contrast agents. Although the present disclosure has been described with reference to exemplary embodiments and implementations thereof, the disclosed systems and methods are not limited to such exemplary embodiments/implementations. Rather, as will be readily apparent to persons skilled in the art from the description provided herein, the disclosed systems and methods are susceptible to modifications, alterations and enhancements without departing from the spirit or scope of the present disclosure. Accordingly, the present disclosure expressly encompasses such modification, alterations and enhancements within the scope hereof.

Claims

CLAIMS:

1. A contrast agent, comprising a nanoparticle -based contrast medium that is adapted to absorb optical energy and to undergo a phase change upon absorption of a requisite level of optical energy, such phase change being effective to generate nano-bubbles and/or micro-bubbles in situ that resonate at a frequency or in a frequency range.

2. The contrast agent according to claim 1 , wherein the contrast medium is coated with a perfiuorocarbon chemical.

3. The contrast agent according to claim 1, wherein the nanoparticle -based contrast medium includes one or more nano-rods, nano-spheres or other nanoparticle element.

4. The contrast agent according to claim 1, wherein the nanoparticle -based contrast medium is fabricated from gold.

5. The contrast agent according to claim 1, wherein the requisite level of optical energy is effective to generate localized heat resulting in a temperature change of only a few degrees Celsius.

6. The contrast agent according to claim 1, wherein the nanoparticle -based contrast medium is encapsulated by a shell layer.

7. The contrast agent according to claim 6, wherein the shell layer is optically transparent and is mechanically elastic.

8. The contrast agent according to claim 7, wherein the mechanical elasticity of the shell layer is sufficient to allow expansion of the nanoparticle -based contrast medium during phase change without rupture.

9. The contrast agent according to claim 1, wherein the nanoparticle -based contrast medium is suspended in a carrier solution for injection with respect to a target tissue region.

10. The contrast agent according to claim 9, wherein the carrier solution is a saline solution such as a phosphate buffered saline.

11. The contrast agent according to claim 1, wherein generation of the nano-bubbles and/or micro-bubbles is associated with a volumetric expansion of the nanoparticle -based contrast medium.

12. The contrast agent according to claim 1 , wherein the resonance of the nano- bubbles and/or micro -bubbles emits an acoustic signal to be received by a detection apparatus.

13. The contrast agent according to claim 12, wherein the detection apparatus is an ultrasound transducer.

14. The contrast agent according to claim 13, wherein the emitted acoustic signal is within (or substantially within) the detectable in-band frequency range of the ultrasound transducer.

15. The contrast agent according to claim 12, wherein the acoustic signal is characterized by a frequency that can be tuned by changing droplet size of the contrast medium.

16. A photoacoustic imaging system comprising:

(a) a transducer adapted to receive ultrasound signals and optionally transmit the signals to an imaging system; (b) a transmission source adapted to transmit laser energy to a target tissue region;

(c) a contrast agent including a nanoparticle -based contrast medium that is adapted to absorb optical energy and to undergo a phase change upon absorption of a requisite level of optical energy, such phase change being effective to generate nano-bubbles and/or micro-bubbles that resonate at a frequency or in a frequency range, the contrast agent being adapted to be injected into a target tissue region; wherein the nano-bubbles and/or micro-bubbles emit an acoustic signal to be received by the transducer.

17. A method for ultrasound imaging using photoacoustic signal transmission comprising the steps of:

(a) injecting a contrast agent into a target tissue region, wherein the contrast agent includes a nanoparticle-based contrast medium that is adapted to absorb optical energy and to undergo a phase change upon absorption of a requisite level of optical energy, such phase change being effective to generate nano-bubbles and/or micro-bubbles that resonate at a frequency or in a frequency range;

(b) optically stimulating the contrast agent by transmitting laser energy from a transmission source to the target tissue region;

(c) providing a transducer to receive ultrasound signals resulting from the optical stimulation and optionally transmitting the signals to an imaging system.

18. An optical pulsing technique, comprising:

(a) generating an optical pulse of predetermined duration that tunes temporal frequency of the absorbed optical energy towards the resonant frequency of activated bubbles; and

(b) repeated pulsing using a burst of said optical pulses directed to a region that includes a contrast agent, the optical pulses substantially matching a temporal bandwidth of absorbed optical energy to that of activated bubbles associated with the contrast agent.

19. The optical pulsing technique, wherein the contrast agent includes a nanoparticle- based contrast medium that is adapted to absorb optical energy and to undergo a phase change upon absorption of a requisite level of optical energy, such phase change being effective to generate nano-bubbles and/or micro-bubbles that resonate at a frequency or in a frequency range.