Abstract:
The position of an imaging catheter in a body structure such as the heart is automatically controlled by a robotic manipulator such that its field of view at all times includes the distal end of a second catheter that is employed to effect a medical procedure. A processor receives signals from position sensors in the catheters. The processor utilizes the information received from the sensors and continually determines any deviation of the second catheter from the required field of view of the imaging catheter. The processor transmits compensation instructions to the robotic manipulator, which when executed assure that the imaging catheter tracks the second catheter.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to sensing the position and orientation of an object placed within a living body. More particularly, this invention relates to stabilizing the position and orientation of an intravascular probe within a moving internal organ of a living body.  
         [0003]     2. Description of the Related Art  
         [0004]     A wide range of medical procedures involve placing objects, such as sensors, tubes, catheters, dispensing devices, and implants, within the body. Realtime imaging methods are often used to assist operators in visualizing the object and its surrounding during these procedures. In most situations, however, realtime three-dimensional imaging is not possible or desirable. Instead, systems for obtaining realtime spatial coordinates of the internal object are often utilized.  
         [0005]     Many such position sensing systems have been developed or envisioned in the prior art. Some systems involve attaching sensors to the internal object in the form of transducers or antennas, which can sense magnetic, electric, or ultrasonic fields generated outside of the body. For example, U.S. Pat. No. 5,983,126, issued to Wittkampf, whose disclosure is incorporated herein by reference, describes a system in which three substantially orthogonal alternating signals are applied through the subject. A catheter is equipped with at least one measuring electrode, and a voltage is sensed between the catheter tip and a reference electrode. The voltage signal has components corresponding to the three orthogonal applied current signals, from which calculations are made for determination of the three-dimensional location of the catheter tip within the body. Similar methods for sensing voltage differentials between electrodes are proposed by U.S. Pat. No. 5,899,860, issued to Pfeiffer, whose disclosure is incorporated herein by reference. In both of these systems, it is necessary to undertake a separate calibration procedure in order to adjust for discrepancies between the apparent position of the catheter tip as measured and its actual position.  
         [0006]     Hybrid catheters are now known that perform ultrasound imaging in conjunction with position sensing. Such devices are disclosed, for example, in U.S. Pat. Nos. 6,690,963, 6,716,166 and 6,773,402, which are herein incorporated by reference. Medical applications include three-dimensional mapping of a cavity of the body, as well as measurement of chamber wall thickness, wall velocity, and mapping of electrical activity. In medical applications, it is common to acquire maps and images of body organs by different modalities, which are to be interpreted in relationship to one another. An example is correlation of an electro-anatomical map of the heart and an image, such as a three-dimensional ultrasound image.  
         [0007]     Commercial electrophysiological and physical mapping systems based on detecting the position of a probe inside the body are presently available. Among them, the Carto-Biosense® Navigation System, available from Biosense Webster Inc., 3333 Diamond Canyon Road Diamond Bar, Calif. 91765, is a system for automatic association and mapping of local electrical activity with catheter location.  
       SUMMARY OF THE INVENTION  
       [0008]     Hybrid catheters, for example, catheters having ultrasound transducers and a location sensor provide real-time visualization of anatomical structures and of surgical procedures. The catheter field of view and the resulting ultrasound images have the form of a two-dimensional “fan,” which opens outward from the catheter tip and provides a sectional image of the tissue that it intersects. If the location or orientation of the tip is incorrect or unstable, the fan may fail to capture a desired structure or may lose the structure during viewing. Disclosed embodiments of the present invention provide methods and systems for directing and stabilizing the orientation of the ultrasound beam. This is particularly useful in imaging an area in which a surgical procedure is being performed. For example, ultrasound imaging can verify that an ablation catheter is in place and in contact with tissue to be ablated. Subsequent to ablation, ultrasound imaging can confirm that ablation was successful because of the change in echogenicity of the tissue. Stabilization of the catheter using the principles of the present invention ensures that the operator has accurate, near realtime visual feedback related to the target of interest. A catheter having the capabilities just described is sometimes referred to herein as an ultrasound catheter or an ultrasound imaging catheter.  
         [0009]     In some aspects of the present invention, convenience of echocardiographic guidance in single operator intracardiac therapeutic procedures is enhanced. By robotically steering an ultrasound catheter to automatically follow the tip of an operative catheter, such as a mapping or ablating catheter, the operator is relieved of the burden of adjusting the imaging catheter to track the mapping or ablation catheter and its target. Realtime visualization of a target site is also enabled during the catheterization procedure, enabling accurate lesion targeting and optimal execution of a therapeutic ablation plan. Other advantages of the invention include monitoring catheter-tissue contact, monitoring the progress of ablation, including detection of bubble and char formation in tissues at the target.  
         [0010]     Although the magnetic-based position and orientation sensor in the ultrasound catheter enables the operator to know the catheter position and orientation at all times, it does not by itself guarantee success in holding the catheter stationary in a desired position. Embodiments of the present invention solve this problem by using automatic control of the ultrasound catheter to ensure that the catheter is correctly positioned, and oriented toward the target. The position sensing system determines desired position and the direction in which the imaging catheter should be pointed and measures any deviations from this position and direction, using the magnetic position sensor in the catheter. It then corrects the imaging catheter position and orientation, using a robotic mechanism. Alternatively, cues are provided for the operator to manipulate the catheter as required.  
         [0011]     According to one disclosed embodiment of the invention a first catheter, e.g., an ultrasound catheter, is controlled in order to keep a second catheter in its field of view.  
         [0012]     The second catheter, which could be an ablating catheter or any catheter for effecting a medical procedure, includes a position sensor. The position sensing system determines the position of the second catheter, using its position sensor, and uses the determined position as a reference point. The first catheter is then controlled to track the movement of the reference point, thereby keeping the second catheter in view. It should be noted that when the echogenic property of a landmark is changing, for example as a result of the medical procedure, image registration may become increasingly difficult. The existence of a reliable reference point, as provided by the invention, then becomes all the more valuable.  
         [0013]     Advantages of the present invention include improved accuracy in utilizing ultrasound imaging to track the progress of medical procedures. It relieves the operator of the continuous distraction of aiming the beam of the imaging catheter while performing a procedure. It can also be used to keep a particular structure or location within the body in the field of view of the catheter.  
         [0014]     The invention provides a method for displaying structural information in a body of a living subject, which is carried out by introducing an imaging catheter into the body, introducing an operative catheter into the body for performing a medical procedure on a target structure, and displacing the operative catheter in the body while performing the medical procedure. While displacing the operative catheter, the method is further carried out by repetitively sensing a current position of the operative catheter, and responsively to the current position of the operative catheter, automatically varying the field of view of the imaging catheter to include a predetermined target.  
         [0015]     According to an aspect of the method, the predetermined target is at least one of a portion of the operative catheter and a portion of the target structure.  
         [0016]     A further aspect of the method includes displaying an image of the field of view of the imaging catheter.  
         [0017]     One aspect of the method displaying an image includes displaying a two-dimensional slice of the field of view of the imaging catheter in registration with a portion of the predetermined target.  
         [0018]     In another aspect of the method, varying the field of view includes maneuvering the imaging catheter in the body.  
         [0019]     In a further aspect of the method, varying the field of view includes fixedly positioning the catheter and scanning an ultrasound beam from the imaging catheter in an oscillatory motion.  
         [0020]     Still another aspect of the method, which is carried out while scanning the ultrasound beam, comprises acquiring a plurality of two-dimensional images of the field of view, constructing a three-dimensional image from the plurality of two-dimensional images, and displaying the three-dimensional image.  
         [0021]     Yet another aspect of the method varying the field of view includes moving the imaging catheter in an oscillatory motion.  
         [0022]     An additional aspect of the method, which is carried out while moving the imaging catheter, comprises acquiring a plurality of two-dimensional images of the field of view, constructing a three-dimensional image from the plurality of two-dimensional images, and displaying the three-dimensional image.  
         [0023]     According to still another aspect of the method, the target structure is a portion of a heart.  
         [0024]     The invention provides a system for displaying structural information in a body of a living subject, including an imaging catheter adapted for introduction into the body, the imaging catheter having a position sensor therein. The system includes an operative catheter adapted for introduction into the body and for effecting a medical procedure on a target structure of the body, the operative catheter having a position sensor therein. The system includes a robotic manipulator operative for maneuvering the imaging catheter in the body, a positioning processor linked to the robotic manipulator, the positioning processor being operative responsively to signals from the position sensor of the operative catheter for repetitively sensing a current position of the operative catheter. The positioning processor is operative responsively to the current position of the operative catheter to transmit control signals to the robotic manipulator to cause the robotic manipulator to maneuver the imaging catheter to maintain a portion of the operative catheter or the target structure in the field of view. The system includes an image processor operative to generate an image of the field of view responsively to image data received from the imaging catheter, and a display for displaying the image.  
         [0025]     According to an additional aspect of the system, the positioning processor is operative to maneuver the imaging catheter responsively to signals produced by the position sensor of the operative catheter.  
         [0026]     According to another aspect of the system, the positioning processor is operative to position the imaging catheter according to predetermined position coordinates.  
         [0027]     According to yet another aspect of the system, the image processor is operative for generating a two-dimensional image of the field of view in registration with the portion of the operative catheter.  
         [0028]     According to a further aspect of the system, the robotic manipulator is operative to maneuver the imaging catheter in an oscillatory motion, and the image processor is operative for generating a plurality of two-dimensional images of the field of view, and a three-dimensional image that is constructed by the image processor from the plurality of two-dimensional images.  
         [0029]     According to one aspect of the system, the imaging catheter is an ultrasound imaging catheter.  
         [0030]     The invention provides a method for displaying structural information in a body of a living subject, which is carried out by introducing an imaging catheter into the body, and positioning the imaging catheter such that its field of view includes a predetermined landmark in the body. The method is further carried out by introducing an operative catheter into the body adapted for performing a medical procedure on a target structure of the body, displacing the operative catheter in the body while performing the medical procedure, automatically adjusting the field of view to maintain the landmark therein, and displaying an image of the landmark.  
         [0031]     One aspect of the method includes constructing a map of the target structure that includes position coordinates of the landmark, wherein positioning the imaging catheter includes directing the field of view according to the position coordinates of the landmark. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]     For a better understanding of the present invention, reference is made to the detailed description of the invention, by way of example, which is to be read in conjunction with the following drawings, wherein like elements are given like reference numerals, and wherein:  
         [0033]      FIG. 1  is an illustration of a system for imaging and mapping a heart of a patient in accordance with a disclosed embodiment of the invention;  
         [0034]      FIG. 2  schematically illustrates an embodiment of the distal end of s catheter used in the system shown in  FIG. 1 , in accordance with a disclosed embodiment of the invention;  
         [0035]      FIG. 3  is a schematic exploded view of a diagnostic image of the heart, in accordance with a disclosed embodiment of the invention;  
         [0036]      FIG. 4  schematically illustrates a control mechanism used by the system shown in  FIG. 1  to maneuver an imaging catheter during a medical procedure in accordance with a disclosed embodiment of the invention; and  
         [0037]      FIG. 5  schematically illustrates a control mechanism used by the system shown in  FIG. 1  to maneuver an imaging catheter during a medical procedure in accordance with an alternate embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0038]     In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent to one skilled in the art, however, that the present invention may be practiced without these specific details. In other instances, well-known circuits, control logic, and the details of computer program instructions for conventional algorithms and processes have not been shown in detail in order not to obscure the present invention unnecessarily.  
         [0039]     Software programming code, which embodies aspects of the present invention, is typically maintained in permanent storage, such as a computer readable medium. In a client-server environment, such software programming code may be stored on a client or a server. The software programming code may be embodied on any of a variety of known media for use with a data processing system. This includes, but is not limited to, magnetic and optical storage devices such as disk drives, magnetic tape, compact discs (CD&#39;s), digital video discs (DVD&#39;s), and computer instruction signals embodied in a transmission medium with or without a carrier wave upon which the signals are modulated. For example, the transmission medium may include a communications network, such as the Internet. In addition, while the invention may be embodied in computer software, the functions necessary to implement the invention may alternatively be embodied in part or in whole using hardware components such as application-specific integrated circuits or other hardware, or some combination of hardware components and software.  
         [0000]     System Overview  
         [0040]     Turning now to the drawings, reference is initially made to  FIG. 1 , which is an illustration of a system  20  for imaging and mapping a heart  24  of a patient, and which is suitable for performing diagnostic or therapeutic procedures involving the heart  24 , in accordance with an embodiment of the present invention. The system comprises a catheter  28 , which is percutaneously inserted by an operator  43 , who is typically a physician, into a chamber or vascular structure of the heart. The catheter  28  typically corpnrises a handle  29  for operation of the catheter by the physician. Suitable controls on the handle enable the physician to steer, position and orient the distal end of the catheter as desired to effect a medical procedure. A second catheter  27  is used for imaging the heart, and for determining the position of the catheter  28  in relation to a target, as described below. The catheter  27  has a steering mechanism  41  that is controlled by a robotic manipulator  31 , and optionally by the operator  43 . The manipulator  31  receives control signals from a positioning processor  36 , located in a console  34 .  
         [0041]     The system  20  comprises a positioning subsystem that measures location and orientation coordinates of the catheter  28 . Throughout this patent application, the term “location” refers to the spatial coordinates of the catheter, and the term “orientation” refers to its angular coordinates. The term “position” refers to the full positional information of the catheter, comprising both location and orientation coordinates.  
         [0042]     In one embodiment, the positioning subsystem comprises a magnetic position tracking system that determines the position and orientation of the catheter  28  and the catheter  27 . The positioning subsystem generates magnetic fields in a predefined working volume its vicinity and senses these fields at the catheter. The positioning subsystem typically comprises a set of external radiators, such as field generating coils  30 , which are located in fixed, known positions external to the patient. The coils  30  generate fields, typically electromagnetic fields, in the vicinity of the heart  24 .  
         [0043]     In an alternative embodiment, a radiator in the catheter, such as a coil, generates electromagnetic fields, which are received by sensors (not shown) outside the patient&#39;s body.  
         [0044]     The position sensor transmits, in response to the sensed fields, position-related electrical signals over cables  33  running through the catheter to the console  34 . Alternatively, the position sensor may transmit signals to the console over a wireless link. The positioning processor  36  that calculates the location and orientation of the catheter  28  based on the signals sent by a position sensor  32 . The positioning processor  36  typically receives, amplifies, filters, digitizes, and otherwise processes signals from the catheter  28 . The positioning processor  36  also provides signal input to the manipulator  31  for maneuvering the catheter  27 .  
         [0045]     Some position tracking systems that may be used for this purpose are described, for example, in U.S. Pat. No. 6,690,963, 6,618,612 and 6,332,089, and U.S. Pat. Application Publications 2002/0065455 A1, 2004/0147920 A1, and 2004/0068178 Al, whose disclosures are all incorporated herein by reference. Although the positioning subsystem shown in  FIG. 1  uses magnetic fields, the methods described below may be implemented using any other suitable positioning subsystem, such as systems based on electromagnetic fields, acoustic or ultrasonic measurements.  
         [0046]     Alternatively, the system  20  can be realized as the above-referenced Carto-Biosense Navigation System, suitably modified to execute the procedures described hereinbelow. For example, the system  20  may employ,  mutatis mutandis , the catheters disclosed in the above-noted U.S. Pat. Nos. 6,716,166 and 6,773,402 in order to acquire ultrasound images for display in near realtime.  
         [0047]     Reference is now made to  FIG. 2 , which schematically illustrates the distal end of the catheter  28  ( FIG. 1 ), in accordance with a disclosed embodiment of the invention. The fields generated by the field generating coils  30  ( FIG. 1 ) are sensed by the position sensor  32  inside the catheter  28 . The catheter  28  also comprises an ultrasonic imaging sensor, which is typically realized as an array of ultrasonic transducers  40 . In one embodiment, the transducers are piezo-electric transducers. The ultrasonic transducers are positioned in or adjacent to a window  41 , which defines an opening within the body or wall of the catheter. The catheter  28  typically has at least one lumen  37 , which can admit a guide wire and guide tube to aid in deployment of a therapeutic device.  
         [0048]     The transducers  40  operate as a phased array, jointly transmitting an ultrasound beam from the array aperture through the window  23 . Although the transducers are shown arranged in a linear array configuration, other array configurations can be used, such as circular or convex configurations. In one embodiment, the array transmits a short burst of ultrasound energy and then switches to a receiving mode for receiving the ultrasound signals reflected from the surrounding tissue. Typically, the transducers  40  are driven individually in a controlled manner in order to steer the ultrasound beam in a desired direction. By appropriate timing of the transducers, the produced ultrasound beam can be given a concentrically curved wave front, to focus the beam at a given distance from the transducer array. Thus, the system  20  ( FIG. 1 ) uses the transducer array as a phased array and implements a transmit/receive scanning mechanism that enables the steering and focusing of the ultrasound beam, so as to produce two-dimensional ultrasound images.  
         [0049]     In one embodiment, the ultrasonic sensor comprises between sixteen and sixty-four transducers  40 , preferably between forty-eight and sixty-four transducers. Typically, the transducers generate the ultrasound energy at a center frequency in the range of 5-10 MHz, with a typical penetration depth of 14 cm. The penetration depth typically ranges from several millimeters to around 16 centimeters, and depends upon the ultrasonic sensor characteristics, the characteristics of the surrounding tissue and the operating frequency. In alternative embodiments, other suitable frequency ranges and penetration depths can be used.  
         [0050]     After receiving the reflected ultrasound echoes, electric signals based on the reflected acoustic signals or echoes are sent by transducers  40  over cables  33  through the catheter  28  to an image processor  42  ( FIG. 1 ) in the console  34 , which transforms them into two-dimensional, typically sector-shaped ultrasound images. The positioning processor  36  in cooperation with the image processor  42  typically computes or determines position and orientation information, displays real-time ultrasound images, performs three-dimensional image or volume reconstructions. and other functions, which will all be described in greater detail below.  
         [0051]     Position sensors and ultrasonic transducers in the catheter  27  ( FIG. 1 ) are similar to those of the catheter  28 , except that the transducers of the catheter  27  may be adapted for imaging applications, rather than delivery of therapeutic ultrasound energy to a target.  
         [0052]     In some embodiments, the image processor  42  uses the ultrasound images and the positional information to produce a three-dimensional model of a target structure of the patient&#39;s heart. The three-dimensional model is presented to the physician as a two-dimensional projection on a display  44 .  
         [0053]     In some embodiments, the distal end of the catheter  28  also comprises at least one electrode  46  for performing diagnostic functions, therapeutic functions or both, such as electro-physiological mapping and radio frequency (RF) ablation. In one embodiment, the electrode  46  is used for sensing local electrical potentials. The electrical potentials measured by the electrode  46  may be used in mapping the local electrical activity at contact points of the endocardial surface. When the electrode  46  is brought into contact or proximity with a point on the inner surface of the heart  24  ( FIG. 1 ), it measures the local electrical potential at that point. The measured potentials are converted into electrical signals and sent through the catheter to the image processor for display as a map reflecting the functional data or activity at each contact point. In other embodiments, the local electrical potentials are obtained from another catheter comprising suitable electrodes and a position sensor, all connected to the console  34 . In some applications, the electrode  46  can be used to determine when the catheter is in contact with a valve, since the electrical potentials are weaker there than in the myocardium.  
         [0054]     Although the electrode  46  is shown as being a single ring electrode, the catheter may comprise any number of electrodes in any form. For example, the catheter may comprise two or more ring electrodes, a plurality or array of point electrodes, a tip electrode, or any combination of these types of electrodes for performing the diagnostic and therapeutic functions outlined above.  
         [0055]     The position sensor  32  is typically located within the distal end of the catheter  28 , adjacent to the electrode  46  and the transducers  40 . Typically, the mutual positional and orientational offsets between the position sensor  32 , electrode  46  and transducers  40  of the ultrasonic sensor are constant. These offsets are typically used by the positioning processor  36  to derive the coordinates of the ultrasonic sensor and of the electrode  46 , given the measured position of the position sensor  32 . In another embodiment, the catheter  28  comprises two or more position sensors  32 , each having constant positional and orientational off-sets with respect to the electrode  46  and the transducers  40 . In some embodiments, the offsets (or equivalent calibration parameters) are pre-calibrated and stored in the positioning processor  36 . Alternatively, the offsets can be stored in a memory device (such as an electrically programmable read-only memory, or EPROM) fitted into the handle  29  ( FIG. 1 ) of the catheter  28 .  
         [0056]     The position sensor  32  typically comprises three non-concentric coils (not shown), such as described in U.S. Pat. No. 6,690,963, cited above. Alternatively, any other suitable position sensor arrangement can be used, such as sensors comprising any number of concentric or non-concentric coils, Hall-effect sensors or magneto-resistive sensors.  
         [0057]     Typically, both the ultrasound images and the position measurements are synchronized with the heart cycle, by gating signal and image capture relative to a body-surface electrocardiogram (ECG) signal or intra-cardiac electrocardiogram. (In one embodiment, the ECG signal can be produced by the electrode  46 .) Since features of the heart change their shape and position during the heart&#39;s periodic contraction and relaxation, the entire imaging process is typically performed at a particular timing with respect to this period. In some embodiments, additional measurements taken by the catheter, such as measurements of various tissue characteristics, temperature and blood flow measurements, are also synchronized to the electrocardiogram (ECG) signal. These measurements are also associated with corresponding position measurements taken by the position sensor  32 . The additional measurements are typically overlaid on the reconstructed three-dimensional model.  
         [0058]     In some embodiments, the position measurements and the acquisition of the ultrasound images are synchronized to an internally generated signal produced by the system  20 . For example, the synchronization mechanism can be used to avoid interference in the ultrasound images caused by a certain signal. In this example, the timing of image acquisition and position measurement is set to a particular offset with respect to the interfering signal, so that images are acquired without interference. The offset can be adjusted occasionally to maintain interference-free image acquisition. Alternatively, the measurement and acquisition can be synchronized to an externally supplied synchronization signal.  
         [0059]     In one embodiment, the system  20  comprises an ultrasound driver  25  that drives the ultrasound transducers  40 . One example of a suitable ultrasound driver, which can be used for this purpose is an AN2300™ultrasound system produced by Analogic Corp. (Peabody, Mass.). In this embodiment, the ultrasound driver performs some of the functions of the image processor  42 , driving the ultrasonic sensor and producing the two-dimensional ultrasound images. The ultrasound driver may support different imaging modes such as B-mode, M-mode, CW Doppler and color flow Doppler, as are known in the art.  
         [0060]     Typically, the positioning processor  36  and image processor  42  are implemented using a general-purpose computer, which is programmed in software to carry out the functions described herein. The software may be downloaded to the computer in electronic form, over a network, for example, or it may alternatively be supplied to the computer on tangible media, such as CD-ROM. The positioning processor and image processor may be implemented using separate computers or using a single computer, or may be integrated with other computing functions of the system  20 . Additionally or alternatively, at least some of the positioning and image processing functions may be performed using dedicated hardware.  
         [0000]     Two-Dimensional Anatomic Imaging  
         [0061]     Referring again to  FIG. 1 , using the catheter  27 , gated images, e.g., ultrasound images, of the heart are created, and registered with location data of the catheter  28 . Suitable registration techniques are disclosed in U.S. Pat. No. 6,650,927, the disclosure of which is herein incorporated by reference.  
         [0062]     Reference is now made to  FIG. 3 , which is a schematic exploded view of a diagnostic image  56  of the heart  24  ( FIG. 1 ), in accordance with a disclosed embodiment of the invention. The view is generated using a bullseye rendition technique. The image  56  comprises a stack of parallel slices  58 , which are perpendicular to an axis  60 . The slices are typically taken at a fixed slice increment along the axis  60 . Each slice shows a section  62 .  
         [0000]     Three-Dimensional Anatomic Imaging  
         [0063]     Referring again to  FIG. 1 , three-dimensional imaging is described in commonly assigned application Ser. No. 11/115,002 filed on Apr. 26, 2005, entitled  Three - Dimensional Cardiac Imaging Using Ultrasound Contour Reconstruction , which is herein incorporated by reference. Essentially, three-dimensional image is constructed by combining multiple two-dimensional ultrasound images, acquired at different positions of the catheter  27  into a single three-dimensional model of the target structure. The catheter  27  may operate in a scanning mode, moving between different positions inside a chamber of the heart  24 . In each catheter position, the image processor  42  acquires and produces a two-dimensional ultrasound image. In one embodiment, the catheter  27  is side-looking, and a partial three-dimensional reconstruction of the heart is obtained by dithering the catheter, using the manipulator  31 , so as to vary its roll angle in an oscillatory manner. Alternatively, the catheter  27  can be dithered so as to vary its pitch or yaw angle. In any case, the result is displayed as a three-dimensional segment of the cardiac chamber, including the catheter  28  and its current target structure.  
         [0064]     Alternatively, the catheter  28  is provided with a two-dimensional array of transducers  40  ( FIG. 2 ), which can be phased in order to sweep the beam in an oscillatory manner and thereby obtain different two-dimensional images of the target structure in a planes, while the catheter  28  is held in a fixed position.  
         [0000]     Tracking and Display  
         [0065]     Referring again to  FIG. 1 , during a medical procedure the system  20  can continuously track and display the three-dimensional position of the catheter  28 , using the catheter  27  to produce near real-time images of the catheter  28  and its target area. The positioning subsystem of the system  20  repetitively measures and calculates the current position of the catheter  28 . The calculated position is stored together with the corresponding slice or slices  58  ( FIG. 3 ). Typically, each position of the catheter  28  is represented in coordinate form, such as a six-dimensional coordinate (X, Y, Z axis positions, and pitch, yaw and roll angular orientations).  
         [0066]     The image processor  42  subsequently assigns three-dimensional coordinates to contours of interest, e.g., features identified in the set of images. The location and orientation of the planes of these images in three-dimensional space are known by virtue of the positional information, stored together with the images. Therefore, the image processor is able to determine the three-dimensional coordinates of any pixel in the two-dimensional images. When assigning the coordinates, the image processor typically uses stored calibration data comprising position and orientation offsets between the position sensor and the ultrasonic sensor, as described above.  
         [0067]     Alternatively, the system  20  can be used for three-dimensional display and projection of two-dimensional ultrasound images, without reconstructing a three-dimensional model. For example, the physician can acquire a single two-dimensional ultrasound image. Contours of interest on this image can be tagged using the procedures described below. The system  20  can then orient and project the ultrasound image in three-dimensional space.  
         [0068]     Reference is now made to  FIG. 4 , which schematically illustrates a mechanism used by the system  20  ( FIG. 1 ) to effect real-time control of an imaging catheter during a medical procedure in accordance with a disclosed embodiment of the invention. The positioning processor  36  uses signals developed by the position sensor  32  ( FIG. 2 ) to determine the location of the catheter  28 , and varies signals that are transmitted to the manipulator  31 . The catheter  27  is then automatically maneuvered by the manipulator  31 , such that the current location of the catheter  28  is always included in a field of view  35  of the catheter  27 . The positioning processor  36  also receives signals from the position sensor (not shown) in the catheter  27  so that it can determine the relative locations of the catheters  27 ,  28 .  
         [0069]     Using the information obtained from the catheters  28 ,  27 , the position sensing system determines the current appropriate location and orientation of the catheter  27 , and measures any deviations. It then automatically signals the manipulator  31  to execute compensatory maneuvers of the catheter  27 . Optionally, an annunciator  39  may audibly or visually cue the operator to override the manipulator  31  and adjust the position of the catheter  27  manually.  
         [0070]     In some embodiments, once the target is in proximity with the catheter  28 , an enhanced mode of operation is enabled. Using images developed by the image processor  42  ( FIG. 1 ), a target  38  is identified, generally by the operator, but alternatively using information obtained from a knowledge base or a pre-acquired map, as described below. The positioning processor  36  then instructs the manipulator  31  not only to include the catheter  28  in the field of view  35 , but also the target  38 . The system  20  ( FIG. 1 ) then displays the catheter  28  and the target  38  on the display  44  in a perspective view that is most helpful to the operator. For example, in endoscopic applications, the display  44  can present complementary angular views as requested by the operator.  
         [0000]     Alternative Embodiments  
         [0071]     The techniques of the present invention may also be used to keep the ultrasound catheter aimed toward a target that is not equipped with a position sensor. Referring again to  FIG. 1 , the catheter  27  may be controlled to aim the ultrasound beam continuously toward a landmark in the heart. There are alternative ways of fixing the location and orientation of the ultrasound beam to include the landmark.  
         [0072]     The operator  43  indicates fixed reference coordinates on a pre-acquired map. A suitable map can be prepared using the methods described in U.S. Pat. No. 6,226,542, whose disclosure is incorporated herein by reference, Essentially, a processor reconstructs a three-dimensional map of a volume or cavity in a patient&#39;s body from a plurality of sampled points on the volume whose position coordinates have been determined. In the case of a moving structure, such as the heart the sampled points are related to a reference frame obtained by gating the imaging data at a point in the cardiac cycle. When acquiring the map, a reference catheter is fixedly positioned in the heart, and the sampled points are determined together with the position of the reference catheter, which is used to register the points.  
         [0073]     Reference is now made to  FIG. 5 , which schematically illustrates a control mechanism used by the system  20  ( FIG. 1 ) to effect real-time tracking and control of an imaging catheter during a medical procedure in accordance with an alternate embodiment of the invention.  FIG. 5  is similar to  FIG. 4 , except now the positioning processor  36  does not receive signals from the location sensor of the catheter  27 . Instead, the position of the catheter  27  is determined automatically by the positioning processor  36  with reference to suitably transformed coordinates of a map  70 , which is shown in  FIG. 5  as a reconstructed heart volume. The map  70  has a plurality of sampled points  72 , which are used to reconstruct a surface  74 . A grid (not shown) is adjusted to form the surface  74 , in which each point on the grid receives a reliability value indicative of the accuracy of the determination. When the map  70  is displayed for the operator  43 , areas of the surface  74  that are covered by relatively less-reliable grid points may be displayed semi-transparently. Alternatively or additionally, different levels of semi-transparency are used together with a multi-level reliability scale.  
         [0074]     Alternatively, the map  70  may indicate coordinates of the target, which are then used as points of reference.  
         [0075]     The embodiments represented by  FIG. 5  may be used to aim the ultrasound catheter toward an important landmark, such as the left atrial appendage or the mitral valve. The purpose of this can be, e.g., to confirm that the area is not being damaged by the medical procedure or that emboli are not developing. As an additional example, the embodiments may be used to confirm the depth of ablation lesions.  
         [0076]     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof that are not in the prior art, which would occur to persons skilled in the art upon reading the foregoing description.