Abstract:
An apparatus, method and software arrangement for imaging a surface of a structure that is in contact with an opaque fluid is provided. The apparatus includes an article of manufacture (e.g., a housing), a fluid delivery arrangement and an imaging arrangement. The housing includes an aperture formed in the article of manufacture. The fluid delivery arrangement is configured to deliver a volume of substantially transparent fluid to the aperture formed in the housing. The imaging arrangement is configured to image the surface of the structure using an imaging modality after the volume of the transparent fluid is delivered to the aperture, wherein the imaging arrangement and/or the article of manufacture is translated along the surface of the structure while imaging the surface of the structure.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a continuation application of, and therefore claims priority from U.S. application Ser. No. 11/211,483, filed on Aug. 24, 2005, now U.S. Pat. No. 8,208,995 issued on Jun. 26, 2012 which claims priority from U.S. Patent Application Ser. No. 60/604,138 filed on Aug. 24, 2004. The entire disclosures of both applications are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to methods and apparatus that use optical radiation for imaging surfaces and, more particularly to a method and apparatus that use optical radiation to image an interior target surface of a blood vessel. 
     BACKGROUND INFORMATION 
     Acute myocardial infarction (“AMI”) is the leading cause of death in the United States and industrialized countries. Research conducted for over the past 15 years has demonstrated that several types of minimally or modestly stenotic atherosclerotic plaques, termed vulnerable plaques, are precursors to coronary thrombosis, myocardial ischemia, and sudden cardiac death. Postmortem studies have identified one type of vulnerable plaque, i.e., the thin-cap fibroatheroma (“TCFA”), as the culprit lesion in approximately 80% of sudden cardiac deaths. Over 90% of TCFA&#39;s are found within the most proximal 5.0 cm segment of each of the main coronary arteries (left anterior descending—LAD; left circumflex—LCx; and right coronary artery—RCA). The TCFA is typically a minimally occlusive plaque characterized histologically by the following features: a) thin fibrous cap (&lt;65 μm) large lipid pool, and c) activated macrophages near the fibrous cap. It is hypothesized that these features predispose TCFAs to rupture in response to biomechanical stresses. Following the rupture, the release of procoagulant factors, such as tissue factor, create a nidus for thrombus formation and the potential for an acute coronary event. While TCFAs are associated with the majority of AMIs, recent autopsy studies have shown that coronary plaques with erosions or superficial calcified nodules may also precipitate thrombosis and sudden occlusion of a coronary artery. 
     Although autopsy studies have been valuable in determining features of culprit plaques, the retrospective nature of these studies may limit their ability to quantify the risk of an individual plaque for causing acute coronary thrombosis. For instance, TCFAs are a frequent autopsy finding in asymptomatic or stable patients, and are found with equal frequency in culprit and non-culprit arteries in acute coronary syndromes. Moreover, disrupted TCFAs have been found in about 10% of non-cardiac deaths. Recent findings of multiple ruptured plaques and increased systemic inflammation in acute patients have challenged the notion of a single vulnerable plaque as the precursor for AMI. A better understanding of the natural history and clinical significance of these lesions may accelerate progress in the diagnosis, treatment and prevention of coronary artery disease. 
     An exemplary approach to studying the evolution of vulnerable plaques is a non-invasive or intracoronary imaging of individual lesions at multiple points in time. Unfortunately, the microscopic features that characterize vulnerable plaque are not reliably identified by the conventional imaging technologies, such as intravascular ultrasound (“IVUS”), catscan (“CT”), and magnetic resonance imaging (“MRI”). While experimental intracoronary imaging modalities such integrated backscatter IVUS, elastography, angioscopy, near-infrared spectroscopy, Raman spectroscopy and thermography have been investigated for the detection of vulnerable plaque, it is believed that no method other than optical coherence tomography (“OCT”) has been shown to reliably identify the characteristic features of these lesions. 
     OCT is an optical analog of ultrasound that provides high-resolution (˜10 μm) cross-sectional images of human tissue. OCT has been established as an accurate method for characterizing the microscopic features associated with vulnerable plaque. This technology can also be used to quantify macrophage content within atherosclerotic plaque. Intracoronary optical imaging using such technology is safe, and images obtained from patients have features substantially identical to those identified ex vivo. Thus, OCT has the ability to provide a large amount of information about plaque microstructure. This technology may play an important role in improving the understanding of vulnerable coronary plaques in patients. 
     Strong attenuation of light in blood may present a significant challenge for intravascular optical imaging methods. To overcome this potential obstacle, intermittent 10 cc flushes of saline through a guiding catheter can provide an average of 2 seconds of clear viewing during which effective images can be captured, as is shown in  FIG. 1B . For example,  FIG. 1B  illustrates an analysis of the time of angiographic lumen attenuation following a 6 cc contrast injection at three separate locations, shown in part A of  FIG. 1B . As can be seen from part B of  FIG. 1B , the angiographic lumen attenuation following the 6 cc contrast injection at a rate of 3 cc/s demonstrates a complete filling for the duration of the purge (approximately 2 seconds) regardless of the location. Additionally, saline flushing of a blood vessel for a limited duration (for example, less than 30 seconds) is safe, and generally does not result in a myocardial ischemia. This approach can provide exceptional cross-sectional images of coronary vasculature. However, the combination of the limited flush duration and low image acquisition rate may reduce comprehensive coronary screening. 
     One proposed solution has been to change the optical properties of blood. The primary mechanism of optical attenuation in blood is optical scattering. For instance, matching the refractive index of the red blood cells, white blood cells and platelets with that of a serum decreases optical scattering. This approach has resulted in a 1.5-fold increase in penetration of OCT when diluting blood with Dextran. Unfortunately, since the optical attenuation of blood is so high, at least a 10-fold improvement would be preferable to allow for effective intracoronary OCT imaging in patients. 
     Another proposed solution is to completely occlude the artery, and replace blood with saline. This technique that is commonly deployed in angioscopic imaging requires proximal balloon occlusion. Following vascular occlusion, all of the remaining blood in the vessel is replaced with saline. This conventional method allows a cross-sectional optical imaging of the entire coronary tree. While this procedure is commonly conducted in Japan, the potential for coronary dissection and myocardial ischemia precludes widespread clinical application of this procedure. 
     Still another proposed solution is to purge the blood vessel with optically transparent blood substitutes. Blood substitutes that are transparent in the infrared can potentially provide clear imaging for an extended duration. This method has achieved significantly improved imaging in murine myocardium by replacing blood with Oxyglobin. Although these compounds may hold promise for future clinical application, they are not yet approved for human use. 
     A further proposed solution is to increase the frame rate of OCT scans. Since the goal is to acquire a sufficient number of images to comprehensively screen coronary arteries, a straightforward approach would be to accept the clear viewing time provided by conventional saline flushing, and increase the frame rate of OCT scans dramatically. Two possibilities exist for increasing the frame rate of OCT scans: a reduction of the number of A-lines per image, and an increase of the radial scan rate. 
     Similarly to many imaging methods, OCT images are acquired in a point sampling fashion and are composed of multiple radial scans or A-lines. To increase the image rate, it is possible to reduce the number of A-lines per image by increasing the catheter rotation rate. Image quality degrades rapidly in such case, however, manifested by a decrease in transverse resolution as can be seen in  FIG. 1A . For example, image A of  FIG. 1A  depicts a sample image generated using OCT imaging at a rate of 4 frames per second having 500 A-line scans per frame. Image B of  FIG. 1A  depicts a sample image generated using OCT imaging at a rate of 40 frames per second having 50 A-line scans per frame. As can be clearly seen, the image quality of Image A far exceeds the image quality of Image B. This degradation is unacceptable for most clinical applications. 
     A second possibility is to increase the radial scan rate. For technical reasons specific to the current OCT paradigm, an increase in A-line rate may result in an unacceptable penalty in signal to noise ratio, and thus, images of sufficient quality for accurate diagnosis cannot be obtained. 
     Therefore, there is a need to provide a method and apparatus that combine quality imaging of internal surfaces of blood vessels and other biological structures and effective imaging of segments of the internal surfaces of the blood vessels. 
     SUMMARY OF THE INVENTION 
     It is therefore one of the objects of the present invention to provide an apparatus and method that combine quality imaging of internal surfaces of blood vessels and other biological structures and effective imaging of segments of the internal surfaces of the blood vessels. Another object of the present invention is to provide an apparatus and method that provide quality images of internal surfaces of segments of blood vessels in order to offer an improved understanding of the natural history and clinical significance of these lesions which will accelerate progress in diagnosis, treatment and prevention of coronary artery disease. 
     These and other objects can be achieved with the exemplary embodiment of the apparatus, method and software arrangement according to the present invention for imaging a structure that is in contact with an opaque fluid. The exemplary apparatus can include a housing, a fluid delivery arrangement and an imaging arrangement. The fluid delivery arrangement is configured to deliver a volume of further fluid to an external location with respect to the housing. And the imaging arrangement is configured to image the structure after the volume of the further fluid is delivered to the external location, wherein the imaging arrangement is translated along a path which approximately corresponds to an axis of extension of a surface while imaging the structure. 
     In another exemplary embodiment of the apparatus, method and software arrangement according to the present invention for imaging a structure that is in contact with an opaque fluid. The exemplary method includes injecting a bolus of transparent or semi-transparent fluid into the vessel, imaging the vessel using rapid circumferential and pull-back imaging to achieve a helical or three-dimensional scan, evaluating image quality during the pull-back, discontinuing imaging when image quality falls below a given level, and taking steps to improve image quality, including repeating the above procedure. 
     According to another exemplary embodiment of the present invention, apparatus, method and software arrangement are provided for imaging a structure (e.g., a blood vessel) that is in contact with a first fluid. A volume of a second fluid is delivered by a fluid delivery arrangement configured to an external location with respect to, e.g., an article of manufacture (e.g., a housing). The structure can be imaged. e.g., using an imaging arrangement during or after the volume of the second fluid (e.g., a transparent fluid) is delivered to the external location. For example, the imaging arrangement or the article of manufacture can be translated along a path which approximately corresponds to an axis of extension of a surface while imaging the structure. The fluid delivery arrangement can be a pump or syringe that is operatively connected to the article of manufacture. a syringe operatively connected to the article of manufacture. 
     The article of manufacture can include an aperture formed in the article of manufacture. The fluid delivery arrangement may include a transparent fluid reservoir containing the second fluid, and a delivery conduit having a first end connected to the second fluid reservoir and a second end connected to the aperture of the article of manufacture. The aperture of the housing can be located at a distal end of the housing or adjacent to the imaging arrangement. The imaging arrangement may include a directing arrangement configured to direct light to a surface of the structure, at least one optical fiber operatively connected to the directing arrangement, and an image processing arrangement operatively connected to the at least one optical fiber. The directing arrangement can include optics at the distal end of the imaging arrangement, and/or a lens and a light directing element. Further, the directing element can be an optical arrangement which is configured to alter at least one direction of light, and the optical arrangement is capable of directing the light from a direction substantially parallel to the greater axis of the housing to a direction substantially perpendicular to the greater axis of the article of manufacture. The lens can focus the light approximately 0.5 mm to 5 mm beyond the article of manufacture. 
     A rotating arrangement can be provided that is operatively connected to the imaging arrangement, and configured to rotate the imaging arrangement. The rotating arrangement may rotate at a rate of at least above approximately 30 rotations/second and at most approximately 1000 rotations/second. The imaging arrangement may be rotated within the article of manufacture while imaging the structure. A pull-back arrangement can be provided that is operatively connected to the imaging arrangement, and configured to translate the imaging arrangement relative to the article of manufacture. The pull-back arrangement may translate the imaging arrangement at a rate of at least approximately 1 mm/second and at most approximately 100 mm/second, and/or at a rate of approximately 10 mm/second. 
     At least a portion of the article of manufacture can be transparent. The imaging modality may be time domain optical coherence tomography, a spectral domain optical coherence tomography or a optical frequency domain imaging. The second fluid can be substantially transparent to radiation utilized by the imaging modality. A guide catheter configured to receive the article of manufacture therein can be provided. The fluid delivery arrangement can deliver the fluid to a proximal end of the guide catheter, and the fluid may flow through an aperture formed through the guide catheter. 
     The imaging arrangement can obtain data associated with the structure, and a processing arrangement may provided to receive the data, and capable of controlling at least one of the fluid delivery arrangement and the imaging arrangement as a function of the data. The processing arrangement may control the fluid delivery arrangement or the imaging arrangement based on information previously received by the processing arrangement. The processing arrangement may also control the translation of the imaging arrangement, the fluid delivery of the fluid delivery arrangement, and/or the translation of the imaging arrangement and the fluid delivery of the fluid delivery arrangement. A catheter can be provided which includes the article of manufacture or the imaging arrangement. The fluid delivery arrangement may deliver the second fluid through an internal portion of the catheter. The imaging arrangement may include imaging optics which emit a beam to obtain the image, the beam being transmitted outside of the catheter. 
     These and other objects, features and advantages of the present invention will become apparent upon reading the following detailed description of embodiments of the invention, when taken in conjunction with the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention and its advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1A  shows images of the interior surface of a blood vessel gathered using OCT imaging at different settings; 
         FIG. 1B  illustrates an analysis of the time of angiographic lumen attenuation following a contrast injection at three separate locations in a given blood vessel; 
         FIGS. 2A-2B  show an exemplary embodiment of an imaging catheter for conducting scans of a segment of a blood vessel; 
         FIG. 3  shows an exemplary embodiment of a flow chart depicting a process for gathering information representative of a helical scan of a segment of a blood vessel using the imaging catheter of  FIGS. 2A-2B ; 
         FIG. 4  shows the imaging catheter of  FIG. 2A  after retraction of a rotateable inner shaft of the imaging catheter; 
         FIG. 5  illustrates an exemplary embodiment of an enlarged section of the imaging catheter of  FIG. 2A  as defined by the dashed box A; and 
         FIG. 6  shows the imaging catheter of  FIG. 2A , enclosed within a guide catheter, whereby a transparent solution is injected into the guide catheter enclosing the imaging catheter. 
     
    
    
     Throughout the drawings, the same reference numerals and characters, unless otherwise stated, are used to denote like features, elements, components, or portions of the illustrated embodiments. Moreover, while the present invention will now be described in detail with reference to the Figures, it is done so in connection with the illustrative embodiments. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 2A ,  2 B,  4 ,  5  and  6  illustrate various exemplary embodiments of an apparatus for obtaining an image of internal surfaces of a segment of an anatomic structure and  FIG. 3  shows an exemplary embodiment of a method to implant the same. Generally, the exemplary method and apparatus according to the present invention perform a helical scan of the internal surfaces of the segment of the anatomic structure after injecting a bolus of transparent or semi-transparent fluid, so as to obtain an image of the internal surfaces of the segment of the anatomic structure using an imaging modality. Such technique combines the efficacy of the imaging modality and the process of injecting a bolus of transparent or semi-transparent fluid with the beneficial effect of imaging an entire segment of the anatomic structure. The exemplary embodiments of the method and apparatus according to the present invention utilize a further paradigm for imaging that provide a significant increase in the image acquisition rate, while preserving a good image quality. According to one exemplary embodiment, the dramatic increase represents at least an approximately 10-fold increase in the image acquisition rate. With this exemplary technology, comprehensive coronary imaging can be achieved using conventional methods of transparent or semi-transparent fluid flushing in conjunction with automatic catheter pullback. In one exemplary embodiment of the present invention, this new paradigm utilizes Optical Frequency Domain Imaging (“OFDI”) as an imaging modality to obtain these images. In another exemplary embodiment, the anatomic structure can be a blood vessel. 
     It should be understood that alternate imaging modalities that detect single scattered light, such as time domain OCT and confocal microscopy, can also be used. 
     In a further exemplary embodiment of the present invention (as shown in  FIG. 2A ), the imaging module  224  utilizes OCT, visible light imaging, spectroscopy and/or thermoography. In another exemplary embodiment, the OCT imaging modality used is time-domain OCT (“TD-OCT”), spectral-domain OCT (“SD-OCT”), optical frequency domain imaging (“OFDI”), and/or low-coherence interferometry. In still another exemplary embodiment of the present invention, the visible light imaging modality used is intracoronary angioscopy, speckle imaging, fluorescence imaging, and/or multi-photon imaging. In yet another exemplary embodiment of the present invention, the spectroscopy modality uses visible light having a spectrum of approximately 0.3-0.7 μm, near infrared light (“NIR”) having a spectrum of approximately 0.7-2.2 μm, infrared light (“IR”) having a spectrum of approximately 2.2-12 μm, Raman scattered light and/or fluorescent light. In a further exemplary embodiment of the present invention, the imaging module  224  utilizes ultrasound, particularly high-frequency ultrasound having a frequency of at least approximately 20 MHz. In a still further exemplary embodiment of the present invention, the imaging assembly  204  includes a lens, mirror and/or prism. 
       FIGS. 2A and 2B  illustrate an exemplary embodiment of an imaging system  200  including a specially modified optical catheter  202  having a distal end  220  and a proximal end  222 . The imaging system  200  is capable of imaging long arterial segments by utilizing a rapid acquisition rate of imaging modalities and implementing automated pullback of the imaging catheter  202 . These efforts may allow a proximal portion of each main coronary artery (LAD, LCx, and RCA) to be comprehensively imaged with a specific longitudinal image spacing, while administering a safe total amount of transparent or semi-transparent fluid to the patient. 
     In one exemplary embodiment of the present invention, long arterial portion that is up to 10 cm in length can be examined. In another exemplary embodiment, the long arterial portion of up to 5.0 cm in length can be examined. In still another exemplary embodiment, the proximal portion of each main coronary artery is up to 10 cm in length which is capable of being examined. In still another exemplary embodiment of the present invention, the proximal portion of each main coronary artery is up to 5 cm in length which is capable of being examined. In still another exemplary embodiment of the present invention, the specific longitudinal imaging spacing is between approximately 100 μm and approximately 150 μm, preferably approximately 125 μm. In still another exemplary embodiment of the present invention, the specific longitudinal imaging spacing matches the transverse spot diameter, and is therefore between approximately 15 μm and approximately 35 μm, preferably approximately 25 μm. In yet another exemplary embodiment of the present invention, the safe total amount of transparent or semi-transparent fluid is at most 150 cc/artery, and preferably no more than 30 cc/artery. In another exemplary embodiment of the present invention, the transparent or semi-transparent fluid can be normal saline, ½ normal saline, ¼ normal saline, lactated ringers solution, phosphate buffered saline, blood substitute such as Oxyglobin, and/or coronary contrast media. In a further exemplary embodiment of the present invention, the imaging system  200  may image segments of any blood vessel including: carotid arteries, iliac arteries, femoral arteries, popliteal arteries, radial arteries, other peripheral arteries and veins. 
     Blood presents a challenge for any light-based intravascular imaging modality. As light propagates in blood, certain information is lost due to both scattering and absorption. At a wavelength of approximately 1.3 μm, the combined attenuation due to scattering and absorption can be minimized Even at this optimal wavelength, however, imaging vascular structure through blood may not be feasible. Particular preferences for imaging may include a high signal-to-noise ratio (“SNR”) and a high image quality requiring substantial amounts of detail. If OCT imaging is utilized as the imaging modality, a large number of A-lines are preferable in each OCT scan. Clear OCT imaging may be achieved for short durations, e.g., on the order of three (3) seconds, by temporarily displacing blood using a bolus injection of transparent or semi-transparent fluid through the catheter  200 . Therefore, at an imaging rate of four (4) frames per second, a single transparent or semi-transparent fluid purge may provide approximately 12 high-quality OCT images before blood reenters the field of view. 
     In a further exemplary embodiment of the present invention, the bolus injection of transparent or semi-transparent fluid can introduce approximately between 1 and 50 cc of transparent or semi-transparent fluid into the blood vessel. In another exemplary embodiment of the present invention, the bolus injection of transparent or semi-transparent fluid introduces approximately 10 cc of transparent or semi-transparent fluid into the blood vessel. 
     According to these embodiments, a modified optical catheter  200 , probe or other instrument may be inserted into a blood vessel (e.g., artery) to image the vessel. When plaque is located, the probe is moved into the proximity of the specific atherosclerotic plaque. Light reflected from the interior wall of the blood vessels and/or from a plaque is collected and transmitted to a detector  236  of an imaging module  224 . 
     In an exemplary embodiment of the present invention, pathologies other than plaque may be imaged, for example, thrombus, dissections, rupture, stents, and the like. 
     Referring to  FIG. 2A , the specially modified optical catheter  202  may include a rotatable inner shaft  210  and an outer sheath  208 . The rotatable inner shaft  210  houses a fiber array  218  and an imaging assembly  204  near the distal end  220  of the catheter  202 . The outer sheath  208  includes an aperture  221  formed therethrough. The aperture  221  is connected to a fluid delivery channel  223 , which is in turn connected to a fluid pump  225 . The fluid pump  225  can cause the injection of a specific volume of transparent or semi-transparent fluid through the fluid delivery channel  223  and out from the aperture  221 , thereby displacing the liquid surrounding the distal end  220  of the catheter  202  with the injected bolus. The fiber array  218  of the catheter  202  connects to a rotary junction  212 , which is in turn connected to a fixed optical fiber  214  that it extends from the catheter  202  proximally to the imaging module  224 . The rotary junction  212  is also connected to a pullback device  215 . The pullback device  215  translates the rotatable inner shaft  210  within the outer sheath  208  when instructed by a processor  240  during imaging, such that a helical scan can be generated.  FIG. 4  illustrates the catheter  202  after pullback of the rotatable inner shaft  210  has been completed, otherwise the system  200  of  FIG. 4  is identical to the system  200  of  FIG. 2A . The process  300  by which the imaging system  200  gathers data representative of a helical scan of a section of the blood vessel is illustrated in  FIG. 3 , and described in more detail herein. 
     In an exemplary embodiment of the present invention, the outer sheath  208  of the catheter  202  is not transparent. For example, during the pull-back the entire catheter  202 , the outer sheath  208  is translated through the blood vessel, while the internal shaft  210  rotates. In this exemplary embodiment, the internal shaft  210  is not translated relative to the outer sheath  208 . In another exemplary embodiment, the fiber array  218  includes a single fiber. In still another exemplary embodiment, the fiber array  218  includes a number of fibers. 
     The catheter  202  can be fabricated using an FDA approved 2.6-3.2F IVUS catheter. The inner core of the IVUS catheter is capable of rotating and obtaining cross-sectional images at, e.g., 40 frames per second. The ultrasound transducer and conductive wire, which are generally used in the IVUS catheter, may be removed and replaced with the imaging assembly  204 , the fiber array  218 , the inner shaft  210 , the fluid delivery channel  223  and an aperture is formed through the aperture of the outer sheath of the IVUS catheter. The newly provided inner shaft  210  of the IVUS catheter rotates to provide circumferential scanning and may be pulled back for screening a segment of a blood vessel. The transparent outer sheath  208 , which incorporates a monorail guide wire (not shown), does not rotate and is plugged at the distal end of the IVUS catheter using an FDA approved polymer. The catheter  202  is also attached to the rotary junction  212 . 
     In an exemplary embodiment, the catheter  202  includes a rotating optical fiber within a flexible inner cable. The flexible inner cable is contained within an outer transparent housing or sheath. The outer housing may include a monorail guide wire. The rotating optical fiber and the flexible inner cable each have a distal end and a proximal end. The rotating optical fiber and the flexible inner cable are oriented such that the distal end of the rotating optical fiber and the distal end of the flexible inner cable are adjacent to one another, and the proximal end of the rotating optical fiber and the proximal end of the flexible inner cable are adjacent to one another. Distal optics including a lens and a beam directing element are attached to the distal end of the flexible inner cable. An optical rotating junction is provided at the proximal end of the rotating optical fiber. The rotating optical fiber couples a static optical fiber to the rotating optical fiber within the flexible inner cable. The optical rotating junction rotates the rotating optical fiber, the flexible inner cable and the distal optics to provide circumferential optical sampling of the luminal surface of the vessel. The optical fiber, inner flexible cable, and distal optics rotate and an image is obtained for each catheter rotation. The inner optical cable is pulled back longitudinally within the outer transparent housing to form a helical scan of the vessel. 
     In an exemplary embodiment, the beam directing element is a prism that directs the beam substantially perpendicular to the catheter axis and the lens focuses the beam to approximately 2 mm from the outer sheath. In another exemplary embodiment, the rotation rate ranges from approximately 10 per second to approximately 100 per second and preferably approximately 30 per second. In still another embodiment, the entire catheter including the transparent outer sheath is pulled back within the lumen of the vessel. In a further preferred embodiment, the pull back rate ranges from approximately 1 mm/second to approximately 20 mm/second and preferably approximately 10 mm/second. 
     In an exemplary embodiment, the monorail guide wire is similar to the guide wire as described in U.S. Pat. No. 5,350,395, entitled “Angioplasty Apparatus Facilitating Rapid Exchanges,” to Paul G. Yock, issued Sep. 27, 1994, and incorporated herein in its entirety. 
     The entire shaft  210  of the catheter  202  can rotate 360 degrees, allowing the catheter  202  to gather images of the subject tissue  250  around the entire circumference of the catheter  202 . In one exemplary embodiment of the present invention, the catheter  202  can obtain images of a plaque around the circumference of an interior vessel wall. 
     In operation, a coherent light, such as laser light, is transmitted from a light source  232  via beam-splitter  234 , through the fixed optical fiber  214  and central fiber  226  and onto the imaging assembly  204 . The light is directed via the imaging assembly  204  to a subject tissue  250  (arrow  206 ). According to an exemplary embodiment of the present invention, the subject tissue  250  may be a layer of static tissue over a layer of moving tissue, such as an atherosclerotic plaque. The outer sheath  208  can be placed directly in contact with the sample  250  and/or can be positioned at a short distance (e.g., 1 mm to 10 cm) away from the sample. For example, light can enter sample  250 , where it is reflected by molecules, cellular debris, proteins, compounds (e.g., cholesterol crystals), and cellular microstructures (such as organelles, microtubules) within the subject tissue  250 . Light remitted from the subject tissue  250  (shown by arrows  228  in  FIG. 2B , the remainder of  FIG. 2B  is identical to  FIG. 2A ) is conveyed through the imaging assembly  204  to the single optical fiber or fibers of the fiber array  218 , and then transmitted by the optical fiber or fiber array  218  to the detection device  236 , via the beam-splitter  234 . In another embodiment, the device transmitting light to the catheter and receiving light from the catheter is an optical circulator. 
     In an exemplary embodiment of the present invention, the fiber array  218  may include one or multiple fibers for detection and illumination. In another exemplary embodiment of the present invention, the detection may occur using a single fiber. Alternatively, the illumination may occur via a fiber array, where each fiber is selectively illuminated to generate multiple focused spots as a function of position on the subject tissue  250 . This exemplary method can provide a scanning of the incident light across the sample, while maintaining the probe in a stationary position. The fibers may be illuminated and/or detected simultaneously or illuminating and/or detecting light from one fiber after another in series. 
     The data produced by the detection device  236  may then be digitized by an analog-digital converter  238 , and analyzed using imaging procedures executed by the processor  240 . The imaging procedures applicable with the exemplary embodiments of the present invention are described in U.S. Provisional Patent Appn. No. 60/514,769, entitled “Method and Apparatus for Performing Optical Imaging Using Frequency-Domain Interferometry,” filed Oct. 27, 2003, and International Patent Application No. PCT/US03/02349 filed on Jan. 24, 2003, the disclosures of which are incorporated by reference herein in their entireties. The processor  240  is also operatively connected to the pullback device  216 , the rotary junction  212  and the fluid pump  225 . 
     The diameter of the catheter can be less than 500 μm. Larger diameters may also be utilized within the scope of the present invention. 
     Other types of instruments can be used to gather image data. For example, the optics of the catheter  200  can be integrated into other types of instruments, such as endoscopes or laparoscopes. The optics can also form a stand-alone unit passed into the accessory port of standard endoscopes or laparoscopes, and/or integrated into another type of catheter, such as dual-purpose intravascular ultrasound catheter. 
     In an exemplary embodiment of the present invention, the detector  236  may be a charge coupled device (“CCD”), a photographic plate, an array of photodetectors, and/or a single detector. In another exemplary embodiment of the present invention, the light source  232  can illuminate the sample with continuous light, continuous broad bandwidth light, wavelength scanning light, or synchronized pulses. 
       FIG. 3  illustrates an exemplary embodiment of a method/process  300  for gathering data representative of a helical scan if a screening segment of the subject tissue  250  according to the present invention. The process  300  begins in step  301  where the catheter  200  is inserted and positioned within the screening segment of the subject tissue  250 . Once the catheter  200  is inserted and positioned properly, the imaging module  232  instructs the fluid pump or operator  225  to inject a bolus of transparent or semi-transparent fluid into the subject tissue  250 , in step  302 . Depending on the size of the subject tissue  250 , the imaging module  232  may alter volume and/or rate of injection of the bolus of transparent or semi-transparent fluid. In an exemplary embodiment, the subject tissue  250  is a blood vessel. 
     In step  304 , the imaging module  232  determines whether the image received from the imaging assembly  204  is of a sufficient quality to begin scanning the subject tissue  250 . If the imaging module  232  determines that the image is not of sufficient quality, the process  300  advances to step  302 . Otherwise, the process  300  advances to step  306 , where the rotary junction  212  begins rotating the rotateable inner shaft  210  and the pullback device  215  begins pulling back the shaft  210 . 
     The imaging module  232  determines whether the image received from the imaging assembly  204  is of sufficient quality to begin scanning by attempting to detect the presence of blood. The imaging module  232  makes this determination by measuring the amount of scattering the imaging modality is experiencing and/or by analyzing the spectroscopy registered by the imaging modality. If the imaging modality is OCT, other methods may be used. 
     When the imaging module  232  measures the amount of scattering experienced by the imaging modality, the imaging module  232  determines whether the light received from the subject tissue  250  is scattered. Saline and other transparent perfusion liquids do not contain an appreciable amount of scattering. Blood on the other hand, is highly scattering. Due to this effect, a method for determining the presence of blood may be to observe the intensity of the reflection of light back to the catheter. Preferentially, certain wavelengths of light may be used that have the property that the absorption penetration depth is small in both water and blood. 
     When the imaging module  232  is utilizing spectroscopy to determine whether blood is present, the imaging module  232  can measure the differential absorption experienced by the imaging modality. Blood adjacent to the subject tissue  250  can be detected by utilizing differential absorption of blood. In blood which is oxygenated, there are several absorption peaks in the visible spectrum, e.g., at 520-590 nm and 800-900 nm. A simple device may obtain the light scattered back from the catheter at these wavelengths, and compare such light to the light scattered back from an adjacent wavelength where blood absorption is low. This comparison can be accomplished by a linear combination of the intensity of light reflected back by the two wavelengths. For example if R(λ 1 ) is the light reflected back to the catheter on the absorption peak and R(λ 2 ) is the light reflected back to the tissue off of the absorption peak, blood can be estimated by several differential/ratiometric combinations of R(λ 1 ) and R(λ 2 ): 
     
       
         
           
             
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     The device for delivery and detection can be a simple side firing single or multi-mode optical fiber. 
     If the imaging modality is OCT, another method for detecting blood can be used. Since OCT is capable of obtaining a cross-sectional image of the lumen, it is potentially more sensitive to the presence of small amounts of blood than is diffuse spectroscopy. The OCT signal from blood is fairly characteristic, and not commonly observed in other tissues. For example, blood can exhibit a rapid attenuation and a homogeneous appearance. As a result, in one exemplary embodiment, it is possible to process the OCT signal to determine if blood is present. A wide variety of image processing techniques known in the art, such as texture discrimination, pattern recognition, etc. can be used to identify blood. In one embodiment, in order to determine whether blood is present, two parameters are determined: (a) the slope of the logarithm of the OCT axial scan data (attenuation); and (b) the standard deviation of the logarithm of the OCT axial scan data (signal variance). These two parameters can differentiate most human tissue types. Other measures of attenuation and signal variance known in the art can also be utilized to differentiate blood from arterial wall tissue. Other measurements including probability distribution function statistics, Fourier domain analysis, high pass filtering, energy and entropy measurements, edge counting, and N-order moments can be utilized to determine the presence of blood in the lumen of the blood vessel. OCT can be combined with spectroscopy (e.g., performing OCT at two wavelengths), birefringence, and Doppler to further enhance the capability of OCT for identifying blood in the lumen. In one exemplary embodiment of the present invention, the fluid pump  225  continues injecting the bolus of transparent or semi-transparent fluid during the step  304 . 
     Turning back to the process  300  of  FIG. 3 , in step  308 , the imaging module  232  determines whether the image received from the imaging assembly  204  is of sufficient quality to continue scanning the subject tissue  250 . If the imaging module  232  determines that the image is of sufficient quality, the process  300  advances to step  306 . Otherwise, the process  300  advances to step  310 . At step  310 , the rotary junction  212  stops rotating the rotateable inner shaft  210 , and the pullback device  215  halts the pullback of the shaft  210 . If the fluid pump  225  is continuing to inject the bolus of transparent or semi-transparent fluid, the fluid pump  225  is instructed to discontinue injecting the transparent or semi-transparent fluid. If there is catheter motion, the catheter may be advanced or retracted substantially along the longitudinal axis of the vessel prior to the next bolus injection in order to ensure that the subsequent pullback imaging process does not skip over any areas of tissue. 
     This process represents a feedback control loop where a measure of image quality is utilized to control the process/conditions under which images are obtained. If the image is not of sufficient quality, action is taken to improve the image quality before additional images are taken. In an exemplary embodiment, when image quality drops below a predetermined measure but image quality is still sufficient to continue imaging, additional transparent or semi-transparent fluid is injected via the fluid pump  225  improving image quality. In another exemplary embodiment, when image quality drops below a predetermined measure and image quality is insufficient to continue imaging, imaging is halted. The feedback control loop may be set up many different ways in order to automate at least a part of the process  300 . A wide variety of image processing techniques known in the art, such as texture discrimination, pattern recognition, etc. can be used to determine whether or not image quality of the vessel wall is sufficient to continue imaging. 
     In an exemplary embodiment, two parameters are determined: (a) the slope of the logarithm of the OCT axial scan data (attenuation); and (b) the standard deviation of the logarithm of the OCT axial scan data (signal variance). These two parameters can differentiate most human tissue types. Other measures of attenuation and signal variance known in the art can also be utilized to identify and characterize the quality of images obtained from arterial wall tissue. These other measurements including image segmentation and blob quantification, morphologic processing, probability distribution function statistics, Fourier domain analysis, high pass filtering, energy and entropy measurements, edge counting, N-order moments. OCT can be combined with spectroscopy (e.g., performing OCT at two wavelengths), birefringence, and Doppler to further enhance the capability of OCT for assessing image quality. 
     In step  312 , the imaging module  232  determines whether the entire length of the screening segment of the subject tissue  250  has been imaged. If additional portions of the screening segment of the subject tissue  250  need to be imaged, the process  300  advances to step  302 . Otherwise, the process  300  advances to step  314  where the imaging module  232  reconstructs the helical or three-dimensional scans of the screening segment of the subject tissue  250  based on the information gathered during the scan. After the helical scans are reconstructed, the imaging module  316  can display the reconstructed data (images) and the process  300  exits. 
       FIG. 6  illustrates an imaging system  600  including the specially modified optical catheter  202  disposed within a guide catheter  602 . The optical catheter  202  is identical to the optical catheter  202  as illustrated in  FIG. 2A , except that the fluid delivery channel  223  and the aperture  221  are not necessarily included in the optical catheter  202  as illustrated in  FIG. 6 . The fluid delivery channel  223  and the aperture  221  are replaced by the fluid delivery channel  606  and the aperture  604 , respectively. In order to use the imaging system  600 , the guide catheter  602  is inserted into a blood vessel and positioned adjacent to a target area to be imaged. 
     The optical catheter  202  is inserted into the guide catheter  602  until the distal end  220  of the optical catheter  202  protrudes beyond the guide catheter  602  (as shown in  FIG. 6 ). Once the guide catheter  602  is positioned relative to the guide catheter  602 , the optical catheter  202  operates in the same manner as discussed above in connection with  FIGS. 2A ,  2 B,  3 ,  4 , and  5 , with the exception that the fluid pump  225  injects transparent or semi-transparent fluid into the fluid delivery channel  606  and through the aperture  604  to the target area instead of utilizing the fluid delivery channel  223  and aperture  221 . 
     In an exemplary embodiment, the guide catheter  602  is used as the fluid delivery channel. The guide catheter  602  does not necessarily include a special purpose fluid delivery channel  606 . The fluid pump  225  is connected directly to the guide catheter  602  and transparent or semi-transparent fluid is provided at the target area via the guide catheter  602 . In another exemplary embodiment, the imaging assembly  204  protrudes beyond the distal end of the guide catheter  602 . The imaging of the target area takes place while the imaging assembly  204  of the optical catheter  202  protrudes from the distal end of the guide catheter  602 . In a further exemplary embodiment, the guide catheter  602  is transparent and the optical catheter  202  is inserted into the guide catheter  602  until it is adjacent to the target area. The imaging assembly  204  of the optical catheter  202  does not protrude beyond the guide catheter  602  and the imaging of the target area takes place while the optical catheter  202  is within the guide catheter  602 . 
     The foregoing merely illustrates the principles of the invention. Various modifications and alterations to the described embodiments will be apparent to those skilled in the art in view of the teachings herein. It will thus be appreciated that those skilled in the art will be able to devise numerous techniques which, although not explicitly described herein, embody the principles of the invention and are thus within the spirit and scope of the invention.