U.S. patent application number 13/185138 was filed with the patent office on 2011-11-03 for system and method for placing and determining an appropriately sized surgical implant.
This patent application is currently assigned to Medtronic Navigation, Inc.. Invention is credited to Kevin T. Foley, David A. Simon, Kurt R. Smith.
Application Number | 20110268248 13/185138 |
Document ID | / |
Family ID | 23050359 |
Filed Date | 2011-11-03 |
United States Patent
Application |
20110268248 |
Kind Code |
A1 |
Simon; David A. ; et
al. |
November 3, 2011 |
System and Method for Placing and Determining an Appropriately
Sized Surgical Implant
Abstract
Digital x-ray images taken before a surgical procedure by a
fluoroscopic C-arm imager are displayed by a computer and overlaid
with graphical representations of instruments being used in the
operating room. The graphical representations are updated in
real-time to correspond to movement of the instruments in the
operating room. A number of different techniques are described that
aid the physician in planning and carrying out the surgical
procedure.
Inventors: |
Simon; David A.; (Boulder,
CO) ; Smith; Kurt R.; (Eldorado Springs, CO) ;
Foley; Kevin T.; (Germantown, TN) |
Assignee: |
Medtronic Navigation, Inc.
Louisville
CO
|
Family ID: |
23050359 |
Appl. No.: |
13/185138 |
Filed: |
July 18, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12581487 |
Oct 19, 2009 |
7996064 |
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13185138 |
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10236013 |
Sep 5, 2002 |
7606613 |
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12581487 |
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09274972 |
Mar 23, 1999 |
6470207 |
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10236013 |
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Current U.S.
Class: |
378/62 |
Current CPC
Class: |
A61B 2034/256 20160201;
A61B 2090/376 20160201; A61B 2034/2055 20160201; G16H 40/63
20180101; A61B 6/12 20130101; A61M 25/0105 20130101; A61B 2034/2072
20160201; A61B 2034/104 20160201; A61B 6/547 20130101; A61B
2034/107 20160201; A61B 2034/105 20160201; A61M 2025/0166 20130101;
A61B 34/20 20160201; A61B 6/463 20130101; A61B 2034/108 20160201;
G06F 19/00 20130101 |
Class at
Publication: |
378/62 |
International
Class: |
A61B 6/02 20060101
A61B006/02 |
Claims
1. An x-ray imaging device comprising: a rotatable C-arm support
having first and second ends; an x-ray source positioned at the
first end for initiating an imaging cycle by generating x-ray
radiation; an x-ray receiving section positioned at the second end
so that x-rays emanating from the x-ray source enter the x-ray
receiving section, the x-ray receiving section generating an image
representing the intensities of the x-rays entering the x-ray
receiving section; a calibration and tracking target; a tracking
sensor for detecting the position, in three-dimensional space, of
the calibration and tracking target; and a computer communicating
with the x-ray receiving section and the tracking sensor, the
computer detecting motion of the C-arm based on changes in the
position detected by the tracking sensor.
2. The imaging device of claim 1, further comprising a dynamic
reference frame marker attached to anatomy of a patient, the
tracking sensor detecting the position, in three-dimensional space,
of the dynamic reference frame marker.
3. The imaging device of claim 1, further comprising means for
detecting the beginning and end of an imaging cycle and
transmitting indications of the detected beginning and end of the
radiation cycle to the computer.
4. The imaging device of claim 3 wherein the computer determines
that an image acquisition cycle is erroneous when the position of
the tracking target moves with respect to the patient during the
imaging cycle.
5. An x-ray imaging device comprising: a rotatable C-arm support
having first and second ends; an x-ray source positioned at the
first end for initiating an imaging cycle by generating x-ray
radiation; an x-ray receiving section positioned at the second end
so that x-rays emanating from the x-ray source enter the x-ray
receiving section, the x-ray receiving section generating an image
representing the intensities of the x-rays entering the x-ray
receiving section; a tracking target associated with the C-arm; a
tracking sensor for detecting the position, in three-dimensional
space, of the tracking target; and a computer communicating with
the x-ray receiving section and the tracking sensor, the computer
detecting motion of the C-arm based on changes in the position
detected by the tracking sensor.
6. The imaging device of claim 5 wherein the receiving section is
operable to convert x-rays directly to digital images, thereby
reducing distortion, which may be introduced by first converting to
visible light.
7. The imaging device of claim 5, further comprising a dynamic
reference frame marker attached to anatomy of a patient, the
tracking sensor detecting the position, in three-dimensional space,
of the dynamic reference frame marker.
8. The imaging device of claim 5, further comprising means for
detecting the beginning and end of an imaging cycle and
transmitting indications of the detected beginning and end of the
radiation cycle to the computer.
9. The imaging device of claim 5 wherein the computer determines
that an image acquisition cycle is erroneous when the position of
the tracking target moves with respect to the patient during the
imaging cycle.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 12/581,487 filed on Oct. 19, 2009, which is a divisional
of U.S. patent application Ser. No. 10/236,013 filed on Sep. 5,
2002, now U.S. Pat. No. 7,606,613 issued Oct. 20, 2009, which is a
continuation of U.S. patent application Ser. No. 09/274,972 filed
on Mar. 23, 1999, now U.S. Pat. No. 6,470,207 issued Oct. 22, 2002.
The disclosures of all of the above applications are incorporated
herein by reference.
[0002] This disclosure is related to U.S. patent application Ser.
No. 09/106,109, entitled "System and Methods for the Reduction and
Elimination of Image Artifacts in the Calibration of X-Ray
Imagers," filed on Jun. 29, 1998, now U.S. Pat. No. 6,118,845.
FIELD
[0003] The present invention is directed generally to image guided
surgery, and more particularly, to systems and methods for using
one or more fluoroscopic X-ray images to assist in instrument
navigation during surgery.
BACKGROUND
[0004] Modern diagnostic medicine has benefitted significantly from
radiology, which is the use of radiation, such as x-rays, to
generate images of internal body structures. In general, to create
an x-ray image, x-ray beams are passed through the body and
absorbed, in varying amounts, by tissues in the body. An x-ray
image is created based on the relative differences in the
transmitted x-ray intensities.
[0005] Techniques are known through which x-ray images are used to
locate the real-time position of surgical instruments in the
patient anatomy represented by the x-ray image without requiring
x-rays to be continually taken. In one such system, as disclosed in
U.S. Pat. No. 5,772,594 to Barrick, light emitting diodes (LEDs)
are placed on a C-arm fluoroscope x-ray imager, on a drill, and on
a reference bar positioned on the bone to be studied. A
three-dimensional optical digitizer senses the position of the
LEDs, and hence the position of the drill, the C-arm fluoroscope,
and the object bone. Based on this information, the real-time
position of the drill in anatomy represented by the x-ray image is
determined, and a corresponding representation of the drill in the
x-ray image is displayed. This allows the surgeon to continually
observe the progress of the surgery without necessitating
additional x-ray images.
[0006] Surgical navigational guidance, as discussed above, can
provide a tool for helping the physician perform surgery. It is an
object of the present invention to provide several enhancements to
traditional surgical navigational guidance techniques.
SUMMARY
[0007] Objects and advantages of the invention will be set forth in
part in the description which follows, and in part will be obvious
from the description, or may be learned by practice of the
invention. The objects and advantages of the invention will be
realized and attained by means of the elements and combinations
particularly pointed out in the appended claims.
[0008] One aspect of the present invention is directed to an x-ray
imaging device comprising a plurality of elements. In particular,
the x-ray imaging device includes an x-ray source for generating
cycles of x-ray radiation corresponding to an image acquisition
cycle; an x-ray receiving section positioned so that x-rays
emanating from the x-ray source enter the x-ray receiving section,
the x-ray receiving section generating an image representing
intensities of the x-rays entering the x-ray receiving section.
Additionally, a computer is coupled to the x-ray receiving section
and radiation sensors are located in a path of the x-rays emitted
from the x-ray source. The radiation sensors detect the beginning
and end of a radiation cycle and transmit the detected beginning
and end of the radiation cycle to the computer.
[0009] Another imaging device consistent with the present invention
includes a rotatable C-arm support having first and second ends.
The first end includes an x-ray source for initiating an imaging
cycle and the second end includes an x-ray receiving section
positioned so that x-rays emanating from the x-ray source enter the
x-ray receiving section. The x-ray receiving section generates an
image representing the intensities of the x-rays entering the x-ray
receiving section. Further, a calibration and tracking target is
included and a tracking sensor detects the position, in
three-dimensional space, of the calibration and tracking target;
and a computer is coupled to the x-ray receiving section and the
tracking sensor. The computer detects motion of the C-arm based on
changes in the position detected by the tracking sensor.
[0010] Another aspect consistent with the present invention is
directed to a surgical instrument navigation system. The system
comprises a computer processor; a tracking sensor for sensing
three-dimensional position information of a surgical instrument and
transmitting the position information to the computer processor; a
memory coupled to the computer processor, the memory including
computer instructions that when executed by the computer processor
cause the processor to generate an icon representing the surgical
instrument and to overlay the icon on a pre-acquired x-ray image,
the icon of the surgical instrument representing the real-time
position of the surgical instrument projected into the pre-acquired
x-ray image and the icon being generated as a first representation
when the surgical instrument is positioned such that it is
substantially viewable in the plane of the pre-acquired image and
the icon being generated as a second representation when the
surgical instrument is positioned such that it is substantially
perpendicular to the plane of the pre-acquired image. Finally, a
display is coupled to the processor for displaying the generated
icon superimposed on the pre-acquired image.
[0011] Yet another system consistent with the present invention
comprises a computer processor and a tracking sensor for sensing
three-dimensional position information of a surgical instrument and
transmitting the position information to the computer processor. A
memory is coupled to the computer processor, the memory including
computer instructions that when executed by the computer processor
cause the processor to generate an icon representing the surgical
instrument positioned in a pre-acquired image of a patient's
anatomy, the icon of the surgical instrument including a first
portion corresponding to an actual position of the surgical
instrument and a second portion corresponding to a projection of
the surgical instrument along a line given by a current trajectory
of the surgical instrument. A display is coupled to the processor
for displaying the generated icon superimposed on the pre-acquired
image.
[0012] Still further, another surgical instrument navigation system
consistent with the present invention comprises a rotatable C-arm
including an x-ray source and an x-ray receiving section for
acquiring x-ray images of a patient, the C-arm being rotatable
about one of a plurality of mechanical axes. A computer processor
is coupled to the rotatable C-arm and a memory is coupled to the
computer processor. The memory stores the x-ray images acquired by
the rotatable C-arm and computer instructions that when executed by
the computer processor cause the computer processor to generate a
line representing a projection of a plane parallel to one of the
plurality of the mechanical axes of the C-arm into the x-ray image,
the line enabling visual alignment of the one of the plurality of
mechanical axes of the C-arm with an axis relating complimentary
image views. A display is coupled to the processor for displaying
the generated line superimposed on the x-ray image.
[0013] Yet another system consistent with the present invention is
for defining a surgical plan and comprises an x-ray imaging device;
a surgical instrument; a tracking sensor for detecting the
position, in three-dimensional space, of the surgical instrument; a
computer processor in communication with the tracking sensor for
defining a point in a virtual x-ray imaging path as the
three-dimensional location of the surgical instrument, the point
being outside of a true x-ray imaging path of the x-ray imaging
device, the computer processor translating position of the surgical
instrument within the virtual x-ray imaging path to a corresponding
position in the true x-ray imaging path; and a display coupled to
the processor for displaying a pre-acquired x-ray image overlaid
with an iconic representation of the surgical instrument, the
position of the iconic representation of the surgical instrument in
the pre-acquired x-ray image corresponding to the translated
position of the surgical instrument.
[0014] Yet another system consistent with the present invention for
defining a surgical plan comprises a combination of elements. The
elements include an x-ray imaging device; a surgical instrument; a
tracking sensor for detecting the position, in three-dimensional
space, of the surgical instrument; a computer processor in
communication with the tracking sensor for calculating a projection
of the trajectory of the surgical instrument a distance ahead of
the actual location of the surgical instrument; and a display
coupled to the processor for displaying a pre-acquired x-ray image
overlaid with an iconic representation of the surgical instrument
and the calculated projection of the trajectory of the surgical
instrument.
[0015] Yet another system consistent with the present invention is
for aligning a first bone segment with a second bone segment in a
patient. The system comprises a first tracking marker attached to
the first bone segment and a second tracking marker attached to the
second bone segment. A tracking sensor detects the relative
position, in three-dimensional space, of the first and second
tracking markers. A computer delineates boundaries of images of the
first and second bone segments in a pre-acquired x-ray image and
when the second bone segment is moved in the patient, the computer
correspondingly moves the delineated boundary of the second bone
segment in the x-ray image. A display is coupled to the computer
and displays the pre-acquired x-ray image overlaid with
representations of the delineated boundaries of the first and
second bone segments.
[0016] Yet another system consistent with the present invention is
directed to a system for placing a surgical implant into a patient.
The system comprises a computer processor; means for entering
dimensions of the implant; a tracking sensor for sensing
three-dimensional position information of a surgical instrument on
which the surgical implant is attached, the tracking sensor
transmitting the position information to the computer processor;
and a memory coupled to the computer processor, the memory
including computer instructions that when executed by the computer
processor cause the processor to generate an icon representing the
surgical instrument and the attached surgical implant, and to
overlay the icon on a pre-acquired two-dimensional x-ray image, the
icon of the surgical instrument representing the real-time position
of the surgical instrument relative to the pre-acquired
two-dimensional x-ray image.
[0017] In addition to the above mention devices and systems, the
concepts of the present invention may be practiced as a number of
related methods.
[0018] An additional method consistent with the present invention
is a method of acquiring a two-dimensional x-ray image of patient
anatomy from a desired view direction. The method comprises
generating the two-dimensional image using an x-ray imager;
specifying a view direction in a three-dimensional image
representing the patient anatomy; generating a two-dimensional
digitally reconstructed radiograph (DRR) image based on the
three-dimensional image and the specified view direction; and
[0019] determining that the two-dimensional x-ray image corresponds
to the desired view direction by matching the DRR image to the
x-ray image.
[0020] Another method consistent with the present invention is a
method of calculating an angle between a surgical instrument and a
plane selected in an x-ray image. The method comprises a number of
steps, including: defining at least two points in the x-ray image;
defining a plane passing through the x-ray image as the plane
including the two points and linear projections of the two points
as dictated by a calibration transformation used to calibrate the
x-ray image for its particular imaging device; sensing a position
of the surgical instrument in three-dimensional space; and
calculating the angle between intersection of a projection of the
surgical instrument in three-dimensional space and the plane.
[0021] Yet another method consistent with the present invention is
a method for aligning a fluoroscopic imager with a view direction
of the medial axis of a patient's pedicle. The method comprises
displaying a three-dimensional image of an axial cross-section of
vertebra of the patient; extracting an angle from the
three-dimensional image corresponding to the angle separating an
anterior/posterior axis and the medial axis of the pedicle;
aligning the fluoroscopic imager with a long axis of the patient;
and rotating the fluoroscopic imager about the long axis of the
patient through the measured angle.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The accompanying drawings, which are incorporated in and
constitute a part of this specification, illustrate several
embodiments consistent with this invention and, together with the
description, help explain the principles of the invention. In the
drawings,
[0023] FIG. 1 is a diagram of an exemplary imaging system used to
acquire x-ray images;
[0024] FIG. 2 is an image illustrating true and distorted
images;
[0025] FIGS. 3A and 3B illustrate a projective transformation in a
fluoroscopic C-arm imager;
[0026] FIG. 4 is a flow chart illustrating methods consistent with
the present invention for performing two-dimensional navigational
guidance;
[0027] FIGS. 5A and 5B are exemplary fluoroscopic x-ray images
illustrating the iconic graphical overlay of a surgical
instrument;
[0028] FIG. 6 is a fluoroscopic image including a "cross hair"
graphical overlay of an instrument;
[0029] FIGS. 7A-7C illustrate images of complementary views and an
axis that relates them;
[0030] FIG. 8 is an image of a lateral view of a patient's
vertebral disc;
[0031] FIG. 9 is an image of a lateral view of a spinal
vertebra;
[0032] FIG. 10 is a diagram illustrating a system for specifying a
planned trajectory of a surgical instrument;
[0033] FIG. 11 is a flow chart illustrating a method for specifying
a planned trajectory of a surgical instrument;
[0034] FIGS. 12A through 12C are images of a fracture of a femur
containing two bone fragments;
[0035] FIG. 13 is a flow chart illustrating methods for aligning
bone fragments consistent with the present invention;
[0036] FIGS. 14A and 14B are images illustrating implantation of an
inter-vertebral cage in the spine of a patient;
[0037] FIGS. 15A through 15C are images used in the acquisition of
an x-ray view of the medial axis of a vertebral pedicle; and
[0038] FIGS. 16A and 16B are images used to illustrate the
measurement of out-of-plane angles based on fluoroscopic
images.
DETAILED DESCRIPTION OF THE DRAWINGS
[0039] As described herein, novel methods and systems improve
surgical navigational guidance using one or more fluoroscopic x-ray
images. The methods and systems may be used for either navigational
guidance using only two-dimensional fluoroscopic images or for
navigational guidance using a combination of two-dimensional
fluoroscopic images and three-dimensional volumetric images, such
as CT or MRI images.
[0040] Reference will now be made in detail to embodiments of the
invention, examples of which are illustrated in the accompanying
drawings. Wherever possible, the same reference numbers will be
used throughout the drawings to refer to the same or like
parts.
[0041] FIG. 1 is a diagram of an exemplary imaging system used to
acquire x-ray images. Fluoroscopic imaging device 100 is a
fluoroscopic C-arm x-ray imaging device that includes C-arm 103,
x-ray source 104, x-ray receiving section 105, a calibration and
tracking target 106, and radiation sensors 107. Calibration and
tracking target 106 includes infrared reflectors (or alternatively
infrared emitters) 109 and calibration markers 111. C-arm control
computer 115 allows a physician to control the operation of imaging
device 100, such as setting imaging parameters.
[0042] One appropriate implementation of imaging device 100 is the
"Series9600 Mobile Digital Imaging System," from OEC Medical
Systems, Inc., of Salt Lake City, Utah, although calibration and
tracking target 106 and radiation sensors 107 are typically not
included in the Series9600 Mobile Digital Imaging System and may
have to be added. The "Series9600 Mobile Digital Imaging System" is
otherwise structurally similar to imaging system 100.
[0043] In operation, x-ray source 104 generates x-rays that
propagate through patient 110 and calibration target 106, and into
x-ray receiving section 105. Receiving section 105 generates an
image representing the intensities of the received x-rays.
Typically, receiving section 105 comprises an image intensifier
that converts the x-rays to visible light and a charge coupled
device (CCD) video camera that converts the visible light to
digital images. Receiving section 105 may also be a device that
converts x-rays directly to digital images, thus potentially
avoiding distortion introduced by first converting to visible
light.
[0044] Fluoroscopic images taken by imaging device 100 are
transmitted to computer 115, where they may further be forwarded to
computer 120. Computer 120 provides facilities for displaying (on
monitor 121), saving, digitally manipulating, or printing a hard
copy of the received images. Three-dimensional images, such as
pre-acquired patient specific CT/MR data set 124 or a
three-dimensional atlas data set 126 (described in more detail
below) may also be manipulated by computer 120 and displayed by
monitor 121. Images, instead of or in addition to being displayed
on monitor 121, may also be displayed to the physician through a
heads-up-display.
[0045] Although computers 115 and 120 are shown as two separate
computers, they alternatively could be variously implemented as
multiple computers or as a single computer that performs the
functions performed by computers 115 and 120. In this case, the
single computer would receive input from both C-arm imager 100 and
tracking sensor 130.
[0046] Radiation sensors 107 sense the presence of radiation, which
is used to determine whether or not imaging device 100 is actively
imaging. The result of their detection is transmitted to processing
computer 120. Alternatively, a person may manually indicate when
device 100 is actively imaging or this function can be built into
x-ray source 104, x-ray receiving section 105, or control computer
115.
[0047] In operation, the patient is positioned between the x-ray
source 104 and the x-ray receiving section 105. In response to an
operator's command input at control computer 115, x-rays emanate
from source 104 and pass through patient 110, calibration target
106, and into receiving section 105, which generates a
two-dimensional image of the patient.
[0048] C-arm 103 is capable of rotating relative to patient 110,
allowing images of patient 110 to be taken from multiple
directions. For example, the physician may rotate C-arm 103 in the
direction of arrows 108 or about the long axis of the patient. Each
of these directions of movement involves rotation about a
mechanical axis of the C-arm. In this example, the long axis of the
patient is aligned with the mechanical axis of the C-arm.
[0049] Raw images generated by receiving section 105 tend to suffer
from undesirable distortion caused by a number of factors,
including inherent image distortion in the image intensifier and
external electromagnetic fields. Drawings representing ideal and
distorted images are shown in FIG. 2. Checkerboard 202 represents
the ideal image of a checkerboard shaped object. The image taken by
receiving section 105, however, can suffer significant distortion,
as illustrated by distorted image 204.
[0050] The image formation process in a system such as fluoroscopic
C-arm imager 100 is governed by a geometric projective
transformation which maps lines in the fluoroscope's field of view
to points in the image (i.e., within the x-ray receiving section
105). This concept is illustrated in FIGS. 3A and 3B. Image 300
(and any image generated by the fluoroscope) is composed of
discrete picture elements (pixels), an example of which is labeled
as 302. Every pixel within image 300 has a corresponding
three-dimensional line in the fluoroscope's field of view. For
example, the line corresponding to pixel 302 is labeled as 304. The
complete mapping between image pixels and corresponding lines
governs projection of objects within the field of view into the
image. The intensity value at pixel 302 is determined by the
densities of the object elements (i.e., portions of a patient's
anatomy, operating room table, etc.) intersected by the line 304.
For the purposes of computer assisted navigational guidance, it is
necessary to estimate the projective transformation which maps
lines in the field of view to pixels in the image, and vice versa.
Geometric projective transformation is well known in the art.
[0051] Intrinsic calibration, which is the process of correcting
image distortion in a received image and establishing the
projective transformation for that image, involves placing
"calibration markers" in the path of the x-ray, where a calibration
marker is an object opaque or semi-opaque to x-rays. Calibration
markers 111 are rigidly arranged in predetermined patterns in one
or more planes in the path of the x-rays and are visible in the
recorded images. Tracking targets, such as emitters or reflectors
109, are fixed in a rigid and known position relative to
calibration markers 111.
[0052] Because the true relative position of the calibration
markers 111 in the recorded images are known, computer 120 is able
to calculate an amount of distortion at each pixel in the image
(where a pixel is a single point in the image). Accordingly,
computer 120 can digitally compensate for the distortion in the
image and generate a distortion-free, or at least a distortion
improved image. Alternatively, distortion may be left in the image,
and subsequent operations on the image, such as superimposing an
iconic representation of a surgical instrument on the image
(described in more detail below), may be distorted to match the
image distortion determined by the calibration markers. The same
calibration markers can also be used to estimate the geometric
perspective transformation, since the position of these markers are
known with respect to the tracking target emitters or reflectors
109 and ultimately with respect to tracking sensor 130. A more
detailed explanation of methods for performing intrinsic
calibration is described in the references B. Schuele et al.,
"Correction of Image Intensifier Distortion for Three-Dimensional
Reconstruction," presented at SPIE Medical Imaging 1995, San Diego,
Calif., 1995 and G. Champleboux et al., "Accurate Calibration of
Cameras and Range Imaging Sensors: the NPBS Method," Proceedings of
the 1992 IEEE International Conference on Robotics and Automation,
Nice, France, May 1992, and U.S. application Ser. No. 09/106,109,
filed on Jun. 29, 1998 by the present assignee, the contents of
which are hereby incorporated by reference.
[0053] Calibration and tracking target 106 may be attached to x-ray
receiving section 105 of the C-arm. Alternately, the target 106 can
be mechanically independent of the C-arm, in which case it should
be positioned such that the included calibration markers 111 are
visible in each fluoroscopic image to be used in navigational
guidance. Element 106 serves two functions. The first, as described
above, is holding calibration markers 111 used in intrinsic
calibration. The second function, which is described in more detail
below, is holding infrared emitters or reflectors 109, which act as
a tracking target for tracking sensor 130.
[0054] Tracking sensor 130 is a real-time infrared tracking sensor
linked to computer 120. Specially constructed surgical instruments
and other markers in the field of tracking sensor 130 can be
detected and located in three-dimensional space. For example, a
surgical instrument 140, such as a drill, is embedded with infrared
emitters or reflectors 141 on its handle. Tracking sensor 130
detects the presence and location of infrared emitters or
reflectors 141. Because the relative spatial locations of the
emitters or reflectors in instrument 140 are known a priori,
tracking sensor 130 and computer 120 are able to locate instrument
140 in three-dimensional space using well known mathematical
transformations. Instead of using infrared tracking sensor 130 and
corresponding infrared emitters or reflectors, other types of
positional location devices are known in the art, and may be used.
For example, a positional location device may also be based on
magnetic fields, sonic emissions, or radio waves.
[0055] Reference frame marker 150, like surgical instrument 140, is
embedded with infrared emitters or reflectors, labeled 151. As with
instrument 140, tracking sensor 130 similarly detects the spatial
location of emitters/reflectors 151, through which tracking sensor
130 and computer 120 determine the three-dimensional position of
dynamic reference frame marker 150. The determination of the
three-dimensional position of an object relative to a patient is
known in the art, and is discussed, for example, in the following
references, each of which is hereby incorporated by reference: PCT
Publication WO 96/11624 to Bucholz et al., published Apr. 25, 1996;
U.S. Pat. No. 5,384,454 to Bucholz; U.S. Pat. No. 5,851,183 to
Bucholz; and U.S. Pat. No. 5,871,445 to Bucholz.
[0056] During an operation, dynamic reference frame marker 150 is
attached in a fixed position relative to the portion of the patient
to be operated on. For example, when inserting a screw into the
spine of patient 110, dynamic reference frame marker 150 may be
physically attached to a portion of the spine of the patient.
Because dynamic reference frame 150 is in a fixed position relative
to the patient anatomy, and instrument 140 can be accurately
located in three dimensional space relative to dynamic reference
frame 150, instrument 140 can also be located relative to the
patient's anatomy.
[0057] As discussed above, calibration and tracking target 106 also
includes infrared emitters or reflectors 109 similar to those in
instrument 140 or dynamic reference frame 150. Accordingly,
tracking sensor 130 and computer 120 may determine the
three-dimensional position of calibration target 106 relative to
instrument 140 and/or dynamic reference frame 150 and thus the
patient position.
[0058] In general, the imaging system shown in FIG. 1 assists
physicians performing surgery by displaying real-time or
pre-acquired images, such as fluoroscopic x-ray images, of patient
110 on display 121. Representations of surgical instruments 140 are
overlaid on pre-acquired fluoroscopic images of patient 110 based
on the position of the instruments determined by tracking sensor
130. In this manner, the physician is able to see the location of
the instrument relative to the patient's anatomy, without the need
to acquire real-time fluoroscopic images, thus greatly reducing
radiation exposure to the patient and to the surgical team.
"Pre-acquired," as used herein, is not intended to imply any
required minimum duration between receipt of the x-ray signals and
displaying the corresponding image. Momentarily storing the
corresponding digital signal in computer memory while displaying
the fluoroscopic image constitutes pre-acquiring the image.
[0059] FIG. 4 is a flow chart illustrating methods consistent with
the present invention for performing two-dimensional navigational
guidance using the system of FIG. 1. The physician begins by
acquiring one or more fluoroscopic x-ray images of patient 110
using imager 100 (step 400). As previously mentioned, acquiring an
x-ray image triggers radiation sensors 107, which informs computer
120 of the beginning and end of the radiation cycle used to
generate the image. For a fluoroscopic x-ray image acquired with
imager 100 to be useable for navigational guidance, imager 100,
when acquiring the image, should be stationary with respect to
patient 110. If C-arm 103 or patient 110 is moving during image
acquisition, the position of the fluoroscope will not be accurately
determined relative to the patient's reference frame. Thus, it is
important that the recorded position of imager 100 reflects the
true position of the imager at the time of image acquisition. If
imager 100 moves during the image acquisition process, or if imager
100 moves after image acquisition but before its position is
recorded, the calibration will be erroneous, thus resulting in
incorrect graphical overlays. To prevent this type of erroneous
image, computer 120 may examine the position information from
tracking sensor 130 while radiation sensors 107 are signaling
radiation detection. If the calibration and tracking target 106
moves relative to dynamic reference frame 150 during image
acquisition, this image is marked as erroneous. (Steps 401 and
402).
[0060] At the end of the radiation cycle, computer 120 retrieves
the acquired image from C-arm control computer 115 and retrieves
the location information of target marker 106 and dynamic reference
frame 150 from tracking sensor 130. Computer 120 calibrates the
acquired image, as described above, to learn its projective
transformation and optionally to correct distortion in the image,
(step 1403), and then stores the image along with its positional
information (step 404). The process of steps 400-404 is repeated
for each image that is to be acquired (step 405).
[0061] Because the acquired images are stored with the positional
information of the calibration and tracking target 106 and dynamic
reference frame 150, the position of C-arm 103, x-ray source 104,
and receiving section 105 for each image, relative to patient 110,
can be computed based upon the projective transformation identified
in the calibration process. During surgery, tracking sensor 130 and
computer 120 detect the position of instrument 140 relative to
dynamic reference frame 150, and hence relative to patient 110.
With this information, computer 120 dynamically calculates, in
real-time, the projection of instrument 140 into each fluoroscopic
image as the instrument is moved by the physician. A graphical
representation of instrument 140 may then be overlaid on the
fluoroscopic images (step 406). The graphical representation of
instrument 140 is an iconic representation of where the actual
surgical instrument would appear within the acquired fluoroscopic
x-ray image if imager 100 was continuously acquiring new images
from the same view as the original image. There is no theoretical
limit to the number of fluoroscopic images on which the graphical
representations of instrument 140 may be simultaneously
overlaid.
[0062] FIGS. 5A and 5B are exemplary fluoroscopic x-ray images
illustrating the iconic graphical overlay of a surgical instrument.
Fluoroscopic image 500, shown in FIG. 5A, is an image of a lateral
view of the lumbar spine. Graphical overlay 502 is the iconic
overlay of a surgical instrument, such as a drill, within image
500. As the physician moves the drill, computer 120 recalculates
and displays the new location of graphical overlay 502. The diamond
shaped end of overlay 502, labeled as area 503, represents the tip
of the instrument. The physician can use image 500 and overlay 502
to visualize the position and orientation of the surgical tool
relative to the patient's anatomy.
[0063] In certain situations, the physician may wish to know where
the tip of the instrument would be if the instrument were projected
along a line given by the instrument's current trajectory.
Consistent with an aspect of the present invention, at the
physician's command, computer 120 may calculate and display this
projection. Area 505 in FIG. 5B is a projection of graphical
overlay 502. As shown, the "look-ahead" trajectory 505 of overlay
502 is displayed in a different line style than overlay 502.
Computer 120 may vary the length of look-ahead trajectory 505 as
directed by the physician through a suitable computer interface
device, such as a keypad, mouse, or touch pad. In this manner,
computer 120 assists the physician in visualizing where the
instrument would be in the patient if it were advanced a
predetermined distance in the patient.
[0064] Although the "look-ahead" technique described above
projected the graphical representation of the instrument into the
image, there is no requirement that the instrument's graphical
representation be in the space of the image for look-ahead
trajectory 505 to be projected into the image. For example, the
physician may be holding the instrument above the patient and
outside the space of the image, so that the representation of the
instrument does not appear in the image. However, it may still be
desirable to project look-ahead portion 505 into the image to
facilitate planning of a surgical procedure.
[0065] When surgical instrument 140 is perpendicular to the plane
of the fluoroscopic image, the graphical overlay of the surgical
instrument essentially collapses to a point, making it difficult to
view. To alleviate this problem, computer 120 may optionally use a
different graphical representation of instrument 140 when the
distance in the image plane between the tip and the tail of
instrument 140 becomes smaller than a fixed distance (e.g., 15
pixels).
[0066] FIG. 6 is a fluoroscopic image including graphical overlay
601 of instrument 140, including a small "cross hair image"
representing tip 602 and a larger cross hair representing tail 603
of instrument 601. Computer 120 automatically switches between the
cross hair representation shown in FIG. 6 and the "straight line"
representation shown in FIG. 5.
[0067] Frequently, the physician would like to acquire two
complementary fluoroscopic images of the patient, such as images
from an anterior/posterior view and a lateral view of the vertebral
discs. The complementary views are related to one another by a
rotation about an axis by a particular amount. For example, an
anterior/posterior view is related to a lateral view by a 90 degree
rotation around the axis running parallel through the length of the
patient. When the mechanical axis of rotation of C-arm 103 is
aligned with the axis relating the complementary views (e.g., when
the mechanical axis is aligned with the axis running through the
length of the patient), the physician can accurately and quickly
switch between the complementary views by simply rotating C-arm 103
through the separation of the complementary views (usually 90
degrees). Generally, however, the axis of rotation of C-arm 103 is
not inherently aligned with the axis that relates the complementary
views, requiring the physician to perform a series of time
consuming trial-and-error based adjustments of the fluoroscope's
position through two or more axes of rotation.
[0068] Consistent with an aspect of the present invention, software
on computer 120 allows the surgeon to easily adjust the
fluoroscope's position so that one of its mechanical rotation axes,
such as the axis of rotation shown by arrows 108 in FIG. 1, is
aligned with the axis of rotation relating the complementary views.
The surgeon may then acquire the second image in the complementary
image set simply by rotating C-arm 103 a certain amount, typically
90 degrees, about the aligned axis.
[0069] Images of complementary views and the axis that relates them
are illustrated in FIGS. 7A-7C. The image of FIG. 7A illustrates a
lateral view of the patient's vertebral disc, in which the view
direction (i.e., the direction of the central ray of fluoroscopic
imager 100) is approximately parallel to the two vertebral end
plates, labeled as endplate 705 and endplate 706. Line 702 is the
projection of the plane substantially parallel to end plates 705
and 706. Similarly, the image shown in FIG. 7B is an
anterior/posterior view of the patient's vertebral disc, in which
the view direction is parallel to plane 702. The axis of rotation
704 that spatially relates the image view of FIG. 7A and the image
view of FIG. 7B is a line perpendicular to plane 702. That is,
rotating the image view of FIG. 7A ninety degrees about the line
perpendicular to plane 702 will result in the image view shown in
FIG. 7B. FIG. 7C is a three-dimensional representation of the
anatomy shown in FIGS. 7A and 7B. The line perpendicular to plane
702 is shown by axis of rotation 704.
[0070] FIG. 8 is an image of a lateral view of the patient's
vertebral disc, similar to FIG. 7A. In FIG. 8, however, computer
120 has drawn line 802, which represents the projection of a plane
that is perpendicular to one of the C-arm's mechanical axes. Line
804 represents the plane that spatially relates the complementary
views. With line 802 visible, the physician may adjust the position
of fluoroscopic imager 100 so that line 802 is lined up with line
804. At this point, switching between the complementary views
simply involves rotating C-arm 103 about a single mechanical
axis.
[0071] Although the alignment of lines 802 and 804, as discussed
above, was illustrated using both lines 802 and 804 drawn on the
fluoroscopic image, in practice, it may only be necessary to
display line 802 in the image. In this case, line 804 is mentally
visualized by the physician. Additionally, although the relation of
complimentary views was discussed using the example of the spine,
complimentary fluoroscopic images of other anatomical regions, such
as, for example, the pelvis, femur, or cranium, may similarly be
obtained by application of the above discussed concepts.
[0072] Before, or during, surgery, the physician may find it
desirable to input an operation "plan" to computer 120. The plan
may, for example, specify a desired trajectory of a surgical
instrument superimposed on a fluoroscopic image. During the
surgical navigation process, the goal of the surgeon would be to
align the graphical icon representing the real-time location of the
surgical instrument with the graphical overlay representing the
planned trajectory.
[0073] FIG. 9 is an image of a lateral view of a spinal vertebra.
Assume the goal of the operation plan is to define a line that
passes along a desired trajectory within the image of the vertebra.
One method of accomplishing this goal is to directly input the
desired trajectory information to computer 120 using traditional
computer input devices. While this method of directly interacting
with computer 120 is possible, it can be cumbersome and disruptive
during surgery. Consistent with an aspect of the present invention,
an alternative method of accomplishing this is for the physician to
position the surgical instrument on the surface of the bone or skin
in the desired orientation, and then project the tip of the
instrument forward using the previously described look-ahead
technique. More specifically, the desired trajectory is specified
by (1) adjusting the position and orientation of the instrument
near the patient with virtual look-ahead active, and (2) adjusting
the length of the virtual look-ahead. FIG. 9 illustrates the iconic
representation of instrument 901 and the virtual look-ahead
projection of the instrument 902. Once the desired trajectory is
achieved, the surgeon may direct computer 120 to "freeze" the
planned trajectory on display 121. The desired trajectory can be
obtained using one or more C-arm fluoroscopic images with two or
more being required to define a specific three-dimensional
trajectory which can then be displayed on any C-arm fluoroscopic
view. The freeze operation may be input to computer 120 through,
for example, a simple input device such as a foot pedal. The
physician may then proceed with the operation, using the overlay of
the planned target as a guide.
[0074] Yet another method consistent with the present invention for
specifying a planned trajectory of a surgical instrument, which,
unlike the method discussed above, does not require positioning the
surgical instrument on or near the patient's bone, is illustrated
in FIGS. 10 and 11.
[0075] As shown in FIG. 10, during the acquisition of an image,
patient 1001 is positioned between C-arm x-ray source 1004 and
x-ray receiving section 1006. Fluoroscopic images of patient 1001
are created by the x-rays emitted from x-ray source 1004 as they
travel in the path generally outlined by cone 1010 through patient
1001. Line 1011, in the center of cone 1010, corresponds to the
origin (i.e., the center point) in the acquired fluoroscopic
images. Physician 1020, standing within the range of tracking
sensor 1030, but away from patient 1001, commands the computer to
create an explicit correspondence between the fluoroscope's imaging
cone 1010 and a "virtual" cone 1012 at an arbitrary position in
space (which is visible to the tracking sensor). Once this virtual
cone has been defined, the surgical instrument 1040 can be
projected from this virtual cone into one or more pre-acquired
fluoroscopic images in the same manner as if the instrument were
located in the actual cone 1010 corresponding to a given image. In
this manner, physician 1020 can plan the trajectory of surgical
instrument 1040 by simply moving the instrument in the coordinate
system established by the virtual cone.
[0076] To define the correspondence between actual and virtual
cones, it is necessary for the physician to define the position of
the virtual cone relative to the tracking sensor. In general, there
are many ways to define a cone in space. For example, the position
and orientation of a cone can be defined by three points, one
corresponding to its apex, one corresponding to a second point
along its central axis, and a third corresponding to the rotation
of the cone about the central axis. Therefore, one way to define
the cone would be to use the tip of the surgical instrument to
define these three points in space relative to the tracking sensor.
Another way to define this correspondence is to use a single
measurement of a surgical instrument. Using this method, the axis
of the instrument corresponds to the axis of the cone, the tip of
the instrument corresponds to a fixed point along the axis of the
cone (which could be the apex, but could also be another point
along the central axis), and the orientation of the instrument
about its axis corresponds to the orientation of the cone about its
axis. In general any set of measurements which define the position
and orientation of a given cone can be used to establish the
correspondence between the actual and virtual cones.
[0077] The operations illustrated in FIG. 10 are shown in the
flowchart of FIG. 11. To begin, the physician holds the surgical
instrument 1040 in the position that defines the virtual cone in
the manner as outlined in the previous paragraph (step 1101).
Computer 120 locates the position of instrument 1040, which
effectively corresponds the position and orientation of the virtual
cone to the actual cone (step 1102). Computer 120 projects
additional movements of instrument 1040 into one or more previously
acquired fluoroscopic images as if the instrument were being moved
in the actual cone corresponding to a given image (step 1103). In
this manner, the physician can align the instrument to particular
points or trajectories within previously acquired images. At the
physician's command, computer 120 "freezes" the position and/or
orientation of the instrument in the displayed fluoroscopic
image(s) and uses those for subsequent processing and plan
generation (step 1104).
[0078] It is also consistent with this invention to provide
automated planning using computer analysis techniques to define an
"optimal" trajectory in the C-arm images. Once the optimal
trajectory is determined, computer 120 overlays the optimal
trajectory in the fluoroscopic image. For example, automated plans
can be generated using computational techniques to reduce a
specified amount of lordosis in spine surgery.
[0079] A common clinical problem, especially in orthopaedic trauma,
is the realignment (reduction) of broken or misaligned bone
fragments. FIG. 12A is a fluoroscopic image of a fracture of the
femur containing two bone fragments 1201 and 1202. The physician's
job is to realign the bone fragments so that the femur can properly
heal.
[0080] FIG. 13 is a flow chart illustrating methods for aligning
bone fragments consistent with the present invention. In general,
one of bone fragments 1201 or 1202 is used as a fixed reference
frame and the other as a dynamic reference frame. When the
physician moves the bone fragment corresponding to the dynamic
reference frame, tracking sensor 130 detects the movement and
updates the x-ray image to reflect the new location of the bone
fragment in the patient.
[0081] To begin the alignment procedure, the physician places a
tracking sensor marker on each of bone fragments 1201 and 1202
(step 1301) and acquires the fluoroscopic images, (step 1302), such
as the image shown in FIG. 12A. Computer 120 processes the acquired
image to obtain positional location information and to calibrate
the image (step 1303, this step is identical to steps 401-403 in
FIG. 4).
[0082] After acquisition of the fluoroscopic image(s), computer 120
uses image detection and extraction techniques to delineate the
boundaries of the bone fragments in the images (step 1304).
Suitable edge detection algorithms for generating the contours are
well known in the art, and may be, for example, the Canny edge
detector, the Shen-Casten edge detector, or the Sobel edge
detector. An edge detected version of FIG. 12A is shown in FIG.
12B, in which the resulting contour corresponding to bone fragment
1201 is labeled as 1203 and the contour corresponding to bone
fragment 1202 is labeled as 1204. Contours 1203 and 1204 may be, as
shown in FIG. 12B, graphically superimposed by computer 120 on the
acquired image(s).
[0083] Overlaying the detected image contours on the fluoroscopic
image allows the physician to easily identify the correspondence
between image contours 1203-1204 and bone fragments 1201-1202. The
physician inputs this correspondence into computer 120 (step 1305).
Alternatively, computer 120 may automatically identify the
correspondence between the image contours and the bone fragments.
Once the correspondence is established, the physician specifies
which contour is to remain fixed and which is to be repositioned.
The tracking sensor marker attached to the fragment to be
repositioned is referred to as the dynamic reference marker and the
tracking sensor marker attached to the fixed fragment is referred
to as the fixed reference frame marker, although physically the
dynamic reference marker and the fixed reference frame marker may
be identical.
[0084] During surgical navigation, the physician moves the bone
fragment having the dynamic reference marker (step 1306). Tracking
sensor 130 detects the position of the dynamic reference frame
marker and the fixed frame marker. With this information and the
previously generated positional location information, computer 120
calculates and displays the new position of the dynamic reference
frame, and hence its corresponding bone fragment, in the
fluoroscopic image (step 1307). FIG. 12C illustrates an updated
version of the fluoroscopic image contour 1203 corresponding to the
fixed bone fragment and contour 1204 corresponding to the new
location of the dynamic reference marker and its bone fragment.
[0085] Methods described above for aligning bone fragments may also
be applied to the proper alignment of multiple vertebral bodies,
for example in the reduction of scoliosis.
[0086] The navigational guidance system consistent with the present
invention is not limited to providing surgical navigational
guidance with two-dimensional fluoroscopic images.
Three-dimensional volumetric data sets may also be overlaid with
graphical representations of a surgical instrument.
Three-dimensional data sets (such as CT or MRI) may be either
pre-acquired or acquired during the operation.
[0087] Two types of three-dimensional data sets are typically used
in surgical navigation: patient-specific image data and non-patient
specific or atlas data. Patient-specific three-dimensional images
are typically acquired prior to surgery using computed tomography
(CT), magnetic resonance (MR), or other known three-dimensional
imaging modalities, although intra-operative acquisition is also
possible. Atlas data is non-patient specific three-dimensional data
describing a "generic" patient. Atlas data may be acquired using
CT, MR or other imaging modalities from a particular patient; and
may even comprise images from several modalities which are
spatially registered (e.g., CT and MR together in a common
coordinate system). Atlas data may be annotated with supplemental
information describing anatomy, physiology, pathology, or "optimal"
planning information (for example screw placements, lordosis
angles, scoliotic correction plans, etc).
[0088] A three-dimensional patient CT or MR data set is shown in
FIG. 1 as data set 124 and atlas data is illustrated in FIG. 1 as
data set 126.
[0089] Before overlaying a three-dimensional image with graphical
representations of surgical instruments, the correspondence between
points in the three-dimensional image and points in the patient's
reference frame must be determined. This procedure is known as
registration of the image. One method for performing image
registration is described in the previously mentioned publications
to Bucholz. Three-dimensional patient specific images can be
registered to a patient on the operating room table (surgical
space) using multiple two-dimensional image projections. This
process, which is often referred to as 2D/3D registration, uses two
spatial transformations that can be established. The first
transformation is between the acquired fluoroscopic images and the
three-dimensional image data set (e.g., CT or MR) corresponding to
the same patient. The second transformation is between the
coordinate system of the fluoroscopic images and an externally
measurable reference system attached to the fluoroscopic imager.
Once these transformations have been established, it is possible to
directly relate surgical space to three-dimensional image
space.
[0090] When performing three-dimensional registration, as with
two-dimensional registration, imager 100, when acquiring the image,
should be stationary with respect to patient 110. If C-arm 103 or
patient 110 is moving during image acquisition, the position of the
fluoroscope will not be accurately determined relative to the
patient's reference frame. Accordingly, the previously described
technique for detecting movement of imager 100 during the image
acquisition process can be used when acquiring fluoroscopic images
that are to be used in 2D/3D registration. That is, as described,
computer 120 may examine the position information from tracking
sensor 130 while radiation sensors 107 are signaling radiation
detection. If the calibration and tracking target 106 moves
relative to dynamic reference frame 150 during image acquisition,
this image is marked as erroneous.
[0091] It may be necessary to acquire complementary fluoroscopic
views (e.g., lateral and anterior/posterior) to facilitate 2D/3D
registration. The techniques previously discussed in reference to
FIGS. 7-8 and relating to the acquisition of complementary views
can be applied here.
[0092] Once registered, computer 120 may use positional information
of instrument 140 to overlay graphical representations of the
instrument in the three-dimensional image as well as the
two-dimensional fluoroscopic images.
[0093] The two-dimensional images generated by imager 100 are not
always able to adequately represent the patient's bone structure.
For example, fluoroscopic x-ray images are not effective when taken
through the length of the patient (i.e., from the point of view
looking down at the patient's head or up from the patient's feet)
because the large number of bones that the x-rays pass through
occlude one another in the final image. However, information
required for planning a surgical procedure which is not otherwise
available based on two-dimensional image data alone may be
extracted from a three-dimensional image data set such as a CT or
MR image data set. The extracted information may then be
transferred to the two-dimensional x-ray images generated by imager
100 and used in surgical navigation. The following examples
describe additional methods for using three-dimensional and
two-dimensional data in surgical navigation.
[0094] FIGS. 14A and 14B are images illustrating the implantation
of an inter-vertebral cage in the spine of a patient. An
inter-vertebral cage is a roughly cylindrical spinal implant that
is inserted in the disc space between adjacent spinal vertebrae.
The physician may find it difficult, if not impossible, to choose
the appropriate length of an inter-vertebral cage based upon
two-dimensional images such as the image of FIG. 14A.
[0095] Rectangle 1401 represents the projection of the cylindrical
inter-vertebral cage into the image. While the long axis of the
cylinder appears to be completely within the bone in this image,
this may not be the case due to curvature of the anterior aspect of
vertebrae 1402. FIG. 14B is an image of a three-dimensional axial
CT cross section of the vertebrae. Corner 1403 of rectangle 1401
protrudes from the bone--a highly undesirable situation that cannot
be reliably detected in x-ray images such as that of FIG. 14A.
Accordingly, when faced with this situation, the appropriate cage
length should be chosen based upon one or more axial CT images,
such as that in FIG. 14B. Selection of the cage length can be
performed automatically by computer 120 or semi-automatically with
the input of the physician.
[0096] Once the cage length has been determined by the physician
and entered into computer 120, the length value can then be used by
computer 120 in properly displaying the graphical overlay in the
associated two-dimensional image. The position of the surgical
instrument used to hold the cage during the insertion process, as
detected by tracking sensor 130, is used to calculate the position
of the cage in FIG. 14A during the two-dimensional navigational
process.
[0097] Although the above discussed example was with a cylindrical
spinal implant, in general, the described concepts could be applied
to any surgical implant.
[0098] In certain clinical procedures, it may be desirable to
acquire a fluoroscopic x-ray image view looking substantially
straight down the medial axis of a vertebral pedicle. For the
purposes of this example, a vertebral pedicle can be thought of as
a cylinder, and the medial axis corresponds to the central axis of
the cylinder.
[0099] FIG. 15A is an x-ray image in which the view direction of
the imager is aligned with the medial axis of the pedicle (i.e.,
the medial axis of the pedicle is into the plane of the image). In
this so-called "owl's eye" view, the pedicle appears as circle 1501
within the image. It is often difficult to precisely acquire this
view using only fluoroscopic x-ray images, as it is difficult to
align the view direction of imager 100 with the medial axis of the
pedicle using only fluoroscopic images.
[0100] Given an anterior/posterior fluoroscopic image view of the
spine, such as the one shown in FIG. 15B, and given that the
mechanical axis of the fluoroscope is aligned with the patient's
long axis (i.e., axis 704 in FIG. 7C), an axial CT cross section of
a vertebra can be used to quickly and easily acquire a high quality
owl's eye view, such as the view of FIG. 15A.
[0101] FIG. 15C is an image of an axial CT cross section of a
vertebra. With this image, computer 120 or the physician may
measure angle 1504 between the anterior/posterior axis 1502 and the
projection of the medial axis 1503 of the pedicle 1501 into the
axial plane. The physician may then rotate imager 100 by the
measured angle about the mechanical rotation axis that is aligned
with the patient's long axis 704. Because most fluoroscopic
imagers, such as imager 100, have angle indicators, rotation by the
desired amount is trivial. However, if the physician requires
additional accuracy in the rotation, tracking sensor 130, because
it detects the position of C-arm 103, can be used to more precisely
measure the rotation angle.
[0102] With conventional fluoroscopic x-ray image acquisition,
radiation passes through a physical media to create a projection
image on a radiation sensitive film or an electronic image
intensifier. Given a 3D CT data set, a simulated x-ray image can
also be generated using a technique known as digitally
reconstructed radiography (DRR). DRR is well known in the art, and
is described, for example, by L. Lemieux et al., "A
Patient-to-Computed-Tomography Image Registration Method Based on
Digitally Reconstructed Radiographs," Medical Physics 21(11), pp
1749-1760, November 1994.
[0103] When a DRR image is created, a fluoroscopic image is formed
by computationally projecting volume elements (voxels) of the 3D CT
data set onto a selected image plane. Using a 3D CT data set of a
given patient, it is possible to create a DRR image that appears
very similar to a corresponding x-ray image of the same patient. A
requirement for this similarity is that the "computational x-ray
imager" and actual x-ray imager use similar intrinsic imaging
parameters (e.g., projection transformations, distortion
correction) and extrinsic imaging parameters (e.g., view
direction). The intrinsic imaging parameters can be derived from
the calibration process.
[0104] A DRR image may be used to provide guidance to the surgeon
in the problem discussed in Example 1 of appropriately placing an
inter-vertebral cage in the patient. Given a 3D CT data set of two
adjacent vertebrae, the physician, interacting with computer 120,
may manually position a 3D CAD model of an inter-vertebral cage in
a clinically desired position in the three-dimensional view of the
vertebrae. The physician may then use the DRR technique to
synthesize an anterior/posterior, lateral, or other x-ray view of
the vertebrae showing the three-dimensional CAD model of the
inter-vertebral cage. Thus, a synthetic fluoroscopic x-ray image
can be created which simulates what a properly placed cage would
look like after implantation.
[0105] The simulated x-ray images may be compared to the actual
images taken by imager 100 during surgery. The goal of the surgeon
is to position the implant such that the intra-operative images
match the DRR images. For this comparison, two types of
intra-operative images may preferably be used. First, conventional
fluoroscopy could be used to acquire an image after the
inter-vertebral cage has been implanted. Second, images acquired
prior to cage placement could be supplemented with superimposed
graphical icons representing the measured cage position. In either
case, the synthetic fluoroscopic image can be used as a template to
help guide the surgeon in properly placing the inter-vertebral
cage.
[0106] Although the above example was described in the context of
implanting an inter-vertebral cage, implants other than the
inter-vertebral cage could also be used.
[0107] The DRR technique can be used to provide guidance to the
physician when acquiring an owl's eye view of a vertebral pedicle.
Given a three-dimensional CT data set containing a vertebra and
associated pedicle, the physician may use computer 120 to manually
locate a three-dimensional representation of the pedicle's medial
axis relative to the three-dimensional images of the vertebrae.
Once this placement has been achieved, it is possible to synthesize
an owl's eye view of the vertebrae based upon the view direction
specified by the physician's selection of the three-dimensional
medial axis. This synthetic image can then be displayed to the
surgeon during surgery and used to guide the acquisition of an
actual owl's eye view using the fluoroscope. By visually comparing
fluoroscopic images taken while positioning the fluoroscope to the
synthetic owl's eye view, the physician can acquire a true
fluoroscopic image with a view direction approximately equal to the
manually selected medial axis. In this manner, a high quality owl's
eye view can be acquired.
[0108] Although the above example was described in the context of
synthesizing a two-dimensional owl's eye view, in general, any
three-dimensional view direction can be selected and a
corresponding two-dimensional image synthesized and used to acquire
a fluoroscopic two-dimensional image.
[0109] It may be desirable to measure the angle between the
trajectory of a surgical instrument and the plane of a fluoroscopic
image (such as a plane aligned with the mid-line of the spine 1502)
during surgery using a pre-acquired fluoroscopic image. This is
useful, as it is often desirable to position or implant a surgical
instrument at a certain angle relative to the plane of the
fluoroscopic image. For example, the surgical instrument may need
to be implanted in the direction aligned with the medial axis of
the pedicle 1503.
[0110] Consider the vertebral cross section shown as an axial CT
image in FIG. 15C. As described above, the angle 1504 between the
anterior/posterior axis of the spine 1502 and the medial axis 1503
of the pedicle can be measured from this CT image. Aligning the
surgical instrument with the medial axis can be accomplished by
dynamically measuring the angle between the trajectory of the
surgical instrument and the plane defined by the mid-line of the
spine 1502. When the dynamically measured angle matches the angle
pre-obtained from the CT image, the surgical instrument is
aligned.
[0111] FIGS. 16A and 16B are figures respectively illustrating an
anterior/posterior fluoroscopic image of the spine and a
corresponding three-dimensional view of the spine. The physician
defines two points along the midline of the spine, such as the
points 1601 drawn on the spinous processes in FIG. 16A (in
non-pathological anatomy a spinous process typically defines the
midline). Computer 120 uses these points to define a line 1602 in
the image, or more generally, the computer defines plane 1603
(shown in FIG. 16B) to include the two points and the linear
projections of these two points dictated by the calibration
transformation. More intuitively, a first order approximation of
plane 1603 can be thought of as the plane passing through the two
points perpendicular to the image plane.
[0112] Plane 1603 defines the midline of the spine in
three-dimensional space. During navigational guidance, the equation
of this plane can be expressed in the coordinate system of either
the dynamic reference frame 150 or the tracking sensor 130.
[0113] Using the tracking sensor 130 to measure the position and
orientation (i.e., the trajectory) of the instrument 140, computer
120 then mathematically projects this trajectory onto the plane
1603. This projection will define a line passing through plane
1603. The angle between this line in plane 1603 and the instrument
trajectory corresponds to the angle to be measured. In other words,
the angle to be measured corresponds to the minimum angle present
between the trajectory of the instrument and the plane 1603. The
angle to be measured can be calculated by computer 120 and
displayed to the physician either in a textual or graphical
format.
[0114] In summary, as described in this example, a single
fluoroscopic image can be used during surgery to position a
surgical instrument at a desired trajectory relative to the plane
of the fluoroscopic image. More generally, the methods described in
this example relate to measuring the angle between the trajectory
of a surgical instrument 140 and a plane (e.g. 1603) defined by two
or more points (e.g., 1601) which have been manually or
automatically selected in a fluoroscopic image. While the
explanation uses a CT for clarity of the example, the measurement
and display of the angle can be achieved without the use of any 3D
image data.
[0115] Although the above five examples used three-dimensional
patient specific data and not atlas data, in certain situations, it
may be possible to use a 2D/3D registration scheme that registers
non-patient specific atlas data to patient specific fluoroscopic
images using deformable registration methods that do not preserve
the rigidity of anatomical structure during the registration
process. In this manner, the patient specific fluoroscopic images
may be used to deform the atlas data to better correspond to the
patient and thereby transfer atlased knowledge to the patient
specific fluoroscopic images.
[0116] The above described systems and methods significantly extend
the conventional techniques for acquiring and using x-ray images
for surgical navigational guidance. It will be apparent to those
skilled in the art that various modifications and variations can be
made to the present invention without departing from the scope or
spirit of the invention. For example, although certain of the
examples were described in relation to spinal examples, many other
regions of body could be operated on.
[0117] Other embodiments of the invention will be apparent to those
skilled in the art from consideration of the specification and
practice of the invention disclosed herein. In particular, an
alternative embodiment of the calibration and tracking target may
allow the calibration component to be detached from the C-arm and
introduced into the C-arm view for calibrations only, and then
removed. It is intended that the specification and examples be
considered as exemplary only, with the true scope and spirit of the
invention being indicated by the following claims.
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