in fig1 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 m of rf receive coils 160 are placed about the subject . in the preferred embodiment , rf receive coils 160 are attached to the x - ray detection means 105 . invasive device 120 is modified to incorporate a small rf transmit coil ( not shown in fig1 ). 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 invasive device 120 ). the calculated position of invasive device 120 is displayed by superposition of a symbol 152 on the x - ray image appearing on video monitor 151 . following the preferred procedure the 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 fig2 a , 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 fig2 b . rf transmitter 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 , 20n , 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 g1 and drive transmit coils 30a , 30b , 30n , respectively . in the preferred embodiment , a number n of these transmit coils 30 a , 30b , 30n 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 the 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 the frame grabber means 54 which captures a single x - ray image from x - ray system 52 . frame grabber means 54 propagates the single x - ray image in video form to the 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 fig1 . 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 fig2 b communicates with a control computer 60 ( fig2 a ) through an interface connection 59 . control computer 60 is also interfaced through a control bus 62 to transmit frequency offset means 20a , 20b , 20n , gating means 21a , 21b , 20n , transmitter amplifier means 23a , 23b , 23n , receive frequency offset 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 x - ray detection means 105 ( fig1 ). control computer 60 is responsive to a 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 fig1 . reciprocity between pickup 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 transmit 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 fig2 a . control computer 60 notifies tracking computer 50 of fig2 b 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 , then a collection of l data points can be acquired from each a / d converter means . the data points are demultiplexed by fourier of 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 the filters of the m receivers . fig3 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 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 , 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 : where θ and φ are the angles shown in fig3 . in the present invention it is convenient to translate the location of the dipole to a position other than the origin , as illustrated in fig4 . 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 0 , y 0 , z 0 ). 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 : 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 an electromagnetic 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 - t 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 ( fig2 b ) is used to solve the ( x , y , z ) position data for the transmit coils acting as a dipole . a flow diagram of a method of solving for the position data is shown in fig5 . step 301 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 ( fig1 ) 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 a iterative non - linear optimization in which a multi - dimensional newton - raphson procedure is employed as explained in numerical recipes , the art of scientific computing , by william h . press , brian p flannery , saul a . teukolsky and william t . vetterling , 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 ( fig2 b ). at step 313 of the flow diagram , a determination is 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 in t 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 in at step 305 . referring now to fig1 automatic placement and alignment of the subject , is facilitated by use of a support arm 101 , within a desired region around invasive device 120 . this is accomplished by transferring the calculated position of the invasive device to tracking computer 50 . the last position of support arm 101 and size of the field of view of medical imaging device 103 are stored in tracking computer 50 . when the position of invasive device 120 begins to move outside of the field of view of medical imaging device 103 , tracking computer causes positioning means 70 to alter the position and orientation of support arm 101 in relation to support table 110 . an x - ray image is initiated whenever invasive device 120 enters a region of the subject for which the x - ray image does not cover . the tracking computer 50 of fig2 b keeps track of the field of view of the imaging system and compares this with the current location and position of the invasive device . this embodiment frees the operator from the task of keeping the invasive device 120 within the field - of - view of the x - ray system 52 and potentially reduces the number of personnel required for the procedure . radiologic images upon which a symbol representing the invasive device is superimposed may be obtained by means other than x - rays . images obtained with magnetic resonance scanners , ultrasound scanners , positron emission tomography scanners and the like , shown as an imaging means 106 of fig6 can 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 .
a tracking system which measures the position and orientation of invasive devices is used to automatically control the location of the x - ray imaging system gantry and / or patient table . the position and orientation of the invasive device , such as a catheter , are measured by detection of a radiofrequency field generated by the invasive device . the invasive device has at least one transmit coil attached near its end and is driven by a low power rf source to produce a dipole electromagnetic field that can be detected by an array of receive coils distributed around a region of interest . automatic gantry placement facilitates medical diagnostic and therapeutic x - ray procedures and in some instances can reduce the number of personnel required to perform these procedures .