Patent Publication Number: US-2023141567-A1

Title: Oct catheter with low refractive index optical material

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority benefit of U.S. Provisional Application Ser. No. 63/165,672, filed Mar. 24, 2021 and U.S. Provisional Application Ser. No. 63/014,110, filed Apr. 22, 2020, which are hereby incorporated by reference in their entirety. This application is also related to U.S. Provisional Application Ser. No. 63/165,673, filed Mar. 24, 2021, which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     Embodiments herein relate generally to endovascular imaging, total-occlusion crossing, and atherectomy devices, and particularly, to interferometric imaging devices with a single element for interventions and imaging, imaging and intervention system, and methods of operation. 
     BACKGROUND 
     Minimally invasive interventions have consistently shown to be of equivalent or greater efficacy and offer lower mortality rates than traditional open surgical interventions. For many such minimally invasive procedures, being able to accurately track the positioning of instruments inserted into the vasculature of the patient is of the utmost importance for surgeons and other medical professionals undertaking such interventions. A majority of minimally invasive procedures involve the use of a flexible guidewire and catheters that are directed to a target vessel site using the guidewire. However, steering the guidewire to the target vessel site can be challenging and fraught with risks. For example, an improperly maneuvered guidewire can cause harmful vascular dissection, perforation, or thrombosis. While some of these risks can be offset by heparinization, the increased use of such anti-coagulants can increase the risk of procedural hemorrhage. 
     Moreover, most guidewire navigation is currently done under X-ray fluoroscopic imaging. However, X-ray imaging often requires the surgeon or other medical professionals to be subjected to long bouts of radiation. 
     Therefore, improved devices, systems, and methods for endovascular imaging are needed which address the challenges faced by current devices on the market. Such a solution should lower the risk of complications for patients and reduce the risk of radiation exposure for operators. Moreover, such a solution should be compatible or easily adapted for use with other minimally invasive surgical devices such as atherectomy catheters. Furthermore, such a solution should reduce the complexity of current devices and be cost-effective to manufacture. The endovascular imaging devices, systems and methods may also be configured to improve imaging quality and/or to reduce various image artifacts. 
     SUMMARY 
     Embodiments of the disclosure are drawn to apparatuses, systems, and methods for a catheter, guidewire or interventional device with a single element tip for both imaging and interventions. The catheter may have a distal tip which is positionable in a patient (e.g., in a lumen of a vessel). The distal tip may have a component or block which includes both an optical feature and a mechanical feature for piercing through and/or insertion across an atherosclerotic plaque. The optical feature may redirect light (e.g., light received along an optical fiber of the catheter) into an imaging beam, which may be directed to a wall of the vessel, while the mechanical feature is configured to facilitate the crossing of an occluded or partially-occluded section of the vessel via an angled tip. The imaging component or assembly may also be configured to reduce one or more imaging artifacts. 
     In one embodiment, an imaging device is provided, comprising an outer shaft with a lumen, an optical fiber located within the lumen of the outer shaft, a reflective element disposed in the lumen of the outer shaft, wherein the reflective element comprises an optical material with an angled surface configured to an imaging beam, and a first optical filler between the distal end of the optical fiber and the reflective element, the first optical filler comprising a refractive index of less than 1.40. The refractive index of the first optical filler may be in the range of 1.30 to 1.40 or 1.33 to 1.38. The optical filler may comprise an aliphatic urethane acrylate and an acrylic monomer. The aliphatic urethane acrylate percentage may be 30% to 70% and the acrylic monomer may 70% to 30%, or the aliphatic urethane acrylate percentage may be 40% to 65% and the acrylic monomer may be 60% to 35%. The viscosity of the first optical filler may be in the range of 1000 to 3000 cps, 1500 to 3000 cps, or 2000 to 2500 cps. The imaging device of claim  1 , further comprising a lens located between the optical fiber and the reflective element. The lens may be a Fresnel lens, GRIN lens, plano-convex or double-convex lens. The lens may be tilted between 0.1 to 2.0 degrees. The imaging device may further comprise a non-clad fiber followed by a GRIN lens between the optical fiber and the reflective element. The first optical filler may be further located between the lens and the reflection element. The imaging device may further comprise a second optical filler located between the optical fiber and the lens. The first optical filler and the second optical filler may comprise different materials, or may comprise the same constituents but at different ratios, and wherein both the first and second optical filler have a refractive index of less than 1.50. The angled surface of the reflection element may comprise a Fresnel diffractive pattern. The Fresnel diffractive pattern may comprise varying degree of collimating or focusing power along its long axis, short axis and in between the long and short axes. The imaging device may further comprise a lens located between the optical fiber and the reflection element. The lens may be a GRIN lens. The first optical filler may be located between the lens and the reflective element. The reflection element may further comprise a tapered distal end protruding from the lumen of the outer shaft, wherein the tapered distal end is configured to penetrate tissue. The first optical filler comprises a UV cured optical material. A collimating lens may be located between the optical fiber and the first optical filler. The collimating lens may be a GRIN lens, and plano-convex, or biconvex or a Fresnel lens. 
     In another example, an imaging device is provided, comprising an outer shaft with a lumen, an optical fiber located within the lumen of the outer shaft, a reflective element disposed in the lumen of the outer shaft, wherein the reflective element comprises an optical material with an angled surface configured to an imaging beam, a beam-collimating element between the optical fiber and a first optical filler, and a first optical filler between the distal end of the beam-collimating element and the reflective element, the first optical filler comprising a refractive index of less than 1.40. An interface between the beam-collimating element and the first optical filler has a return loss or reference signal between −15 dB and −28 dB, −20 dB and −35 dB, or −23 dB and −40 dB. The beam-collimating element may be a Fresnel lens or a GRIN lens. 
     In still another example, an imaging device is provided, comprising an outer shaft with a lumen, an optical fiber located within the lumen of the outer shaft, a reflective element disposed in the lumen of the outer shaft, wherein the reflective element comprises an optical material with an angled surface configured to an imaging beam, a beam-collimating segment between the optical fiber and a first optical filler comprising of a non-clad fiber and a GRIN lens, and a first optical filler between the distal end of the GRIN lens and the reflective element, the first optical filler comprising a refractive index of less than 1.40. An interface between the beam-collimating element and the first optical filler may have a return loss or reference signal between −15 dB and −28 dB, −20 dB and −35 dB, or −23 dB and −40 dB. An interface between the optical fiber and the non-clad fiber, and the interface between the non-clad fiber and the GRIN lens may have reflection artifacts weaker than −40 dB. 
     In another variation, an imaging device is provided, comprising an outer shaft with a lumen, an optical fiber located within the lumen of the outer shaft, a reflective element disposed in the lumen of the outer shaft, wherein the reflective element comprises an optical material with an angled surface configured to an imaging beam, a beam-collimating segment between the optical fiber and a first optical filler consisting a GRIN lens, and a first optical filler between the distal end of the GRIN lens and the reflective element, the first optical filler comprising a refractive index of less than 1.40. An interface between the beam-collimating element and the first optical filler may have a return loss or reference signal between −15 dB and −28 dB, −20 dB and −35 dB, or −23 dB and −40 dB. 
     In still another variation, an imaging device is provided, comprising an outer shaft with a lumen, an optical fiber located within the lumen of the outer shaft, a reflective element disposed in the lumen of the outer shaft, wherein the reflective element comprises diffractive patterns on a flat surface of an optical material with an angled surface configured to an imaging beam, and a first optical filler between the optical fiber and the reflective element, the first optical filler comprising a refractive index of less than 1.40. The imaging device may further comprise a lens located between the optical fiber and the reflective element. The lens may be a Fresnel lens, plano-convex or double-convex lens. The lens may be tilted between 0.1 to 2.0 degrees. The lens may be a GRIN lens. The lens may comprise a non-clad fiber followed by a GRIN lens. 
     In another embodiment, an imaging device is provided, comprising an outer shaft with a lumen, an optical fiber located within the lumen of the outer shaft, a reflective element disposed in the lumen of the outer shaft, wherein the reflective element comprises diffractive patterns on a flat surface having varying degree of collimating and focusing power between its long and short axes, with an angled surface configured to an imaging beam, and a first optical filler between the optical fiber and the reflective element, the first optical filler comprising a refractive index of less than 1.40. The imaging device may further comprise a lens located between the optical fiber and the reflective element. The lens may be a Fresnel lens, plano-convex or double-convex lens. The lens may be tilted between 0.1 to 2.0 degrees. The lens may also be a GRIN lens, or comprise a non-clad fiber followed by a GRIN lens 
     In another example, an imaging device is provided, comprising an optical fiber, a reflective element with an angled surface configured to an imaging beam, a beam-collimating GRIN lens having at least 0.75 pitch or more in unit length, and a first optical filler between the distal surface of the GRIN lens and the reflective element, the first optical filler comprising a refractive index of less than 1.40. An interface between the optical fiber and the GRIN lens produces a reflection artifact stronger or weaker than −40 dB. 
     In still another embodiment, an imaging device is provided, comprising an optical fiber, a reflective element with an angled surface configured to an imaging beam, a beam-collimating GRIN lens having at least 0.75 pitch or more in unit length, having a reflection artifact at the interface between the optical fiber and the GRIN lens that is stronger than −40 dB, and a first optical filler between the distal surface of the GRIN lens and the reflective element, the first optical filler comprising a refractive index of less than 1.40. 
     In still another embodiment, an imaging device is provided, comprising an outer shaft with a lumen, an optical fiber located within the lumen of the outer shaft, a reflective element disposed in the lumen of the outer shaft, wherein the reflective element comprises diffractive patterns on a flat surface having varying degree of collimating and focusing power between its long and short axes, with an angled surface configured to an imaging beam, a beam-collimating element between the optical fiber and a first optical filler, and a first optical filler between the distal end of the beam-collimating element and the reflective element, the first optical filler comprising a refractive index of less than 1.40. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic diagram of an imaging and intervention system according to some embodiments of the present disclosure. 
         FIG.  2    is a cross-sectional diagram of a distal end of an imaging crosser device according to some embodiments of the present disclosure. 
         FIG.  3    is a cross-sectional diagram of a distal end of another embodiment of exemplary imaging crosser device. 
         FIG.  4    is a cross-sectional diagram of a distal end of another embodiment of an imaging crosser device. 
         FIGS.  5 A and  5 B  are schematic diagram of exemplary imaging crosser devices with angled tips. 
         FIGS.  6 A and  6 B  are schematic cross-sectional and perspective views of another exemplary imaging crosser device. 
         FIGS.  7 A to  7 E  are side elevational, side perspective, rear perspective, front perspective and top perspective views, respectively, of the optical element in  FIGS.  6 A and  6 B . 
         FIGS.  8 A and  8 B  are schematic perspective and side views of the imaging crosser device in  FIGS.  6 A and  6 B  in a retracted position;  FIGS.  8 C and  8 D  are schematic perspective and side views of the image crosser device in an extended position. 
         FIG.  9    is a schematic longitudinal cross-sectional view of another exemplary embodiment of an imaging crosser device. 
         FIG.  10    is a schematic longitudinal cross-sectional view of another exemplary embodiment of an imaging crosser device. 
         FIG.  11    is a schematic longitudinal cross-sectional view of another exemplary embodiment of an imaging crosser device. 
         FIG.  12 A  depicts an exemplary guide catheter configured for use with the imaging crosser device.  FIG.  12 B  is schematic view of the proximal hub and proximal shaft of the guide catheter in  FIG.  12 A . 
         FIGS.  13  and  14    schematically depict different variations of marker bands that may be provided on the guide catheter. 
         FIGS.  15  and  16    are schematic side views of different guide catheter tips that may be used. 
         FIGS.  17 A and  17 B  are proximal elevational and perspective views of the guide catheter tip in  FIG.  15   . 
         FIGS.  18 A to  18 C  are OCT images depicting various types of artifacts that may result from refractive index mismatches between the imaging system and the surrounding fluid and tissue. 
         FIGS.  19 A to  19 C  are schematic cross-sectional views of a side-viewing OCT imaging system, depicting back reflections from different optical filler configuration. 
         FIGS.  19 D and  19 E  are schematic cross-sectional views of a side-viewing OCT imaging system with optical filler configurations that reduce back reflection. 
         FIGS.  20 A to  20 C  are OCT images with reduced artifacts resulting from a low refractive index optical filler material in the imaging system. 
         FIGS.  21 A and  21 B  are schematic cross-sectional views of an OCT imaging system with a low refractive index optical filler, with and without an outer shaft, respectively. 
         FIGS.  22 A and  22 B  are schematic depictions of variations of the OCT imaging assembly comprising a GRIN lens, without and with a non-clad fiber segment, respectively.  FIGS.  22 C and  22 D  schematically depict an OCT imaging assembly with a GRIN lens configured to produce a collimating beam.  FIGS.  22 E and  22 F  schematically depict an OCT imaging assembly with a GRIN lens configured to produce a focusing beam. 
         FIGS.  23 A and  23 B  are schematic depictions of other variations of the OCT imaging system with a GRIN lens. 
         FIG.  24    is a schematic depiction of a variation of the OCT imaging system comprising a spherical or aspheric lens with collimated light. 
         FIG.  25    is a schematic depiction of a variation of the OCT imaging system comprising a spherical or aspheric lens with collimated or non-collimated light. 
         FIGS.  26 A and  26 C  schematically depicts an embodiment of an exemplary OCT imaging system with a Fresnel lens;  FIG.  26 B  is a perspective component view of the Fresnel lens in  FIG.  26 A . 
         FIG.  27 A  is a schematic depiction of a Fresnel reflector in an exemplary OCT imaging system.  FIGS.  27 B to  27 E  depict various beam profiles of the OCT imaging system in  FIG.  27 A  at various focal lengths and their respective equivalent radii of curvature.  FIG.  27 F  is a superior perspective view of a Fresnel reflector. 
         FIG.  28 A  is a schematic depiction of an exemplary OCT imaging system with a GRIN lens and a Fresnel reflector.  FIGS.  28 B and  28 C  are schematic side and end views, respectively, of the beam profile of the OCT imaging system of  FIG.  28 A .  FIG.  28 D  depicts a beam profile of the OCT imaging system of  FIG.  28 A . 
         FIGS.  29 A and  29 B  schematically depict the mechanical coupling of an optical fiber and GRIN lens. 
     
    
    
     DETAILED DESCRIPTION 
     The following description of certain embodiments is merely exemplary in nature and is in no way intended to limit the scope of the disclosure or its applications or uses. In the following detailed description of embodiments of the present systems and methods, reference is made to the accompanying drawings which form a part hereof, and which are shown by way of illustration specific embodiments in which the described systems and methods may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice presently disclosed systems and methods, and it is to be understood that other embodiments may be utilized and that structural and logical changes may be made without departing from the spirit and scope of the disclosure. Moreover, for the purpose of clarity, detailed descriptions of certain features will not be discussed when they would be apparent to those with skill in the art so as not to obscure the description of embodiments of the disclosure. The following detailed description is therefore not to be taken in a limiting sense, and the scope of the disclosure is defined only by the appended claims. 
     Minimally invasive medical interventions may involve insertion of a catheter through the lumen of one or more vessels of a patient. For example, during atherectomy, a catheter or guidewire may be advanced through a blood vessel of a patient and one or more intervention tools inserted into the working lumen of the catheter or over the guidewire to remove (or otherwise decrease the volume of) an atherosclerotic plaque. In some variations, the progress of the catheter advancement through the patient may be monitored, for example to reduce the risk of piercing or dissection of the vascular wall, and/or to monitor the progress of plaque or clot removal. 
     The imaging device includes an optical assembly or element which allows for imaging of the walls of the vessel that the imaging device is disposed in. In general, the imaging device may be coupled to an optical system, which may direct light along the imaging device (e.g., along one or more optical fibers of the imaging device). The reflecting surface may be located at a distal region of the imaging device and may redirect the light to illuminate the vessel lumen and walls. The reflecting surface may also redirect light received from the vessel lumen and walls back along the imaging device to the optical system. For example, the imaging device may be an interferometric imaging device which may perform optical coherence tomography (OCT) by scanning light along the walls of a vessel, generating an interferometric pattern by combining light reflected (or scattered) from the vessel walls with light reflected from a reference surface, and imaging the vessel walls based on the interferometric pattern. The imaging device may also be configured to with an angled or tapered geometry to facilitate crossing of occluded vasculature. This interventional feature and the reflecting surface may be integrated together. It may be useful to reduce the size and bulk of the imaging device to increase its ability to travel through relatively narrow or occluded vessels. This may be achieved, for example, using an optical structure that includes the reflection surfaces and the angled or tapered surfaces for crossing occlusions. 
     The present disclosure is directed to an interferometric imaging device with single element for interventions and imaging, imaging and intervention system, and methods of operation thereof. The interferometric imaging device includes an optical component at a distal end of the imaging device. The optical component includes both an imaging feature and crossing element. For example, the imaging feature may be a reflective surface of the optical component, at an angle relative to the long axis of the imaging device to redirect light between the long axis of the imaging device and a side surface of the imaging device. The optical component may also include a geometric configuration which may be an angled, tapered, cutting or penetrating surface to facilitate crossing or navigation through a plaque, occlusion or clot as the imaging device passes along the vessel. The mechanical geometry may also be configured with a tissue contact surface to penetrate, cut or abrade the plaque. The tissue contact surface of the optical component may be adjustably extendable from an end or a portion of the imaging device. In some embodiments, the tissue contact surface which forms the tapered distal end and the reflective surface which forms the reflecting surface may be the same surface of the optical component. 
     For example, the distal tip component may be a monolithic, optically transparent component or structure in which the optical feature is an angled internal reflection surface of distal geometry region of the structure. The structure may also comprise a tapered geometry to facilitate selective penetration and passage through an occluded portion of the vasculature, and wherein the distal geometry region of the structure comprises an angled external surface corresponding to the angled internal reflection surface. The corresponding internal and external points, surfaces or regions on the structure may be the opposing internal and external geometries, the parallel internal and external geometries, and/or the immediately orthogonal or normal internal and external points, surfaces or regions. The distal geometry region and/or other surfaces of the structure may comprise a coating to augment the reflection or other optical properties of the structure. 
       FIG.  1    is a schematic diagram of an imaging and intervention system according to some embodiments of the present disclosure. The imaging system  100  includes a guidewire, catheter or imaging device  102 , all or a portion of which may be inserted into a patient. A distal tip  104  of the imaging device  102  may be used for imaging of tissues of the patient and/or one or more interventions in the patient. A proximal end of the imaging device  102  is coupled to a control unit  110  which may be used to perform the imaging and/or intervention through the imaging device  102 . An optional computing device  130  may be coupled to the control unit  110  in order to operate or interface with the control unit  110  feeding imaging data from the optical unit and human interface devices/position unit back to the control unit  110 , and/or record/store/interpret/augment data (e.g., imaging data) generated by the control unit  110 . Control unit  110  may optionally consist of a graphical processing unit to enhance data received from the optical unit and human interface devices/position unit using a neural network to improve visualization and image guidance for the physician through the Display  136 . 
     The inset of  FIG.  1    depicts an expanded schematic view of a distal tip  104  of the imaging device  102 . The imaging device  102  includes an outer shaft  108 , and an inner member  105  which includes an optical component  106 . The optical component  106  may include a reflecting surface which is used for imaging tissue  107  around the distal tip  104  of the imaging device  102  and also includes a tapered distal end which is used to perform one or more interventions. For example, the optical component  106  may include a crossing element for crossing a narrowing or obstruction in a vessel, such as the total or partial occlusion  109 . The tissue  107  may be a wall of a vessel that the distal tip  104  is positioned in. For example, the tissue  107  may be the wall of a coronary blood vessel. 
     The control unit  110  includes a position unit  120  and an optical unit  112 . Although shown as a single control unit  110  in the example of  FIG.  1   , it should be understood that the human interface devices/position unit  120  and optical unit  110  may be separate components of the system  100  in some embodiments. The human interface devices/position unit  120  may be used to control and/or track the position of the imaging device  102  and/or one or more components of the imaging device  102  relative to the imaging device  102 . The optical unit  110  may image the tissue  107  around the distal tip  104  by providing and receiving light through the imaging device. 
     The optical unit  112  may be used to perform optical coherence tomography (OCT). The optical unit  112  includes a light source  116  which provides transmitted light Tx. The light source  116  may be a laser, a light emitting diode (LED), an arc lamp, incandescent source, fluorescent source, other source of light, or combinations thereof. The transmitted light Tx may have a relatively narrow bandwidth centered on a particular frequency, may be broadband source (e.g., white light), or combinations thereof. In some embodiments, the light source  116  may be a swept source laser, and a center frequency of the transmitted light Tx may change over time. In some embodiments, the center frequency of the transmitted light may be chosen to penetrate tissue. For example, the center frequency may be in the near infra-red (NIR) window where tissue has a relatively low extinction (e.g., between about 800 nm and 1400 nm). 
     The optical unit  112  includes an interferometer  114 , which receives the transmitted light Tx and provides it along the imaging device  102 . For example, the interferometer  114  may couple the transmitted light into an optical fiber which runs along the length imaging device  102 . The interferometer may also receive light from the imaging device  102  (e.g., received light along the optical fiber) and provide the received light to a detector  118 . The received light may include a portion of receive light Rx which has interacted with the tissue  107  and a local oscillator LO portion of the light which was reflected from a reference surface. The interferometer  114  may include a Faraday isolation device, such as a Faraday Effect optical circulator which may separate the optical paths of the returning received light Rx and local oscillator LO light from the transmitted light Tx. The separated Rx and LO light may then be directed to the detector  118 . In some embodiments, the optical unit  112  may include additional components (e.g., lenses, filters, etc.) which may improve the performance of the optical unit  112  and/or add additional functionality. 
     In some embodiments, the system  100  may be a common-path OCT system, and the reference surface may be an end of the optical fiber in component  104 , where Fresnel reflection causes a portion of the transmitted light Tx to reflect off the end of the fiber, while another portion of the transmitted light exits the fiber to interact with the optical component  104  (and from there the tissue  107 ). In other embodiments, the system  100  may consist of a reference arm as the reference surface separately from the optical fiber in component  104 . 
     The transmitted light Tx may interact with the tissue  107 , and a portion of the light may be redirected (e.g., scattered, reflected, or combinations thereof) along an optical path which causes the redirected light to re-enter the catheter  102  and return to the optical unit  112  as received light Rx. The received light Rx may travel a longer distance the LO light, and so there may be a difference of frequency, phase, and/or time between the Rx and LO light. These differences may be used by the detector  118  (and/or computing device  130 ) to determine properties of the tissue  107 . For example, the received light Rx and local oscillator LO light may interfere with each other to generate an interference pattern at the detector  118 . The interference pattern may be interpreted (e.g., by the computing device  130 ) to extract information about the difference between the distance the received light Rx travelled compared to the distance the LO light travelled. In the case of swept source OCT, the difference may be encoded as a beat frequency heterodyned on the carrier reference beam. 
     The detector  118  may convert light incident on the detector  118  (e.g., the received light Rx and LO light) into an electrical signal. For example, the detector  118  may be an array detector (e.g., a CCD or CMOS) which provides a signal based on an amount and/or color of light incident on each pixel of the array. 
     The outer shaft  108  can be a long flexible tube configured to allow components such as a guidewire, an imaging component, a drive shaft, sensor wires or fibers, imaging wires or fibers, cables, protective sheaths, parts therein, or a combination thereof to extend or pass through the imaging device lumens of the imaging device  102 . The human interface devices/position unit  120  may track and control both the position of the outer shaft  108 , and also the inner member  105  relative to the outer shaft  108 . For example, the position unit  120  may extend/retract the inner member  105  relative to the outer shaft  108  and/or rotate the inner member  105  relative to the outer shaft  108 . 
     The human interface devices/position unit  120  can include a number of electromechanical devices or sensors that convert the translational or angular/rotational motion of the imaging device  102 , the inner member  105 , or a combination thereof into digital signals or data. For example, the position unit  120  can comprise one or more linear encoders  122 , rotary encoders  124 , or a combination thereof. Human control and sensory feedback from the human interface devices/position unit  120  may be used to augment image visualization through the use of and integration with a neural network in order to provide improved guidance to the physician during a procedure. 
     The one or more linear encoders  122  can be optical linear encoders, mechanical linear encoders, magnetic linear encoders, inductive linear encoders, capacitive linear encoders, or a combination thereof. The linear encoders  122  can be absolute encoders, incremental encoders, or a combination thereof. The one or more linear encoders  122  can track or encode the longitudinal movement/translation or displacement of the imaging device  102  and/or inner member  105 . For example, the one or more linear encoders  122  can track or encode the longitudinal movement/translation or displacement of the proximal segments of the inner member  105 . 
     In these and other embodiments, the position unit  120  can also include one or more rotary encoders  124 . The one or more rotary encoders  124  can be absolute rotary encoders, incremental rotary encoders, or a combination thereof. The one or more rotary encoders  124  can be optical rotary encoders, mechanical rotary encoders, magnetic rotary encoders, capacitive rotary encoders, or a combination thereof. The one or more rotary encoders  124  can track or encode the rotation or angular position of the imaging device  102  and/or inner member  105 . 
     The position unit  120  can also include a motor and drive assembly  126 . The motor and drive assembly  126  can be configured to translate the imaging device  102 , the inner member  105 , or a combination thereof in a longitudinal direction (e.g., in a distal direction, a proximal direction, or a combination thereof). For example, the motor and drive assembly  126  can provide torque or rotate a proximal segment of the inner member  108  while the outer shaft  108  may remain fixed. This may rotate the optical component  106  and scan the transmitted light Tx around the distal tip  104 . The rotation of the inner member  108  relative to the outer shaft  108  may also allow a tapered distal end of the optical component  106  to cut or otherwise ‘de-bulk’ tissue in order to remove and/or cross occlusions or partial occlusions such as the partial occlusion  109 . 
     While the imaging device  102  is generally described in terms of imaging and an intervention such as crossing, it should be understood that these operations may only be a portion of the imaging device&#39;s  102  functionality. For example, the inner member  105  and outer shaft  108  may extend along one lumen of the imaging device  102 , while other lumens are used for other tools or functions. For example, the imaging device  102  can also be used to deliver or otherwise introduce fluids, pharmaceutical compositions including small molecules and biologics, contrast media, biomarkers, or a combination thereof to the distal tip  104 , a target treatment site in proximity to the distal tip  104  (e.g., a target vessel site within the patient&#39;s body), or a combination thereof. 
     The control unit  110  may be coupled to a computing device  130 . In some embodiments, the computing device  130  may be a separate component from the control unit  110 . In some embodiments, the control unit  110  may be integral with the computing device  130 . In some embodiments, the control unit  110  can be configured as a handle or handheld unit. In other embodiments, the control unit  110  can be configured as a control box or tabletop unit. In some embodiments, the computing device  130  can be a desktop computer, a laptop computer, a tablet device, or a combination thereof. The computing device  130  can comprise a processor  132 , such as a central processing unit (CPU) and a memory  134 . The processor  132  may have a 32-bit processor data bus or a 64-bit processor data bus. The processor  132  can be a dual core, quad core, or other multi-core processors. The processor  132  can operate at speeds of 3 GHz or more. The memory  134  can comprise random access memory (RAM) and read-only memory (ROM). More specifically, the memory units can comprise dynamic RAM (DRAM), static RAM (SRAM), sync DRAM (SDRAM), double data rate (DDR) SDRAM, double data rate 2 (DDR2) SDRAM, or a combination thereof. An optional GPU may consist of 8 GB or more memory for the implementation of a neural network for analyzing images realtime in order to enhance their visualization with machine or deep learning, resulting in more visually intuitive images for instance with warnings, notations, and augmented reality. 
     The computing device  130  can process and store images captured by the optical unit  112 . The optical unit  112  and the computing device  130  can be combined with other devices to make up part of an OCT imaging system. For example, the OCT imaging system can be a common-path OCT system, a time domain OCT system, a spectral ore frequency domain OCT system, or a combination thereof. The computing device  130  can be coupled to a detector  118 . The computing device  130  may receive a signal, such as a raw data, from the detector  118  and record a sequence of raw data over time in the memory  134 . The processor  132  may perform one or more image processing steps to extract an image from the raw data. 
     The processor  132  may work together with the control unit  110  to image the walls of vessel that the distal tip  104  is located in. For example, the position unit  120  may rotate the inner member  105 , which in turn may ‘scan’ the field of view of the optical unit around the walls of the vessel as the optical component  106  is rotated. The raw data generated by the detector  118  may be associated with a particular angular position of the inner member  105  relative to the outer shaft  108  (e.g., as reported by the rotational encoder  124 ). The processor  132  may use the angular information and the raw data to reconstruct an angular view of the vessel walls which is larger than a single field of view of the detector  118 . In some embodiments the processor  132  may generate a 360 degree view of the vessel wall. In a similar fashion, the processor  132  may also work with the position unit  120  to build an image which extends along a longitudinal axis which is larger than a single field of view by moving the inner member  105  in a proximal/distal direction while imaging. 
     In some embodiments, the images generated by the processor  132  may be displayed by a user of the system  100  (e.g., displayed on a screen). In some embodiments, the processor  132  may generate a display images at a rate (e.g., a video rate) which allows for “real-time” imaging of the vessel walls. In some embodiments, the use of the tapered distal end may be based on the images generated. 
     The computing device  130  can also be configured to perform image registration on images captured by the optical unit  112 . For example, image registration can involve establishing correspondence between features in sets of images and using one or more transformation models to infer correspondence of additional features away from such features. Imaging registration can also be referred to as image alignment. Image registration can also be done to align or map images obtained from different imaging modalities (e.g., OCT with intravascular ultrasound (IVUS) or OCT with X-ray fluoroscopy). 
       FIG.  2    is a cross-sectional diagram of a distal end of an imaging device according to some embodiments of the present disclosure. The imaging device  200  may represent a distal end of an imaging device which may be used as part of an imaging and intervention system such as the imaging and intervention system  100  of  FIG.  1   . 
     The imaging device  200  includes an outer shaft  220 , and an inner element  210 . The inner element  210  may rotate relative to the outer shaft  220 , and may be used for both imaging and interventions. The inner element  210  includes an optical element  202  which includes a tapered distal end  204  and a reflecting surface  206 . The reflecting surface  206  may be configured to redirect light between one or more optical fibers  212  of the inner element  210  and an imaging target (e.g., a field of view). As shown in the example of  FIG.  2   , the reflecting surface  206  is redirecting light received from the optical fiber  212  (e.g., transmitted light Tx of  FIG.  1   ) into an imaging beam  208  which extends out of a periphery of the imaging device  200 . The reflecting surface  206  may also redirect received light (e.g., Rx of  FIG.  1   ) back into the optical fiber  212 . The tapered distal end  204  may include one or more tissue contact surfaces (here a conical tissue contact surface) which can be extended up to a distance ‘d’ from a front surface of the outer shaft  220 . 
     The outer shaft  220  may be an outer body of the imaging device  200 . The outer shaft  220  may have an outer diameter of around 0.020 inches (0.508 mm) or less. In another embodiment, the outer shaft  220  may have an outer diameter of around 0.045 inches (1.143 mm) or less. The outer shaft  220  may be roughly the size of a guidewire. The outer shaft  220  may be fixed (e.g., to a control or optical unit) relative to the inner element  210 . Accordingly, the outer shaft  220  may be a torque shaft which may remain fixed while the inner element  210  rotates within the outer shaft  220 . 
     The outer shaft  220  may have a body  222  and a cap  224 . The cap  224  may be located at a distal end of the outer shaft  220 . The cap  224  may allow light to pass from an outside of the outer shaft  220  to a lumen of the outer shaft  220  (e.g., may allow light to pass from outside the outer shaft  220  to the inner element  210 ). In some embodiments, the cap  224  may be a separate component attached to an end of the body  222 . In some embodiments, the cap  224  may be integral to the body  222 , and the cap  224  may refer to a distal end of the body  222 . The body  222  may be formed from a coil or modified hypotube. The body  222  and cap  224  may both be generally tubular members, with a lumen which the inner element  210  is disposed within. The body  222  and cap  224  may, in some embodiments, have similar outer diameters and thicknesses. In some embodiments, the body  222  and cap  224  may have different outer diameters and/or thicknesses from each other. 
     In some embodiments, the cap  224  may be an optically transparent material. For example, the cap  224  may be made from a different material than the body  222  of the outer shaft  220 . In some embodiments, the cap  224  may include a window, for example such as a laser cut window through the material of the cap  224 . In some embodiments, the laser cut window may include a cover made from an optically transparent material. In some embodiments, the window may be uncovered. In some embodiments, the body  222  of the outer shaft  220  may be made from an optically transparent material, and a separate cap  224  may be unnecessary. 
     In some embodiments, the cap  224  may include a hard stop  226 , which limits the motion of the inner member  210  relative to the outer body  220 . The hard stop  226  may limit the longitudinal motion of the inner member  210  relative to the outer body  220  (e.g., the motion along a long axis of the imaging device  200 ). For example, an inner diameter of the cap  224  at the hard stop  226  may be less than an outer diameter of the inner member  210 . In some embodiments, the hard stop  226  may be a tapered section of the cap  224  and/or a step on the inner wall of the cap  224 . 
     The inner member  210  may be a generally cylindrical member which is disposed in a lumen of the outer shaft  222 . The inner member  210  may have an outer diameter of around 0.010 inches (0.254 mm) or less. In another embodiment, the outer shaft  210  may have an outer diameter of around 0.018 inches (0.457 mm) or less. The inner member  210  may be movable relative to the fixed outer shaft  220 . The inner member  210  may be extendable/retractable relative to the outer shaft  220  along a longitudinal axis of the imaging device  200  and may be rotatable relative to the outer shaft  220 . For example, a motor/drive assembly (e.g.,  126  of  FIG.  1   ) may move the inner member  210  relative to the outer shaft  220 . 
     The inner member  210  includes an optical fiber  212 . The optical fiber  212  may transmit light between an optical unit (e.g.,  112  of  FIG.  1   ) coupled to a proximal end of the imaging device  200  and the distal end of the imaging device  200 . The optical fiber  212  may be a single fiber or a multi-core fiber. The optical fiber  212  may be surrounded by a cladding material  211 , which may support the optical fiber  212  and provide optical conditions (e.g., an index mismatch) which enables the transmission of light along the optical fiber  212 . The cladding material  211  may in turn be surrounded by a fiber reinforcement  216 . The fiber reinforcement  216  may support the optical fiber  212  and cladding material  211 . In some embodiments, the fiber reinforcement  216  may be a coil or a modified hypotube. A portion of the optical component  202  may extend beyond a distal end of the fiber reinforcement. 
     The optical fiber  212  (and cladding material  211 ) may terminate at a reference surface  214 . The reference surface  214  may be at a distal end of the optical fiber. In some embodiments, the reference surface  214  may generally be a flat surface (e.g., a cut in the fiber) which is at a right angle to the long axis of the imaging device  200 . The reference surface  214  may reflect a portion of the transmitted light along the optical fiber  212 . The light reflected from the reference surface  214  may be a local oscillator (LO) portion of the light, which may be interferometrically combined with received light in an optical unit (e.g., optical unit  112  of  FIG.  1   ). 
     The inner member  210  includes an optical component  202 . The optical component  202  may be a single element which includes a reflecting surface  206  and a tapered distal end  204 . The optical component  202  may be single piece of material which has a one or more surfaces shaped to form the reflecting surface  206  and one or more surfaces shaped to form the tapered distal end  204 . In the example embodiment of the optical component  202 , the reflecting surface  206  and tapered distal end  204  may be separate surfaces of the optical component  202 . 
     The optical component  202  may be a generally cylindrical element, a portion of which may be disposed in the fiber reinforcement  216 . The optical component  202  may be formed from a single material. For example, the optical component  202  may be formed of aluminum oxide, zirconium oxide, silicon carbide, diamond, or combinations thereof. In some embodiments, the optical component  202  may be a metal, such as SS 304 . 
     The tapered distal end  204  may be a tool for coronary interventions, such as a crossing tool. The tapered distal end  204  may be a tissue contact surface which extends a distance d from a distal tip of the outer shaft  220  when the inner member  210  is extended to the hard stop  226 . In some embodiments the distance d may be about 1 mm or less. The tapered distal end  204  may be formed along the distal end of the optical component  202 . For example, the tapered distal end  204  may be a conical surface extending to a point. In some embodiments, the point of the tissue contact surface may be along a long axis of the imaging device  200 . In some embodiments, the point of the conical surface may be offset from the long axis of the imaging device  200 . 
     The reflecting surface  206  of the optical component  202  may be reflective surface which redirects light from an optical axis generally aligned with the long axis of the fiber, and an optical axis directed to a side of the imaging device  200 . For example, the reflecting surface  206  may redirect light at about a right angle. The reflecting surface  206  may be a slanted surface at a proximal end of the optical component  202 . The angle of the optical  206  with respect to a long axis of the imaging device  200  may be chosen based on the desired deflection of light which reflects from the reflecting surface  206 . For example, in some embodiments, the reflecting surface  206  may have an angle of about 45 degrees. In some embodiments, the reflecting surface  206  may be coated with a reflective material. For example, the reflective surface of the reflecting surface  206  (e.g., the proximal surface of the optical component  202 ) may be gold coated. 
     As shown by the example rays of the imaging beam  208 , transmitted light through the optical fiber  212  may diverge as it leaves the optical fiber  212  at the reference surface  214 . The divergent rays of the imaging beam  208  may then be redirected by the reflecting surface  206  such that the imaging beam  208  forms a divergent cone of transmitted light which extends out a side of the imaging device  200 . The fiber reinforcement  216  may include a window (and/or be made of a transparent material) to allow the imaging beam  208  to pass out of the inner member  210  (and from there out of the imaging device  200 ). 
     Accordingly, as the inner member  210  rotates relative to the outer shaft  220 , the imaging beam  208  may be swept around the perimeter of the imaging device  200 . The inner member  210  may also be movable along a longitudinal axis of the imaging device  200  relative to the outer shaft  220 . The field of view represented by the imaging beams  208  may be swept around the imaging device  200  in a 360 degree arc, and may be moved longitudinally relative to a position of the imaging device along an imaging range L. The imaging range may be defined by a furthest extension of the inner member  210  within the outer shaft  220  (e.g., when the inner member  210  abuts the hard stop  226 ) and by a length of the transparent portion of the outer shaft  220  (e.g., the transparent portion of the cap  224 ). In some embodiments the imaging range L may be about 5-25 mm. 
     The imaging beam  208  may interact with the environment surrounding the imaging device  200 . For example, when the imaging device  200  is in a blood vessel, the imaging beam may illuminate a portion of the blood vessel wall around the imaging device  200 . As the inner member  210  rotates, the imaging beam  208  may be swept around to illuminate a strip of the vessel wall. Light may be returned from the illuminated tissue to the imaging device  200 . For example, a portion of the illumination light may be scattered and/or reflected by one or more components and structures of the tissue. Some of the illumination light may penetrate a distance into the tissue before being returned to the imaging device  200 . A portion of the returned light may follow an optical path which reflects off the imaging tool  206  and is coupled into the optical fiber  212 . The returned light which is coupled into the fiber may form the received light (e.g., received light Rx of  FIG.  1   ). 
     The inner member  210  may include a filler material  218  between a distal end of the optical fiber  212  (e.g., the reference surface  214 ) and a proximal surface of the optical component  202  (e.g., the reflecting surface  206 ). Since the reference surface  214  and reflecting surface  206  may be at different angles with respect to a long axis of the imaging device  200 , there may be a gap between the reference surface  214  and the optical component  202 . The filler material  218  may an optically transparent material to allow the imaging beam  208  (and received light) to pass through the filler material  218 . In some embodiments, the filler material  218  may have an index of refraction chosen to match an index of refraction of the optical fiber  212  to prevent refraction of the light at the optical fiber/filler interface. For example, the filler material may have an index of refraction of about 1.55. In other examples, the filler material may have an index of refraction in the range of 1.40 to 1.80, 1.50 to 1.60, or 1.55 to 1.65. In some embodiments, the filler material  218  may be an adhesive, and may help couple the optical component  202  to the optical fiber  212  and cladding  211 . In some embodiments, the filler material  218  may also fill a window in the fiber reinforcement  216 . 
       FIG.  3    is a cross-sectional diagram of a distal end of an imaging device according to some embodiments of the present disclosure. The imaging device  300  may represent a distal end of an imaging device which may be used as part of an imaging and intervention system such as the imaging and intervention system  100  of  FIG.  1   . The imaging device  300  may include many similar features to the imaging device  200  of  FIG.  2   . For the sake of brevity, features and operations previously described with respect to the imaging device  200  will not be repeated with respect to the imaging device  300  of  FIG.  3   . 
     The imaging device  300  includes an optical component  302  which includes a single distal surface  304  which acts as both the reflecting surface and tapered distal end. The optical component  302  may be generally cylindrical, with a generally trapezoidal longitudinal cross section. The optical component  302  may be formed from an optically transparent material (e.g., sapphire), and light may pass through a proximal surface  307  of the optical component  302  and into the material of the optical component  302 . The proximal surface  307  may be angled (relative to the long axis of the imaging device  300 ) to minimize the reflection of light as it passes from the filler material  318  into the material of the optical component  302 . 
     The distal surface  304  of the optical component  302  may extend a distance d from a distal tip of the outer shaft  320 . The distal surface  304  may be a generally flat surface at an angle relative to a long axis of the imaging device  300 . The distal surface  304  may come to a point a distance d beyond the outer shaft  320  which may act as a crossing tool. Light from the optical fiber  312  may reflect from the distal surface  304  within the optical component  302 . In some embodiments, the distal surface  304  may be angled to set up a total internal reflection (TIR) condition at the distal surface  304 . In some embodiments, the distal surface  304  may be coated with a reflective material (e.g., gold). Accordingly, the optical component  302  has a single surface (e.g., distal surface  304 ) which acts as both a tapered distal end and a reflecting surface. 
       FIG.  4    is a cross-sectional diagram of a distal end of an imaging device according to some embodiments of the present disclosure. The imaging device  400  may represent a distal end of an imaging device which may be used as part of an imaging and intervention system such as the imaging and intervention system  100  of  FIG.  1   . The imaging device  400  may include many similar features to the imaging device  200  of  FIG.  2    and the imaging device  300  of  FIG.  3   . For the sake of brevity, features and operations previously described with respect to the imaging device  200  and/or the imaging device  300  of  FIG.  3    will not be repeated with respect to the imaging device  400  of  FIG.  4   . 
     Similar to the optical component  302  of  FIG.  3   , the optical component  402  includes a distal surface  404  which is both a tapered distal end and reflecting surface. The imaging device  400  also includes a lens  409  disposed between a reference surface  414  of the optical fiber  412  and a proximal surface  407  of the optical component  402 . The lens  409  may shape the imaging beam  408 . For example, the lens  409  may focus the imaging beam  408  so that the imaging beam  408  focuses at a focal point. The focal length of the lens  409  may be chosen to focus the imaging beam  408  at focal region at or within a wall of the vessel (e.g., based on an expected distance from the imaging device  400  to the vessel wall). In some embodiments, the lens  409  may be a gradient-index (GRIN) lens. In the imaging device  400 , the reference surface  414  may be at a distal end of the lens  409 . 
     In this and in other variations of the exemplary embodiments described herein, the lens  409  may be but not limited to a Fresnel lens, spherical lens, aspheric lens, or an anamorphic lens. The lens may also be selected to either focus light at a selected distance from the exterior exit surface of the imaging beam, as depicted in  FIGS.  22 E and  22 F , or may be selected to provide a collimated beam profile, as depicted in  FIGS.  22 C and  22 D , or may be selected to produce varying degrees of focusing power including collimation at different angular orientation across the beam. The lens  409  may also comprise a lens assembly with two or more lenses. Additional features regarding lenses in the OCT imaging system are provided later below. 
       FIGS.  5 A and  5 B  are schematic diagrams of an exemplary imaging device with a bend or angle region according to some embodiments of the present disclosure. FIGS.  5 A and  5 B depict of imaging devices  502  and  504 , each which includes a pre-shaped bend which may aid in directional crossing with the tapered distal end. The imaging device  502  may be an imaging device, such as the imaging devices  300  of  FIG.  3  and  400    of  FIG.  4    where the optical component has a single surface which acts as both the tapered distal end and the reflecting surface. The imaging device  504  may be an imaging device, such as the imaging device  200  of  FIG.  2   , where the optical component has a first surface which acts as the reflecting surface and a second surface which acts as a tapered distal end. 
       FIGS.  6 A and  6 B  are schematic diagrams of another embodiment of an imaging crosser device  600 , comprises a shaped or molded optical element  602 . The molded optical element  602  may comprise one or more structures  604  and  606  that are configured to facilitate handling, manipulation, and/or alignment of the optical element  602  during manufacturing and/or during usage. The device  600  may further comprise optical fiber  608  to transmit light between a proximal light source or light receiver and the optical element  602 . The optical fiber  608  may be single-mode fiber or multi-modal fiber. A lens  610  may be provided to optically couple the fiber  608  and the optical element  602 . The lens  610 , for example, may be a gradient-index lens that is fused at its proximal end  612  to the distal end of the fiber  608 , and is configured to collimate or focus light from the light source and fiber  608  to the reflecting surface  614  of the optical element  602 . The fiber  608 , lens  610  and optical element  602  may be housed in a hypotube  616  of the device  600 . The device  600  may be used in conjunction with a catheter  618 . In the particular example depicted in  FIGS.  6 A and  6 B , the hypotube  616  has an outer diameter of 0.010″ or 0.017″ or 0.022″, but in other examples the hypotube have an outer diameter in the range of 0.009″ to 0.022″ or 0.012″ to 0.020″. The catheter  618  may comprise an inner diameter in the range 0.010″ to 0.013″, or 0.018″ to 0.020″ or 0.020″ to 0.024″ or otherwise configured with a diameter to receive the hypotube  616  of the device  600 . The outer diameter of the catheter  618  may be in the range of 0.016″ to 0.020″, or 0.016″ to 0.017″ or 0.018″ to 0.045″. The inner and/or outer surfaces of the hypotube  616  and catheter  618  may comprise a lubricious coating to facilitate rotational or longitudinal movement of the device  600  or catheter  618 . 
     Referring still to  FIGS.  6 A and  6 B , the optical element  602  comprises first and second protruding flanges or structures  604  and  606 . The protruding structure  604  and  606  may be used to grasp and manipulate the optical element  602  while reducing the risk of damage to the optical properties of the optical element  602 , and/or may be used to facilitate alignment In this particular example, structure  604  comprises a smaller cross-sectional shape than structure  606 , with a transverse width and/or longitudinal length that is smaller than the transverse width or dimension of the other structure  606 . In some examples, however, the structures  604  and  606  may be located at and proximally aligned and contiguous with the proximal end  620  of the optical element  602 . These structures  604  and  606  may be sized and shaped to facilitate insertion and/or alignment of the optical element  602  with the hypotube  616 . Referring to  FIGS.  8 A to  8 D , the hypotube  616  comprises an inner lumen  622  to receive the fiber  608  and the optical element  602 , as well as a proximal opening (not shown), and distal opening  623  from which the optical element  602  can partially protrude. The hypotube  616  further comprises an insertion opening  626  through which the optical element  602  may be inserted into the inner lumen  622 . The insertion opening  626  may be configured with a length sufficient to receive the optical element  602  such that the distal end of the optical element  602  can protrude from the distal opening  624  of the hypotube  616 , and to have the structure  604  to be seated in an alignment recess opening  628  that is opposite the insertion opening  626 . The alignment opening  628  and the protruding structure  604  may be configured to receive the protruding structure  604  or to form a complementary interfit between the opening  628  and structure  604 . When the optical element  602  is fully seated in the hypotube  616 , the distal surface  630  of the protruding structure  606  may abut the distal surface  632  of the insertion opening  626 . The protruding structures  604  and  606  may further comprise cylindrically rounded side surfaces  634  and  636 , which may align with the outer diameter of the hypotube  616  when the optical element  602  is fully seated. As depicted in  FIG.  6 A , the lens  610  may be pushed distally to abut and couple to the proximal surface of the optical element  602 . 
     Referring now to  FIGS.  7 A to  7 E , optical element  602  may an inner core  700  and an outer molded shell  702 . The material for the inner core  700  may be high refractive index polymer such as but not limited to polyurethane, polyvinylpyrrolidone, polystyrene, polycarbonate, zeonex, or NAS-21, and the material for the outer molded shell  702  may be the same as that of the inner core  700  or optical polymer such as but not limited to polymethyl methacrylate (PMMA), optorez or cellulose. The inner core  700  may comprise a refractive index of around 1.55, but in other examples, may have an index in the range of 1.40 to 1.80, or 1.50 to 1.60, or 1.55 to 1.65. The length of the optical element may be approximately 500 microns, but in other examples may be in the range of 400 microns to 800 microns, or 500 microns to 600 microns, or 450 microns to 550 microns for example. The optical element  602  comprises a cylindrical body  706  that has a diameter that is about 150 microns, but in other embodiments may be in the range of 140 microns to 200 microns, or 140 microns to 160 microns. The distal end of the cylindrical body  706  may comprise a reflecting surface  708  that has a 45 degree orientation relative to the longitudinal optical axis  710  of the optical element  602 . The cylindrical body  706  may further comprise a side interface surface  712  from which light reflected from the reflecting surface  708  exits the optical element  602 , and from which light reflected back from the surrounding tissue structures are received. The side interface surface  712  may be formed or be oriented at a 5 degree to 8 degree angle inward from the outer diameter of the cylindrical body  706 , and may have a maximum width of about 60 microns and a length of 135 microns. In other examples, the width may be in the range of 40 microns to 80 microns, and the length may be in the range of 100 microns to 180 microns. As depicted in  FIG.  7 E , the side interface surface  712  has a planar configuration with a parabolic shape, with an apex  714  that is proximal to a distal edge  716  that located at the proximal end of the optical element  602 . The side interface surface  712  and reflecting surface  708  may share a common edge  720 , as shown in  FIG.  7 D . 
     The inner core  702  may comprise a generally cylindrical shape with an orthogonal proximal surface  722  and an angled distal surface  724  that is aligned with the reflecting surface  708 . In other examples, however, the core may comprise a flattened surface distally. The inner core  702  may have a diameter in the range of 100 to 150 microns, or 110 microns to 130 microns. In some variations, the inner core  702  may have a variable diameter or transverse shape. In  FIGS.  7 C and  7 D , for example, the inner core  702  may have a diameter of about 110 microns at its proximal end, and a diameter of about 125 microns at its distal end. 
     The protruding structures  604  and  606  are located on opposite sides of the cylindrical body  706 . The structures  604  and  606  may protrude the same or a different distance from the surface of the cylindrical body  706 . In some variations, the total protruding distance may be about 100 microns, or in the range of 90 microns to 120 microns. Individually, each structure  604  and  606  may have a protruding distance in the range of 40 to 60 microns, or about 50 to 55 microns. 
     As depicted in  FIG.  6 A , light traveling through the fiber  608  is collimated or focused by the lens  610  onto the reflecting surface  708  of the optical element  602 . The reflecting surface  708  is an interior surface of the optical element, opposite or complementary to an outer surface  726  of the tapered or angled distal end  728 . In some variations, the outer surface  726  may further comprise a reflecting material, such as a gold or other metallic coating to facilitate reflections. To facilitate bonding of the coating, the surface may be mechanically or chemically treated to roughen the surface to enhance bonding. 
     In other examples, however, the reflecting surface of the optical element may comprise an outer reflecting surface.  FIG.  9    depicts another example of an imaging crosser device  900 , comprising an optical element  902  with a cylindrical body  904  with a length in range of 500 microns to 1000 microns, and a reflecting surface  906  located at the proximal end of the optical element  902  and is spaced apart from the tapered or angled penetrating distal end  908  of the optical element  900 . In this embodiment, the device  900  also comprise a hypotube  908 , fiber  910  and lens  912 , but wherein the hypotube  908  comprise an imaging cavity  914  that is filled with an optically transparent polymer or adhesive  916 . The lens  912  is configured to collimate or focus the light from the fiber  910  to the surrounding tissue at a different focal length, because of the closer location of the reflecting surface  906  in this device  900  compared to the more distal reflecting surface in device  600  of  FIGS.  6 A and  6 B . The reflecting surface  906  may also be treated and/or coated with a reflecting material as described for the reflecting surface  708  of device  600 . In embodiments comprising an outer reflecting surface, the material of the optical elements does not need to be optically transparent, and may comprise a metal such as steel, and may comprise a polished reflecting surface, or a material such a ceramic or glass, including sapphire or sapphire fiber. 
     Many optical adhesives or curable materials that may be used as an optically transparent polymer, adhesive or filler material have a refractive index of greater than 1.5, but this results in a significant index mismatch with saline, blood and/or plasma, which may have a refractive index in the range 1.33 to 1.38. This results in visible artifacts, including ghost images, light streaks and/or ringing.  FIG.  18 A , for example, depicts an OCT image with a ring artifact  1802  from interface mismatch and also a light streak artifact  1804  that results from a strong refractive index mismatch that saturates the detector.  FIG.  18 B  is another exemplary OCT image with a ring artifact  1812  and light streaks  1814 .  FIG.  18 C  is still another exemplary OCT image with a ring artifact  1816  and a ghost image  1818 . To potentially ameliorate these artifacts, the contour of the filler material may be modified during manufacture from a cylindrical shape to a concave or convex shape along the length of the filler material, to reflect away the refractive index mismatch signal from the detector.  FIG.  19 A  depicts the back reflection  1902  from the mismatch filler/fluid interface  1904  between the optical filler  1806  and the surrounding fluid/tissue  1906 , while  FIGS.  19 B and  19 C  depicts a concave and convex interfaces  1914 ,  1924 , respectively, centered about the lateral optical axis.  FIGS.  19 D and  19 E  depict concave and convex interfaces  1934 ,  1944  that are proximal or offset from the lateral optical axis, which reduces back reflection,  1932 ,  1942 . 
     In some variations, the optically transparent polymer, adhesive or filler material may be selected with a refractive index of less than or equal to 1.5. For example, an aliphatic urethane acrylate with an acrylic monomer may be used, with a refractive index in the range of 1.30 to 1.40, for example. Such a material may improve manufacturing tolerances, repeatability of manufacturing process and manufacturability because such materials are less sensitive to reflection variations caused by meniscal concave/convex shapes and interfaces of high refractive index mismatch, and thereby minimizing or eliminating undesirable detector saturation and ghost images. 
     In some variations, the filler material may be a UV curable optical adhesive comprising mixtures of aliphatic urethane acrylate and acrylic monomers. Examples include Norland Optical Adhesives 133 and 13775 (Norland Products, Inc. Cranbury, N.J.). NOA 133 has a refractive index of 1.33 and a pre-cure viscosity of 15 cps, and comprises a mixture of 1-15% aliphatic urethane acrylate and 85-99% acrylic monomer, while NOA 13775 has a refractive index of 1.3775 and a pre-cure viscosity of 4000 cps, and is a mixture of 80-99% aliphatic urethane acrylate and 1-20% acrylic monomer. In some variations, the selection of the filler material used may be selected based on the desired refractive index, as a well as manufacturing characteristics, such as the viscosity. A viscosity of 15 cps may be too thin to easily manufacture, while 4000 cps may be too thick or viscose to handle at smaller volumes, for example. In some variations, a viscosity of the filler material may be in the range of 1000 to 3000 cps, 1500 to 3000 cps, 2000 to 2500 cps, for example. Notwithstanding aliphatic urethane acrylate and acrylic monomers, other polymeric materials with refractive index 1.30 to 1.40, a pre-curing viscosity between 1000 to 3000 cps, and post curing Shore D Hardness above 30 may be applicable. Another example of a low refractive index polymer material or filler are combinations of hexafluoroacetone and 3-aminopropyltriethoxysilane. 
     While the viscosity of the filler material may be further selected or modified by, for example, selecting materials with a different relative amounts of aliphatic urethane acrylate and acrylic monomer, the optical properties may or may not exhibit a linear relationship based on the relative percentages or ratios of the constituents. For example, a mixture comprising 50% by volume NOA 133 and 50% by volume NOA 13775 (40% to 57% aliphatic urethane acrylate and 43% to 60% acrylic monomer) had a refractive index of 1.274 at 1310 nm, while a mixture comprising 40% by volume NOA 133 and 60% by volume NOA 13775 (48% to 65% aliphatic urethane acrylate and 35% to 52% acrylic monomer) had a refractive index of 1.345 at 1310 nm. In other variations, the percentage of aliphatic urethane acrylate is in the range of 30% to 70% and the percentage of acrylic monomer is in the range of 70% to 30%, while in still other variations, the percentage of aliphatic urethane acrylate is in the range of 40% to 50% and the percentage of acrylic monomer is in the range of 60% to 50%. 
     Two or more mixture of optical adhesive with different refractive indices may be combined before curing to achieve the desirable or optical refractive index. For example, a mixture comprising 50% by volume NOA 133 and 50% by volume NOA 13775 (40% to 57% aliphatic urethane acrylate and 43% to 60% acrylic monomer) had a refractive index of 1.274 at 1310 nm, while a mixture comprising 40% by volume NOA 133 and 60% by volume NOA 13775 (48% to 65% aliphatic urethane acrylate and 35% to 52% acrylic monomer) had a refractive index of 1.345 at 1310 nm. The mix ratio to achieve the desirable refractive index may or may not be linear or proportional. The pre-curing and post-curing refractive index may or may not differ. A post-curing refractive index of 1.33 to 1.35 may index-match with water, saline and fluid, while a post-curing refractive index of 1.37 to 1.40 may index-match with soft tissue. 
       FIGS.  20 A to  20 C  depict various OCT images from imaging system using a UV cured filler material comprising a 40/60 mixture of NOA 133 and 13775. As depicted, the signal intensity from the ring artifacts  2002 ,  2006 ,  2010 ,  2012  resulting from the filler/fluid interface is substantially reduced, compared to those in  FIGS.  18 A to  18 C . This in turn reduces detector saturation and the corresponding light streaks found in  FIGS.  18 A and  18 B , in addition to reducing the incidence of ghost images. 
     The low refractive index filler materials described herein may be used with any of a variety of OCT imaging devices and systems, whether used or not in an aqueous or biologic environment, and is not limited to the various exemplary OCT imaging embodiments described herein. For example, the low refractive index filler materials described herein may be used an OCT imaging probe that may or may not include the cutting tip or distal tapered ends or edges  204 ,  304 ,  404 ,  716  or  1004  described herein. 
       FIG.  21 A  is a cross-sectional diagram of an exemplary distal end of an OCT imaging device  2100  with a low refractive-index filler material  2118 , located between the distal end of the optical fiber  2114  and a reflecting surface  2106  of a tip structure  2102 . The imagine device  2100  may include an inner core  2110  located in a lumen  2115  of an outer tubular shaft  2120 . The inner element  2110  may be configured to rotate relative to the outer shaft  2120 , and includes an optical fiber  2112  which transmits lights from a light source and receives light signals returning from the target structures. Light from the light source travels distally through the optical fiber  2112  and exits from the end  2114  of the optical fiber  2112  and through the optical filler  2118  before reflecting off of the reflecting surface  2106  of the tip structure  2102 , to redirect the imaging beam  2108  toward the target structure. 
     The outer shaft  2120  may have an outer diameter of around 0.020 inches or less, 0.045 inches or less, or may comprise diameters sized according to common guidewire dimensions, including 0.014 inch, 0.018 inch and 0.35 inch. The outer shaft  2120  may comprise a unibody design, or may comprise a body  2122  and cap  2124  that are joined during the manufacturing process. The body  2122  may be formed from a coil or hypotube. The body  2122  and cap  2124  may have similar or different outer diameters and wall thicknesses. As with other variations described herein, the cap  2124  may comprise a different material, e.g., an optically transmissive material to facilitate light transmission through the cap  2124 . In some embodiments, the cap  2124  may include a window, for example such as a laser cut window through the material of the cap  2124 . In some embodiments, the laser cut window may include a cover made from an optically transparent material. In some embodiments, the window may be uncovered. In some embodiments, the body  2122  of the outer shaft  2120  may be made from an optically transparent material, and a separate cap  2124  may be unnecessary. 
     The inner core  2110  may be a generally cylindrical member which is disposed in a lumen  2115  of the outer shaft  2120 . The inner core  2110  may have an outer diameter of around 0.010 inches (0.254 mm) or less. The inner core  2110  may be extendable/retractable relative to the outer shaft  2120  along a longitudinal axis of the imaging device  2100  and may be rotatable relative to the outer shaft  2120 . For example, a motor/drive assembly (e.g.,  126  of  FIG.  1   ) may move the inner core  2110  relative to the outer shaft  2120 . This may facilitate movement of the imaging area without requiring movement of the outer shaft  2120 . 
     The optical fiber  2112  is configured to transmit light between an optical unit (e.g.,  112  of  FIG.  1   ) coupled to a proximal end of the imaging device  2100  and the distal end of the imaging device  2100 . The optical fiber  2112  may be a single fiber or a multi-core fiber. The optical fiber  2112  may be surrounded by a cladding material  2111 , which may support the optical fiber  2112  and provide optical conditions (e.g., an index mismatch) which facilitate the transmission of light along the optical fiber  2112 . The cladding material  2111  may in turn be surrounded by a fiber reinforcement  2116 . The fiber reinforcement  2116  may support the optical fiber  2112  and cladding material  2111 . In some embodiments, the fiber reinforcement  2116  may be a coil or a modified hypotube. 
     The end  2114  (and cladding material  2111 ) of the optic fiber  2114  may be a flat surface (e.g., a cut in the fiber) which is at a right angle to the long axis of the imaging device  2100 , but in other variations, may comprise a non-orthogonally oriented surface. Some light traveling distally through the fiber  2112  may be reflected back at the end  2114  of the fiber  2112 , to function as the local oscillator (LO) portion of the light, which is interferometrically combined with received light in an optical unit (e.g., optical unit  112  of  FIG.  1   ). 
     The reflecting surface  2106  of the tip structure  2102  is configured to redirects light from an optical axis generally aligned with the long axis of the fiber  2112 , and an optical axis directed to a side of the imaging device  2100 . For example, the reflecting surface  2106  may redirect light at about a right angle. The reflecting surface  2106  may be a slanted surface at a proximal end of the tip structure  2102 . The angle of the reflecting surface  2106  with respect to a long axis of the imaging device  200  may be configured based on the desired deflection of light which reflects from the reflecting surface  2106 . For example, in some embodiments, the reflecting surface  2106  may have an angle of about 45 degrees. In some embodiments, the reflecting surface  2106  may be coated with a reflective material, such as gold. As depicted in  FIG.  21 A , the low refractive index filler material  2118 , described herein, located between the end  2114  of the optic fiber  2112  (or lens located at the end  2114  of the optic fiber  2112  and the reflecting surface  2106  of the tip structure  2102 . The refractive index mismatch between the outer surface  2130  of the filler material  2118  and the surrounding fluid or tissue is reduced along with image artifacts as a result of the low refractive index material used. 
     In other variations, as depicted in  FIG.  21 B , an outer tubular shaft is not provided or required, as the imaging device  2150  is configured for insertion and use inside a lumen of a catheter or other tool, to provide OCT imaging, but may be otherwise similar to the inner core structure  2110  of the imaging device  2100  in  FIG.  21 A . 
     Referring to  FIG.  22 A , in other variations, the OCT imaging system  2200  may comprise an optical fiber  2202 , lens  2204 , a mirror or reflector  2206  and a low refractive index filler material  2208  between the lens  2204  and reflector  2206 . As noted elsewhere, the optical fiber  2202  may be a single-mode fiber, but in other variations may be a multi-mode fiber. The distal surface  2210  of the fiber  2202  and proximal surface  2212  of the lens  2204  may be mechanically spliced or fusion spliced. The surfaces  2210  and  2212  are cut or cleaved with complementary geometries and then mechanically aligned. This may be flat surfaces that are each perpendicular to the axis of the fiber  2202 , but in other variations may comprise non-flat surfaces or non-perpendicular orientations of the surfaces.  FIG.  29 A  depicts an example of an OCT imaging system  2900  that has been mechanically spliced with an optical fiber  2902  with a distal surface  2904  that is flat and comprising a 90-degree orientation with respect to the longitudinal axis of the fiber  2902 . This surface  2904  is against a corresponding proximal surface  2906  of a lens  2908 , where the surface  2906  is also flat and comprising a 90-degree orientation. As further depicted in  FIG.  29 A , the distal region  2910  of the optical fiber  2902  may have its jacket  2912  removed. In some further variations, as depicted in  FIG.  29 B , after splicing the fiber  2902  and lens  2204  may be re-coated and partially or fully re-jacketed, which may improve light streaking, ringing, ghost images and other image artifacts. In further variations, an optical adhesive with a refractive index similar to that of the fiber  2902  and/or lens  2904  may be used, or the fiber  2202  and lens  2204  may be fused or welded together. This fusion may result in a loss of single-mode coupling and form an intermediate zone  2914  of optical functionality between the optical fiber  2902  and lens  2904 . 
     Referring back to  FIG.  22 A , the lens  2204  may be a GRIN lens. In this particular embodiment, the length of the GRIN lens corresponds to one-quarter of a full sinusoidal path or pitch of the lens  2204 , but in other variations may comprise a length equal to or corresponding to any odd-multiple of a quarter pitch, e.g. % pitch length, one and ¼ pitch length  2300  ( FIG.  23 A ), one and % pitch length  2302  ( FIG.  23 B ), or more. One potential benefit of the increased length is that the ring artifacts (e.g., artifacts  2004 ,  2008 ,  2012  in  FIGS.  20 A,  20 B and  20 C ) resulting from the reflective interface between the optical fiber  2304 ,  2306  and the GRIN lens  2308 ,  2310 , respectively, will exhibit increased diameters in the image, such that the ring artifact may be enlarged to a size where it lies outside of the effective field of view. This reflective artifact is typically stronger or larger than −40 dB. 
     As depicted in  FIG.  22 A , the GRIN lens  2204  may output a collimated beam  2220  through the low refractive index material  2208  and as reflected by the mirror or reflector  2206 . The beam diameter may be configured to be nominally in the range of 20 μm to 200 μm. The distal surface  2214  of the GRIN lens  2204  and the polymer material  2208  may also be at a 90-degree surface angle relative to the axis of the optical fiber  2202 . This may ensure that any light rays reflecting at the lens/polymer interface will have the same reflective path of partial reflection, which may provide improved consistency/reproducibility during manufacturing and provide a stable and optimal return loss (RL) or reference signal. In some variations, the RL at the lens/polymer interface may be in the range of −14 dB to −28 dB, or −15 dB to −35 dB, or −20 dB to −40 dB, for example. 
     For example,  FIGS.  22 C and  22 D  depict general and close-up schematic views of one example an OCT imaging assembly  2260  configured with a collimating beam  2270 , comprising a single mode optical fiber  2262 , GRIN lens  2264 , low index polymer material  2266  and mirror  2268 . The light beam  2270  will exit the single mode optical fiber  2262  and is collimated by the GRIN lens  2262 , but at the interface between the GRIN lens  2262  and the low refractive index polymer  2266 , a partial reflection of the collimated beam  2270  occurs, with a RL typically in the range of −14 dB to −35 dB at the interface. Depending on the specific design, RL range of −14 dB to −40 dB, or −15 dB to −35 dB, or −20 dB to 35 dB or −25 dB to −35 dB may be achieved. As this reflected beam  2272  travels back toward the optical fiber  2262 , the GRIN lens  2264  will focus the reflected beam back into the single mode fiber core  2274 , which may have a size of 10 μm or less, 9 μm or less, or 8 μm or less, for example. This may result in a high effective RL of the reference signal. This high RL is achieved at local maxima of the optimal GRIN length for beam collimation, which can be found at or around 0.5 pitch unit length, 0.75 pitch unit length, 1.25 pitch unit length, 1.75 pitch unit length, and so forth, This may be beneficial for improving manufacturability and repeatability, allowing for +/−10% tolerance about the optimal GRIN unit length. 
     In comparison, an OCT imaging assembly  2280  configured with a focusing light beam  2290 , illustrated in  FIGS.  22 E and  22 F , and comprising a single mode optical fiber  2282 , GRIN lens  2284 , low refractive index polymer material  2286  and mirror  2288 . As the light beam  2290  exits the single mode optical fiber  2282 , it is partially focused by the GRIN lens  2284 , and then is partially reflected at the junction of the GRIN lens  2284  and the low refractive index polymer material  2286 . This partial reflection  2292  may result in low RL in the range of −35 dB to −60 dB. As the reflected beam returns toward the optical fiber  2282 , however, it is not refocused back into the fiber core  2294 , resulting in a low and lossy, ineffective RL of the reference signal. As the reflective reference when the GRIN length is configured to focusing the beam, it is centered away from local maxima of optimal GRIN length for beam collimation, resulting in unpredictable RL and less manufacturability. 
     The GRIN lenses in the exemplary OCT imaging systems described herein may be coated or uncoated. They may comprise a length in the range of 500 um to 700 um, 600 um to 800 um or 700 um to 1100 mm, 800 um to 5000 um, a diameter in the range of 65 um to 130 um, 100 um to 200 um or 125 um to 500 um, and a pitch length of 0.500 mm to 1.200 mm, 0.600 mm to 1.500 mm or 0.700 mm to 2.000 mm (at a wavelength in the range 750 nm to 1380 nm, for example). 
     The reflector  2206  may comprise a mechanical and/or electropolished surface that is typically oriented at a 45-degree angle relative to the longitudinal axis of the optical fiber. The angle may be deliberately varied between 35 and 65 degrees. The reflector  2206  may comprise a layer of gold, silver, tin, nickel chromium, aluminum, dielectric or multilayer dielectric or other coatings, with the coating process involving chemical vapor deposition, physical vapor deposition, sputtering, electroplating, etc. The coating thickness may be between 50 nm to 300 nm, or 100 nm to 250 nm, or 100 nm to 200 nm. The base material for the reflector may be optical glass, ceramic, semiconductor materials, or polymer. 
       FIG.  24    depicts another embodiment of an OCT imaging system  2400 , wherein the optical fiber  2402  comprises a distal end or surface  2404  that was cleaved or configured with an angle that was not orthogonal to the longitudinal axis of the fiber  2402 , and instead comprises a surface angle that is angled at least 8, 10, 12, 15 or more degrees from the orthogonal orientation, which may reduce or eliminate reflection artifacts. A spherical or aspheric lens  2406  may be spaced apart from the distal surface  2404  of the optical fiber  2402  to receive and collimate the light beam  2408 . The light beam  2408  then continues through the low refractive index material  2410  and is reflected by the mirror or reflector  2412  out of the materials  2410  and out of the imaging system  2400 . The spherical or aspheric lens  2406  may comprise a refractive index in the range of 1.50 to 1.85, for example, with and a plano-convex configuration with a convex proximal surface  2414  having a radius of curvature in the range of 0.20 to 1.00 mm and a flat distal surface  2416 . The RL or reference signal at the interface between the lens and polymeric materials  2410  may be in the range of −14 dB to −40 dB, or −15 dB to −35 dB, or −20 dB to 35 dB or −25 dB to −35 dB. The cavity or space between the fiber  2402  and the lens  2406  may be filled with the same material as  2410  or a different material, e.g. a polymeric material with a higher refractive index in the range of 1.45 to 1.48, 1.40 to 1.50, or 1.5 to 1.85, or a different low refractive index material, for example, e.g. a refractive index in the range 1.30 to 1.48. The refractive index of the material in the space between the fiber and  2402  and the lens  2406  shall be lower than that of the lens  2406 . The spacing between the end of the optical fiber and the lens may be in the range of 0.1 mm to 4.0 mm, or 0.3 mm to 3.0 mm, or 0.5 mm to 2.00 mm, for example. The spacing between the distal surface of the lens and the center of the reflector may be in the range of 0.025 mm to 0.250 mm, 0.050 to 0.500 mm, 0.1 mm to 3.0 mm, or 0.2 mm to 2.0 mm, or 0.3 mm to 2.00 mm, for example. 
       FIG.  25    depicts another variation of an OCT imaging system  2500  that is similar to the OCT imaging system  2400  of  FIG.  24   , except that instead of a plano-convex lens, a double-convex lens  2502  is provided. The radius of curvature of the proximal convex surface  2504  may also be in the range of 0.20 mm to 1.00 mm and the refractive index may be in the range of 1.50 to 1.85. In this particular example, the lens  2502  may comprise a lens tilt  2506  in the range of 0.10 degrees to 2.00 degrees. The tilt minimizes or eliminate a reflection artifact by deflecting any interface reflection from the lens  2502  away from the optical fiber. In this embodiment, the return loss or reference signal is collected from the interface between the optical fiber and the polymeric material between the optical fiber and the lens  2502  in the range of −14 dB to −40 dB, or −15 dB to −35 dB, or −20 dB to 35 dB or −25 dB to −35 dB. The lens  2502  may be configured to provide a non-collimated light beam  2508 , as depicted in  FIG.  25   , or a collimated light beam. For non-collimated embodiments, the focal distance of the OCT system may be 0.250 mm to 4.00 mm from the longitudinal axis of the OCT imaging system  2500  or center of the mirror or reflector  2510 . 
       FIGS.  26 A and  26 C  depict additional exemplary embodiments of an OCT imaging system  2600 ,  2650  wherein instead of a GRIN lens or spherical/aspheric lens, a Fresnel lens  2602  is provided. The Fresnel lens  2602  may be configured with a grating or diffractive pattern that is optically equivalent to a radius of curvature of 0.20 mm to 1.00 mm of a convex lens. Like some other embodiments described herein the Fresnel lens  2602  may be spaced apart from the end  2604 ,  2652  of the optical fiber  2606 ,  2654  in the range of 0.1 mm to 4.00 mm, 0.2 mm to 3.00 mm, or 0.5 mm to 2.5 mm, for example. The gap or cavity  2608 ,  2656  between the end  2604 ,  2652  of the optical fiber  2606 ,  2654  and the Fresnel lens  2602  may be filled with a polymeric material  2608 ,  2656 . In some further variations, the polymeric material  2608 ,  2656  may be a low-index material with n&lt;1.50 or in the range of 1.30 to 1.48, for example. The polymeric material  2610  between the lens  2602  and the reflector  2612  may be in the range of 1.30 to 1.40, or between 1.30 to 1.48, or 1.30 to 1.30, or 1.32 to 1.38, and may be the same or different from the proximal polymeric materials  2608 ,  2656 . In some variation, the polymeric material  2610  may be higher refractive index in the range of 1.50 to 1.70, or 1.52 to 1.65. The interface between the optical fiber  2606 ,  2654  and the proximal polymeric material  2608 ,  2656  may be flat and may be at a 90-degree angle as depicted in  FIG.  26 A , or may be offset from the 90-degree angle as depicted in  FIG.  26 C , by at least 8, 10, 12, 15 or more degrees from the orthogonal orientation, for example.  FIG.  26 A  depicts an embodiment with a RL or reference signal from interface  2604  in the range of −14 dB to −40 dB, or −15 dB to −35 dB, or −20 dB to 35 dB or −25 dB to −35 dB; it is preferable the refractive index of the Fresnel lens and the distal polymeric material  2610  are matched or similar.  FIG.  26 C  depicts an embodiment with a RL or reference signal from the interface between the Fresnel lens and the distal polymeric material  2610 ; it is preferable the refractive index of the distal polymeric material  2610  is mismatched or different such that the RL is in the range of −14 dB to −40 dB, or −15 dB to −35 dB, or −20 dB to 35 dB or −25 dB to −35 dB.  FIG.  26 B  depicts an exemplary Fresnel lens  2602  that may be used in the OCT imaging systems in  FIGS.  26 A and  26 C . The lens  2602  may comprise a circular lens body with a plurality of concentric grates or gratings  2660 . In this particular example, the lens  2602  is radially symmetrical with respect to a first axis  2662  and a second axis  2664  of the lens  2602 , wherein both axes  2662 ,  2664  are transverse to the longitudinal axis of the optical fiber  2606 ,  26054  and are orthogonal to each other. In other variations, however, the configuration of the diffractive grating pattern may be different along the two axes  2662 ,  2664  to produce varying focusing power, or combination of beam focusing and collimation. This may be provided to adjust the beam profile to ensure a confocal beam, without requiring the use of an anamorphic lens. 
     In still another embodiment, depicted in  FIG.  27 A , the OCT imaging system  2700  may lack a lens but comprises a reflector  2702  with a Fresnel diffractive grating formed or etched on its reflective surface. Like the mirrors or reflectors described herein, the Fresnel reflector  2702  may comprise of a layer of gold, silver, tin, nickel chromium, aluminum, dielectric or multilayer dielectric or other coatings thereof. Between the distal end  2704  of the optical fiber  2706  and the Fresnel reflector  2702 , a low refractive index polymer  2708  as described herein may be provided, as described elsewhere herein. As the light beam  2710  exits the fiber  2706  and travel through the polymeric material  2708 , it is reflected and collimated by the diffractive pattern of the Fresnel reflector  2702 , and then exits the polymeric material  2708  to the target imaging location. The Fresnel reflector  2702  may be configured with an equivalent radius of curvature (R) in the range of −0.25 mm to −2.00 mm (concave), or −0.20 mm to −1.00 mm (concave); an inactive pitch angle in the range of 0 degrees to 75 degrees, a pitch frequency (pitch/mm) of 20 or greater, and a focal length (f) between the fiber to the reflector surface in the range of 200 μm to 1000 μm. The reflector  2702 , as illustrated in  FIG.  27 F , like the Fresnel lens in  FIG.  26 B , may comprise a circular reflector body with a plurality of concentric circular grates, gratings or diffractive patterns  2760 . In this particular example, the reflector  2702  is radially symmetrical with respect to a long axis  2762  and a short axis  2764  of the reflector  2702 , but in other variations as discussed below, the configuration of the grating pattern may be different along the two axes  2762 ,  2764 . The orientation of the long axis  2762  lies in the same plane as the longitudinal axis of the optical fiber  2706  and the reflected beam  2712 , while the short axis  2764  is transverse and orthogonal to the long axis  2762  and to the longitudinal axis of the optical fiber  27 . This may be provided to adjust the beam profile to ensure a confocal beam or beam waist, without requiring the use of an anamorphic lens having a concave reflective surface. In an embodiment of a Fresnel reflector  2702 , the diffractive grating pattern imprinted on a flat surface with parameters including but not limited to the inactive pitch angle and pitch frequency may be gradually varied from the long axis  2762  to the short axis  2764  to ensure a confocal beam waist. In yet another embodiment of the Fresnel reflector  2702 , the diffractive grating pattern imprinted on a flat surface may appear asymmetrical and elliptical with the long axis of the elliptical pattern matching the long axis  2762  and the short axis of the elliptical pattern matching the short axis  2764  to match the cross section of the incidence beam  2710  in the Fresnel reflector  2702 . 
       FIGS.  27 B to  27 E  depict various cross section beam profiles of the light beam  2710  in  FIG.  27 A .  FIGS.  27 B and  27 C  depicts the beam profile  2712 ,  2714  at a focal length (fiber to reflector surface distance) of 200 μm and at radii of curvature of −0.4 mm (concave) and a focal length of 250 μm and at radii of curvature of −0.5 mm (concave), respectively.  FIG.  27 D  depicts the beam profile  2716  at a focal length of 300 μm and at a radius of curvature of −0.6 mm, while  FIG.  27 E  depicts the beam profile  2718  at a focal length of 500 μm and a radius of curvature of −1.1 mm. 
       FIG.  28 A  depicts another embodiment of an OCT imaging system  2800  comprising both a GRIN lens  2802  and a Fresnel reflector  2804 . In this example, the light beam  2814  exiting the optical fiber  2806  then is collimated by the GRIN lens  2802  and then enters the low refractive index polymeric material  2808  and then reflected by the Fresnel reflector  2804  before exiting the low reflective index polymeric material  2808  to reach the target location. As described with other embodiments herein, the end  2812  of the optical fiber  2806  may be a flat surface with an orientation 90 degrees to the longitudinal axis of the fiber  2806 , or may be offset. The collimated light beam  2816  may have a diameter in the range of 20 μm to 200 μm, and the RL at the interface between the GRIN lens  2802  and the low refractive index material may be in the range of −14 dB to −28 dB, −14 dB to −40 dB, or −15 dB to −35 dB, or −20 dB to 35 dB or −25 dB to −35 dB. 
     Depending on the desired configuration, the Fresnel reflector  2804  may be configured with an equivalent radius of curvature in the range of −0.25 mm to −30 mm to provide a focusing beam, or an equivalent radius of curvature in the range of −30 mm to −infinity for a collimated beam. As noted previously, the diffractive pattern of the Fresnel reflector  2804  may be configured with a radially symmetric or an asymmetric pattern. Whether the light beam from the GRIN lens is converging (focusing), diverging, collimated or adequately collimated, a Fresnel reflector  2702  or  2804  placed at 45 degrees as depicted in  FIG.  27 A  and  FIG.  28 A  will introduce astigmatism exhibiting a deviation or an offset of the beam focus or combination of beam collimation/focus along the long axis  2762  and short axis  2764 . In variations where some correction to an astigmatic light beam is desired, an elliptical or asymmetric diffractive pattern may be provided. Depending on the axis of the astigmatism, the long axis or the short axis of the Fresnel reflector  2804  may be adjusted to provide an elliptical diffractive pattern to compensate for the astigmatism to match the incidence beam at the 45-degree tilt of the reflector  2804 .  FIGS.  28 B and  28 C  are schematic side and end views of the reflector  2804  and the profile of the reflected light beam  2816 , with an exemplary astigmatic beam profile  2818  depicted in  FIG.  28 D . To compensate for the asymmetric profile  2818 , the long axis  2762  and the short axis  2764  of a reflector  2702 , as depicted in  FIG.  27 F , may be adjusted to an elliptical shape to compensate for any astigmatism in the incidence beam, so that the reflected beam is sufficiently collimated, or confocal if it were a focusing beam. Alternatively, the diffractive grating pattern imprinted on a flat surface with parameters including but not limited to the inactive pitch angle and pitch frequency may be gradually varied from the long axis  2762  to the short axis  2764  to ensure a confocal beam waist. The use of a Fresnel reflector  2702  or  2804  avoids the need for an anamorphic lens having a concave reflective surface. 
       FIG.  10    depicts another variation of the imaging cross device  1000 , similar to device  900  but wherein the optical element  1002  comprise a tapered distal end  1004  with an enlarged diameter body  1006  that is external to the hypotube  1008 , and a smaller diameter proximal stem  1010  with a proximal reflecting surface  1012 . The proximal reflecting surface  1012  may be angled at 45 degree, or off 45 degree such as 48 degree but no more than 45+/−5 degrees. As with device  900 , a hypotube  1014 , optical fiber  1016 , lens  1018  and adhesive  1020  are provided. 
       FIG.  11    depicts still another embodiment of an imaging crosser device  1100 , wherein the optical element  1102  comprises a proximal angled surface  1104  on a stem  1106  of an enlarged cutting head  1108 , but with the addition of a prism  1110  inserted between the lens  1112  and the proximal angled surface  1104 . The prism  1110  comprises an orthogonal proximal surface  1114  that is optically coupled to the lens  1112 , and a 45-degree angled distal end  1116  that is bonded to the proximal angled surface  1104  of the stem  1106 , with the prism having the longest dimension no larger than 255 um. The angled distal end (or the hypotenuse)  1116  of the prism  1110  may be coated or bonded to the stem  1106  using an aluminum or other metallic reflecting coating. 
     In use, the imaging crosser devices described herein may be used in conjunction with a traditional guiding catheters for accessing the desired target locations for diagnostic assessment and/or therapeutic treatment. In other variations of the imaging crosser devices, a dual-lumen guide catheter may be used. Referring to  FIG.  12 A , the guide catheter  1200  may comprise a proximal hub  1202 , a catheter body  1204  and a distal tip  1206 . Referring to  FIG.  12 B , the proximal hub  1204  may comprise one or more proximal openings  1206  which communicate with two or more catheter body lumens  1208 ,  1210  of the catheter body  1204 . As depicted in  FIG.  12 B , the catheter body  1204  is partially located inside the distal lumen  1212  and distal opening  1214  of the hub  1202 . The hub may further comprise an optional flushing port  1216 , which may be configured to be in fluid communication with one or both of the catheter body lumens  1208 ,  1210 . In some variations, the catheter body  1204  may comprise separate tubular bodies for each catheter body lumens  1208 ,  1210 , which located inside an overtube  1218 , as depicted in  FIG.  12 B . In these variations, the flush port may be in fluid communication in the tubular lumen of the overtube  1218  but outside catheter body lumens  1208 ,  1210 . The catheter lumens  1208 ,  1210  may have the same or different diameters. In the particular example depicted in  FIG.  12 B , catheter lumen  1208  comprises a smaller diameter of 0.016″ for use with a guidewire, and catheter lumen  1210  comprises a larger diameter of 0.022″ or 0.025″ to receive the imaging crosser device. In  FIGS.  13  and  14    depicting variations of the distal embodiment of the dual-lumen guide catheter  1300  and  1400 , the last 2 to 6 inches (50.8 to 152.4 mm) of the distal end comprise optically transparent materials. In another embodiment, 3 to 5 inches (76.2 to 127.0 mm) of the distal end  1300  and  1400  comprise optically transparent materials. 
     The distal end  1220  of the catheter body  1204  may be attached to a catheter tip  1220 . Referring to  FIG.  15   , the catheter tip  1220  may comprise a cylindrical base  1222  with a proximal surface  1224  and comprises a corresponding opening for tip lumens  1226  and  1228 , which are configured to be in fluid communication with catheter body lumens  1208  and  1210 . In this particular catheter tip  1220 , tip lumen  1226  extends distally a greater distance from the proximal surface  1224  than tip lumen  1228 , along an extension tube  1230  that projects distally from the cylindrical base  1222 . One or more angled transition surfaces  1232  may be provided on the outer surface of the cylindrical base  1222  in order to provide a smooth transition from to the extension tube  1230 . 
     In some variations of the guide catheters used with the imaging crosser devices, one or more side openings along the catheter body and/or catheter tip may be provided so that the guidewire may be manipulated to exit the guide catheter proximally to the distal opening  1334  of the guidewire tip lumen  1228 . In  FIG.  16   , for example, a side opening  1602  is provided in the catheter tip  1600 . This may be in addition to catheter body openings that are provided, e.g. at 20 cm, 40 cm and/or 60 cm distances along the catheter body as measured from the hub. 
     As shown in  FIGS.  15  and  16   , in some variations, the guidewire tip lumens  1226  and  1604  may comprise a generally linear configuration, where the lumen extends axially without any angle or deviation from the longitudinal axis. In other examples, or with the imaging crosser device tip lumens  1228  and  1606 , however, the lumens  1228  and  1606  may be provided with angled regions to direct the inserted devices outwardly angle, away from the extension tube  1230 . This may help with facilitating diagnosis or treatment of eccentric lesions on the anatomy. In other examples, however, the lumens  1228  and  1606  may have a straight orientation. 
       FIGS.  13  and  14    depict additional features of the guide catheter  1300  and  1400 , which may include radiopaque marker bands  1302  and  1402  located along the catheter body  1304  and  1404 , respectively. The marker band  1302  comprises a ring-like configuration with uniform length, while the marker band  1402  comprises an asymmetric configuration, which may facilitate assessment of the catheter orientation and/or location. 
     The operation of the optical systems is generally described with respect to light being emitted by the optical system towards a target area. However, one of skill in the art would appreciate that since optical paths may typically be reversible, the beam path may also represent a field of view ‘seen’ by the optical system (e.g., reach a receiver of the optical system). Thus, an imaging beam emitted by an optical fiber may also represent (all or part of) a pathway along which received light may return to the fiber. 
     Certain materials have been described herein based on their interaction with light (e.g., opaque, reflective, transmissive, refractive, etc.). These descriptors may refer to that material&#39;s interactions with a range of wavelength(s) emitted by the system and/or that the receiver is sensitive to. It would be understood by one of skill in the art that a given material&#39;s properties vary at different ranges of wavelengths and that different materials may be desirable for different ranges of wavelengths. The description of a particular example material is not intended to limit the disclosure to a range of wavelengths over which that particular example material has the desired optical properties. Similarly, the description of a particular wavelength is not intended to limit the system to only those wavelengths. The term ‘light’ may be used throughout the spectrum to represent electromagnetic radiation, and is not intended to limit the disclosure to electromagnetic radiation within the visible spectrum. The term ‘light’ may refer to electromagnetic radiation of any wavelength. 
     Of course, it is to be appreciated that any one of the examples, embodiments or processes described herein may be combined with one or more other examples, embodiments and/or processes or be separated and/or performed amongst separate devices or device portions in accordance with the present systems, devices and methods. 
     Finally, the above-discussion is intended to be merely illustrative of the present system and should not be construed as limiting the appended claims to any particular embodiment or group of embodiments. Thus, while the present system has been described in particular detail with reference to exemplary embodiments, it should also be appreciated that numerous modifications and alternative embodiments may be devised by those having ordinary skill in the art without departing from the broader and intended spirit and scope of the present system as set forth in the claims that follow. Accordingly, the specification and drawings are to be regarded in an illustrative manner and are not intended to limit the scope of the appended claims.