The present invention relates to remote manipulation of a probe such as a catheter and, more particularly, to a method for determining the position and orientation, with respect to a coordinate system external to an opaque body, of a probe within the opaque body.  Typically, the opaque body is a patient undergoing a medical procedure and the probe is a catheter. 
It is known to navigate a catheter through body cavities of a patient by electromagnetic means.  See, for example, U.S. Pat.  No.  5,558,091, to Acker et al.,  PCT Application WO 96/05768, to Ben-Haim et al.,  and PCT Application IL99/00371.  In particular, PCT Application IL99/00371, which is incorporated by reference for all purposes as if fully set forth herein, teaches devices and methods for navigating a catheter through body cavities of a patient while simultaneously obtaining, in real time, images of the portion of the patient through which the catheter is inserted, and superposing on the images a representation of the catheter in true position and orientation.  FIG.  1 illustrates a C-mount fluoroscope 80 modified according to the teachings of IL99/00371 for simultaneous real-time image acquisition and intrabody navigation.  Fluoroscope 80 includes the conventional components of a C-mount fluoroscope: an x-ray source 82 and an image acquisition module 84 mounted on opposite ends of a C-mount 78, and a table 86 whereon the patient lies.  Image acquisition module 84 converts x-rays that transit the patient on table 86 into electronic signals representative of a 2D image of the patient.  C-mount 78 is pivotable about an axis 76 to allow the imaging of the patient from several angles, thereby allowing the reconstruction of a 3D image of the patient from successive 2D images.  In addition, either a three-component electromagnetic field receiver 114 or a three-component electromagnetic field transmitter 24 is rigidly mounted on C-mount 78.  Receiver 114 or transmitter 24 serves to define a coordinate system that is fixed relative to C-mount 78.  Transmitter 24, and another three-component electromagnetic field transmitter 24xe2x80x2 that is not rigidly mounted on C-mount 78, are driven by driving circuitry 32.  The electromagnetic waves generated by transmitter 24 or by transmitter 24xe2x80x2 are received by receiver 114 and by another receiver 14 inside a probe 10.  The signals from receiver 14 that correspond to the electromagnetic waves generated by transmitter 24 or by transmitter 24xe2x80x2 are sent to reception circuitry 34.  The signals from receiver 114 that correspond to the electromagnetic waves generated by transmitter 24 or by transmitter 24xe2x80x2 are sent to reception circuitry 134.  Reception circuitries 34 and 134 and driving circuitry 32 are controlled by a controller/processor 36 that directs the generation of transmitted signals by driving circuitry 32 and the reception of received signals by reception circuitries 34 and 134 In addition, controller processor 36 implements the algorithm of PCT IL99/00371 to infer the position and orientation of probe 10 relative to transmitter 24 or to infer the positions of probe 10 and receiver 114 relative to transmitter 24xe2x80x2.  Controller/processor 36 also directs the acquisition of an image of the patient by image acquisition module 84 of fluoroscope 80. 
By determining the position and orientation of probe 10 relative to the coordinate system defined by transmitter 24, controller/processor 36 determines the position and orientation of probe 10 relative to each acquired 2D image.  Alternatively, the electromagnetic signals are transmitted by transmitter 24xe2x80x2, and controller/processor 36 determines the position and orientation of probe 10 relative to the 2D images by determining the positions and orientations of receivers 14 and 114 relative to transmitter 24xe2x80x2.  Controller/processor 36 synthesizes a combined image that includes both the 3D image of the patient acquired by fluoroscope 80 and an icon representing probe 10 positioned and oriented with respect to the 3D image of the patient in the same way as probe 10 is positioned and oriented with respect to the interior of the patient.  Controller/processor 36 then displays this combined image on a monitor 92. 
As noted in PCT IL99/00371, the methods taught therein for intrabody navigation are suitable for use in conjunction with a wide variety of devices for acquiring 2D or 3D images of the interior of the patient, in modalities including CT, MRI and ultrasound in addition to fluoroscopy. 
Another family of systems, for determining the location of a probe such as a catheter within the body of a patient, uses a plurality of transducers on the surface of the patient""s body and at least one similar transducer in the tip of the probe.  The surface transducers define a reference system of coordinates.  The signals produced by the various transducers are distinguishable from one another, and the character of the signals is diagnostic of the position on or within the patient""s body at which the signals are received.  Typically, either signals from the surface transducers are received by the probe transducers, or, exploiting the principal of reciprocity, signals from the probe transducers are received by the surface transducers.  From these signals, the position of the tip of the probe relative to the reference coordinate system is determined.  It should be noted that in this family of systems, the reference coordinate system, with respect to which the position of the probe is determined, is defined only with reference to the patient""s body itself.  This family of systems is incapable of determining the location of the probe with respect to a coordinate system external to, or independent of, the patient""s body. 
One example of such a system is taught by Smith et al.  in U.S. Pat.  No.  5,515,853, which is incorporated by reference for all purposes as if fully set forth herein.  Ultrasonic signals are broadcast by a transducer (a piezoelectric crystal) at the tip of a catheter.  These signals are received by a plurality of similar transducers on the patient""s body, to measure the acoustic travel time from the tip of the catheter to the surface transceivers.  The location of the tip of the catheter within the patient""s body is determined by triangulation. 
Another example of such a system is taught by Wittkampf in U.S. Pat.  No.  5,697,377, incorporated by reference for all purposes as if fully set forth herein.  Three substantially orthogonal alternating electrical currents, each of a different frequency, are applied through the patient""s body via six electrodes mounted on the surface of the patient""s body.  A measuring electrode at the tip of a catheter measures the local electrical potential at each of the three frequencies.  The displacement of the measuring electrode relative to lines connecting the pairs of surface electrodes is assumed to be linear in the three measured potentials.  The surface electrodes define a (not necessarily orthogonal) reference coordinate system, and the three measured potentials are proportional to the coordinates of the measuring electrode in this coordinate system. 
One drawback of the teachings of IL99/00371 is the relatively high cost and complexity of electromagnetic receivers 14 and 114 relative to the ultrasonic transceivers of Smith et al.  and to the electrical transceivers of Wittkampf.  It would be highly advantageous to have a method of intrabody navigation that relies on an electromagnetic technique to determine the position and orientation of the patient relative to the imaging device, while using some other technique, such as the ultrasonic technique of Smith et al.  or the electrical technique of Wittkampf to determine the position and orientation of the probe relative to the patient""s body. 
The term xe2x80x9ctransducerxe2x80x9d usually is used to refer to a device that can interact with a physical field in either of two ways: by transmitting signals into the field or by receiving signals from the field.  The piezoelectric crystals of Smith et al.  are examples of transducers that interact in this manner with an acoustic field.  The electrodes of Wittkampf are examples of transducers that interact in this manner with an electric potential field.  As used herein, the term xe2x80x9ctransducerxe2x80x9d also includes devices that interact with a physical field only by transmitting signals into the field and devices that interact with a physical field only by receiving signals from the field.  The term xe2x80x9ctransducerxe2x80x9d, as used herein, thus includes within its scope receivers 14 and 114 that are used only for reception of electromagnetic signals and not for transmission of electromagnetic signals.  It should be noted that receivers 14 and 114 typically include wire coils that could be used either for transmission or for reception, but the small dimensions of probe 10 preclude the application of sufficiently high power levels to the coils of receiver 14 to allow receiver 14 to be used for transmission. 
Note that the field with which the transceiver interacts may be a scalar field, such as the acoustic field of Smith et al.  or the electric potential field of Wittkampf, or may be a vector field such as the electromagnetic field of PCT IL99/00371. 
Two or more transducers are said to have a xe2x80x9cfixed spatial relationshipxe2x80x9d to each other if the transducers are deployed so that their only allowed movements are collective translations or collective rigid rotations.  In the preferred embodiments discussed below, transducers are provided with fixed spatial relationships to each other by being attached rigidly to a common substrate such as a catheter.  Similarly, a transducer is said to have a xe2x80x9cfixed spatial relationshipxe2x80x9d to another body (e.g., a the body of a patient) if the transducers are attached to the body so that as the body moves, the position and orientation of the transducer mimics or reflects the position and orientation of the moving body. 
According to the present invention there is provided a method for determining a position and an orientation of a workpiece relative to a primary coordinate system, including the steps of: (a) providing a primary reference transducer that interacts with a primary field; (b) providing a plurality of secondary reference transducers that interact with a secondary field different from the primary field, the secondary transducers being in a fixed mutual spatial relationship so as to define a secondary coordinate system, the primary reference transducer being in a fixed spatial relationship to the secondary transducers; (c) measuring at least one value of the primary field, using the primary reference transducer; (d) inferring, from the at least one value of the primary field, a position and an orientation of the reference transducers relative to the primary coordinate system, thereby determining a position and an orientation of the secondary coordinate system relative to the primary coordinate system; (e) providing the workpiece with at least one workpiece transducer that interacts with the secondary field; (f) measuring a plurality of values of the secondary field, using the secondary reference transducers and the workpiece transducer; (g) inferring, from the values of the secondary field, a first measure of a position of the workpiece relative to the secondary coordinate system; and (h) combining the first measure of the position of the workpiece relative to the secondary coordinate system with the position and the orientation of the secondary coordinate system relative to the primary coordinate system to provide the position and the orientation of the workpiece relative to the primary coordinate system. 
According to the present invention there is provided a method of navigating a probe inside a body, including the steps of: (a) providing a primary reference transducer that interacts with a primary field; (b) providing a plurality of secondary reference transducers that interact with a secondary field different from the primary field, the secondary transducers being in a fixed mutual spatial relationship and in a fixed spatial relationship to the body, the primary reference transducer being in a fixed spatial relationship to the secondary transducers; (c) providing a device for acquiring an image of the body; (d) simultaneously: (i) acquiring the image of the body, (ii) measuring at least one value of the primary field; using the primary reference transducer, and (iii) inferring, from the at least one value of the primary field, a position and an orientation of the reference transducers relative to the device, thereby determining a position and an orientation of the body relative to the image; (e) determining a position of the probe relative to the body by: (i) providing the probe with at least one probe transducer that interacts with the secondary field; (ii) measuring a plurality of values of the secondary field, using the secondary reference transducers and the probe transducer, and (iii) inferring, from the values of the secondary field, the position of the probe relative to the body; and (f) displaying the image of the body with a representation of the probe superposed thereon according to the positions and the orientation. 
According to the present invention there is provided a system for navigating a probe through an opaque body, including: (a) a primary reference transducer that interacts with a primary field; (b) a plurality of secondary reference transducers that interact with a secondary field; (c) a mechanism for deploying the reference transducers in a fixed mutual spatial relationship; and (d) at least one probe transducer, having a fixed spatial relationship to the probe, that interacts with the secondary field. 
According to the present invention there is provided a method for determining a position and an orientation of a first workpiece relative to both a second workpiece and a primary coordinate system, including the steps of: (a) providing the second workpiece with at least one primary transducer that interacts with a primary field and that defines a workpiece coordinate system that is fixed with respect to the second workpiece; (b) providing a mechanism for generating a secondary field and for defining a secondary coordinate system; (c) providing the first workpiece with at least one secondary transducer that interacts with the secondary field; (d) providing a transformation from the secondary coordinate system to the workpiece coordinate system; (e) measuring at least one value of the secondary field, using the at least one workpiece transducer; (f) inferring, from the at least one value of the secondary field, a position of the first workpiece relative to the secondary coordinate system; (g) measuring at least one value of the primary field, using the at least one primary transducer; (h) inferring, from the at least one value of the primary field, a position and an orientation of the at least one reference transducer relative to the primary coordinate system, thereby providing a transformation from the workpiece coordinate system to the primary coordinate system; (i) combining the position of the first workpiece relative to the secondary coordinate system with the transformation from the secondary coordinate system to the workpiece coordinate system to provide the position and the orientation of the first workpiece relative to the second workpiece; and (j) combining the position and the orientation of the first workpiece relative to the second workpiece with the transformation from the workpiece coordinate system to the primary coordinate system to provide the position and the orientation of the first workpiece relative to the primary coordinate system. 
According to the present invention there is provided a method of navigating a probe inside a body, including the steps of: (a) providing a device for acquiring an image of the body; (b) providing the body with at least one body transducer that is in a fixed spatial relationship to the body and that interacts with a primary field; (c) simultaneously: (i) acquiring the image of the body, (ii) measuring at least one value of the primary field, using the at least one body transducer, and (iii) inferring, from the at least one value of the primary field, a position and an orientation of the at least one body transducer relative to the device, thereby determining a position and an orientation of the body relative to the image; (d) determining a position of the probe relative to the body by: (i) providing the probe with at least one probe transducer that interacts with a secondary field; (ii) measuring a plurality of values of the secondary field, using the at least one probe transducer, and (iii) inferring, from the values of the secondary field, the position of the probe relative to the body; and (e) displaying the image of the body with a representation of the probe superposed thereon according to the positions and the orientation. 
According to the present invention there is provided a system for navigating a probe through an opaque body, including: (a) at least one primary transducer that interacts with a primary field; (b) a generating mechanism for generating a secondary field; (c) a securing mechanism for securing the at least one primary transducer and the generating mechanism in a fixed spatial relationship to the opaque body; and (d) at least one probe transducer, having a fixed spatial relationship to the probe, that interacts with the secondary field. 
In the context of intrabody navigation, the imaging device defines a primary coordinate system, and the physical field that is used to determine positions and orientations relative to the imaging device is considered to be a primary field.  For example, in the prior art system of FIG.  1, the primary field is an electromagnetic field, and receiver 14 interacts with the primary field.  According to the present invention, receiver 14 is replaced with one or more transducers of a secondary field.  According to one aspect of the present invention, the patient is provided with a plurality of secondary reference transducers that also interact with the secondary field and with a primary reference transducer that interacts with the primary field.  The reference transducers are in mutual fixed spatial relationship, typically by being rigidly attached to a reference probe that is inserted into the body cavity along with probe 10.  The secondary reference transducers define a secondary coordinate system for determining positions and optionally orientations relative to the patient""s body.  The primary reference transducer is used to determine the position and orientation of the secondary coordinate system of the patient""s body, with respect to the primary coordinate system of the imaging device.  The secondary reference transducers are used along with the transducers that replace receiver 14 to determine the position and optionally the orientation of probe 10 relative to the secondary coordinate system.  The positions and orientation or orientations thus determined are combined by well-known methods to provide the position, and optionally the orientation, of probe 10 relative to the images provided by the imaging device. 
According to a second aspect of the present invention, the mechanism for generating the secondary field itself defines the secondary coordinate system.  Under this aspect of the present invention, which lacks the reference probe, one or more primary transducers are attached to the patient""s body to provide a body coordinate system.  For calibration, the catheter is provided with a calibration transducer, having a fixed spatial relationship to the secondary field transducers, that also interacts with the primary field.  Simultaneous measurements of the catheter position with respect to the body, using the primary transducers and the calibration transducer, provides a transformation from secondary coordinates to body coordinates.  During the calibration, the body coordinates coincide with the secondary coordinates.  Subsequently, field measurements made using the primary transducers provide transformations from body coordinates to primary coordinates, and the two transformations are combined to provide the position and the orientation of the catheter in primary coordinates.