CROSS REFERENCE TO RELATED APPLICATIONS 
     This patent application claims priority to commonly-owned U.S. Provisional Patent Application with Ser. No. 60/216,047 filed on Jul. 5, 2000, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to medical devices, and more particularly to a brake assembly for a steerable catheter that allows both dynamic locking and swapping between a locked mode of operation and an unlocked mode of operation. 
     2. Description of Related Art 
     Medical practitioners frequently use catheters to access internal regions of a patient&#39;s body for a variety of medical procedures. The use of catheters advantageously reduces or eliminates the need for more invasive procedures. Medical catheters may be used to access internal body regions with a fiberoptic scope, light bundles, and/or other surgical instruments or devices, for a variety of diagnosis, treatment and/or material delivery purposes. For example, U.S. Pat. No. 5,658,263 to Dang, et al. discloses a multi-segmented guiding catheter typically utilized for internal vascular access. 
     Steerable catheters have been developed to provide improved access to internal tissue. These catheters typically include a flexible catheter shaft and steering wires or other steering means for controlling the flexure of the catheter shaft. An example of a steerable catheter is shown by U.S. Pat. No. 5,199,950 to Schmitt, et al. U.S. Pat. No. 5,454,794 to Narcisco, et al. shows a steerable light-diffusing catheter for treating luminal surfaces with photodynamic therapy. A mechanism for steering a catheter is disclosed by U.S. Pat. No. 5,456,664 to Heinzelman, et al. 
     For certain applications, it has been found desirable by some practitioners to provide a steerable catheter with a locking steering mechanism. As the steering mechanism flexes the catheter shaft, the locking mechanism retains the shaft in the selected flexed position even after the steering dial is released. This can be accomplished, for example, by providing ridges or notches on the outer circumferential face of the steering dial, and providing a spring actuated pawl or other retaining element for engagement with the ridges or notches. 
     These locking steering mechanisms suffer the disadvantage that the steering mechanism locks only in discrete, spaced-apart positions, typically corresponding to the spaced ridges or notches on the outer circumferential face of the steering dial. Adjacent locking positions may be further apart than would be desirable to the practitioner. Thus, it has been found that a need exists for a fully adjustable steering lock mechanism that permits the user to lock the catheter shaft in any position within a continuous range. 
     Although locking steering mechanisms are often desirable, it is sometimes preferable to have a freewheeling steering mechanism that does not lock the shaft in position, but instead permits the catheter shaft to return to a generally straight configuration when the steering dial is released. Previously known locking steering mechanisms typically cannot convert into freewheeling mechanisms, which implicitly limits use to applications where freewheeling is not desired. It has therefore been discovered that needs exist for a steerable catheter having a steering mechanism that permits the practitioner to selectively engage or disengage a steering brake assembly. Further needs exist for a catheter whereby the user can choose to retain the shaft in a selected flexed position by locking the steering mechanism after releasing the steering dial, or alternatively, can allow freewheeling of the steering dial such that the catheter shaft returns to a generally straight configuration when the steering dial is released. 
     It is to the provision of a steerable catheter and steering brake assembly meeting these and other needs that the present invention is primarily directed. 
     SUMMARY OF THE INVENTION 
     Briefly described, in a preferred form, the present invention is a brake assembly for a steerable catheter that allows both dynamic locking and swapping between a locked mode of operation and an unlocked mode of operation. Dynamic locking enables very fine adjustment of the catheter shaft position, and locking of the shaft in an infinite number of positions through the entire range of motion of the catheter shaft. This advantageously frees the user from maintaining torque on the steering dial to maintain catheter tip positioning. Consequently, the user can concentrate on diagnosis, avoid hand fatigue, and use the hands for other tasks. 
     In addition to dynamic locking, preferred forms of the present invention allow swapping between a locked mode of operation and an unlocked mode of operation. Thus, a user can either lock the catheter shaft in a given position or allow freewheeling. During freewheeling, the catheter returns to a generally straight position after a user releases the steering dial. As a result, a user can more effectively redirect the direction of the catheter for further investigation. Thus, the invention also advantageously improves stability of catheter shaft positioning, which improves imaging and recording. Moreover, swapping between modes of operation results in a more multi-purpose catheter that can be used in both environments where locking catheters are needed as well as environments where non-locking catheters are needed. 
     In one preferred form, the invention is a brake assembly for dynamically locking a steerable catheter having a steering dial rotatable through a plurality of positions. The brake assembly comprises a brake shoe positioned for slidable movement between a locked position contacting the steering dial and an unlocked position not contacting the steering dial. At least one spring biases the brake shoe toward the locked position. 
     In another preferred form, the invention is a brake assembly for dynamically locking a steerable catheter having a steering dial rotatable through a plurality of positions. The brake assembly comprises a base assembly connected to the catheter and positioned proximate the steering dial. A spring-biased brake shoe is positioned in sliding relation to the base assembly and movable between a locked position engaging the steering dial and an unlocked position not engaging the steering dial. A toggle is pivotally connected to the base assembly and movable between a first position corresponding to the locked position of the brake shoe and a second position corresponding to the unlocked position of the brake shoe. 
     In another preferred form, the invention is a steerable catheter that comprises a catheter body, a flexible catheter shaft extending from the catheter body, and a steering dial rotationally mounted to the catheter body for manipulating the shaft. A brake assembly is also included. The brake assembly comprises a base assembly connected to the catheter body and positioned proximate the steering dial. A brake shoe is positioned in sliding relation to the base assembly and having a contact face slidably mounted to the catheter for selectively engaging and disengaging with the steering dial. The brake assembly further comprises at least one spring for biasing the contact face of the brake shoe against the steering dial. 
     In yet another preferred form, the invention is a steerable catheter comprising a housing, a flexible shaft extending from the housing, a steering dial for steering the shaft, and a brake assembly. The brake assembly preferably comprises a brake element frictionally engaged between the steering disc and the housing. In one embodiment of the invention, the brake element is a brake sleeve frictionally engaged between a socket in the housing and a boss on the steering dial. In another embodiment, the brake element is a brake shoe biased into frictional engagement with a face of the steering dial. 
     These and other features and advantages of preferred forms of the present invention are described herein with reference to the drawing figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     FIG. 1 is a top cutaway view of a steerable catheter that includes a brake assembly according to a preferred form of the present invention. 
     FIG. 2 is a side cutaway view of the steerable catheter body with the brake assembly shown in FIG.  1 . 
     FIG. 3 is a perspective view illustrating a base assembly for the brake assembly shown in FIG.  1 . 
     FIG. 4 is a perspective view illustrating a spring biased brake shoe for the brake assembly shown in FIG.  1 . 
     FIG. 5 is a perspective view illustrating a brake toggle for controlling the operation of the brake assembly of FIG.  1 . 
     FIG. 6 is a perspective view illustrating the assembly of the brake shoe of FIG.  4  and the toggle of FIG.  5 . 
     FIG. 7 is a perspective view of the brake assembly of FIG. 1 illustrating the position of the springs. 
     FIG. 8 is an exploded perspective view of another embodiment of the invention, according to preferred form. 
    
    
     DETAILED DESCRIPTION 
     Referring now to the figures, in which like numerals refer to like elements through the several figures, FIG. 1 is a top cutaway view of a steerable catheter that includes a brake assembly. The catheter  10  includes a catheter body  12  and a steerable catheter shaft  14 . Though not shown in FIG. 1, the catheter body  12  preferably includes upper and lower half housings. Consequently, the shaft  14  is preferably secured within a recess formed between the upper and lower housing members. Typically, the recess and hence the catheter shaft  14  extend from one end of the body  12 . The catheter shaft  14  preferably is any one of various types of flexible catheter shafts, such as the catheter shaft disclosed in U.S. Pat. No. 6,213,974 to Smith, which patent is hereby incorporated herein by reference. 
     Typically, steering wires extend through the shaft  14  and connect to a steering dial  16 . As mentioned above, upper and lower half housing members preferably secure the steering dial  16  therebetween. The steering dial  16  is rotationally mounted within the catheter body  12  and coupled to the steering wires such that rotation of the steering dial  16  clockwise correspondingly flexes the shaft  14  to the right, and counterclockwise rotation of the dial flexes the shaft to the left. For example, rotating the steering dial  16  flexes the catheter shaft  14  to the right. Thus flexibility of the catheter shaft  14  and the rotational limits of the steering dial  16  jointly define the range of motion of the catheter shaft  14 . 
     FIG. 1 also illustrates that the catheter  10  includes a brake assembly  30  according to one form of the present invention. As more clearly seen in FIG. 2, the brake assembly  30  preferably includes a toggle  80 , which is described in detail with reference to FIG.  5 . However, the design of the catheter body  12  ergonomically positions both steering dial  16  and toggle  80  within easy reach of the user. Consequently, effective single-handed operation of the catheter  10  is achieved. 
     The components of the brake assembly  30  are shown in greater detail in FIGS. 3-7. FIG. 3 is a perspective view illustrating a base assembly  40  for the brake assembly shown  30 . The base assembly  40  is mounted to the catheter body  12  adjacent to the steering dial  16 . The base assembly  40  can be attached to the catheter body  12  by adhesive, fasteners, or integrally forming the base  40  with the body  12 . Alternatively or in addition, recesses  42  formed in the base  40  can engage bosses  44  (see FIG. 1) of the catheter body  12 . 
     As shown in FIG. 3, the base assembly  40  resembles a generally U-shaped yoke. Alternatively, this base assembly could be V-shaped, arced, or some other suitable shape. The base assembly preferably comprises an end piece  46 , and a first arm  48  and a second arm  50  that extend from opposite ends of end piece  46 . Each of these arms includes an inwardly projecting rib  52  that extends lengthwise along its inner face and away from the end piece  46 . In addition, each of the arms  48 ,  50  also defines an opening  54  distal the end piece  46 . Though not shown, the end piece  46  can include one or more recesses or projections on the inner face of for retaining springs  68 , which are described with reference to FIG.  4 . Using the projecting rib  52 , opening  54  and recesses (not shown), the base assembly effectively and securely couples with the other components of the brake assembly  30 , as described below. 
     FIG. 4 is a perspective view illustrating a spring biased brake shoe  60  for the brake assembly  30 . Opposite sides of the brake shoe  60  preferably include grooves  62  for slidable engagement along the projecting ribs  52  of the base assembly  40 . Thus, these ribs only allow generally linear movement of the brake shoe  60  with reference to the base  40 . The brake shoe  60  also includes an arcuate face  64  contoured to generally match the curvature of the steering dial  16 . Consequently, sliding the arcuate face  64  into contact with the steering dial  16  frictionally secures the dial in place. In a face opposite the arcuate face  64 , the brake shoe  60  includes recesses, or projections,  66  that retain springs  68 . Recesses or projections on the inner face of the end piece  46  of the base can also retain the springs  68 , such that the springs are captive between the base  40  and the brake shoe  60 . An upper face of the brake shoe  60  includes a recess or opening  70  that cooperates with an actuator arm  84  of the brake toggle  80 , as described with reference to FIG.  5 . 
     During operation, the brake shoe  60  moves either towards or away from the end piece  46 . In a retracted (“brake off”) position, the brake shoe  60  slides towards the end piece  46 , and away from and out of contact with the steering dial  16 . In an engaged (“brake on”) position, the brake shoe  60  slides away from the end piece  46  towards, and into contact with the steering dial  16 . Pressure is applied by the brake shoe  60  against the steering dial  16  sufficient to generate a frictional force capable of maintaining the steering dial  16  in position and thereby overcoming the resilience of the catheter shaft and fixing the shaft in any selected position. When the brake shoe  60  is engaged, the arcuate face  64  preferably locks the steering dial  16  to an extent sufficient to prevent freewheeling but permit the user to adjust the steering dial with moderate finger pressure. Because the steering dial  16  is generally circular, the matched curvature of the arcuate face  64  and the steering dial can secure the dial in an infinite number of positions throughout the entire range of motion of the catheter shaft. 
     To achieve dynamic locking, the brake assembly  30 , in one form of the invention, comprises only the base assembly  46 , the brake shoe  60 , and at least one spring  68 . Alternatively, the brake shoe  60  and the spring  68  are replaced by a spring-biased brake shoe. The springs  68  act in compression between the brake shoe  60  and the base  40 , which biases the brake shoe  60  towards the engaged position. Hence, the arcuate face  64  maintains contact with the steering dial  16 . As a user rotates the steering dial  16 , the friction fit between the steering dial  16  and the arcuate face  64  hinder the catheter shaft  14  from moving after the dial is released. Thus, the catheter shaft  14  remains in a locked position. 
     FIG. 5 is a perspective view illustrating a brake toggle  80  for controlling the operation of the brake assembly  30 . The brake toggle  80  includes pivot lugs  82  that extend outwardly from opposite sides of this toggle. These lugs are engaged and pivotally mounted within the openings  54  of the first arm  48  and the second arm  50  (see FIG. 6) of the base assembly. Using pivotal mounting, the brake toggle  80  rock back and forth between an “on” position and an “off” position. The pivot lugs  82  are preferably positioned approximately midway along the length of the toggle  80 , which facilitates an easy change of position. Typically, the brake toggle  80  will include labeling that indicates the on and off positions 
     In addition to the pivot lugs  82 , the toggle  80  includes an actuator arm  84  that extends from a lower face. A recess  70  in the brake shoe  60  receives the actuator arm  84 . The actuator arm  84  acts as a cam to slide the brake shoe  60  away from the steering dial  16  into the retracted (“brake off”) position. Generally, the actuator arm  84  moves in response to movement of the brake toggle  80 . For example, moving the brake toggle  80  into its “off” position causes the actuator arm to move the brake shoe  60  out of contact with the steering dial  16 . Consequently, the catheter shaft  14  becomes unlocked, which allows freewheeling. Moving the brake toggle  80  into the “on” position transfers control of the brake assembly  30  from the actuator arm  84  to the springs  68  (see FIG.  7 ). As mentioned above, the springs  68  bias the brake shoe  60  towards an engaged position. Thus, the spring(s)  68  drive the brake shoe  60  into contact with the steering dial  16 , which locks the steering dial  16  to prevent freewheeling. The spring constant and compression distance of the spring(s) result in the application of a normal force between the shoe  60  and the dial  16 , which together with the coefficient of friction between the shoe and dial, generates a frictional force sufficient to resist the resilience or memory of the shaft that would otherwise cause the shaft to return to a straight configuration. 
     The actuator arm  84  is normally self-locking. For example, putting the brake toggle  80  in an “on” position keeps the toggle  80  in that position until the user applies sufficient force to pivot it to the “off” position. In this manner, a user can put the brake toggle  80  into its “off” position for freewheeling operation of the steering dial  16 . When the catheter shaft  14  is flexed to a desired position, the user then switches the brake toggle  80  into its “on” position, which locks the steering dial  16  to prevent freewheeling. Locking the steering dial  16  maintains the catheter shaft  14  in the desired flexure orientation until the brake is unlocked. Preferably, the user can adjust the shaft flexure when the brake is applied in the “on” position by application of moderate finger pressure to the steering dial sufficient to overcome the frictional force of the brake. 
     In view of the foregoing, it will be appreciated that the present invention provides a brake assembly  30  for locking a steerable catheter  10 . By using the spring biased brake shoe  60 , the brake assembly  30  can dynamically lock the catheter  10  in an infinite number of positions throughout the catheter shaft&#39;s range of motion. Using the toggle  80 , the brake assembly  30  can switch from freewheeling operation to locked operation. These features result in freeing the user&#39;s steering hand to carry out other tasks. 
     FIG. 8 shows a catheter having a dynamic braking system according to another preferred form of the present invention. In this embodiment of the invention, the braking mechanism remains locked, and is not switchable to permit freewheeling of the steering dial. The catheter  100  preferably comprises a housing formed by upper and lower half housings  102 ,  104 , a steerable flexible catheter shaft  106  defining one or more lumens extending through its length, and a rotationally mounted steering dial  108  for selectively controlling the flexure of the shaft. The shaft  106  is preferably mounted to a manifold  110  providing fluid communication with one or more inlet housings  112  via manifold extension tubes  114 . The manifold  110  preferably comprises a mounting ring for engagement between the upper and lower half housings  102 ,  104  to secure the shaft  106  to the housing. One or more steering wires  124  extend through at least a portion of the shaft  106 , whereby tension applied to the steering wire(s) causes flexure of the shaft. End(s) of the steering wire(s) are affixed to the steering dial  108 , preferably by a set screw  126  engaged within an insert  128  attached to the steering dial a distance from the dial&#39;s axis of rotation. 
     The steering dial  108  preferably comprises an upper face having an outwardly projecting central hub  116  for rotational engagement within a cooperating recess or opening  118  provided in the upper half housing  102 . The hub  116  preferably comprises a directional indicator for indicating the corresponding direction and degree of flexure of the shaft  106 . The steering dial  108  preferably further comprises a generally cylindrical boss  120  projecting outwardly from a lower face thereof, opposite the hub  116 , for rotational engagement within a cooperating socket  122  of the lower half housing  104 . Assembly of the upper and lower half housings  102 ,  104  by coupling of crash pins or other attachment means captures the steering dial  108  within the housing and constrains the dial to rotation about an axis through the hub  116  and the boss  120 . The steering dial  108  preferably comprises one or more wings  130  extending radially outward from a circumferential face of the dial and projecting externally of the housing for easier manipulation of the dial by a user. A stop  132  is optionally mounted to the housing and extends through a cutout section  134  of the circumferential face of the steering dial  108  to limit the extent of rotation permitted by the steering dial. The cutout section  134  is preferably sized to span an arc of a selected angle, such that the steering dial  108  cannot rotate beyond that selected angle due to contact between the stop  132  and the edges of the dial surrounding the cutout section. The cutout section can be located along the circumferential face of the steering dial  108  to permit steering of the shaft  106  in one direction only, to an equal extent in either direction, or to a greater extent in a first direction than in a second direction. In a preferred embodiment, the dial is permitted to steer in one direction only, and a single steering wire leg  124  is provided to effect steering in that direction. The resilience or “memory” of the shaft  106  biases the shaft to return to a straight orientation when tension is released from the single steering wire  124  by returning the dial to its neutral position. 
     Dynamic braking of the steering mechanism is accomplished by a brake assembly comprising a brake sleeve  136  frictionally engaged between the steering dial boss  120  and the socket  122  in the housing. The inner and outer diameters of the sleeve  136  form interference fits with the outer diameter of the steering disc boss  120  and the inner diameter of the socket  122 , respectively. The tightness or degree of interference of these fits is selected to permit smooth rotation of the steering dial  108  by moderate finger pressure, and to produce friction sufficient to resist the tendency of the shaft  106  to return to a straight configuration as a result of the shaft material&#39;s memory. The brake sleeve  136  is preferably a split cylinder of metal, plastic or other material having sufficient rigidity and wear-resistance to maintain the frictional interference fits with the steering disc boss  120  and the socket  122  during the anticipated life of the catheter device, and having the desired coefficient of friction with the materials of the steering disc boss and the socket. 
     While the invention has been described in its preferred forms, it will be readily apparent to those of ordinary skill in the art that many additions, modifications and deletions can be made thereto without departing from the spirit and scope of the invention.
A brake assembly for a steerable catheter that allows both dynamic locking and swapping between a locked mode of operation and an unlocked mode of operation is described. Dynamic locking enables fine adjustment of the catheter shaft position, while swapping between modes results in a more multipurpose catheter. Generally, the brake assembly includes a brake shoe positioned for slidable movement between a locked position contacting the steering dial and an unlocked position not contacting the steering dial. At least one spring biases the brake shoe toward the locked position. A catheter including a brake assembly is also described.