Photoacoustic catheter and imaging system using same

A photoacoustic catheter includes an elongated catheter body and a housing positioned near a distal end of the elongated catheter body. A length of multimode fiber extends through the elongated catheter body and has a distal end that is beveled at about 45° relative to a longitudinal axis of the multimode fiber and is positioned in the housing. An ultrasonic transducer, electrically connected to an electrical wire extending along the elongated catheter body, is positioned within the housing. A mirror element is also positioned within the housing and includes a mirror surface beveled at about 45° relative to the longitudinal axis of the multimode fiber. The catheter is operable to deliver an optical wave through the multimode fiber and to deliver an ultrasonic wave collinearly from the housing and out of an aperture of the housing to obtain optical data and ultrasonic data within a mammalian luminal organ.

FIELD OF THE INVENTION

The present application relates to a photoacoustic catheter, and more particularly to a photoacoustic catheter incorporating collinear alignment of optical and acoustic waves.

BACKGROUND

Cardiovascular disease has been the leading cause of death in the United States and many other developed countries over the past century. Atherosclerosis, a major form of cardiovascular disease, is caused by the chronic accumulation of lipids and fibrous elements within the wall of an artery. This plaque can grow and become clinically symptomatic if it significantly encroaches and obstructs the lumen of the artery. A plaque may also rupture and result in acute coronary syndrome or even sudden death. Therefore, the early detection of plaques that are vulnerable for rupture is essential in the diagnosis, treatment, and prevention of cardiovascular diseases. Non-invasive modalities such as X-ray angiography, magnetic resonance, and computed tomography angiography have been used to visualize obstructive stenosis in coronary arteries. However, vulnerable plaques prone to rupture are often non-obstructive or moderately obstructive, thus evading detection by these modalities. Intravascular ultrasound (IVUS) can provide important morphologic information of arteries including lumen geometry, plaque burden, and vessel structure. However, the sensitivity and specificity for differentiation of plaque composition is limited, partly due to the lack of chemical contrast with IVUS. Intravascular optical coherence tomography has been reported, but these optical imaging modalities fail to provide necessary imaging depth and chemical specificity for vulnerable plaque detection. Near-infrared spectroscopy provides chemical selectivity, but it lacks the spatial resolution to define the lipid core size and its detection sensitivity is compromised by scattered photons.

Catheter-based intravascular photoacoustic (IVPA) imaging, on the basis of converting the overtone vibrational absorption in an arterial tissue into thermoelastic waves detectable with an ultrasonic transducer, is an emerging modality with potential of bridging the aforementioned gaps. IVPA imaging offers the following advantages. First, the optical absorption-induced contrast provides a unique approach to differentiate chemical composition of arteries. Second, the imaging depth of IVPA is extended beyond the ballistic regime owing to the diffused photon absorption and 2-3 orders of magnitude lower acoustic scattering in tissues compared to optical scattering. Third, by sharing the same detector, IVUS is inherently compatible with IVPA imaging. Such a hybrid modality provides complementary information of the tissue.

The desirable characteristics of a clinically feasible IVPA catheter include having a small diameter, being flexible, and being capable of imaging through blood and of acquiring images with high sensitivity and chemical specificity at an acceptable frame rate. These requirements collectively render the design and fabrication of a high-performance IVPA probe to be one of the most challenging tasks in the photoacoustic imaging field. A number of groups have reported IVPA catheters with diameters approaching the clinical target of about 1 millimeter (mm). Specifically, the Emelianov group reported two designs of IVPA catheters, one based on side fire fiber and the other based on mirror reflection. Both designs were based on a front-to-back arrangement of the light delivery element and ultrasonic transducer. The Chen group introduced another design of an IVPA catheter based on parallel arrangement of side-firing fiber and transducer, where two different frequencies, 35 MHz and 80 MHz, of the transducer were performed to demonstrate an outstanding axial resolution of 35 microns (μm). The Xing group introduced an intravascular confocal photoacoustic probe with a dual-element ultrasound transducer. The Song group reduced the diameter of an IVPA catheter probe to 1.1 mm by carefully arranging the positions of the optical and acoustic elements. Most recently, the inventors further reduced the probe diameter of a conventional IVPA catheter to 0.9 mm.

Despite these advances, sufficient arterial imaging depth has not been shown for these single-element transducer-based IVPA catheters, largely because the optical and ultrasonic waves were cross-overlapped in a very limited space. Although the overlap range can be altered by changing the coupling angle, it is hard to maintain the photoacoustic sensitivity constant along the millimeter-scale imaging depth. Furthermore, the IVUS and IVPA images in these non-collinear designs are not truly co-registered along the imaging depth, which may lead to poor localization of artery and plaque features. Further, assembly of such non-collinear designs is not trivial, as all the components must be constrained to a limited space. To maximize the overlap of an incident optical field and generated acoustic wave, the inventors recently demonstrated a coaxial design based on a ring-shaped transducer. However, at 2.9 mm, the outer diameter of the probe needed to be further reduced for clinical compatibility. Accordingly, there remains a need for further contributions in this area of technology.

BRIEF SUMMARY OF THE INVENTION

At least one exemplary embodiment of the present disclosure includes a photoacoustic catheter, including an elongated catheter body having a lumen defined therethrough and a housing positioned at or near a distal end of the elongated catheter body, the housing defining an aperture therethrough, a length of multimode fiber extending through at least part of the lumen of the elongated catheter body, the multimode fiber having an axis along its length, whereby a distal end of the multimode fiber is beveled at or about 45° to the axis and is located within the housing, an electrical wire extending along the elongated catheter body, an ultrasonic transducer electrically connected to the electrical wire, whereby at least a portion of the ultrasonic transducer is positioned within the housing, and a mirror element positioned within the housing and including a mirror surface beveled at or about 45° to the axis of the multimode fiber, whereby the catheter is operable to deliver an optical wave and an ultrasonic wave collinearly from the housing and out of the aperture to obtain optical data and ultrasonic data within a mammalian luminal organ. The ultrasonic wave reflects from the distal end of the multimode fiber and the optical wave and the ultrasonic wave each reflect collinearly from the mirror surface of the mirror element and out of the aperture. The optical data and the ultrasonic data are each indicative of a plaque within the mammalian luminal organ.

Another aspect of the present disclosure includes disclosure of a method to obtain optical data and ultrasonic data within a mammalian luminal organ using a photoacoustic catheter, similar to the photoacoustic catheter described above. The method includes steps of introducing at least a portion of the photoacoustic catheter into the mammalian luminal organ, transmitting an optical wave from the multimode fiber and toward the mirror surface of the mirror element, and transmitting an ultrasonic wave from the ultrasonic transducer and toward the distal end of the multimode fiber. The method also includes redirecting the ultrasonic wave from the distal end of the multimode fiber and toward the mirror surface of the mirror element, and redirecting the optical wave and the ultrasonic wave from the mirror surface and the mirror element and collinearly from the housing and out of an aperture through the housing to obtain optical data and ultrasonic data within the mammalian luminal organ.

Another aspect of the present disclosure includes disclosure of an imaging system including a photoacoustic catheter, which is similar to that described above. The imaging system includes an optical excitation source operatively connected to the photoacoustic catheter via the multimode fiber, and a pulser/receiver operatively connected to the photoacoustic catheter via the ultrasonic transducer. The photoacoustic catheter is operable to deliver an optical wave through the multimode fiber and to deliver an ultrasonic wave collinearly from the housing and out of an aperture through the housing. The optical wave and the ultrasonic wave are detected by the ultrasonic transducer and received by the pulser/receiver. The imaging system also includes a data acquisition device operatively connected to the photoacoustic catheter via the pulser/receiver to digitize signals received at the pulser/receiver.

Corresponding reference characters indicate corresponding parts throughout the several views. Although the drawings represent embodiments of the present invention, the drawings are not necessarily to scale and certain features may be exaggerated in order to better illustrate and explain the full scope of the present invention. The flow charts and screen shots are also representative in nature, and actual embodiments of the invention may include further features or steps not shown in the drawings. The exemplification set out herein illustrates an embodiment of the invention, in one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of the present disclosure, reference will now be made to the embodiments illustrated in the drawings, and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of this disclosure is intended, with any additional alterations, modifications, and further applications of the principles of this disclosure being contemplated hereby as would normally occur to one skilled in the art. Accordingly, this disclosure is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of this disclosure as defined by the appended claims. While this technology may be illustrated and described in a preferred embodiment, the systems, methods, and techniques hereof may comprise many different configurations, forms, materials, and accessories.

The present disclosure includes a photoacoustic catheter probe that uses collinear alignment of optical and acoustic waves to overcome the drawbacks in aforementioned conventional IVPA catheters. In at least one embodiment, an optical beam delivered through a 365-μm-core multimode fiber (MMF) with a low numerical aperture of 0.22 enables quasi-uniform illumination along the imaging depth. An outer diameter of 1.6 mm was obtained for the catheter tip through novel arrangement of the optical and acoustic elements. The disclosed collinear catheter probe ensures an efficient overlap between optical and photoacoustic waves over a6mm imaging depth. The capability of the collinear catheter probe was evaluated through ex vivo high-speed IVPA imaging of a diseased porcine carotid artery and a human coronary artery, with optical excitation via a lab-built optical parametric oscillator outputting optical pulses at 1.7 μm wavelength and 500 Hz repetition rate.

A catheter probe10, also referred to as a photoacoustic catheter, according to at least one embodiment of the present disclosure is shown inFIGS. 1A-1D. The catheter probe10includes a housing12. The housing12includes a lumen24formed therethrough, a signal chamber22formed therein, and a transducer chamber26formed in or near an exterior wall of the housing12. The housing12may further include a wire passage28. The housing12may be fabricated by molding, casting or additive manufacturing techniques, such as a micro-resolution stereolithography process (e.g., such as practiced by Proto Labs, Inc.).

The catheter probe10includes a multimode fiber14capable of transmitting and emitting an optical wave70of electromagnetic energy (i.e., a light beam). The multimode fiber14is at least partially disposed within the lumen24of the housing12. In at least one embodiment, the multimode fiber14may include a core/cladding diameter of 365/400 μm, NA of 0.22 (e.g., multimode fiber FG365LEC, Thorlabs, Inc.). The multimode fiber14includes a fiber axis40defined along the length of the multimode fiber14and a distal end42. The distal end42terminates at a reflection face44configured at approximately a 45° angle to the fiber axis40. The multimode fiber14includes a proximal end (not shown) that may terminate at an inlet face (not shown) configured at substantially a 90° angle to the fiber axis40. The reflection face44and the inlet face may be polished with a fiber polisher (e.g., NANOpol, ULTRA TEC Manufacturing, Inc.). The distal end42of the multimode fiber14may be disposed within the lumen24of the housing12such that the reflection face44enters the signal chamber22, as shown inFIG. 1B.

The catheter probe10further includes a transducer16, also referred to as an ultrasonic transducer, disposed at least partially within the transducer chamber26of the housing12and capable of sensing acoustic waves72directed toward a sensing area68of the transducer16. A wire18electrically connected to the transducer16may be disposed in the wire passage28. The wire18enables signals from the transducer16to be communicated to and from an imaging system as described further herein. The transducer16may be any suitable ultrasonic transducer. In certain embodiments, the transducer16may be a single-element ultrasonic transducer having a relatively small form factor. In at least one embodiment, the transducer16may be a single-element ultrasonic transducer with dimensions of 0.5×0.6×0.2 mm3, center frequency of 42 MHz and bandwidth of 60% (e.g., as sold by Blatek, Inc.).

The transducer16may be oriented within the transducer chamber26such that the sensing area68of the transducer16faces the reflection face44of the multimode fiber14. The transducer16and the multimode fiber14may be positioned and oriented to ensure the collinearity between the optical wave70emitted from the multimode fiber14and the acoustic waves72transmitted through the catheter probe10, as shown in FIGS.1C and1D. Accordingly, the refection face44may be positioned to lie in the acoustic reflection plane of the transducer16. The housing12also includes a signal aperture74formed therethrough, as shown inFIG. 1D, to enable the optical wave70and acoustic waves72to be transmitted through the catheter probe10.

The catheter probe10further includes a mirror element20disposed in a mirror passage30formed in a distal end of the housing12, as shown inFIGS. 1A and 1B. As shown inFIG. 1A, the mirror element20includes a mirror face66capable of reflecting the optical wave70and acoustic waves72. The mirror element20is positioned and oriented to ensure the optical wave70and acoustic waves72are emitted radially from the catheter probe10, as shown inFIG. 1C. In at least one embodiment, the mirror element20may be a mirror rod in which the mirror face66is disposed at a mirror rod distal end64. In such an embodiment, the mirror element20may be a mirror rod having a diameter of I mm (e.g., as sold by Edmund Optics, Inc.). The relative positions among each of the components may be optimized by monitoring the photoacoustic signal in real time under an aqueous environment.

The catheter probe10may further include a torque coil62attached to the housing12, as shown inFIG. 1D. The torque coil62may enclose the multimode fiber14and the wire18extending from the transducer16. The catheter probe10may include various connectors and accessories as needed, for example, a fiber connector installed on a proximal end of the multimode fiber14. The catheter probe10may further include a solution, such as an aqueous solution, disposed and contained within the signal chamber22, where the solution is capable of transmitting the optical wave70and acoustic waves72.

The multimode fiber14delivers the optical wave70to the catheter probe10. The fiber distal end42of the multimode fiber14may be polished to 45° for reflecting the ultrasonic wave72, while the optical wave70still propagates forward after the polished end when the multimode fiber14is submerged in an aqueous environment. The transducer16is disposed relative to the multimode fiber14such that the sensing area68of the transducer16faces the polished reflection face44. Therefore, the optical and ultrasonic paths are collinear after encountering the reflection face44, as shown inFIG. 1C. The 45° mirror face66of the mirror element20disposed opposite the multimode fiber14redirects both the optical wave70and ultrasonic waves72perpendicularly for side-view illumination and imaging. It should be noted that the ultrasound trace after the mirror element20is designed to be perpendicular to its receiving plane within the housing12to prevent direct ultrasound wave venting from the transducer16as shown inFIG. 1C, which may cause errant image reconstruction. The disclosed catheter probe10ensures that optical and acoustic waves70and72are collinear within a large tissue depth. The components involved are installed in the housing12having an outer diameter reasonably compatible in clinical settings, thus greatly simplifying the catheter assembly process. An embodiment of the catheter probe10is shown inFIG. 1D, with its outer diameter measured to be 1.6 mm.

An imaging system100according to at least one embodiment of the present disclosure is shown inFIG. 2. The imaging system100includes the catheter probe10connected to an optical parametric oscillator (OPO)102. In at least one embodiment, the OPO102may be a potassium titanyl phosphate (KTP)-based OPO emitting at 1.7 μm with a repetition rate of 500 Hz and pulse width of approximately 13 ns, which provides an optical excitation source for photoacoustic imaging using the imaging system100. The pulse energy at the fiber distal end42of the multimode fiber14may be controlled to approximately 120 μJ, corresponding to an energy density of approximately 30 mJ/cm2at the tissue surface, which is below the 1.0 J/cm2ANSI safety standard for skin at 1.7 μm.

Light generated by the OPO102may be coupled to the catheter probe10via the multimode fiber14via an optical rotary joint104and a slip ring108. The optical rotary joint104together with the slip ring108may control the rotational scanning of the catheter probe10. The optical rotary joint104may be mounted to a pullback stage106to enable 3-D imaging.

Sequential photoacoustic and ultrasound signals may be generated and detected with a proper time delay. A trigger signal provided by a Q-switch of the OPO102synchronizes the data acquisition of the optical wave70and acoustic wave72signals. A time delay of approximately10is may be applied to an ultrasound pulser/receiver112via a delay generator110. Both the optical wave70and acoustic wave72signals are sequentially detected by the transducer16and received by the pulser/receiver112. A data acquisition card114may be used to digitize and transfer the generated signals to a computer116, which may employ data acquisition software such as LabView® software. The imaging system100may include the delay generator110(e.g., delay generator 37000-424 from Datapulse, Inc.), the pulser/receiver112(e.g., pulser/receiver 5073PR from Olympus, Inc.), the data acquisition card114(e.g., data acquisition card ATS9462 PCI express digitizer from AlazerTech, Canada), and/or the computer16. In certain embodiments, the pulser/receiver112may employ an amplification factor of 39 dB, and the data acquisition card114may employ 16-bit digitization and a 180 MS/s sampling rate. In at least one embodiment, the imaging system100may have an imaging speed of approximately 1 frame per second, which is around 50 times faster than conventional IVPA imaging systems based on 10-Hz Nd:YAG lasers.

Exemplary embodiments of the catheter probe10and imaging system100were characterized for performance evaluation and validated with ex vivo artery imaging as described in the following experiments.

Experiment 1: Characteristics of Spatial Resolution and Imaging Depth

The spatial resolution of an exemplary embodiment of the catheter probe10and the imaging system100was evaluated by photoacoustically imaging a carbon fiber with 7-μm diameter as a first test sample. The carbon fiber serves as a model target to determine the spatial resolution of the imaging system100due to its strong optical absorption and well-defined thin diameter. The carbon fiber was positioned parallel to the catheter probe10with a variable distance controlled by a translation stage. The experiments were performed in deuterium oxide (D20) medium because of its lower optical absorption at 1.7 μm compared to water.FIG. 3Ashows a reconstructed cross-sectional photoacoustic image200of a carbon fiber202with a rotational catheter scanning. An inset204shows the zoom-in view of the carbon fiber image.

The generated photoacoustic signals along the axial and lateral directions centered at the carbon fiber position are plotted in respective graphical plots206and208ofFIGS. 3B and 3Cto determine the spatial resolution. The axial and lateral resolutions are derived from the full width at half maximum of Gaussian fit of these results. An axial resolution of 81 μm and lateral resolution of 372 μm were obtained at a radial distance of 2.2 mm. Spatial resolutions for photoacoustic imaging at different axial distances were obtained similarly by changing the position of the carbon fiber as displayed in a graphical plot210ofFIG. 3D. The axial resolutions are found to fluctuate around 80 μm, which are primarily determined by the bandwidth of the transducer, while lateral resolutions are found to vary from 350 μm to 430 μm, which may be due to the non-focus property of the ultrasonic transducer. The magnitude of the photoacoustic signals at different axial distances is plotted as well in a graphical plot212ofFIG. 3E.FIG. 3Eshows an approximate exponential decay along the axial direction. Notably, the overlap range between optical beam and ultrasonic wave was found to be over 6 mm, which has not been achieved for non-collinear catheter designs previously reported. This imaging depth is sufficient for intravascular applications.

Experiment 2: Chemical Specificity Validation with a Lipid-Mimicking Phantom

A lipid-mimicking phantom comprised of a butter rod and a portion of porcine intramuscular fat were employed for photoacoustic imaging to evaluate the sensitivity and validate the chemical specificity of our system as a second test sample. Similar to pathologic lipid deposition in atherosclerosis, both butter and intramuscular fat are abundant in CH2 groups, which exhibit strong absorptions at their first overtone transitions around 1.7 μm. Porcine intramuscular fat serves as a reliable model of pathologic lipid deposition, thus validating the feasibility of our photoacoustic catheter probe to perform intravascular imaging. The second test sample was prepared from a 2.5% agarose gel made from agar powder and D20 approximately mimics the tissue environment. A butter rod with a diameter of about 1.5 mm and a small piece of intramuscular fat were embedded in the agarose gel as imaging targets. A central hole in the phantom was reserved for catheter insertion. The phantom was fully submerged in D20 during imaging experiment to ensure a lower optical loss at 1.7 μm.

Both photoacoustic and ultrasound images of the phantom are shown in respective images300,306, and308ofFIGS. 4A-4C. Both butter302and fat304can be identified from both photoacoustic and ultrasound images, with strong association between them on position and morphology, as highlighted inFIG. 4A. The signal-to-noise ratios for butter and fat in photoacoustic image were calculated to be38and18, respectively, while the signal-to-noise ratios are30and46for butter and fat in ultrasound mode. The photoacoustic signals are specific for the density of CH2 bond in these two targets, while the ultrasound signals are related to the overall structural properties. These results from the lipid-mimicking phantom presented inFIGS. 4A-4Cvalidate the performance of photoacoustic and ultrasonic imaging of lipid using the catheter probe10, indicating the imaging system100can be used for reliable IVPA and IVUS imaging of an artery.

Experiment 3: IVPA Imaging of Lipid-Laden Carotid Artery Excised from Ossabaw Swine

The performance of our IVPA imaging system was validated by ex vivo imaging of a diseased porcine carotid artery. The porcine atherosclerotic carotid artery was harvested from a miniature Ossabaw swine and fixed in 10% formalin. A segment of artery with suspected plaque was selected and cut as a region of interest with the aid of a microscope. The artery segment was then held by agarose gel and submerged under D20 for imaging experiment.

A segment of the artery with suspected plaque (shown as artery stenosis402in a cross-sectional photograph400FIG. 5D) was selected as the imaging target. Co-registered and merged IVPA/IVUS images404,406, and410were obtained as shown inFIGS. 5A-5C. From the IVUS image406inFIG. 5B, the characteristic three-layer appearance and luminal area of the carotid artery can be visualized, with the suspected plaque region408and inner and outer boundaries of the artery inscribed, which agree well with gross inspection at the plaque position (seeFIG. 5D). Strong photoacoustic signal within the plaque region shown inFIG. 5Aindicates a possible lipid-rich core of the plaque. The merged image410inFIG. 5Cshows the overlap between the photoacoustic and ultrasonic signal at the plaque area. The imaged cross-sectional region was further sectioned and stained for histology, as shown in a histology412ofFIG. 5E. The lumen size and arterial structure were verified by the histology. The plaque position was highlighted in the detailed section inset. The lipid deposition, which might have been leached out during the histology process, is suggested by the blank area. Some debris of the lipid core can still be visualized in the zoom-in view indicated by black lines at414.

Experiment 4: IVPA Imaging of Fresh Coronary Artery Excised from Human Patient

The performance of the catheter probe10and the imaging system100were further validated by ex vivo imaging a perfused fresh right coronary artery from a human patient. The fresh right coronary artery was harvested from an explanted human heart at the time of transplant. The vessel segment was excised from the ostium to 6 cm distally, leaving approximately 5 mm of surrounding perivascular fat attached. The ostium was cannulated with an 8F introducer sheath and side b ranches were ligated to allow for pressure perfusion. The artery was then pinned in a Sylgard® 184 Silicone Elastomer tray, submerged in phosphate-buffered saline at room temperature, and perfused to mimic physiologic pressure during imaging.

The artery segment was imaged in 3-D using the optical rotary joint104and a linear pullback stage106. At a particular longitudinal position, a region of interest was identified with a strong photoacoustic signal in the arterial wall, which could possibly indicate lipid depositions500as shown in respective images504,506, and508ofFIGS. 6A-6C. Furthermore, an intense photoacoustic signal peripheral from the vessel wall with an imaging depth of 4.3 mm was observed, suggesting that the imaging system100is able to penetrate through the entire arterial wall to reach surrounding perivascular fat502that was retained on the excised vessel.

In the experiments described herein, the catheter probe10and imaging system100demonstrated greatly improved overlap between optical and acoustic waves. The catheter probe10and imaging system100provided optimal photoacoustic sensitivity over an imaging depth over 6 mm, allowing reliable access of the deeper component information in the entire arterial wall, including perivascular fat. Even so, the photoacoustic signal along an A-line still decayed exponentially as shown inFIG. 3E. This decay may be affected by a number of factors including optical beam divergence, optical absorption/scattering in imaging environment, acoustic loss in medium, and unfocused transducer. Some approaches to reduce signal decay include integrating a gradient-index lens in the catheter to improve the optical beam focusing, introducing a n external wavefront shaping method to focus the light beam deeper inside the tissue, and using a quasi-focused transducer to enhance the acoustic receiving efficiency.

In at least one embodiment of the present disclosure, the diameter of the catheter probe10is 1.6 mm, which is affected by the size of the mirror element20(i.e., rod mirror having a 1 mm diameter). In certain embodiments, the mirror element20may have a reduced diameter of 0.5 mm (e.g., using a rod mirror with a 0.5 mm diameter). In such an embodiment, the catheter probe10may be further reduced to about 1 mm in diameter, which is similar to the size of certain conventional commercially available IVUS catheter probes.

In at least one embodiment of the present disclosure, the imaging speed of the imaging system100is 1 frame per second, which is based on the 500 Hz repetition rate of the OPO102and one revolution per second rotation speed of the catheter probe10. Considering the lateral resolution of approximately 425 μm at an axial distance of 5 mm, the number of A-lines for each cross-sectional image may be reduced to 75, which would enable a maximum imaging speed over 6 frames per second. In certain embodiments, the OPO102includes a laser system having a higher repetition rate of 2 kHz, which further improves the imaging speed of the imaging system100to approach that of conventional commercial in vivo intravascular imaging systems.

The present disclosure includes a miniature IVPA catheter probe with collinear overlap between the optical and acoustic fields. The catheter probe enables high-quality IVPA imaging of the entire artery wall from lumen to perivascular fat. A lab-fabricated collinear photoacoustic catheter was evaluated for spatial resolution characterization with a 7-μm carbon fiber and chemical composition validation by using a lipid-mimicking phantom. The axial and lateral resolutions were found to be around 80 μm and 400 μm, respectively, over an imaging depth larger than 6 mm. With a co-registered IVPA/IVUS imaging system based on a lab-built 500 Hz OPO at 1.7 μm, the catheter probe was used to image a diseased carotid artery and a human coronary artery ex vivo, resulting in IVPA/IVUS images showing a lipid-rich plaque that corresponds with gross inspection.

Further, in describing representative embodiments, the present disclosure may have presented a method and/or a process as a particular sequence of steps. However, to the extent that the method or process does not rely on the particular order of steps set forth therein, the method or process should not be limited to the particular sequence of steps described, as other sequences of steps may be possible. Therefore, the particular order of the steps disclosed herein should not be construed as limitations of the present disclosure. In addition, disclosure directed to a method and/or process should not be limited to the performance of their steps in the order written. Such sequences may be varied and still remain within the scope of the present disclosure.