Abstract:
The invention relates to an apparatus for in vivo imaging. More specifically, the present invention relates to a catheter that incorporates an Optical Coherence Tomography (OCT) system and an Intravascular Ultrasound (“IVUS) system for concurrent imaging of luminal systems, such as imaging the vasculature system, including, without limitation, cardiac vasculature, peripheral vasculature and neural vasculature.

Description:
CROSS-REFERENCE TO RELATED INVENTIONS 
       [0001]    This application claims priority pursuant to 35 U.S.C. §119(e) from U.S. Provisional Application Ser. Nos. 60/949,472 and 60/949,511, both filed Jul. 12, 2007 and incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates generally to an apparatus for in vivo imaging. More particularly, the present invention relates to a catheter that incorporates an Optical Coherence Tomography (OCT) system and an Intravascular Ultrasound (IVUS) system for concurrent imaging of luminal systems, such as imaging the vasculature system, including, without limitation, cardiac vasculature, peripheral vasculature and neural vasculature. 
         [0003]    Myocardial infarction or heart attack remains the leading cause of death in society. Until recently, many investigators believed that the primary mechanism for myocardial infarction was coronary arteries critically blocked with atherosclerotic plaque that subsequently progressed to total occlusion. Recent evidence from many investigational studies, however, clearly indicates that most infarctions are caused by sudden rupture of non-critically stenosed coronary arteries resulting from sudden plaque rupture. For example, Little et al. (Little, W C, Downes, T R, Applegate, R J. The underlying coronary lesion in myocardial infarction: implications for coronary angiography.  Clin Cardiol  1991, 14: 868-874, incorporated by reference herein) observed that approximately 70% of patients suffering from an acute plaque rupture were initiated on plaques that were less than 50% occluded as revealed by previous coronary angiography. This and similar observations have been confirmed by other investigators (Nissen, S. Coronary angiography and intravascular ultrasound.  Am J Cardiol  2001, 87 (suppl): 15A-20 A, incorporated by reference herein). 
         [0004]    The development of technologies to identify these unstable plaques holds the potential to decrease substantially the incidence of acute coronary syndromes that often lead to premature death. Unfortunately, no methods are currently available to the cardiologist that may be applied to specify which coronary plaques are vulnerable and thus prone to rupture. Although treadmill testing has been used for decades to identify patients at greater cardiovascular risk, this approach does not have the specificity to differentiate between stable and vulnerable plaques that are prone to rupture and frequently result in myocardial infarction. Inasmuch as a great deal of information exists regarding the pathology of unstable plaques (determined at autopsy), technologies based upon identifying the well-described pathologic appearance of the vulnerable plaque offers a promising long term strategy to solve this problem. 
         [0005]    The unstable plaque was first identified and characterized by pathologists in the early 1980&#39;s. Davis noted that with the reconstruction of serial histological sections in patients with acute myocardial infarctions associated with death, a rupture or fissuring of athermanous plaque was evident (Davis M J, Thomas A C. Plaque fissuring: the cause of acute myocardial infarction, sudden death, and crescendo angina.  Br Heart J  1985; 53: 3 63-37 3, incorporated by reference herein). Ulcerated plaques were further characterized as having a thin fibrous cap, increased macrophages with decreased smooth muscle cells and an increased lipid core when compared to non-ulcerated atherosclerotic plaques in human aortas (Davis M J, Richardson E D, Woolf N. Katz O R, Mann J. Risk of thrombosis in human atherosclerotic plaques: role of extracellular lipid, macrophage, and smooth muscle cell content, incorporated by reference herein). Furthermore, no correlation in size of lipid pool and percent stenosis was observed when imaging by coronary angiography. In fact, most cardiologists agree that unstable plaques progress to more stenotic yet stable plaques through progression via rupture with the formation of a mural thrombus and plaque remodeling, but without complete luminal occlusion (Topol E J, Rabbaic R. Strategies to achieve coronary arterial plaque stabilization.  Cardiovasc Res  1999; 41: 402-417, incorporated by reference herein). Neovascularization with intra-plaque hemorrhage may also play a role in this progression from small lesions, i.e., those less than about 50% occluded, to larger significant plaques. Yet, if the unique features of unstable plaque could be recognized by the cardiologist and then stabilized, a dramatic decrease may be realized in both acute myocardial infarction and unstable angina syndromes, and in the sudden progression of coronary artery disease. 
         [0006]    The present invention uses depth-resolved light reflection or Optical Coherence Tomography to identify the pathological features that have been identified in the vulnerable plaque. In OCT, light from a broad band light source or tunable laser source is split by an optical fiber splitter with one fiber directing light to the vessel wall and the other fiber directing light to a reference mirror. The distal end of the optical fiber is interfaced with a catheter for interrogation of the coronary artery during a heart catheterization procedure. The reflected light from the plaque is recombined with the signal from the reference mirror forming interference fringes (measured by a photovoltaic detector) allowing precise depth-resolved imaging of the plaque on a micron scale. 
         [0007]    OCT uses a superluminescent diode source or tunable laser source emitting a 400-2000 nm wavelength, with a 50-250 nm band width (distribution of wave length) to make in situ tomographic images with axial resolution of 2-20 μm and tissue penetration of 2-3 mm. OCT has the potential to image tissues at the level of a single cell. In fact, the inventors have recently utilized broader bandwidth optical sources so that axial resolution is improved to 4 um or less. With such resolution, OCT can be applied to visualize intimal caps, their thickness, and details of structure including fissures, the size and extent of the underlying lipid pool and the presence of inflammatory cells. Moreover, near infrared light sources used in OCT instrumentation can penetrate into heavily calcified tissue regions characteristic of advanced coronary artery disease. With cellular resolution, application of OCT may be used to identify other details of the vulnerable plaque such as infiltration of monocytes and macrophages. In short, application of OCT can provide detailed images of a pathologic specimen without cutting or disturbing the tissue. 
         [0008]    An OCT catheter to image coronary plaques has been built and is currently being tested by investigators. (Jang I K, Bouma B E, Hang O H, et al. Visualization of coronary atherosclerotic plaques in patients using optical coherence tomography: comparison with intravascular ultrasound.  JACC  2002; 3 9: 604-609, incorporated by reference herein). The prototype catheter consists of a single light source and is able to image over a 360 degree arc of a coronary arterial lumen by rotating a shaft that spins the optical fiber. Because the rotating shaft is housed outside of the body, the spinning rod in the catheter must rotate with uniform angular velocity so that the light can be focused for equal intervals of time on each angular segment of the coronary artery. 
         [0009]    While OCT imaging provides high resolution (2-20 μm) tomographic visualization of coronary arteries, OCT, however, lacks penetration with a maximum penetration depth of only 2-3 mm into the tissue. The present invention overcomes this disadvantage by incorporating an ultrasound transducer suitable for performing intravascular ultrasound (“IVUS”) into an OCT catheter to form an OCT-IVUS catheter. The present invention uses IVUS imaging to identify the pathological features that have been identified in the vulnerable plaque. A particularly valuable tool, IVUS technology uses high frequency sound waves to detect blood vessel blockages and other problems such as aneurysms. 
         [0010]    Ultrasound imaging systems can be equipped with a 38 mm aperture, broadband (5-10 MHz) linear array transducer. Cells can be imaged in color power Doppler, power Doppler, M-mode and B-scan modes. B-scan sonogram images, also called the grayscale mode, are the typical ultrasound method to monitor or examine the human body using backscattering of acoustic waves. M-mode ultrasound employs a sequence of scans at a fixed ultrasound beam over a given time period. M-mode is used for visualizing rapidly moving subjects, such as heart valves. Compared to conventional B-scan images, Doppler ultrasound is used to assess changes in the frequency of reflected acoustic waves. Color power Doppler converts reflected acoustic waves that are Doppler shifted into colors that overlay the conventional B-scan images and can indicate the speed and direction of moving objects. Power Doppler ultrasound is most commonly used to evaluate moving objects and has higher sensitivity than the color power Doppler mode. The gain of the color power and Doppler imaging mode can be manually adjusted to suppress the background noise. If the settings of the ultrasound instrumentation remain unchanged, objective comparisons of each can be made. Additional disclosure explaining how OCT and ultrasound imaging systems work and cooperate can be found in commonly assigned and co-pending U.S. patent application Ser. No. 11/550,771, filed Oct. 18, 2006 and published as U.S. Publication No. US 2008-0097194 on Apr. 24, 2008, which is incorporated herein by reference in its entirety. 
         [0011]    IVUS is a widely available clinical tool for guiding percutaneous interventions and/or intraluminal imaging. While IVUS uses frequencies from 20 to 40 MHz and provides good depth penetration, it lacks sufficient resolution (˜120 μm) to study thin-cap thrombus or atheroma lesions and other fine details with the vasculature. Conversely, while OCT provides high resolution (2-20 μm) tomographic visualization of coronary arteries, OCT, however, lacks penetration with a maximum penetration depth of only 2-3 mm. However, it has been found that OCT can image behind calcifications clearly while ultrasounds are intensely reflected. The current high resolution capabilities of OCT are well suited for imaging vulnerable plaques but poor depth penetration hamper full characterization of coronary lesions and plaque burden. Because IVUS penetrates deeper into the media and adventitia, combining OCT and IVUS modalities will enhance quantitative analysis of coronary arteries significantly. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention relates to a catheter that incorporates an Optical Coherence Tomography (OCT) system and an Intravascular Ultrasound (“IVUS) system for concurrent imaging of luminal systems, such as imaging the vasculature system, including, without limitation, cardiac vasculature, peripheral vasculature and neural vasculature. An OCT assembly and an IVUS transducer are positioned at the distal end of the catheter. 
         [0013]    A combined OCT and IVUS catheter imaging system has been developed to provide cross-sectional structural images of blood vessels, including coronary arteries. A catheter assembly surrounding an OCT assembly and an IVUS transducer at the distal end of an imaging core is used to accomplish such imaging. The catheter assembly is positioned within a blood vessel at the site of interest (i.e., the location of a stenosis). The OCT assembly and IVUS transducer generate a series of pulses which are transmitted outward from the OCT assembly and the IVUS transducer as they are rotated. Echo pulses reflected from the surrounding tissues are received by the OCT assembly and the IVUS transducer and collected by a control apparatus coupled to the proximal end of the sheath. The collected data is then combined, transformed, and displayed as a cross-sectional image of the vessel and surrounding tissue. 
         [0014]    The scope of the invention is indicated in the appended claims. It is intended that all changes or modifications within the meaning and range of equivalents are embraced by the claims. 
         [0015]    The foregoing and other features and advantages of the invention are apparent from the following detailed description of exemplary embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention rather than limiting, the scope of the invention being defined by the appended claims and equivalents thereof. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0016]      FIG. 1  is a diagrammatic side view of the present invention&#39;s OCT-IVUS system. 
           [0017]      FIG. 2  is a side view of an actuation system for rotating the OCT-IVUS catheter in accordance with the present invention. 
           [0018]      FIG. 3  is a side view of the distal end optics of the OCT assembly in accordance with the present invention taken from circle  3  in  FIG. 1 . 
           [0019]      FIG. 4  is a side view of the distal end optics and ultrasound transducer of one embodiment of the invention. 
           [0020]      FIG. 5  is a side view of the distal end optics and ultrasound transducer of another embodiment of the invention. 
           [0021]      FIG. 6  is perspective view of a combined ferrule-ultrasound transducer in accordance with an embodiment of the present invention. 
           [0022]      FIG. 7  is a side view of the distal end optics and ultrasound transducer of another embodiment of the invention. 
           [0023]      FIG. 8  is a perspective view of one embodiment of an optical fiber in accordance with the present invention. 
           [0024]      FIG. 9  is a perspective view of another embodiment of an optical fiber in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    In the accompanying figures, like elements are identified by like reference numerals among the several preferred embodiments of the present invention. 
         [0026]    In the present invention, a distal end assembly including an ultrasound transducer  120  and an optical coherence tomography (“OCT”) optical assembly (as hereinafter described) are positioned longitudinally adjacent or in close proximity to each other at or near a distal end of a catheter assembly  111 . Both the ultrasound transducer  120  and the OCT optical assembly are coupled to a rotary drive system (as hereinafter described) that rotates both the OCT optical assembly and ultrasound transducer  120  about their longitudinal axis and within a catheter sheath. In use, the OCT-IVUS catheter assembly  111  is carefully maneuvered through a patient&#39;s body to a point of interest such as within a blood vessel to position the distal end in imaging proximity with the point of interest. 
         [0027]    The ultrasound transducer  120  may be a single-element crystal or probe that is mechanically scanned or rotated back and forth to cover a sector over a selected angular range. OCT and acoustic signals are then transmitted and echoes (or backscatter) from these OCT and acoustic signals are received. The ultrasound transducer  120  and the OCT assembly may be oriented to direct their respective energies such that the optical signal and the ultrasound signal scan the same or at least partially overlapping spatial areas. Alternatively, the optical signal and the ultrasound signal may be aligned by employing appropriate computer processing software to adjust for spatial discrepancies between the two signals and permit simultaneous display of at least partially coordinated optical images and the ultrasound images to the physician. The backscatter data can be used to identify the type of a scanned tissue. As the probe is swept through the sector, many OCT and acoustic lines are processed building up a sector-shaped image of the patient. After the data is collected, an image (e.g., a combined OCT and IVUS image) of the blood vessel and any associated intraluminal structures, such as plaque, thrombus, stent, etc., can be reconstructed using well-known techniques. This image is then visually analyzed by a physician to assess the vessel components and plaque content. 
         [0028]    Image analysis of data collected from use of the present invention includes determining the size of the lumen and amount of plaque in the vessel. This is performed by generating an image of the vessel (e.g., combined OCT and IVUS image) and manually drawing contoured boundaries on the image where the clinician believes the luminal and the medial-adventitial borders are located. In other words, the luminal border, which demarcates the blood-intima interface, and the medial-adventitial border, which demarcates the external elastic membrane or the boundary between the media and the adventitia, are manually drawn to identify the plaque-media complex that is located there between. 
         [0029]    As illustrated generally in  FIG. 1 , the OCT-IVUS system  100  of the present invention includes a proximal end and a distal end. The proximal end of the OCT-IVUS system  100  includes a rotary optical fiber connector  101 , sometimes also referred to in the art as a fiber optical rotary joint (“FORJ”). The optical fiber connector  101  joins two optical fibers, one optical fiber  105  that is stationary and proximal to the optical fiber connector  101  and another optical fiber  106  that is rotatable and distal to the optical fiber connector  101 . Different types of optical fiber connectors  101 , e.g., SC-APC plug-in connector, have been developed in the art for various applications and are encompassed by the present invention. A distal end catheter assembly  111 , shown in greater detail in  FIG. 3 , consists generally of the OCT optical assembly and the ultrasound transducer  120  positioned with a housing  110 , that is coupled at its proximal end to a rotary drive shaft  104 , that, in turn, is coupled to a rotary drive actuator, such as that illustrated in and described with reference to  FIG. 2 . 
         [0030]    Turning to  FIG. 2 , a rotary actuation system, which includes a rotary motor  102 , an optical fiber connector  101 , and gears  103 , is illustrated rotating a driveshaft  104 . The driveshaft  104  itself is comprised of a fitting (not shown) that overlies and houses the optical fiber  106  distal to the optical fiber connector  101 . This rotary actuation system may also include a linear actuation system (not shown) to facilitate manual or automated pull-back of the catheter. 
         [0031]      FIG. 3  provides a closer side view of the distal end of the OCT-IVUS system  100  depicted in  FIG. 1 . The catheter assembly  111  includes an outer catheter sheath  113  that terminates at a distal end thereof in a distal tip  120  having a guidewire lumen  161  configured for rapid guidewire exchange. The outer catheter sheath  113  is preferably at least partially, preferably near totally, transparent to both optical and ultrasound energy to permit transmission of optical and ultrasonic energy to and from the IVUS-OCT catheter assembly  111 . Co-pending, commonly assigned U.S. Provisional Patent Application Ser. No. 60/949,511, filed Jul. 12, 2007, from which priority is claimed, describes a monolithic catheter construct and rotary drive system well-suited for use with the present invention, and is hereby incorporated by reference thereto as if fully set forth herein. 
         [0032]    A driveshaft  104  extends from the proximal end of the OCT-IVUS system  100  into the catheter assembly  111 . In one embodiment, the outer surface of the catheter assembly  111  is covered and protected by an outer sheath formed of PFA (perfluoroalkoxy). To avoid damage to blood vessels, sheaths may also be formed of other flexible plastic-type materials, having high hoop strengths or with reinforcements, to help stop this kinking and bending while reducing tissue damage during catheter introduction. Other appropriate materials known in the art for use as an outer sheath are also within the scope of the present invention. 
         [0033]    The catheter assembly  111  includes, but is not limited to, a protection bearing  110 , a ferrule  114 , a GRIN lens assembly  116 , a prism  118 , an OCT imaging port  117 , an optical fiber  106 , and an IVUS transducer  120 , all of which are housed within the protection bearing  110 . The catheter assembly  111  may also include a rapid exchange section  160  which has a guidewire lumen  161  that extends between a guidewire entrance port  162  and a guidewire exit port  164 . The protection bearing  110  serves as shield for OCT and IVUS components. The protection bearing  110  is preferably a tubular housing and may be formed of metal or other suitable material. The protection bearing  110  has at least one transparent portion  117 , which is preferably an opening through the protection bearing  110  that is transparent to both optical and ultrasonic energy to permit transmission of such energy to and from the OCT optical system and the ultrasound transducer  110 . The at least one transparent portion  117  may be positioned through a wall of the protection bearing  110  or through an end of the protection bearing  110 . 
         [0034]      FIG. 4  illustrates one embodiment of the invention, wherein an ultrasound transducer  120  is positioned proximal to the OCT assembly (i.e., ferrule  114 , GRIN lens assembly  116 , and prism  118 ). In this embodiment, the ultrasound transducer  120  possesses a cylindrical structure with a hollow core to provide access for an optical fiber  106  to pass through to the OCT assembly. Electrical conduits or wires  122  provide electrically couple the ultrasound transducer  120  to a power source (not shown) and to an ultrasound receiver (not shown). 
         [0035]      FIG. 5  illustrates another embodiment of the invention, wherein an ultrasound transducer  120  is formed on or as a part of the ferrule  114  of the OCT assembly.  FIG. 6  provides a perspective view of the ferrule  114  with the ultrasound transducer  120  embedded with a wall of the ferrule  114 . Similar to the embodiment illustrated in  FIG. 4 , in this embodiment, conduits or wires  122  provide power to the ultrasound transducer  120  and electrically connect the transducer to the ultrasound receiver (not shown). 
         [0036]      FIG. 7  illustrates another embodiment of the invention, wherein the ultrasound transducer  120  is positioned distal to the OCT assembly and is generally forward-looking. In this embodiment, wires  122  extend across the OCT assembly and connect to the ultrasound transducer  120  to provide power. 
         [0037]    Turning to  FIGS. 8 and 9  the optical fiber  106  described above is formed of a core  140  and two layers, a cladding  142  layer and a buffer  144  layer, surrounding the core  140 . At least two electrical conduits or wires  146  are operably associated with wall surfaces of the buffer layer  144 , but may, alternatively, be associated with the cladding layer  142  or the core  140 . 
         [0038]    In the preferred embodiment of the present invention, the optical fiber core  140  is formed of glass made from silica. Nonetheless, other materials known in the art, such as fluorozirconate, fluoroaluminate, and chalcogenide glasses, which are used for longer-wavelength infrared applications, are also within the scope of the present invention. Like other glasses, these glasses have a refractive index of about 1.5. Typically the difference between core and cladding is less than one percent. 
         [0039]    As known in the art, the cladding  142  may be formed of a material has a slightly lower refractive index (faster speed) in order to keep the light in the core. The cladding  142  and core  140  make up an optical waveguide. 
         [0040]    The cladding  142  is usually coated with a tough resin buffer  144  layer, which may be further surrounded by a jacket layer (not shown), usually plastic. As known in the art, the buffer material surrounding the cladding of a fiber may be a soft plastic material that protects the core  140  from damage. These layers add strength to the fiber but do not contribute to its optical wave guide properties. Rigid fiber assemblies sometimes put light-absorbing (“dark”) glass between the fibers, to prevent light that leaks out of one fiber from entering another. This reduces cross-talk between the fibers, or reduces flare in fiber bundle imaging applications. 
         [0041]      FIG. 8  illustrates one embodiment of the optical fiber  106 , wherein the wires  122  (shown in  FIGS. 4 ,  5 , and  7  to be interwinding) are embedded or otherwise coupled on top of the buffer  144  layer of the optical fiber  106 . The wires may be formed of metal films vacuum deposited onto the optical fiber or otherwise operably associated with the optical fiber. In another embodiment, as illustrated in  FIG. 9 , the wires  146  are embedded within the buffer  144  layer of the optical fiber  106 . 
         [0042]    The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. Therefore, the scope of the invention is not limited to the specific exemplary embodiment described above. All changes or modifications within the meaning and range of equivalents are intended to be embraced herein. 
         [0043]    As used in this application, the articles “a” and “an” refer to one or more than one (i.e., to at least one) of the grammatical objects of the article. By way of example, “an element” means one element or more than one element.