Abstract:
A method of inhibiting a flow of a purging fluid from a tissue visualization device comprises positioning an imaging hood attached to a distal end of a flexible elongate shaft in proximity to a tissue region to be visualized The method also comprises introducing the purging fluid into an inflation member suspended within an open area of the imaging hood. The inflation member includes at least one aperture on a proximal portion of the inflation member. The method also comprises inflating the inflation member to a partially inflated state in which the inflation member is circumferentially separated from an inner surface of the hood to form an annular flow path for the purging fluid from the at least one aperture, between the inner surface of the hood and the inflation member, and into fluid communication with an environment external to the hood. The method also comprising inflating the inflation member to a blocking inflated state in which the inflation member expands into contact with the imaging hood to block the flow from the at least one aperture to the annular flow path.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Continuation of U.S. patent application Ser. No. 12/499,681, filed Jul. 8, 2009 which claims the benefit of priority to U.S. Prov. Pat. App. 61/079,414 filed Jul. 9, 2008, which is incorporated herein by reference in its entirety. U.S. patent application Ser. No. 12/499,681 is also a continuation-in-part of U.S. patent Application Ser. No. 11/259,498, filed Oct. 25, 2005 (now U.S. Pat. No. 7,860,555), which is incorporated by reference herein in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to medical devices used for accessing, visualizing, and/or treating regions of tissue within a body. More particularly, the present invention relates to methods and apparatus for efficiently purging opaque fluids from an intravascular visualization system to facilitate visualization and/or treatment of the tissue. 
       BACKGROUND OF THE INVENTION 
       [0003]    Conventional devices for accessing and visualizing interior regions of a body lumen are known. For example, various catheter devices are typically advanced within a patient&#39;s body, e.g., intravascularly, and advanced into a desirable position within the body. Other conventional methods have utilized catheters or probes having position sensors deployed within the body lumen, such as the interior of a cardiac chamber. These types of positional sensors are typically used to determine the movement of a cardiac tissue surface or the electrical activity within the cardiac tissue. When a sufficient number of points have been sampled by the sensors, a “map” of the cardiac tissue may be generated. 
         [0004]    Another conventional device utilizes an inflatable balloon which is typically introduced intravascularly in a deflated state and then inflated against the tissue region to be examined. Imaging is typically accomplished by an optical fiber or other apparatus such as electronic chips for viewing the tissue through the membrane(s) of the inflated balloon. Moreover, the balloon must generally be inflated for imaging. Other conventional balloons utilize a cavity or depression formed at a distal end of the inflated balloon. This cavity or depression is pressed against the tissue to be examined and is flushed with a clear fluid to provide a clear pathway through the blood. 
         [0005]    However, many of the conventional catheter imaging systems lack the capability to provide therapeutic treatments or are difficult to manipulate in providing effective therapies. For instance, the treatment in a patient&#39;s heart for atrial fibrillation is generally made difficult by a number of factors, such as visualization of the target tissue, access to the target tissue, and instrument articulation and management, amongst others. 
         [0006]    Conventional catheter techniques and devices, for example such as those described in U.S. Pat. Nos. 5,895,417; 5,941,845; and 6,129,724, used on the epicardial surface of the heart may be difficult in assuring a transmural lesion or complete blockage of electrical signals. In addition, current devices may have difficulty dealing with varying thickness of tissue through which a transmural lesion is desired. 
         [0007]    Conventional accompanying imaging devices, such as fluoroscopy, are unable to detect perpendicular electrode orientation, catheter movement during the cardiac cycle, and image catheter position throughout lesion formation. The absence of real-time visualization also poses the risk of incorrect placement and ablation of structures such as sinus node tissue which can lead to fatal consequences. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    A tissue imaging and manipulation apparatus that may be utilized for procedures within a body lumen, such as the heart, in which visualization of the surrounding tissue is made difficult, if not impossible, by medium contained within the lumen such as blood, is described below. Generally, such a tissue imaging and manipulation apparatus comprises an optional delivery catheter or sheath through which a deployment catheter and imaging hood may be advanced for placement against or adjacent to the tissue to be imaged. 
         [0009]    The deployment catheter may define a fluid delivery lumen therethrough as well as an imaging lumen within which an optical imaging fiber or assembly may be disposed for imaging tissue. When deployed, the imaging hood may be expanded into any number of shapes, e.g., cylindrical, conical as shown, semi-spherical, etc., provided that an open area or field is defined by the imaging hood. The open area is the area within which the tissue region of interest may be imaged. The imaging hood may also define an atraumatic contact lip or edge for placement or abutment against the tissue region of interest. Moreover, the distal end of the deployment catheter or separate manipulatable catheters may be articulated through various controlling mechanisms such as push-pull wires manually or via computer control 
         [0010]    The deployment catheter may also be stabilized relative to the tissue surface through various methods. For instance, inflatable stabilizing balloons positioned along a length of the catheter may be utilized, or tissue engagement anchors may be passed through or along the deployment catheter for temporary engagement of the underlying tissue. 
         [0011]    In operation, after the imaging hood has been deployed, fluid may be pumped at a positive pressure through the fluid delivery lumen until the fluid fills the open area completely and displaces any blood from within the open area. The fluid may comprise any biocompatible fluid, e.g., saline, water, plasma, Fluorinert™, etc., which is sufficiently transparent to allow for relatively undistorted visualization through the fluid. The fluid may be pumped continuously or intermittently to allow for image capture by an optional processor which may be in communication with the assembly. 
         [0012]    In an exemplary variation for imaging tissue surfaces within a heart chamber containing blood, the tissue imaging and treatment system may generally comprise a catheter body having a lumen defined therethrough, a visualization element disposed adjacent the catheter body, the visualization element having a field of view, a transparent fluid source in fluid communication with the lumen, and a barrier or membrane extendable from the catheter body to localize, between the visualization element and the field of view, displacement of blood by transparent fluid that flows from the lumen, and an instrument translatable through the displaced blood for performing any number of treatments upon the tissue surface within the field of view. The imaging hood may be formed into any number of configurations and the imaging assembly may also be utilized with any number of therapeutic tools which may be deployed through the deployment catheter. 
         [0013]    More particularly in certain variations, the tissue visualization system may comprise components including the imaging hood, where the hood may further include a membrane having a main aperture and additional optional openings disposed over the distal end of the hood. An introducer sheath or the deployment catheter upon which the imaging hood is disposed may further comprise a steerable segment made of multiple adjacent links which are pivotably connected to one another and which may be articulated within a single plane or multiple planes. The deployment catheter itself may be comprised of a multiple lumen extrusion, such as a four-lumen catheter extrusion, which is reinforced with braided stainless steel fibers to provide structural support. The proximal end of the catheter may be coupled to a handle for manipulation and articulation of the system. 
         [0014]    To provide visualization, an imaging element such as a fiberscope or electronic imager such as a solid state camera, e.g., CCD or CMOS, may be mounted, e.g., on a shape memory wire, and positioned within or along the hood interior. A fluid reservoir and/or pump (e.g., syringe, pressurized intravenous bag, etc.) may be fluidly coupled to the proximal end of the catheter to hold the translucent fluid such as saline or contrast medium as well as for providing the pressure to inject the fluid into the imaging hood. 
         [0015]    In clearing the hood of blood and/or other bodily fluids, it is generally desirable to purge the hood in an efficient manner by minimizing the amount of clearing fluid, such as saline, introduced into the hood and thus into the body. As excessive saline delivered into the blood stream of patients with poor ventricular function may increase the risk of heart failure and pulmonary edema, minimizing or controlling the amount of saline discharged during various therapies, such as atrial fibrillation ablation, atrial flutter ablation, transseptal puncture, etc. may be generally desirable. 
         [0016]    One variation of an imaging hood may incorporate an internal diaphragm, which may be transparent, attached to the inner wall of the hood about its circumference. The diaphragm may be fabricated from a transparent elastomeric membrane similar to the material of the hood (such as polyurethane, Chronoflex™, latex, etc) and may define one or more apertures through which saline fluid introduced into the hood may pass through the diaphragm and out through the main aperture to clear blood from the open field within the hood. The one or more apertures may have a diameter of between, e.g., 1 mm to 0.25 mm. 
         [0017]    Flow of the saline fluid out of the hood through the main aperture may continue under relatively low fluid pressure conditions as saline is introduced from the catheter shaft, through the diaphragm apertures, and out of the main aperture. Upon the application of a relatively higher fluid pressure, the diaphragm may be pushed distally within the hood until it extends or bulges distally to block the main aperture until fluid flow out of the hood is reduced or completely stopped. With the aperture blocked, the hood may retain the purging fluid within to facilitate visualization through the fluid of the underlying tissue. Thus, the hood may be panned around a target tissue region with sustained visualization to reduce the amount of saline that is introduced into a patient&#39;s heart or bloodstream. Once the fluid pressure of the purging fluid is reduced, the diaphragm may retract to unblock the aperture and thus allow for the flow of the purging fluid again through the diaphragm apertures and main aperture. Alternatively, one or more unidirectional valves may be positioned over the diaphragm to control the flow of the purging fluid through the hood and out the main aperture. Other variations may incorporate an internal inflation member or pouch which may be positioned within the hood and which controls the outflow of the purged saline based on the fluid pressure within the pouch. 
         [0018]    In yet other variations, one or more portions of the hood support struts may extend at least partially within the distal chamber such that the saline within the distal chamber can be electrically charged, such as with RF energy, when the support struts are coupled to an RF generator. This allows the saline encapsulated in the distal chamber to function as a virtual electrode by conducting the discharged energy to the underlying visualized tissue for treatments, such as tissue ablation. 
         [0019]    Yet another variation may incorporate an electrode, such as ring-shaped electrode, within the hood which defines a central lumen therethrough. The central lumen may define one or more fluid apertures proximally of the electrode which open to the hood interior in a circumferential pattern around an outer surface of the lumen. With the positioning of, e.g., a fiberscope, within the lumen and its distal end positioned adjacent to or distal to the electrode, the distal opening of the lumen may be obstructed by the fiberscope such that the purging fluid introduced through the lumen flows in an annular space between the fiberscope and the lumen and is forced to flow sideways into the hood through the one or more fluid apertures while the distal opening of the lumen remains obstructed by the fiberscope. 
         [0020]    Another variation of the hood may incorporate one or more protrusions or projections extending from a distal membrane over the hood. These protrusions or projections may extend distally adjacent to a corresponding unidirectional valve which has overlapping leaflets. As the hood is filled with the purging fluid, flow through the valves is inhibited or prevented by the overlapping leaflets but as the distal face of the hood membrane is pressed against a surface of tissue to be visualized and/or treated, the protrusions or projections pressing against the tissue surface may force the valve leaflets to separate temporarily, thus allowing the passage of saline out through the valves to clear any blood within the hood as well as any blood between membrane and the tissue surface. 
         [0021]    In yet another variation, an imaging hood may be configured to form a recirculating flow inside the hood. The purging fluid may be introduced (e.g., injected) as well as withdrawn from the imaging hood interior through two different lumens in the catheter shaft. For instance, the fluid may be introduced by an inlet lumen which injects the fluid along a first path into the hood while the recirculating fluid may be withdrawn by suction through a separate outlet lumen. By keeping a relatively higher volume flow rate in the inlet lumen for injecting the purging fluid than the flow rate in the outlet lumen for withdrawing it, a considerable amount of purging fluid may be conserved resulting in efficient hood purging. Another variation may incorporate a suction lumen, e.g., a pre-bent lumen, extending from the catheter directly to the main aperture. This particular variation may allow for the direct evacuation of blood through a lumen opening at a particular location along the main aperture where the in-flow of blood (or other opaque fluids) is particularly high. 
         [0022]    Yet another variation may utilize a hood partitioned into multiple chambers which are in fluid communication with individual corresponding fluid lumens defined through the catheter. Each of the chambers may be separated by corresponding transparent barriers which extend along the length of hood. Each of the different chambers may have a corresponding aperture. Efficient purging and reduction of saline discharged may achieved when purging can be selectively stopped once a particular chamber establishes optical clarity. This can be done manually by the operator or through automation by a processor incorporated within the system. 
         [0023]    Another variation may incorporate an expandable distal membrane which projects distally from the hood and is sufficiently soft to conform against the underlying contacted tissue. With the membrane defining one or more hood apertures, the purging fluid may enter within the hood and exit through the hood apertures into an intermediate chamber. The purging fluid may exit the intermediate chamber through at least one central aperture. In the event that the hood contacts against a surface of tissue at a non-perpendicular angle, the distal membrane may still conform to the tissue surface. 
         [0024]    In yet another variation, an imaging hood may have a distal membrane without an aperture and which may be filled with the purging fluid once desirably positioned within the subject&#39;s body in proximity to the tissue region to be visualized and/or treated. Once a tissue region to be treated has been located by the hood, a piercing instrument may be advanced through the hood from the catheter to puncture through the distal membrane at a desired site. This may form a puncture aperture through which the purging fluid may escape. Hence, purging is only performed at locations where instruments are passed out of the imaging hood thus reducing the amount of saline discharged out of the hood. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1A  shows a side view of one variation of a tissue imaging apparatus during deployment from a sheath or delivery catheter. 
           [0026]      FIG. 1B  shows the deployed tissue imaging apparatus of Fig. lA having an optionally expandable hood or sheath attached to an imaging and/or diagnostic catheter. 
           [0027]      FIG. 1C  shows an end view of a deployed imaging apparatus. 
           [0028]      FIGS. 2A and 2B  show one example of a deployed tissue imager positioned against or adjacent to the tissue to be imaged and a flow of fluid, such as saline, displacing blood from within the expandable hood. 
           [0029]      FIGS. 3A and 3B  show examples of various visualization imagers which may be utilized within or along the imaging hood. 
           [0030]      FIGS. 4A and 4B  show perspective and end views, respectively, of an imaging hood having at least one layer of a transparent elastomeric membrane over the distal opening of the hood. 
           [0031]      FIGS. 5A and 5B  show perspective and end views, respectively, of an imaging hood which includes a membrane with an aperture defined therethrough and a plurality of additional openings defined over the membrane surrounding the aperture. 
           [0032]      FIGS. 6A and 6B  show side views of one variation of the imaging hood having an internal diaphragm in which the flow of the purging fluid through the hood can be controlled or stopped by varying its fluid pressure. 
           [0033]      FIGS. 7A and 7B  show side views of another variation of the hood having an inflatable membrane positioned within the hood which may be used to control or stop the purging fluid. 
           [0034]      FIG. 8  shows a side view of another variation having an internal membrane which comprises one or more unidirectional valves to control the flow of the purging fluid. 
           [0035]      FIGS. 9A and 9B  show side views of the one or more unidirectional valves in an opened and closed configuration, respectively, for controlling the fluid flow therethrough. 
           [0036]      FIGS. 10A and 10B  show perspective and cross-sectional perspective views, respectively, of yet another variation where one or more side ports may be defined within the hood for uniformly purging blood from the hood interior. 
           [0037]      FIG. 11  shows a side view of another variation of the hood having a distal membrane which defines one or more projections which extend distally to actuate the opening of one or more corresponding valves. 
           [0038]      FIGS. 12A and 12B  show detail side views of the one or more projections which actuate the opening of a corresponding valve when contacted against a tissue surface. 
           [0039]      FIG. 13  shows a side view of another variation which incorporates a recirculating flow within the hood. 
           [0040]      FIG. 14  shows a side view of another variation which incorporates a suction lumen for selective evacuation at or near the main aperture. 
           [0041]      FIG. 15  shows a side view of another variation which comprises multiple chambers each defining an aperture to form a uniform flow through the hood. 
           [0042]      FIG. 16  illustrates a flow chart of one example for automating the selective purging of individual chambers to reduce saline discharge from the hood. 
           [0043]      FIGS. 17A and 17B  show side views of another variation which incorporates a distal chamber to facilitate the efficient purging of blood from the hood when contact against a tissue surface is at an angle. 
           [0044]      FIG. 18  shows a side view of another variation which comprises a piercing instrument, such as a needle, for forming an aperture through which the purging fluid may escape. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0045]    A tissue-imaging and manipulation apparatus described herein is able to provide real-time images in vivo of tissue regions within a body lumen such as a heart, which is filled with blood flowing dynamically therethrough and is also able to provide intravascular tools and instruments for performing various procedures upon the imaged tissue regions. Such an apparatus may be utilized for many procedures, e.g., facilitating transseptal access to the left atrium, cannulating the coronary sinus, diagnosis of valve regurgitation/stenosis, valvuloplasty, atrial appendage closure, arrhythmogenic focus ablation, among other procedures. 
         [0046]    One variation of a tissue access and imaging apparatus is shown in the detail perspective views of  FIGS. 1A to 1C . As shown in  FIG. 1A , tissue imaging and manipulation assembly  10  may be delivered intravascularly through the patient&#39;s body in a low-profile configuration via a delivery catheter or sheath  14 . In the case of treating tissue, it is generally desirable to enter or access the left atrium while minimizing trauma to the patient. To non-operatively effect such access, one conventional approach involves puncturing the intra-atrial septum from the right atrial chamber to the left atrial chamber in a procedure commonly called a transseptal procedure or septostomy. For procedures such as percutaneous valve repair and replacement, transseptal access to the left atrial chamber of the heart may allow for larger devices to be introduced into the venous system than can generally be introduced percutaneously into the arterial system. 
         [0047]    When the imaging and manipulation assembly  10  is ready to be utilized for imaging tissue, imaging hood  12  may be advanced relative to catheter  14  and deployed from a distal opening of catheter  14 , as shown by the arrow. Upon deployment, imaging hood  12  may be unconstrained to expand or open into a deployed imaging configuration, as shown in  FIG. 1B . Imaging hood  12  may be fabricated from a variety of pliable or conformable biocompatible material including but not limited to, e.g., polymeric, plastic, or woven materials. One example of a woven material is Kevlar® (E. I. du Pont de Nemours, Wilmington, Del.), which is an aramid and which can be made into thin, e.g., less than 0.001 in., materials which maintain enough integrity for such applications described herein. Moreover, the imaging hood  12  may be fabricated from a translucent or opaque material and in a variety of different colors to optimize or attenuate any reflected lighting from surrounding fluids or structures, i.e., anatomical or mechanical structures or instruments. In either case, imaging hood  12  may be fabricated into a uniform structure or a scaffold-supported structure, in which case a scaffold made of a shape memory alloy, such as Nitinol, or a spring steel, or plastic, etc., may be fabricated and covered with the polymeric, plastic, or woven material. Hence, imaging hood  12  may comprise any of a wide variety of barriers or membrane structures, as may generally be used to localize displacement of blood or the like from a selected volume of a body lumen or heart chamber. In exemplary embodiments, a volume within an inner surface  13  of imaging hood  12  will be significantly less than a volume of the hood  12  between inner surface  13  and outer surface  11 . 
         [0048]    Imaging hood  12  may be attached at interface  24  to a deployment catheter  16  which may be translated independently of deployment catheter or sheath  14 . Attachment of interface  24  may be accomplished through any number of conventional methods. Deployment catheter  16  may define a fluid delivery lumen  18  as well as an imaging lumen  20  within which an optical imaging fiber or assembly may be disposed for imaging tissue. When deployed, imaging hood  12  may expand into any number of shapes, e.g., cylindrical, conical as shown, semi-spherical, etc., provided that an open area or field  26  is defined by imaging hood  12 . The open area  26  is the area within which the tissue region of interest may be imaged. Imaging hood  12  may also define an atraumatic contact lip or edge  22  for placement or abutment against the tissue region of interest. Moreover, the diameter of imaging hood  12  at its maximum fully deployed diameter, e.g., at contact lip or edge  22 , is typically greater relative to a diameter of the deployment catheter  16  (although a diameter of contact lip or edge  22  may be made to have a smaller or equal diameter of deployment catheter  16 ). For instance, the contact edge diameter may range anywhere from 1 to 5 times (or even greater, as practicable) a diameter of deployment catheter  16 .  FIG. 1C  shows an end view of the imaging hood  12  in its deployed configuration. Also shown are the contact lip or edge  22  and fluid delivery lumen  18  and imaging lumen  20 . 
         [0049]    As seen in the example of  FIGS. 2A and 2B , deployment catheter  16  may be manipulated to position deployed imaging hood  12  against or near the underlying tissue region of interest to be imaged, in this example a portion of annulus A of mitral valve MV within the left atrial chamber. As the surrounding blood  30  flows around imaging hood  12  and within open area  26  defined within imaging hood  12 , as seen in  FIG. 2A , the underlying annulus A is obstructed by the opaque blood  30  and is difficult to view through the imaging lumen  20 . The translucent fluid  28 , such as saline, may then be pumped through fluid delivery lumen  18 , intermittently or continuously, until the blood  30  is at least partially, and preferably completely, displaced from within open area  26  by fluid  28 , as shown in  FIG. 2B . 
         [0050]    Although contact edge  22  need not directly contact the underlying tissue, it is at least preferably brought into close proximity to the tissue such that the flow of clear fluid  28  from open area  26  may be maintained to inhibit significant backflow of blood  30  back into open area  26 . Contact edge  22  may also be made of a soft elastomeric material such as certain soft grades of silicone or polyurethane, as typically known, to help contact edge  22  conform to an uneven or rough underlying anatomical tissue surface. Once the blood  30  has been displaced from imaging hood  12 , an image may then be viewed of the underlying tissue through the clear fluid  30 . This image may then be recorded or available for real-time viewing for performing a therapeutic procedure. The positive flow of fluid  28  may be maintained continuously to provide for clear viewing of the underlying tissue. Alternatively, the fluid  28  may be pumped temporarily or sporadically only until a clear view of the tissue is available to be imaged and recorded, at which point the fluid flow  28  may cease and blood  30  may be allowed to seep or flow back into imaging hood  12 . This process may be repeated a number of times at the same tissue region or at multiple tissue regions. 
         [0051]      FIG. 3A  shows a partial cross-sectional view of an example where one or more optical fiber bundles  32  may be positioned within the catheter and within imaging hood  12  to provide direct in-line imaging of the open area within hood  12 .  FIG. 3B  shows another example where an imaging element  34  (e.g., CCD or CMOS electronic imager) may be placed along an interior surface of imaging hood  12  to provide imaging of the open area such that the imaging element  34  is off-axis relative to a longitudinal axis of the hood  12 , as described in further detail below. The off-axis position of element  34  may provide for direct visualization and uninhibited access by instruments from the catheter to the underlying tissue during treatment. 
         [0052]    In utilizing the imaging hood  12  in any one of the procedures described herein, the hood  12  may have an open field which is uncovered and clear to provide direct tissue contact between the hood interior and the underlying tissue to effect any number of treatments upon the tissue, as described above. Yet in additional variations, imaging hood  12  may utilize other configurations. An additional variation of the imaging hood  12  is shown in the perspective and end views, respectively, of  FIGS. 4A and 4B , where imaging hood  12  includes at least one layer of a transparent elastomeric membrane  40  over the distal opening of hood  12 . An aperture  42  having a diameter which is less than a diameter of the outer lip of imaging hood  12  may be defined over the center of membrane  40  where a longitudinal axis of the hood intersects the membrane such that the interior of hood  12  remains open and in fluid communication with the environment external to hood  12 . Furthermore, aperture  42  may be sized, e.g., between 1 to 2 mm or more in diameter and membrane  40  can be made from any number of transparent elastomers such as silicone, polyurethane, latex, etc. such that contacted tissue may also be visualized through membrane  40  as well as through aperture  42 . 
         [0053]    Aperture  42  may function generally as a restricting passageway to reduce the rate of fluid out-flow from the hood  12  when the interior of the hood  12  is infused with the clear fluid through which underlying tissue regions may be visualized. Aside from restricting out-flow of clear fluid from within hood  12 , aperture  42  may also restrict external surrounding fluids from entering hood  12  too rapidly. The reduction in the rate of fluid out-flow from the hood and blood in-flow into the hood may improve visualization conditions as hood  12  may be more readily filled with transparent fluid rather than being filled by opaque blood which may obstruct direct visualization by the visualization instruments. 
         [0054]    Moreover, aperture  42  may be aligned with catheter  16  such that any instruments (e.g., piercing instruments, guidewires, tissue engagers, etc.) that are advanced into the hood interior may directly access the underlying tissue uninhibited or unrestricted for treatment through aperture  42 . In other variations wherein aperture  42  may not be aligned with catheter  16 , instruments passed through catheter  16  may still access the underlying tissue by simply piercing through membrane  40 . 
         [0055]    In an additional variation,  FIGS. 5A and 5B  show perspective and end views, respectively, of imaging hood  12  which includes membrane  40  with aperture  42  defined therethrough, as described above. This variation includes a plurality of additional openings  44  defined over membrane  40  surrounding aperture  42 . Additional openings  44  may be uniformly sized, e.g., each less than 1 mm in diameter, to allow for the out-flow of the translucent fluid therethrough when in contact against the tissue surface. Moreover, although openings  44  are illustrated as uniform in size, the openings may be varied in size and their placement may also be non-uniform or random over membrane  40  rather than uniformly positioned about aperture  42  in  FIG. 5B . Furthermore, there are eight openings  44  shown in the figures although fewer than eight or more than eight openings  44  may also be utilized over membrane  40 . 
         [0056]    Additional details of tissue imaging and manipulation systems and methods which may be utilized with apparatus and methods described herein are further described, for example, in U.S. patent application Ser. No. 11/259,498 filed Oct. 25, 2005 (U.S. Pat. Pub. 2006/0184048 A1), which is incorporated herein by reference in its entirety. 
         [0057]    In utilizing the devices and methods above, various procedures may be accomplished. One example of such a procedure is crossing a tissue region such as in a transseptal procedure where a septal wall is pierced and traversed, e.g., crossing from a right atrial chamber to a left atrial chamber in a heart of a subject. Generally, in piercing and traversing a septal wall, the visualization and treatment devices described herein may be utilized for visualizing the tissue region to be pierced as well as monitoring the piercing and access through the tissue. Details of transseptal visualization catheters and methods for transseptal access which may be utilized with the apparatus and methods described herein are described in U.S. patent application Ser. No. 11/763,399 filed Jun. 14, 2007 (U.S. Pat. Pub. 2007/0293724 A1), which is incorporated herein by reference in its entirety. Additionally, details of tissue visualization and manipulation catheter which may be utilized with apparatus and methods described herein are described in U.S. patent application Ser. No. 11/259,498 filed Oct. 25, 2005 (U.S. Pat. Pub. 2006/0184048 A1), which is incorporated herein by reference in its entirety. 
         [0058]    In clearing the hood of blood and/or other bodily fluids, it is generally desirable to purge the hood in an efficient manner by minimizing the amount of clearing fluid, such as saline, introduced into the hood and thus into the body. As excessive saline delivered into the blood stream of patients with poor ventricular function may increase the risk of heart failure and pulmonary edema, minimizing or controlling the amount of saline discharged during various therapies, such as atrial fibrillation ablation, atrial flutter ablation, transseptal puncture, etc. may be generally desirable. 
         [0059]      FIGS. 6A and 6B  show side views of one variation of an imaging hood incorporating an internal diaphragm  50 , which may be transparent, attached to the inner wall of the hood  12  about its circumference  54  such that diaphragm  50  is suspended circumferentially within the hood  12 . The diaphragm  50  may be fabricated from a transparent elastomeric membrane similar to the material of hood  12  (such as polyurethane, Chronoflex™, latex, etc) and may define one or more apertures  52  through which saline fluid introduced into hood  12  may pass through diaphragm  50  and out through main aperture  42  to clear blood from the open field within hood  12 . The one or more apertures  52  may have a diameter of between, e.g., 1 mm to 0.25 mm. 
         [0060]    Flow of the saline fluid out of the hood  12  through main aperture  42  may continue under relatively low fluid pressure conditions as saline is introduced from the catheter shaft  16 , through the diaphragm apertures  52 , and out of the main aperture  42 , as shown in  FIG. 6A . Upon the application of a relatively higher fluid pressure, the diaphragm  50  may be pushed distally within hood  12  until it extends or bulges distally to block the main aperture  42  until fluid flow out of the hood  12  is reduced or completely stopped, as shown in  FIG. 6B . With aperture  42  blocked, the hood  12  may retain the purging fluid within to facilitate visualization through the fluid of the underlying tissue. Thus, hood  12  may be panned around a target tissue region with sustained visualization to reduce the amount of saline that is introduced into a patient&#39;s heart or bloodstream. Once the fluid pressure of the purging fluid is reduced, diaphragm  50  may retract to unblock aperture  42  and thus allow for the flow of the purging fluid again through diaphragm apertures  52  and main aperture  42 . 
         [0061]    Another variation is shown in the side views of  FIGS. 7A and 7B , which show an internal inflation member or pouch  60 , which may be transparent, positioned within hood  12  which also controls the outflow of the purged saline based on fluid pressure. When internal inflation member of pouch  60  is at least partially inflated via the purging fluid introduced within, an annular flow path  62  may be defined between an external surface of pouch  60  and an inner surface of hood  12 , as illustrated in  FIG. 7A . The purging fluid may be initially introduced into pouch  60  and then flow through one or more apertures  64  defined along the surface of pouch  60  and out through the main aperture of hood  12  to clear blood therefrom. Each of the one or more apertures  64  may have a diameter ranging from, e.g., 1 mm to 0.25 mm, and the apertures  64  may be located along a proximal side of the pouch  60  along the annular flow path  62  in apposition to an interior surface of hood  12 . When the pressure of the purging fluid is increased, the internal pouch  60  may expand into contact against the inner surface of hood  12  to block flow path  62  and pouch apertures  64  against hood  12 , as depicted in  FIG. 7B , thereby reducing or stopping saline outflow from the imaging hood  12 . As the fluid pressure of the purging fluid is reduced, the size of internal pouch  60  may retract to thus unblock apertures  64  and flow path  62  and again allow for the flow of the fluid therethrough. 
         [0062]    In yet another variation shown in the side view of  FIG. 8 , hood  12  may be sectioned into a proximal chamber  74  and a distal chamber  76  separated by a transparent diaphragm  70  suspended within the hood  12 , as previously described. In this embodiment, one or more unidirectional valves  72  may be positioned over the diaphragm  70  through which the purging fluid may flow from the proximal chamber  74  to the distal chamber  76  and through the main aperture. Because of the unidirectional flow of the fluid through valves  72 , the purging fluid may exit proximal chamber  74  but may not flow back through the valves  72 . In this manner, while the main aperture on the base of the hood  12  may temporarily allow the entry of blood back into the distal chamber  76 , the blood is prevented from filling the entire hood  12  by the valves  72  and the proximal chamber  74  may be purged once to initially clear away blood to obtain optical clarity. Hence the volume required for constant clearing of opaque fluids, such as blood, is reduced thus reducing the amount of saline required to establish visualization.  FIGS. 9A and 9B  show detail cross-sectional side views of the one or more unidirectional valves  72 . As shown in  FIG. 9A , flow  82  from the proximal chamber  74  may open overlapping valve leaflets  80 , which extend distally into distal chamber  76 . Once the flow  82  is stopped or reduced, the backflow  84  from distal chamber  76  may collapse the valve leaflets  80  upon themselves, thus inhibiting backflow through the valves  72  and into proximal chamber  74 , as shown in  FIG. 9B . 
         [0063]    As further shown in  FIG. 8 , one or more portions  78  of the hood support struts which extend at least partially within the distal chamber  76  may be internally exposed such that the saline within the distal chamber  76  can be electrically charged, such as with RF energy, when the support struts are coupled to an RF generator. This allows the saline encapsulated in the distal chamber  76  to function as a virtual electrode by conducting the discharged energy to the underlying visualized tissue for treatments, such as tissue ablation. Details of electrode ablation of visualized tissue which may be utilized with apparatus and methods described herein are described in further detail in U.S. patent application Ser. No. 12/118,439 filed May 9, 2008 (U.S. Pat. Pub. 2009/0030412 A1), which is incorporated herein by reference in its entirety. 
         [0064]      FIGS. 10A and 10B  show perspective and cross-sectional perspective views of yet another variation which incorporates an electrode  90 , such as ring-shaped electrode, within hood  12  which defines a central lumen  92  therethrough. The central lumen  92  may define one or more fluid apertures  94  proximally of electrode  90  which open to the hood interior in a circumferential pattern around an outer surface of lumen  92 . Central lumen  92  may also be sized to allow for the introduction and advancement of a fiberscope (or other instrument) therethrough. With the positioning of, e.g., a fiberscope, within lumen  92  and its distal end positioned adjacent to or distal to electrode  90 , the distal opening of lumen  92  may be obstructed by the fiberscope. As the purging fluid  98  is introduced through lumen  92  and flows in an annular space between the fiberscope and the lumen  92 , the purging fluid may be forced to flow sideways into hood  12  through the one or more fluid apertures  94  while the distal opening of lumen  92  remains obstructed by the fiberscope. The purging fluid  98  forced through apertures  94  may flow  96  along the interior surface of hood  12  in a uniform manner, e.g., much like a “shower head”, to uniformly purge blood (or other opaque fluids) from within the hood  12 , consequently reducing the amount of saline required to establish visualization. Moreover, creating such a flow effect may prevent jets of the purging fluid from being purged distally in the event that fluid pressure and flow rate becomes too high as such jets of purging fluid may directly exit the hood aperture without thoroughly purging the hood  12 . 
         [0065]      FIG. 11  shows yet another variation of a hood incorporating one or more protrusions or projections  100  extending from a distal membrane over hood  12 . These protrusions or projections  100  may extend distally adjacent to a corresponding unidirectional valve  102  which has overlapping leaflets  104 . The protrusions or projections  100  may comprise hemispherical protrusions made of a transparent elastomeric material (e.g., can be the same or different material as the imaging hood  12 ). In use, as hood  12  is filled with the purging fluid, flow through the valves  102  is inhibited or prevented by the overlapping leaflets  104 , as shown in the cross-sectional detail view of  FIG. 12A . As the distal face of the hood membrane  40  is pressed against a surface of tissue T to be visualized and/or treated, the protrusions or projections  100  pressing against the tissue surface may force the valve leaflets  104  to separate temporarily, thus allowing the passage of saline  106  out through the valves  102  to clear any blood within the hood  12  as well as any blood between membrane  40  and the tissue surface, as shown in the side view of  FIG. 12B . Lifting hood  12  from the tissue may again allow the valve leaflets  104  to coapt and thus close the valve to prevent the blood from re-entering the hood  12  thus effectively reducing the amount of saline that is introduced into the patient&#39;s heart or bloodstream. 
         [0066]    In yet another variation,  FIG. 13  shows an imaging hood  12  which is configured to form a recirculating flow inside the hood  12 . The purging fluid may be introduced (e.g., injected) as well as withdrawn from the imaging hood  12  interior through two different lumens in the catheter shaft  16 . For instance, the fluid may be introduced by an inlet lumen  110  which injects the fluid along a first path into hood  12  while the recirculating fluid  114  may be withdrawn by suction through a separate outlet lumen  112 . By keeping a relatively higher volume flow rate in inlet lumen  110  for injecting the purging fluid than the flow rate in outlet lumen  112  for withdrawing it, a considerable amount of purging fluid may be conserved resulting in efficient hood purging. 
         [0067]      FIG. 14  shows a side view of another variation incorporating a suction lumen  120 , e.g., a pre-bent lumen, extending from catheter  16  directly to the main aperture  42 . This particular variation may allow for the direct evacuation of blood through a lumen opening  122  at a particular location along the main aperture  42  where the in-flow of blood (or other opaque fluids) is particularly high. For instance, when visualizing a pulmonary vein ostium in the left atrium of a patient&#39;s heart, the constant in-flow of blood into the imaging hood  12  from the pulmonary vein may occur. In order to overcome such a high in-flow rate, a higher flow rate or pressure of the purging fluid may be required to maintain visualization consequently increasing the amount of saline discharged into the patient&#39;s body. With the suction lumen  120 , visualization can be achieved with a lower purging fluid flow rate or pressure when the suction lumen  120  is extended slightly out of the main aperture  42  or within the main aperture  42  to evacuate the in-flowing blood. The lumen opening  122  of suction lumen  120  can also be moved around the aperture space by torquing the suction lumen  120 . The suction lumen  120  may also be used for evacuating any thrombosis or coagulated residue that may be formed during a therapeutic procedure, such as ablation. 
         [0068]    Yet another variation is shown in the side view of  FIG. 15  which illustrates a hood  12  partitioned into multiple chambers, e.g., chambers A, B, C, D, which are in fluid communication with individual corresponding fluid lumens defined through catheter  16 . Each of the chambers A, B, C, D may be separated by corresponding transparent barriers  132 ,  134 ,  136  which extend along the length of hood  12 . Although four chambers are shown in this example, fewer than four or greater than four chambers may be utilized. Each of the different chambers A, B, C, D may have a corresponding aperture  130 A,  130 B,  130 C,  130 D. The imaging element may be positioned inside the hood  12  to allow visualization of the tissue region. 
         [0069]    Efficient purging and reduction of saline discharged may achieved when purging can be selectively stopped once a particular chamber establishes optical clarity. This can be done manually by the operator or through automation by a processor incorporated within the system. If automation is used, optical clarity of each individual chamber can be determined by quantifying the amount of red (co-related to amount of blood in chamber) through the Red:Green:Blue ratio of the image captured of a particular sector that corresponds to the particular chamber.  FIG. 16  shows an example of a flow chart for the automated selective purging of individual chambers A, B, C, D. As all of the chambers A, B, C, D are initially purged  140 , each chamber may be monitored as to whether it has obtained sufficient optical clarity  142 A,  142 B,  142 C,  142 D, i.e., whether a sufficient amount of blood (or other opaque bodily fluid) has cleared from the chamber to allow direct visualization of the underlying tissue region as detected either by the operator or automatically. If the respective chamber has not yet obtained sufficient optical clarity, the process of purging the chamber may be repeated or continued until sufficient optical clarity has been reached. Once the sufficient level of optical clarity has been reached, flow of the purging fluid into the respective chamber A, B, C, D may be stopped  144 A,  144 B,  144 C,  144 D. As different parts of the hood  12  may be cleared at different rates, flow of the purging fluid may be selectively stopped and/or continued in different chambers depending on the optical clarity and thus potentially reducing the amount of purging fluid released into the body. 
         [0070]    In yet another variation,  FIGS. 17A and 17B  show side views of a hood  12  which may incorporate an expandable distal membrane  150  which projects distally from hood  12  and is sufficiently soft to conform against the underlying contacted tissue. As shown, distal membrane  150  may define an intermediate chamber  158  between membrane  154  of hood  12  and the distal membrane  150 . With membrane  154  defining one or more hood apertures  156 , the purging fluid may enter within hood  12  and exit through hood apertures  156  into intermediate chamber  158  which may extend distally as shown in  FIG. 17A . The purging fluid may exit intermediate chamber  158  through at least one central aperture  152 . In the event that hood  12  contacts against a surface of tissue T at a non-perpendicular angle, as indicated by angle of incidence θ defined between the catheter longitudinal axis  160  and tissue surface, distal membrane  150  may conform to the tissue surface despite the angle of incidence θ. As result, distal membrane  150  may still establish contact with the tissue yielding a more efficient purging effect, as shown in  FIG. 17B . 
         [0071]    In yet another variation, as shown in the side view of  FIG. 18 , an imaging hood  12  may have distal membrane without an aperture and which may be filled with the purging fluid once desirably positioned within the subject&#39;s body in proximity to the tissue region to be visualized and/or treated. As the distal membrane is closed, the underlying tissue may be contacted and visualized through the hood  12 . Once a tissue region to be treated has been located by hood  12 , a piercing instrument, such as a transseptal or transmural needle  170 , may be advanced through hood  12  from catheter  16  to puncture through the distal membrane at a desired site. This may form a puncture aperture  172  through which the purging fluid may escape  174 . Hence, purging is only performed at locations where instruments are passed out of the imaging hood  12  thus reducing the amount of saline discharged out of the hood  12 . A plurality of puncture apertures can be made across the distal membrane according to the needs of the procedure or the operator. Details of transseptal needles which may be utilized with apparatus and methods described herein are described in U.S. patent application Ser. No. 11/763,399 filed Jun. 14, 2007 (U.S. Pat. Pub. No. 2007/0293724 A1), which has been incorporated hereinabove. Details of transmural needles which may be utilized with apparatus and methods described herein are described in U.S. patent application Ser. No. 11/775,837 filed Jul. 10, 2007 (U.S. Pat. Pub. No. 2008/0009747 A1), which is incorporated herein by reference in its entirety. 
         [0072]    The applications of the disclosed invention discussed above are not limited to certain treatments or regions of the body, but may include any number of other treatments and areas of the body. Modification of the above-described methods and devices for carrying out the invention, and variations of aspects of the invention that are obvious to those of skill in the arts are intended to be within the scope of this disclosure. Moreover, various combinations of aspects between examples are also contemplated and are considered to be within the scope of this disclosure as well.