Stereoscopic X-ray fluoroscopy system using radiofrequency fields
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to applications &#34;Tracking System To Follow the Position and Orientation of a Device with Radiofrequency Field Gradients, &#34; Ser. No. 07/753,565, &#34;Tracking System to Follow the Position and Orientation of a Device with Radiofrequency Fields&#34; Ser. No. 07/753,563, &#34;Automatic Gantry Positioning for Imaging Systems&#34; Ser. No. 07/753,567 and &#34;Multi-Planar X-ray Fluoroscopy System using Radiofrequency Fields&#34; Ser. No. 07/753,566 all by Charles L. Dumoulin, all filed simultaneously with this application, and all assigned to the present assignee. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to medical procedures in which an invasive device such as a catheter, guide wire, biopsy needle, endoscope, laparoscope or the like is inserted into a body, and more particularly concerns the tracking of such a device stereoscopically, in three dimensions, without the use of X-rays. 
     X-ray fluoroscopes are used routinely to monitor the placement of invasive devices during diagnostic and therapeutic medical procedures. Conventional X-ray fluoroscopes are designed to minimize X-ray dosage. Nevertheless, some procedures can be very long and the accumulated X-ray dose to the subject can become significant. The long term exposure of the attending medical staff is of even greater concern since they participate in these procedures regularly. Consequently, it is desirable to reduce the X-ray dose during these procedures. 
     One consequence of the need to minimize X-ray dose is the use of minimal fields-of-view. Smaller fields-of-view necessitate frequent movement of the X-ray detection system or of the subject to follow the invasive devices during a procedure. Automatic positioning of the field-of-view of the imaging system over the invasive device within may shorten the procedure and reduce the number of required personnel need to perform the procedure. 
     Another limitation on the use of X-ray fluoroscopes is that the technique is projective in nature and produces a single two-dimensional image. Information concerning the depth of an object within the field-of-view is not available to the operator. It is often desirable to obtain this information during invasive procedures. Stereoscopic X-ray fluoroscopes utilizing dual X-ray beams have been devised for this purpose. Dual beam X-ray systems, however, double the X-ray dose to the subject and are more complicated. 
     SUMMARY OF THE INVENTION 
     Tracking of catheters and other invasive devices without X-rays is accomplished using RF transmitters and receivers. An invasive device such as a guide wire, catheter, endoscope, laparoscope or biopsy needle is modified by attaching a small RF transmit coil near its end. This transmit coil is driven by a low power RF source and creates a dipole electromagnetic field. The dipole field induces currents and voltages in an array of receive coils distributed about the region of interest. These voltage signals are digitized and sent to a tracking computer for analysis. The tracking computer utilizes non-linear iterative methods to solve for the position and orientation of the transmitting coil. This positional information is then superimposed on a stereoscopic X-ray image to give the operator real-time information on the three-dimensional location of the invasive device within the subject. Because the X-ray images are updated less frequently than conventional catheter tracking systems and a single X-ray source is requires instead of two X-ray sources, the X-ray dose to the subject is greatly reduced. 
     OBJECTS OF THE INVENTION 
     It is an object of the present invention to provide stereoscopic tracking of the three-dimensional location of an invasive device without using X-rays. 
     It is another object of the present invention to provide stereoscopic tracking of an invasive device, in three dimensions, which minimizes any X-ray dose to the subject and medical staff. 
     It is another object of the present invention to provide an interactive stereoscopic image of an invasive device superimposed upon another medical diagnostic image. 
     It is another object of the present invention to provide an interactive stereoscopic image of an invasive device enabling visual depth perception as an aid in placement of an invasive device inside a subject. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself, however, both as to organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawing in which: 
     FIG. 1 is a perspective view of one embodiment of the present invention in operation, tracking the location and orientation of an invasive device in a subject. 
     FIGS. 2A and 2B together are a schematic block diagram of a radiofrequency tracking system according to the present invention. 
     FIG. 2C is a diagram illustrating how FIGS. 2A and 2B are assembled. 
     FIG. 3 is a vector representation of an electromagnetic dipole located at the origin. 
     FIG. 4 is a vector representation of an electromagnetic dipole located at a position in space other than the origin. 
     FIG. 5 is a flowchart of the method used to compute the position and orientation of an invasive device. 
     FIG 6 is a perspective view of another embodiment of the present invention employing alternate imaging means. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 1, a support arm 101 capable of being rotated about at least one axis 102 and translated by gantry control means 70 is shown. Support arm 101 holds an X-ray source 103 that emits a substantially collimated beam of X-rays 104 suitable for X-ray imaging and X-ray fluoroscopy. Support arm 101 also holds an X-ray detection means 105 aligned with the propagation direction of X-rays 104 emitted by X-ray source 103. X-rays 104 penetrate a subject support table 110 and a subject 112. An invasive device 120 is inserted into the subject by an operator 140. The location of the invasive device 120 is visible on the display of an X-ray image on a display monitor 151 of display means 150 driven by a tracking/display unit 108. In fluoroscopic usage, this image is acquired and displayed several (12 to 60) times a second. 
     According to the invention, a plurality of M RF receive coils 160 are placed about the subject. In the preferred embodiment of this invention RF receive coils 160 are attached to X-ray detection means 105. Invasive device 120 is modified to incorporate a small RF transmit coil (not shown in FIG. 1). The transmit coils may be attached to several invasive devices, with at least one coil per invasive device to determine its position and at least two coils per invasive device to determine its orientation. Tracking/display unit 108 provides power to the RF transmit coil to create a dipole electromagnetic field which is detected by RF receive coils 160. The signals detected by receive coils 160 are used by tracking/display unit 108 to calculate the position and orientation of the transmit coil (and therefore the invasive device 120). The calculated position of invasive device 20 is displayed by superposition of a symbol 152 on the X-ray image appearing on video monitor 151. 
     Following the preferred procedure, operator 140 initiates the acquisition of X-ray image only when it is deemed necessary, in order to minimize X-ray dose to the subject 112 and operator 140. The instantaneous location of invasive device 120 is updated several times per second (ideally 12 to 60 times per second) and provides an approximation of the fluoroscopic image of invasive device 120 that operator 140 would expect to see with a conventional X-ray fluoroscopic system. 
     Tracking/display unit 108 is comprised of an RF transmitter 5 and an RF receiver 7 as shown in FIG. 2A, and analog-to-digital (A/D) converters 48a, 48b, 48m, a tracking computer 50, a frame grabber 54, and a superposition means 56 as shown in FIG. 2B. RF transmitter 5 employs a master oscillator 10 that generates a signal at a selected frequency. This signal is propagated to a plurality of N transmit frequency offset means 20a, 20b, 20n which generate a plurality of N signals of selected different frequencies. Each transmit frequency offset means 20a, 20b, 20n propagates its signal to a gating means 21a, 21b, 21n, respectively, which either passes the signal to an amplifier means 23a, 23b, 23n, respectively, or blocks the signal thereto. Amplifier means 23a, 23b, 23n boost the signals by a selected gain Gl and drive transmit coils 30a, 30b, 30n, respectively. In the preferred embodiment, a number N of these transmit coils 30a, 30b, 30 n are situated on invasive device 120. 
     The signals generated by the transmit coils are detected by a plurality of M receive coils 40a, 40b, 40m placed at known locations and with known orientation about the subject. Each receive coil 40a, 40b, 40m detects signals emitted by all transmit coils. The amplitudes and phases of these detected signals are a function of the relative placement and orientations of the transmit and receive coils. The signals detected by each receive coil 40a, 40b, 40m are propagated to low-noise amplifiers 42a, 42b, 42m, respectively, in RF receiver 7, where they are amplified by a selected gain factor G2. The amplified signals are passed from low-noise amplifiers 42a, 42b, 42m to quadrature phase detectors 44a, 44b, 44m, respectively, where they are mixed with a reference signal from a receive frequency offset means 43 that is driven by master oscillator 10. Mixing two signals in each quadrature phase detector results in a signal having a component at a frequency equal to the sum of the input frequencies, and a component at a frequency equal to the difference of the input frequencies. The component of interest in the preferred embodiment of this invention is the component equal to the difference of the input frequencies. The signals are propagated in quadrature fashion (i.e. as pairs of signals having a 90° degree phase difference) to filters 46a, 46b, 46m, respectively, where the low frequency component is selected and propagated to A/D converters 48a, 48b, 48m, respectively. The A/D converters convert the low frequency signals in each quadrature pair to digital form. This digital information is sent to tracking computer 50 through a data bus 51. The tracking computer calculates the positions and orientations of the N transmit coils using the digitized signals derived from the M receive coils. The calculated positions and orientations of the N transmit coils are transformed to display coordinates by computer 50. 
     X-ray imaging and fluoroscopy system 52 generates a video signal that is propagated to frame grabber means 54 which captures a single X-ray image from X-ray system 52. Frame grabber means 54 propagates the single image in video form to superposition means 56 which overlays a symbol 152 on the video signal supplied by frame grabber means 54. The composite video signal is propagated to a suitable display means 150 such as video monitor 151 shown in FIG. 1. The X-ray image will detect the invasive device. The tracking computer 50 is initialized by placing the invasive device at an origin marked on the table 110, and setting the position to zero. The X-ray system, likewise is adjusted to coincide with the origin marked on the table. Tracking computer 50 of FIG. 2b communicates with a control computer 60 (FIG. 2A) through an interface connection 59. Control computer 60 is also interfaced through a control bus 62 to transmit frequency offset means 20a, 20b, 20n, transmitter gating means 21a, 21b, 21n, transmitter amplifier means 23a, 23b, 23n, receive frequency means 43, and filters 46a, 46b, 46m. Furthermore, tracking computer 50 is interfaced through an interface connection 75 to a gantry control means 70 which is capable of changing the relative position and orientation of the subject and the X-ray detection means 105 (FIG. 1). Control computer 60 is responsive to the timing signal from master oscillator 10. 
     In the preferred embodiment of this invention, transmit coils 30a, 30b, 30n are placed on invasive device 120 illustrated in FIG. 1. Reciprocity between receive and transmit coils exists such that placement of receive coils 40a, 40b, 40m on invasive device 120 and placement of transmit coils 30a, 30b, 30n outside the subject is possible. 
     In the illustrated embodiment of the invention, a minimum of N=1 coils and M=5 receive coils is required to unambiguously determine the location and orientation of invasive device 120. It can be advantageous, however, to have N&gt;1 transmit coils to provide location and orientation for multiple points on the invasive device and/or multiple invasive devices. 
     Several methods for detecting signals from multiple transmit coils are possible. One method requires that only one of the N gating means be allowed to propagate signal at any instant. Selection of the propagating gating means is made by control computer 60 of FIG. 2A. Control computer 60 notifies tracking computer 50 of FIG. 2B of a gating means selected. The process is repeated for each of the N coils. Thus, tracking computer 60 is able to calculate the position of N transmit coils. 
     An alternative embodiment requires that all N transmit coils be active at once, each transmitting at a different frequency. If all N transmit frequencies are within the selected bandwidth of each filter means, then a collection of L data points can be acquired from each A/D converter means. The data points are demultiplexed by Fourier or Hadamard transformation to separate the individual frequency components arising from each transmit coil. Alternatively, M receivers can be constructed for the N transmitters if each transmit frequency is within the bandwidth of filter means of M receivers. 
     FIG. 3 is a vector representation of an electromagnetic dipole 200 located in a three-dimensional coordinate system comprising an X axis, Y axis and Z axis, and having an origin 201. The strength of the electromagnetic field generated by the dipole at a given position 205 (given as x, y and z) in three-dimensional space is a function of position 205, the orientation of the dipole, here defined by the rotation angles θ and φ and the physical constant μ o  known as the permeability of free space, and can be expressed as: ##EQU1## 
     In this equation the electromagnetic field at a selected position 205 in space is divided into three orthogonal components defined by the unit vector quantities i, j and k. R represents the distance between the location of the dipole and the selected position, and is defined as: ##EQU2## 
     The quantities M x , and M y  and M z  represent the vector components of the unit dipole along the x, y and z axes. These quantities can be expressed as: 
     
         M.sub.z =cos(θ)                                      [3] 
    
     
         M.sub.y =sin(φ)sin(θ)                            [4] 
    
     
         M.sub.x =cos(φ)sin(θ)                            [5] 
    
     where θ and φ are the angles shown in FIG. 3. 
     In the present invention it is convenient to translate the location of the dipole to a position other than the origin, as illustrated in FIG. 4. Since the coordinate system is simply translated and not rotated, the rotation angles in the new coordinate system θ&#39; and φ&#39; are identical to the original rotation angles θ and φ. The translated origin is 201&#39; (x o , y o , z o ). The translated dipole 200&#39; creates an electromagnetic field at a selected receive coil, i, in space at position 205 (x i , y i , z i ) which can be calculated from equation 1 using the following substitutions for x, y and z: 
     
         x=x.sub.i -x.sub.o                                         [6] 
    
     
         y=y.sub.i -y.sub.o                                         [7] 
    
     
         z=z.sub.i -z.sub.o.                                        [8] 
    
     Each receive coil is positioned at a predetermined location, with receive coil 1 being located at (x 1 , y 1 , z 1 , receive coil 2 at (x 2 , y 2 , z 2 ) etc. Receive coil 1 experiences field of flux density B 1  at location (x 1 , y 1 , z 1 ) from the transmit coil transmitting from point (x 0 , y 0 , z 0 ), translated from the origin. 
     The same transmit coil at the same point (x 0 , y 0 , z 0 ), causes receive coil 2 to experience a magnetic field of flux density B 2  at a location (x 2 , y 2 , z 2 ). This is true for all receive coils at a given instant. 
     Since x=x i  -x 0 , y=y i  -y 0 , and z=z i  -z 0 , x, y and z in equation 1 can be replaced by known quantities resulting in: ##EQU3## for each of i=1 to M receive coils. These equations can be generalized as: ##EQU4## which is a set of M equations with 5 unknowns. These equations are solvable provided M≧5, meaning that there must be at least 5 receive coils. 
     Tracking computer 50 (FIG. 2B) is used to solve for the (x, y, z) position data. A flow diagram of a method employed by the tracking computer for determining the position data is shown in FIG. 5. Step 301 of the method serves as the entry into the method. Step 303 initializes variables to appropriate values. Step 304 provides an initial guess for the location and orientation (θ, φ, x 0 , y 0 , z 0 ) of the N transmit coils being tracked. This initial guess is obtained by placing the invasive device at a predetermined position which is marked on table 110 (FIG. 1) as being (x 0 ,y 0 ,z 0 ) location, and aligning the invasive device at a predetermined orientation (θ, φ), also marked on the table, at the beginning of the tracking process. 
     At step 305, data are acquired from the M receive coils. At step 307, the N components contained in each of the M sets of data acquired by the receive coils are separated. This separation can be accomplished by Fourier or Hadamard transformation or any other appropriate method. At step 309, the position of each of the N transmit coils is computed by using equation [9]. Step 309 can be performed by any suitable mathematical method, although our preferred method is an iterative non-linear optimization in which a multi-dimensional Newton-Raphson procedure is employed, as explained in NumericaI Recipes, The Art of Scientific Computing, by William H. Press, Brian P Flannery, Saul A. Teukolsky and William T. Vetterli Cambridge University Press, 1986, chapter 9.4 &#34;Newton-Raphson Method Using Derivatives&#34; hereby incorporated by reference. The Newton-Raphson procedure is applied to minimize the difference between the data acquired from the M receive coils and the predicted data calculated by using equations [1-9] for the best guess of the N transmit coil positions (x 0 , y 0 , z 0 ) and orientations (θ, φ). At step 311 of the flow diagram, the calculated position (x 0 , y 0 , z 0 ) of the N transmit coils 30a, 30b, 30n is displayed by the superposition means 56 and display means 150 (FIG. 2B). Step 313 of the flow diagram, a determination made as to whether the tracking process is complete. If the tracking process is complete, step 315 of the flow diagram is taken and the process stops, otherwise a new guess of the position (x 0 , y 0 , z 0 ) and orientation (θ, φ) for each of the N transmit coils is made at step 317 of the flow diagram. The presently preferred method to make the guess at step 317 is a linear extrapolation of the position and orientation based on the two immediately prior positions and orientations calculated for each of the N coils. After step 317 of the flow diagram is complete, the process continues with the acquisition of new data at step 305. 
     Automatic placement and alignment of the subject of FIG. 1 within a desired region around the location of invasive device 120 is facilitated by use of a support arm 101. The calculated position of the invasive device from tracking computer 50 (FIG. 2B) is supplied to a positioning means 70 which controls the position and orientation of support arm 101 in relation to support table 110. An X-ray image can be also initiated whenever invasive device 120 enters a region of the subject for which an additional X-ray image is required. This embodiment frees the operator from the task of keeping invasive device 120 within the field-of-view of the X-ray system 52 and potentially reduces the number of assistants that the operator requires. 
     Referring again to FIG. 1, the present invention generates a stereoscopic X-ray display for the purpose of improved visualization of tracking device 120 and its surroundings. The operator initiates acquisition of a first X-ray image at a selected position and orientation of support arm 101. Support arm 101 is then rotated a predetermined angle about a rotation axis 102 and a second X-ray image is obtained. Alternatively, an X-ray system incorporating a second X-ray detection system and two X-ray beams can be used to generate the first and second X-ray images. The first image is displayed to be viewed by only one of the operator&#39;s eyes with the second image is displayed to be viewed by only the other eye. This is accomplished by generating two images upon a display device which are coded such that a viewer wearing glasses 156 will see a different image with each eye creating a stereoscopic image for the operator. A first lens of glasses 156 filters out the first image and passes the second image to a first eye of the viewer. Similarly, a second lens of glasses 156 filters out the second image and passes the first image to a second eye of the viewer. A symbol 152 representing the location of invasive device 120 is superimposed on each of the first and second images. The resulting perception by the operator is one of a three-dimensional structure representing the subject in which the symbol 152 representing invasive device 120 can be perceived to move in any direction including depth within the screen. 
     Medical diagnostic images may be obtained by means other than X-rays such as the use of Magnetic Resonance scanners, Ultrasound scanners, Positron Emission Tomography scanners and the like shown as an imaging means 106 of FIG. 6. These images may be used in place of the X-ray images. 
     While several presently preferred embodiments of the novel radiofrequency tracking system have been described in detail herein, many modifications and variations will now become apparent to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and variations as fall within the true spirit of the invention.