Stereoscopic X-ray fluoroscopy system using radiofrequency fields

During an X-ray procedure, the position and orientation of an invasive device, such as a catheter are measured with radio frequency fields and displayed stereoscopically. Instantaneous three-dimensional positions of the invasive device are displayed by superposition of a graphic symbol on static X-ray images obtained at two different view angles. The X-ray images are obtained only when deemed necessary by the operator to minimize X-ray dose. A single X-ray source and detector may be implemented since it is not necessary to obtain the X-ray images simultaneously.

CROSS REFERENCE TO RELATED APPLICATIONS 
This application is related to applications "Tracking System To Follow the 
Position and Orientation of a Device with Radiofrequency Field Gradients, 
" Ser. No. 07/753,565, "Tracking System to Follow the Position and 
Orientation of a Device with Radiofrequency Fields" Ser. No. 07/753,563, 
"Automatic Gantry Positioning for Imaging Systems" Ser. No. 07/753,567 and 
"Multi-Planar X-ray Fluoroscopy System using Radiofrequency Fields" 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.

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. 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 .theta. and 
.phi. and the physical constant .mu..sub.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.sub.x, and M.sub.y and M.sub.z represent the vector 
components of the unit dipole along the x, y and z axes. These quantities 
can be expressed as: 
EQU M.sub.z =cos(.theta.) [3] 
EQU M.sub.y =sin(.phi.)sin(.theta.) [4] 
EQU M.sub.x =cos(.phi.)sin(.theta.) [5] 
where .theta. and .phi. 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 .theta.' and .phi.' are 
identical to the original rotation angles .theta. and .phi.. The 
translated origin is 201' (x.sub.o, y.sub.o, z.sub.o). The translated 
dipole 200' creates an electromagnetic field at a selected receive coil, 
i, in space at position 205 (x.sub.i, y.sub.i, z.sub.i) which can be 
calculated from equation 1 using the following substitutions for x, y and 
z: 
EQU x=x.sub.i -x.sub.o [6] 
EQU y=y.sub.i -y.sub.o [7] 
EQU 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.sub.1, y.sub.1, z.sub.1, receive coil 2 at 
(x.sub.2, y.sub.2, z.sub.2) etc. Receive coil 1 experiences field of flux 
density B.sub.1 at location (x.sub.1, y.sub.1, z.sub.1) from the transmit 
coil transmitting from point (x.sub.0, y.sub.0, z.sub.0), translated from 
the origin. 
The same transmit coil at the same point (x.sub.0, y.sub.0, z.sub.0), 
causes receive coil 2 to experience a magnetic field of flux density 
B.sub.2 at a location (x.sub.2, y.sub.2, z.sub.2). This is true for all 
receive coils at a given instant. 
Since x=x.sub.i -x.sub.0, y=y.sub.i -y.sub.0, and z=z.sub.i -z.sub.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.gtoreq.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 (.theta., .phi., x.sub.0, y.sub.0, z.sub.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.sub.0,y.sub.0,z.sub.0) location, and aligning the 
invasive device at a predetermined orientation (.theta., .phi.), 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 "Newton-Raphson 
Method Using Derivatives" 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.sub.0, y.sub.0, z.sub.0) and orientations (.theta., .phi.). 
At step 311 of the flow diagram, the calculated position (x.sub.0, 
y.sub.0, z.sub.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.sub.0, y.sub.0, z.sub.0) and orientation (.theta., .phi.) 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'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.