The present disclosure relates to position recognition systems, and in particular, the registration of a medical image to a three-dimensional tracking space.
Surgical navigation requires registration of a medical image or a set of medical images to a three-dimensional tracking space, such that a surgical device detected as a rigid body in the tracking space can then be displayed as a graphic overlaid on the medical images in the correct position relative to the patient's anatomy. This process is one-way registration of the image to the tracking coordinate system. In robotic navigation, two-way co-registration of the image and tracking coordinate systems is also necessary to allow positioning of a rigid body from the image coordinate system to the tracking coordinate system. In practice, a desired trajectory is overlaid on the medical image, defining a medical object (e.g., surgical screw) as a rigid body in the medical image coordinate system, where co-registration allows the rigid body to be simultaneously defined in the tracking space. Since the robot is calibrated so that its position in the tracking space is known, the robot can then move to a known position along the desired trajectory to hold a tool for surgery that will allow a screw, needle, or other instrument to follow the desired trajectory.
The process of co-registering a three-dimensional image volume such as a computed tomography (CT) scan with the tracking space is possible because the CT image volume and the camera space are both well defined. The positions of fiducials in the image volume can be detected accurately by image processing and the positions of tracking markers in the camera space can be detected accurately by stereophotogrammetry. A registration fixture containing both tracking markers and imaging fiducials in a known relative position can therefore provide the required co-registration when working with a three-dimensional image. However, when working from multiple two-dimensional (2D) images the process of registering these images with the tracking space becomes challenging.
In the current pre-operative CT robotic/navigation workflow, a preoperative 3D CT is registered to the tracking camera's coordinate system using a pair of 2D tracked fluoro images. For each fluoro image, the locations of the image plane and emitter are optically tracked via a fixture attached to the fluoro unit. The algorithm works by generating synthetic fluoro shots (digitally reconstructed radiographs ((DRRs)) mathematically by simulating the X-ray path through the CT volume. When a match is found between the actual X-ray images and DRRs, registration is achieved because the locations of the image plane and emitter are simultaneously known relative to the CT volume and relative to the cameras.
One part of the above algorithm for registering the CT image to the tracking cameras is to accurately track the location of the image plane and emitter relative to the patient tracker so that the DRRs are generated with correct corresponding configuration of the virtual emitter and virtual collector relative to the CT volume. The way that the existing software tracks the image plane is by tracking a fixture directly attached to the face of the image collector of the C-arm. The emitter location is then accurately determined by implementing a commonly used “pinhole camera” algorithm where the shadows of two planes of BBs passing through the fixture are analyzed to determine where the emitter must have been relative to the collector to create such shadows.
One underlying assumption of the CT-fluoro registration process is that the far BB plane of the tracked fluoro fixture, which is up against the physical collector of the C-arm, can be considered to be coincident with the X-ray image plane, even though the BB plane is really several centimeters from the hardware that detects the X-rays. The reason this simplification is valid is that the only effect moving the BB plane away from the imaging hardware has been to magnify the image uniformly. That is, if it were possible to move the collector linearly away from the emitter, leaving the fluoro fixture in the same place relative to the emitter, equivalent shadows of BBs would be generated at different collector distances with the only difference between the images from the different collector locations being the increasing image magnification.
Some embodiments of the present disclosure are directed to a registration fixture is provided for registration of a medical image to a three-dimensional tracking space. The registration fixture includes a base frame, an orientation plate, and a registration plate. The base frame is adapted to be mounted to an x-ray medical imaging device. The orientation plate is attached to the base frame and has a first set of radiopaque markers arranged along at least four radial lines which extend from a center location. The radial lines are angularly spaced apart about the center location. At least three of the radiopaque markers are arranged along each of the radial lines. The registration plate is attached to the base frame and spaced apart from the orientation plate. The registration plate has a second set of radiopaque markers arranged along a circle.
Some embodiments of the present disclosure are directed to a surgical navigation system for registration of a medical image to a three-dimensional tracking space. The surgical navigation system includes at least one processor and at least one memory storing instructions executable by the at least one processor to perform operations. The operations include to obtain an image generated by an x-ray medical imaging device. The operations identify in the image a first set of radiopaque markers based on recognizing the first set of radiopaque markers are arranged along at least four radial lines which extend from a center location, based on recognizing the radial lines are angularly spaced apart about the center location, and based on recognizing at least three of the radiopaque markers are arranged along each of the radial lines. The operations identify in the image locations of a second set of radiopaque markers based on recognizing the second set of radiopaque markers are arranged along a circle, where the first set of radiopaque markers are on an orientation plate, the second set of radiopaque markers are on a registration plate spaced apart from the orientation plate, and the orientation plate and the registration plate are attached to a base frame of a fixture mounted to the x-ray medical image device. The operations register the image to a three-dimensional tracking space based on comparison of locations of the first set radiopaque markers in the image to locations of the second set radiopaque markers in the image.
These and other systems, methods, objects, features, and advantages of the present invention will be apparent to those skilled in the art from the following detailed description of the preferred embodiment and the drawings. All documents mentioned herein are hereby incorporated in their entirety by reference.
The invention and the following detailed description of certain embodiments thereof may be understood by reference to the following figures:
While the invention has been described in connection with certain preferred embodiments, other embodiments would be understood by one of ordinary skill in the art and are encompassed herein.
It is to be understood that the present disclosure is not limited in its application to the details of construction and the arrangement of components set forth in the description herein or illustrated in the drawings. The teachings of the present disclosure may be used and practiced in other embodiments and practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings.
The following discussion is presented to enable a person skilled in the art to make and use embodiments of the present disclosure. Various modifications to the illustrated embodiments will be readily apparent to those skilled in the art, and the principles herein can be applied to other embodiments and applications without departing from embodiments of the present disclosure. Thus, the embodiments are not intended to be limited to embodiments shown, but are to be accorded the widest scope consistent with the principles and features disclosed herein. The following detailed description is to be read with reference to the figures, in which like elements in different figures have like reference numerals. The figures, which are not necessarily to scale, depict selected embodiments and are not intended to limit the scope of the embodiments. Skilled artisans will recognize the examples provided herein have many useful alternatives and fall within the scope of the embodiments.
Turning now to the drawing,
With respect to the other components of the robot 102, the display 110 can be attached to the surgical robot 102 and in other exemplary embodiments, display 110 can be detached from surgical robot 102, either within a surgical room with the surgical robot 102, or in a remote location. End-effector 112 may be coupled to the robot arm 104 and controlled by at least one motor. In exemplary embodiments, end-effector 112 can comprise a guide tube 114, which is able to receive and orient a surgical instrument 608 (described further herein) used to perform surgery on the patient 210. As used herein, the term “end-effector” is used interchangeably with the terms “end-effectuator” and “effectuator element.” Although generally shown with a guide tube 114, it will be appreciated that the end-effector 112 may be replaced with any suitable instrumentation suitable for use in surgery. In some embodiments, end-effector 112 can comprise any known structure for effecting the movement of the surgical instrument 608 in a desired manner.
The surgical robot 102 is able to control the translation and orientation of the end-effector 112. The robot 102 is able to move end-effector 112 along x-, y-, and z-axes, for example. The end-effector 112 can be configured for selective rotation about one or more of the x-, y-, and z-axis, and a Z Frame axis (such that one or more of the Euler Angles (e.g., roll, pitch, and/or yaw) associated with end-effector 112 can be selectively controlled). In some exemplary embodiments, selective control of the translation and orientation of end-effector 112 can permit performance of medical procedures with significantly improved accuracy compared to conventional robots that utilize, for example, a six degree of freedom robot arm comprising only rotational axes. For example, the surgical robot system 100 may be used to operate on patient 210, and robot arm 104 can be positioned above the body of patient 210, with end-effector 112 selectively angled relative to the z-axis toward the body of patient 210.
In some exemplary embodiments, the position of the surgical instrument 608 can be dynamically updated so that surgical robot 102 can be aware of the location of the surgical instrument 608 at all times during the procedure. Consequently, in some exemplary embodiments, surgical robot 102 can move the surgical instrument 608 to the desired position quickly without any further assistance from a physician (unless the physician so desires). In some further embodiments, surgical robot 102 can be configured to correct the path of the surgical instrument 608 if the surgical instrument 608 strays from the selected, preplanned trajectory. In some exemplary embodiments, surgical robot 102 can be configured to permit stoppage, modification, and/or manual control of the movement of end-effector 112 and/or the surgical instrument 608. Thus, in use, in exemplary embodiments, a physician or other user can operate the system 100, and has the option to stop, modify, or manually control the autonomous movement of end-effector 112 and/or the surgical instrument 608. Further details of surgical robot system 100 including the control and movement of a surgical instrument 608 by surgical robot 102 can be found in co-pending U.S. patent application Ser. No. 13/924,505, which is incorporated herein by reference in its entirety.
The robotic surgical system 100 can comprise one or more tracking markers 118 configured to track the movement of robot arm 104, end-effector 112, patient 210, and/or the surgical instrument 608 in three dimensions. In exemplary embodiments, a plurality of tracking markers 118 can be mounted (or otherwise secured) thereon to an outer surface of the robot 102, such as, for example and without limitation, on base 106 of robot 102, on robot arm 104, or on the end-effector 112. In exemplary embodiments, at least one tracking marker 118 of the plurality of tracking markers 118 can be mounted or otherwise secured to the end-effector 112. One or more tracking markers 118 can further be mounted (or otherwise secured) to the patient 210. In exemplary embodiments, the plurality of tracking markers 118 can be positioned on the patient 210 spaced apart from the surgical field 208 to reduce the likelihood of being obscured by the surgeon, surgical tools, or other parts of the robot 102. Further, one or more tracking markers 118 can be further mounted (or otherwise secured) to the surgical tools 608 (e.g., a screwdriver, dilator, implant inserter, or the like). Thus, the tracking markers 118 enable each of the marked objects (e.g., the end-effector 112, the patient 210, and the surgical tools 608) to be tracked by the robot 102. In exemplary embodiments, system 100 can use tracking information collected from each of the marked objects to calculate the orientation and location, for example, of the end-effector 112, the surgical instrument 608 (e.g., positioned in the tube 114 of the end-effector 112), and the relative position of the patient 210.
In exemplary embodiments, one or more of markers 118 may be optical markers. In some embodiments, the positioning of one or more tracking markers 118 on end-effector 112 can maximize the accuracy of the positional measurements by serving to check or verify the position of end-effector 112. Further details of surgical robot system 100 including the control, movement and tracking of surgical robot 102 and of a surgical instrument 608 can be found in co-pending U.S. patent application Ser. No. 13/924,505, which is incorporated herein by reference in its entirety.
Exemplary embodiments include one or more markers 118 coupled to the surgical instrument 608. In exemplary embodiments, these markers 118, for example, coupled to the patient 210 and surgical instruments 608, as well as markers 118 coupled to the end-effector 112 of the robot 102 can comprise conventional infrared light-emitting diodes (LEDs) or an Optotrak® diode capable of being tracked using a commercially available infrared optical tracking system such as Optotrak®. Optotrak® is a registered trademark of Northern Digital Inc., Waterloo, Ontario, Canada. In other embodiments, markers 118 can comprise conventional reflective spheres capable of being tracked using a commercially available optical tracking system such as Polaris Spectra. Polaris Spectra is also a registered trademark of Northern Digital, Inc. In an exemplary embodiment, the markers 118 coupled to the end-effector 112 are active markers which comprise infrared light-emitting diodes which may be turned on and off, and the markers 118 coupled to the patient 210 and the surgical instruments 608 comprise passive reflective spheres.
In exemplary embodiments, light emitted from and/or reflected by markers 118 can be detected by camera 200 and can be used to monitor the location and movement of the marked objects. In alternative embodiments, markers 118 can comprise a radio-frequency and/or electromagnetic reflector or transceiver and the camera 200 can include or be replaced by a radio-frequency and/or electromagnetic transceiver.
Similar to surgical robot system 100,
Input power is supplied to system 300 via a power source 548 which may be provided to power distribution module 404. Power distribution module 404 receives input power and is configured to generate different power supply voltages that are provided to other modules, components, and subsystems of system 300. Power distribution module 404 may be configured to provide different voltage supplies to platform interface module 406, which may be provided to other components such as computer 408, display 304, speaker 536, driver 508 to, for example, power motors 512, 514, 516, 518 and end-effector 310, motor 510, ring 324, camera converter 542, and other components for system 300 for example, fans for cooling the electrical components within cabinet 316.
Power distribution module 404 may also provide power to other components such as tablet charging station 534 that may be located within tablet drawer 318. Tablet charging station 534 may be in wireless or wired communication with tablet 546 for charging table 546. Tablet 546 may be used by a surgeon consistent with the present disclosure and described herein. Power distribution module 404 may also be connected to battery 402, which serves as temporary power source in the event that power distribution module 404 does not receive power from input power 548. At other times, power distribution module 404 may serve to charge battery 402 if necessary.
Other components of platform subsystem 502 may also include connector panel 320, control panel 322, and ring 324. Connector panel 320 may serve to connect different devices and components to system 300 and/or associated components and modules. Connector panel 320 may contain one or more ports that receive lines or connections from different components. For example, connector panel 320 may have a ground terminal port that may ground system 300 to other equipment, a port to connect foot pedal 544 to system 300, a port to connect to tracking subsystem 532, which may comprise position sensor 540, camera converter 542, and cameras 326 associated with camera stand 302. Connector panel 320 may also include other ports to allow USB, Ethernet, HDMI communications to other components, such as computer 408. [0057] Control panel 322 may provide various buttons or indicators that control operation of system 300 and/or provide information regarding system 300. For example, control panel 322 may include buttons to power on or off system 300, lift or lower vertical column 312, and lift or lower stabilizers 520-526 that may be designed to engage casters 314 to lock system 300 from physically moving. Other buttons may stop system 300 in the event of an emergency, which may remove all motor power and apply mechanical brakes to stop all motion from occurring. Control panel 322 may also have indicators notifying the user of certain system conditions such as a line power indicator or status of charge for battery 402.
Ring 324 may be a visual indicator to notify the user of system 300 of different modes that system 300 is operating under and certain warnings to the user.
Computer subsystem 504 includes computer 408, display 304, and speaker 536. Computer 504 includes an operating system and software to operate system 300. Computer 504 may receive and process information from other components (for example, tracking subsystem 532, platform subsystem 502, and/or motion control subsystem 506) in order to display information to the user. Further, computer subsystem 504 may also include speaker 536 to provide audio to the user.
Tracking subsystem 532 may include position sensor 504 and converter 542. Tracking subsystem 532 may correspond to camera stand 302 including camera 326 as described with respect to
Moreover, system 300 may provide for automatic movement of vertical column 312, upper arm 306, and lower arm 308 through a user indicating on display 304 (which may be a touchscreen input device) the location of a surgical instrument or component on three dimensional image of the patient's anatomy on display 304. The user may initiate this automatic movement by stepping on foot pedal 544 or some other input means.
A tracking array 612 may be mounted on instrument 608 to monitor the location and orientation of instrument tool 608. The tracking array 612 may be attached to an instrument 608 and may comprise tracking markers 804. As best seen in
Markers 702 may be disposed on or within end-effector 602 in a manner such that the markers 702 are visible by one or more cameras 200, 326 or other tracking devices associated with the surgical robot system 100, 300, 600. The camera 200, 326 or other tracking devices may track end-effector 602 as it moves to different positions and viewing angles by following the movement of tracking markers 702. The location of markers 702 and/or end-effector 602 may be shown on a display 110, 304 associated with the surgical robot system 100, 300, 600, for example, display 110 as shown in
For example, as shown in
In addition, in exemplary embodiments, end-effector 602 may be equipped with infrared (IR) receivers that can detect when an external camera 200, 326 is getting ready to read markers 702. Upon this detection, end-effector 602 may then illuminate markers 702. The detection by the IR receivers that the external camera 200, 326 is ready to read markers 702 may signal the need to synchronize a duty cycle of markers 702, which may be light emitting diodes, to an external camera 200, 326. This may also allow for lower power consumption by the robotic system as a whole, whereby markers 702 would only be illuminated at the appropriate time instead of being illuminated continuously. Further, in exemplary embodiments, markers 702 may be powered off to prevent interference with other navigation tools, such as different types of surgical instruments 608.
The manner in which a surgeon 120 may place instrument 608 into guide tube 606 of the end-effector 602 and adjust the instrument 608 is evident in
With respect to the locating coupling, robot arm 604 may comprise mounting plate 1216, which may be non-magnetic material, one or more depressions 1214, lip 1218, and magnets 1220. Magnet 1220 is mounted below each of depressions 1214. Portions of clamp 1204 may comprise magnetic material and be attracted by one or more magnets 1220. Through the magnetic attraction of clamp 1204 and robot arm 604, balls 1208 become seated into respective depressions 1214. For example, balls 1208 as shown in
With respect to the reinforcing coupling, portions of clamp 1204 may be configured to be a fixed ground link and as such clamp 1204 may serve as a five bar linkage. Closing clamp handle 1206 may fasten end-effector 602 to robot arm 604 as lip 1212 and lip 1218 engage clamp 1204 in a manner to secure end-effector 602 and robot arm 604. When clamp handle 1206 is closed, spring 1210 may be stretched or stressed while clamp 1204 is in a locked position. The locked position may be a position that provides for linkage past center. Because of a closed position that is past center, the linkage will not open absent a force applied to clamp handle 1206 to release clamp 1204. Thus, in a locked position end-effector 602 may be robustly secured to robot arm 604.
Spring 1210 may be a curved beam in tension. Spring 1210 may be comprised of a material that exhibits high stiffness and high yield strain such as virgin PEEK (poly-ether-ether-ketone). The linkage between end-effector 602 and robot arm 604 may provide for a sterile barrier between end-effector 602 and robot arm 604 without impeding fastening of the two couplings.
The reinforcing coupling may be a linkage with multiple spring members. The reinforcing coupling may latch with a cam or friction based mechanism. The reinforcing coupling may also be a sufficiently powerful electromagnet that will support fastening end-effector 102 to robot arm 604. The reinforcing coupling may be a multi-piece collar completely separate from either end-effector 602 and/or robot arm 604 that slips over an interface between end-effector 602 and robot arm 604 and tightens with a screw mechanism, an over center linkage, or a cam mechanism.
Referring to
In order to track the position of the patient 210, a patient tracking device 116 may include a patient fixation instrument 1402 to be secured to a rigid anatomical structure of the patient 210 and a dynamic reference base (DRB) 1404 may be securely attached to the patient fixation instrument 1402. For example, patient fixation instrument 1402 may be inserted into opening 1406 of dynamic reference base 1404. Dynamic reference base 1404 may contain markers 1408 that are visible to tracking devices, such as tracking subsystem 532. These markers 1408 may be optical markers or reflective spheres, such as tracking markers 118, as previously discussed herein.
Patient fixation instrument 1402 is attached to a rigid anatomy of the patient 210 and may remain attached throughout the surgical procedure. In an exemplary embodiment, patient fixation instrument 1402 is attached to a rigid area of the patient 210, for example, a bone that is located away from the targeted anatomical structure subject to the surgical procedure. In order to track the targeted anatomical structure, dynamic reference base 1404 is associated with the targeted anatomical structure through the use of a registration fixture that is temporarily placed on or near the targeted anatomical structure in order to register the dynamic reference base 1404 with the location of the targeted anatomical structure.
A registration fixture 1410 is attached to patient fixation instrument 1402 through the use of a pivot arm 1412. Pivot arm 1412 is attached to patient fixation instrument 1402 by inserting patient fixation instrument 1402 through an opening 1414 of registration fixture 1410. Pivot arm 1412 is attached to registration fixture 1410 by, for example, inserting a knob 1416 through an opening 1418 of pivot arm 1412.
Using pivot arm 1412, registration fixture 1410 may be placed over the targeted anatomical structure and its location may be determined in an image space and navigation space using tracking markers 1420 and/or fiducials 1422 on registration fixture 1410. Registration fixture 1410 may contain a collection of markers 1420 that are visible in a navigational space (for example, markers 1420 may be detectable by tracking subsystem 532). Tracking markers 1420 may be optical markers visible in infrared light as previously described herein. Registration fixture 1410 may also contain a collection of fiducials 1422, for example, such as bearing balls, that are visible in an imaging space (for example, a three dimension CT image). As described in greater detail with respect to
At step 1504, an imaging pattern of fiducials 1420 is detected and registered in the imaging space and stored in computer 408. Optionally, at this time at step 1506, a graphical representation of the registration fixture 1410 may be overlaid on the images of the targeted anatomical structure.
At step 1508, a navigational pattern of registration fixture 1410 is detected and registered by recognizing markers 1420. Markers 1420 may be optical markers that are recognized in the navigation space through infrared light by tracking subsystem 532 via position sensor 540. Thus, the location, orientation, and other information of the targeted anatomical structure is registered in the navigation space. Therefore, registration fixture 1410 may be recognized in both the image space through the use of fiducials 1422 and the navigation space through the use of markers 1420. At step 1510, the registration of registration fixture 1410 in the image space is transferred to the navigation space. This transferal is done, for example, by using the relative position of the imaging pattern of fiducials 1422 compared to the position of the navigation pattern of markers 1420.
At step 1512, registration of the navigation space of registration fixture 1410 (having been registered with the image space) is further transferred to the navigation space of dynamic registration array 1404 attached to patient fixture instrument 1402. Thus, registration fixture 1410 may be removed and dynamic reference base 1404 may be used to track the targeted anatomical structure in both the navigation and image space because the navigation space is associated with the image space.
At steps 1514 and 1516, the navigation space may be overlaid on the image space and objects with markers visible in the navigation space (for example, surgical instruments 608 with optical markers 804). The objects may be tracked through graphical representations of the surgical instrument 608 on the images of the targeted anatomical structure.
There are methods for displaying the simulated projection of a surgical tool overlaid on a fluoroscopic image to assist in surgery through one-way registration of the medical image to the tracking space. For example, a calibrating fixture may be attached to the image intensifier of a fluoroscope, such as illustrated in
Although this method maps 3D tool position to two-dimensional (2D) medical images, it is not necessary for that application to map points detected on the 2D medical images to the 3D tracking space, i.e., to co-register the medical image space with the tracking space. However, it is possible to obtain such mapping by considering the vectors extending from emitter to collector.
Considering a case where one x-ray view is taken from a perspective substantially different than another x-ray view, such as depicted in
Implicit assumptions are that the positions of the collector plate and emitter source are known in 3D during both shots. For instance, the collector plate may have a tracking array attached and its 3D position would therefore be directly tracked. It may also be possible to place a tracker on the emitter. However, there are disadvantages to doing so, mainly that the tracking field needs to be very large to observe both trackers, where the distance between collector and emitter may typically be on the order of one meter. Tracking systems such as optical trackers may only have a tracking field less than one cubic meter. Additionally, the emitter could be positioned out of view for some clinically typical x-ray shots. The emitter source location could instead be calibrated relative to the collector array, but the extrapolated accuracy in defining the emitter location may be low with a large distance between collector and emitter and the different amounts of sag when the fluoroscope is oriented differently. Alternatively, the emitter source distance and direction relative to the collector may be calculated from fluoroscopic images of two parallel planes of BBs with known spacing dab 1712, such as is shown in
where,
Therefore, if the spacing between the planes is known and the spacing between the BBs is known, the distance from the emitter to collector can be determined through image processing of an x-ray image containing these BBs. Note that
This method for defining the position of the emitter relative to the collector makes use of the two parallel plates in defining the direction of the emitter as well as the distance. With the registration fixture mounted to the collector, the direction from emitter to collector is assumed to be perpendicular to the plane of the collector and the planes containing BBs. If this assumption is untrue, the projections of BBs from the near and far field planes will not be symmetrically overlaid on x-ray images. For a given amount of angular deviation of the x-ray plane from the BB planes, the amount of offset of BB shadows from a symmetrical projection is proportional to the distance between BB planes, with larger plane separation manifesting as larger lateral displacements of the projections on the x-ray images. Through geometry, the lateral offsets of BB shadows can be used to determine accurately the actual orientation of the BB planes relative to the collector plane and therefore the position of the emitter in 3D, or used to manually or automatically adjust the orientation of the registration fixture on the image intensifier until BB planes and collector plate are truly coplanar.
The scaling factor k is present in the above equation, but this factor is necessary for subsequent 3D to 2D mapping of the 3D coordinates of a generalized point. In general, to map a 3D point with coordinates x, y, z onto a 2D x-ray image that has the X and Y axes of the image aligned with the x and y axes of the Cartesian coordinate system, Equation 3 holds.
Where X is the coordinate axis of the 2D x-ray image aligned with the Cartesian X-axis, Y is the coordinate axis of the 2D x-ray image aligned with the Cartesian y-axis, and z is the Cartesian axis perpendicular to the x-ray image.
If two fluoroscope shots are taken at different orientations up to 90 degrees apart, such as one common clinical anteroposterior shot and one common clinical lateral shot, then (X1, Y1) could be defined as the x-ray coordinates of a point (x, y, z) as the point appears on an x-ray image 1 (e.g., an anteroposterior image). The Cartesian coordinates of the point in a local coordinate system aligned with that x-ray plane could be defined as (x1, y1, z1). Similarly, (X2, Y2) could be defined as the x-ray coordinates of the same point as it appears on an x-ray image 2 (e.g., a lateral image). The Cartesian coordinates of the point in a local coordinate system aligned with that x-ray plane could be defined as (x2, y2, z2). Because a tracking system may be used to detect the 3D position of the x-ray collector while the fluoroscope is in each orientation, the transformation T12 from Cartesian coordinate system 1 to Cartesian coordinate system 2 is known, with T12 being a standard 4×4 transformation matrix as is commonly used in the field. There is therefore a unique solution that results in Equation 4.
Note that the two coordinate systems are each oriented with their z-axis perpendicular to each x-ray plane, the origin of their z-axis at each x-ray plane, and the origins of their x and y axes at the center of the x-ray plane. The direction of x and y relative to the x-ray planes may be arbitrary. By tracking the locations of the x-ray planes in 3D, using for example 3D optical tracking, the transformation from the first to the second 3D coordinate system (T12) can be determined.
A method for defining the 3D Cartesian coordinate system associated with two fluoroscopic views may assume that the BBs are projected uniformly onto the image intensifier. However, distortion is commonly associated with images obtained from fluoroscopes, such as pincushion distortion, s-distortion, and the like. These types of distortion may be corrected using image processing before applying the methods described herein. Distortion correction may make use of the fact that the BBs are arranged in a symmetrical pattern on the registration device. Therefore, the x-rays projected through the known symmetrical pattern should create an image with matching symmetry. Spacing between BBs and alignment of rows of BBs may be determined through image processing and compared to the expected projections of the BBs. Algorithms commonly known in image processing such as affine transformations and the like may be used to force the projected x-ray image to match the known and expected symmetry. Since the same correction may also be applied to anatomical images on the x-rays, the resulting x-ray images should represent undistorted projections and should allow valid calculation of registration as described herein.
In embodiments, the symmetrical pattern used for correcting distortion could be a square pattern as depicted in
The process of 2D to 3D mapping of images relies on the correct interpretation of direction on the x-ray image. For example, if a 2D x-ray image is an anteroposterior image, it must be known if the 2D image represents a shot with emitter anterior and collector posterior or a shot with emitter posterior and collector anterior. Additionally, since fluoroscopic images are commonly round, there must be a way to precisely determine from the BB shadows which direction points left, right, up or down. The fixture's plane of BB's may provide information for alignment correction, such as using BB's located closest to the x-ray collector to provide information to orient the x-ray image rotationally and also with regard to reflection, e.g., the BB pattern may determine whether the positive z direction extends off the front or back of the visible plane. In embodiments, the fixture may contain an outer ring of large BBs that are arranged so it uniquely identifies aspects of alignment, such as the rotation and flip of the image. The pattern of BBs for identifying orientation and/or flip may be chosen based on the ability of the pattern to provide a unique combination of image rotation and flip and on the ability of pattern to provide redundant BB s for increased reliability of detection. Redundant BBs may be important because it is possible that not all BBs would be visible on any given x-ray shot due to obstruction of the BB shadow from tools or implants, or poor x-ray penetration through parts of the image.
In embodiments, a ring of BBs of varying spacing around the perimeter of a standard circular fluoroscopic image may be employed, such as illustrated in
In embodiments, orientation matching may utilize a point match algorithm (e.g., the Kabsch point match algorithm) or other suitable point match algorithm that assumes both point sets are scaled the same. The algorithm may then determine the transformation between two point sets, where one point set comes from the orientation BB detection and the other point set comes from a fixture 3D model. The fixture's orientation markers may then be projected into image space. Since both point sets need to be scaled the same, the algorithm tests a range of projection scaling to find the best match. Once the best match is found the transform is scaled appropriately, and the algorithm assigns point correspondence between the detected image markers and the physical fixture markers. The resulting transform can then be applied to the image to rotate and/or flip it to produce alignment with the fixture.
Ring Registration Fixture:
As an alternative to an array of BBs to establish orientation, it is possible to use rings or other shapes, such as formed from radio-opaque materials such as metal wire, as fiducials in a registration fixture. Referring to
When an x-ray image is taken of the ring registration fixture 2300 while centered on a collector plate, it should appear as two concentric circles 2402 and 2404, such as shown on
In addition, tracking markers 2302A-D may be used as references for ring positioning of the ring registration fixture 2300 in 3D, such as utilizing an array of optical markers, a magnetic sensor, or other like 3D tracking method. For reference it may be convenient to define a local coordinate system on the registration fixture 2300. For example, a reference local coordinate system may have its origin at the center of the ring that is closer to the x-ray emitter, with the second (e.g., parallel) ring closer to the collector, and the x-axis and y-axis may be coincident with the crosshairs that identify the first ring's center, where the z-axis is coincident with the vector joining the centers of the two rings.
In embodiments, mapping points from 3D to 2D using a ring registration fixture may utilize vectors through known points on both rings that are created to form a conical pattern, where this pattern is then used to interpolate vectors through regions of interest.
In embodiments, a sequence of common transformations may be applied (i.e., rotations, translations, magnification), such as with the transformation parameters estimated from features on the images. As an example, consider a 3D coordinate system based on a two-ring fixture such that the coordinate system is centered on a first ring nearer to the emitter as a near field ring and a second ring that is nearer to the collector as a far field ring. In this example, near and far field rings may be the same diameter, where in order to map a point from this coordinate system to the coordinate system of the collector plate, a number of transformations may be applied.
A non-limiting example set of illustrative transformations are depicted in
In this example, on the final image the near field ring appears more magnified than the far field ring since points on the near field ring have larger z values than the points on the far field ring. Additionally, rotation of the rings in x-y plane may appear different depending on how far the rings are from x, y=0, 0.
Thus, to go from a point in 3D that is specified in a coordinate system attached to the ring registration fixture to a point in 2D on the x-ray plane, a sequence of transformations is applied where there are five unknowns: θ, α, ϕ, dx, and dy. It is possible to use image processing to estimate these five unknowns from the rings themselves. Therefore, once these five parameters are defined and registration is established, any new point specified as x, y, z in the reference coordinate system may be directly mapped to the x-ray image coordinates.
For many of the calculations for determining the five parameters from the images, the ratio of dec/k is needed, as was described respect to the BB fixture. This ratio may be determined similarly from an x-ray image taken of the rings while oriented parallel to the collector plate.
Where
Non-limiting examples of how the five unknowns (θ, α, ϕ, dx, and dy) may be determined will now be described.
Calculating Angle of Incidence α:
Consider a conical beam hitting a plane of arbitrary angle relative to the cone and projecting the image on to the collection plate, as viewed from the perspective depicted in
Based on
Referring to
To solve for zea (and zeb) consider the other perspective of the ring, as viewed across the widest part as illustrated in
For ring a
For ring b
Plugging into equation 7,
Where
Equation 11 dictates that the widest and narrowest projections of the rings be measured, and slight variations may lead to discrepancies in α. It is useful to seek an equation based on displacement of the ring centers instead, which is less sensitive to error. If the lower ring is “magnified” to match the known ratio of diameters of upper and lower rings (e.g., ratio of 1 if the rings are the same diameter), it would be the same as if the ring were moved up in the vertical direction (z direction) since scaling is done for values of each point on the ring in the coordinate system where ring points are offset from zero.
This z position becomes the z position of the major axis of both ellipses. Image processing enables the scaling of the image of the far field ellipse about the center of the image until the far field ellipse diameter relative to the near field ellipse diameter match the expected ratio. For example, if the far field ring and near field ring physically have identical diameters, the x-ray projected far field ellipse will appear smaller than the near field ellipse. Points on the far field ellipse may then be scaled so that a new image of the far field ellipse would have the same diameter as the near field ellipse. In particular, the only point that needs to be scaled may be the center of the far field ellipse, as defined by the intersection of the far field ring's crosshairs. With the far field ellipse scaled and the near field ellipse not scaled, the offset in the centers of the ellipses represents the measurement in image coordinates of the side opposite in a triangle with hypotenuse equal to the distance between rings. At this z position, the hypotenuse may be determined in image coordinates, where it is the distance between rings times the ratio of near field ellipse major axis over the near field ring diameter (or times the ratio of the scaled far field ellipse major axis over the far field ring diameter, which is by definition of the scaling factor the same). With the side opposite and hypotenuse, α can be found with an arcsine function.
Where:
It might appear that the azimuth angle ϕ (the angle required to put the axis of rotation for ring incidence on the y-axis) is just the angle relative to the major axis of one of the ellipses. For example,
An additional consideration is that for small angles, it may be difficult to accurately assess the exact direction in which the diameter is largest, thus a method for using the orientation of the major axis of an ellipse to find θ may produce a lower accuracy result. In embodiments, a method using the length of the major axis of an ellipse should produce better results.
In the scaling exercise described in reference to
The angle θ is the angle of the near field vertical crosshair relative to Y or horizontal crosshair relative to X after accounting for the perspective. Thus, finding the locations of the intersections of crosshairs with the ellipses and then applying an inverse incidence angle would give the intersection points in a flat plane, allowing the angle θ to be determined from an arctangent of the x, y coordinates of the crosshair intersection point.
Note that it is important to know which crosshair is aligned with X or Y and which direction of a crosshair points to +X or +Y. This information can be determined from additional features on the fixture that appear on x-ray images, such as a BB or wire near the reference crosshair's positive axis or any other suitable feature.
The offset position dx, dy is the offset of the x, y coordinate of the center of the near field ring. This point can be directly tracked based on the tracker on the fixture and the corresponding point seen on the resulting x-ray. This point can serve as a registration check. That is, if a navigated probe is pointed at the center of the near field ring, an image of a probe with its tip at the projected ellipse's crosshair intersection should be seen.
If the registration fixture is attached very precisely to the image intensifier, then several of the parameters referenced above go to zero. That is, the incidence angle α, axis of rotation reference ϕ, displacements dx and dy all go to zero, simplifying the registration process. Thus, as with the BB fixture, the locations of intersections of crosshairs and rings on x-rays could be used as adjustment tools instead of for extracting transformation parameters. That is, if an x-ray shows disparity in the intersections of the ring centers and edges, such as depicted in
When using rings in a registration fixture, correction of distortion may be achieved in a way that is similar to the correction applied for a BB fixture. In order for distortion correction to be effective, the crosshairs on the ring fixture require an additional feature in which evenly spaced markings are placed along each crosshair. These markings could be hatch marks, circles, gaps, or any such feature that appears on a visible projection on the x-ray image. Then, by considering both the linearity of the crosshairs and the spacing between indices on the crosshairs, pincushion and s-distortion may be accounted for and corrected. The
3D Surgical Planning in 2D:
In the planning of medical procedures, such as in conjunction with a surgical robot platform, planning for placement of medical objects such as surgical screws may be provided in 3D based on the 2D images. For instance, in such planning a line segment drawn to represent a screw in one of the 2D views may be assumed to have a certain dimension into and out of the plane in which it is drawn (e.g., in the z dimension). It can also be assumed to have a certain starting and ending z coordinate into and out of the plane in which it is drawn. For example, if a pedicle screw is being planned on an anteroposterior and lateral x-ray image, an appropriate assumption for the z coordinates could be that the dimension of the screw on the lateral x-ray represents the maximum length of the screw. That is, the screw is not angled into or out of the plane and therefore the z coordinate of the tip and tail of the screw on the lateral image's local coordinate system are the same. The z coordinate, which is equal for tip and tail, could be assumed to be a value that would be appropriate to place the screw at the center of the anteroposterior image. That is, for the user selecting x- and y-coordinates in the lateral planar view, whatever z coordinate in the lateral image causes the screw image to appear at the center of the screen on the anteroposterior image would be used.
In embodiments, other means could be used for improved initial guesses on the unknown planning plane. For example, anteroposterior and lateral images could be used for planning, where the top of both images could be oriented to represent the rostral anatomical direction. If it is known through software prompting that the user is about to place the left screw by dropping it on the lateral image, the starting location of the screw on the anteroposterior image could be toward the left side of the screen, assuming left screen is left anatomical direction.
Once an initial position is dictated by the user in one view and guessed or otherwise specified by software in the other view, any subsequent repositioning of the screw in either view may be mapped to the other view through satisfying the forward mapping of the 3D coordinates to 2D. For example, the user may have defined the x, y, z coordinates of a screw's tip in a local Cartesian coordinate system associated with the registration fixture during a lateral x-ray. If, through software interactions, the user then selects and drags the representation of the screw tip, they must be moving the tip in the x-y plane of that Cartesian coordinate system, not in its z direction, since the x-y plane of the Cartesian coordinate system is parallel to the image plane. The x and y movements (with z movement=0) in that local coordinate system can be updated through the user interaction. Then, because the transformation between the local coordinate systems of the anteroposterior and lateral x-rays are known through tracking, the resulting x, y, z coordinates associated with the anteroposterior image's local coordinate system can also be updated, allowing mapping of the planned tip of the screw on to a new position in the anteroposterior image. Through a sequence of updating one image and then the other, the user can move the screw into a 3D position that is known relative to both tracked positions of the registration fixture and therefore known to the camera space and to the robot. The robot can then move to a position to allow the screw to be placed accurately.
Note that if the two coordinate systems of the images are perpendicular, one representing the anteroposterior and one representing the lateral x-ray, then movement of a planned representation of a screw tip or tail on the anteroposterior view in the rostrocaudal direction through software interaction would have the effect of causing the screw tip or tail representation to move rostrocaudally in the lateral view by the same amount. However, movement of the screw tip or tail left or right in the anteroposterior view may have no effect on the planned tip or tail position in the lateral image. Conversely, movement anterior or posterior of the screw tip or tail in the lateral image would have no effect on the screw tip or tail position in the anteroposterior image, but movement of the screw tip or tail position rostrally or caudally in the lateral image would cause the representation of the screw tip or tail in the anteroposterior image to change rostrocaudally by the same amount. If the two x-rays are not taken perpendicular, then movement left, right, up or down in one view of the planned screw tip or tail will cause the representation in the other view to move by at least some amount.
Although it has been described that two views are used for planning, such as one anteroposterior and one lateral x-ray, since the mapping of 3D to 2D can be created for any x-ray image it is possible to simultaneously display and update a plan on any number of x-ray images as long as the registration fixture's tracking information is acquired at the time the image is taken. For example, four images shot at 45-degree increments could be displayed in four quadrants of the screen and a planned screw could be registered to each view. Using software interactions to update the planned position in one view would cause the image in each of the other views to change.
The registration fixture 2 includes a base frame 6, first and second side frames 8,10 and a kinematic mount 12 that detachably attaches to the base frame 6. By definition, the kinematic mount restrains all six degrees of freedom of the side frames 8,10 relative to the base frame 6.
The base frame 6 is composed of an aluminum frame of minimized volume and weight, but can be fabricated from any of a number of reasonable cost, low density, high strength and stiffness materials.
A radiolucent plate 14 is attached to the base frame 6. In the embodiment shown in
Each plate is manufactured from a radiolucent material (e.g., carbon fiber, Rohacell foam, acrylic, ABS, or similar material) and houses embedded radiopaque markers 17,19 oriented in unique configurations for image processing and navigation purposes. In the embodiment shown, the radiopaque markers 17,19 are ⅛ inch stainless steel balls, but could be composed of any number of radiopaque materials and of a variety of geometries.
In one embodiment, the plates 14, 16 are mounted to a precision flat surface on the base frame 6 and a separation distance between the two plates is between 10 mm and 75 mm. In another embodiment, the range is between 25 mm and 50 mm, which may minimize obstruction within a surgical work area for the clinical team. Nevertheless, the plate separation distance can be increased or decreased to improve accuracy.
The orientation and registration plates 14, 16 are aligned with precision dowel pins 20 through a hole and slot configuration to achieve optimal plate to plate alignment precision. The hole, slot, pin configuration for the two plates are unique to prevent incorrect installation.
The second set of markers 19 in the registration plate 18 includes a plurality of radiopaque markers that are equally spaced from each other in a circular pattern. In the embodiment shown, there are 24 uniformly spaced markers in the registration plate 18.
The first set of markers 17 in the orientation plate 16 includes a set of markers that are spaced from each other in a circular pattern although the spacing among them is non-uniform. The circle formed by the markers 17 is smaller in diameter than the one defined by the second set of markers 19 in the registration plate 18. The two circles defined by the markers (smaller circle defined by markers 17 and larger circle defined by markers 19) are coaxial and concentric with one another.
The first set of markers 17 also includes markers (e.g., two shown for each of the corresponding non-uniformly spaced markers) that extend radially outwardly from the corresponding marker in the small circle defined by the non-uniform markers such that an imaginary line from the center of the circle crosses the radially extending markers and the corresponding marker in the circle. In the embodiment shown, there are 24 markers 17 in the orientation plate 16 (8 markers lying on the small circle and 8 subsets of 2 radially extending markers from the corresponding marker in the circle). It is important to note that the number of markers in both the registration plate 18 and the orientation plate 16 are the same at 24.
All radiopaque markers may be in the form of stainless steel balls or BBs' although they may be of any suitable radiopaque material.
The placement, size and the number of the radiopaque markers in the registration and orientation plates as described above provide optimal parameters for navigation accuracy, collimation requirements (i.e., ability to detect enough markers and place surgical implants accurately even with the presence of collimation which will truncate the patterns), minimization of anatomical obstruction to the surgeon during navigated procedures, and detection of orientation to allow navigation tracking software to deterministically detect whether the image is not flipped by 180-degrees or 90-degrees.
Although the radiolucent plate 14 is described with reference to a flat panel registration fixture 2, they may be implemented as part of a registration fixture (such as shown in
The side frame has a plurality of optical tracking markers and is adapted to detachably mount to the base frame 6 without piercing a sterilizing drape to be interposed between the base frame 6 and the side frame.
As shown in
The side frames 8,10 may be bead blasted and black anodized to reduce potential reflections, among many surface treatment options. In the embodiment shown, the two side frames 8,10 are oriented 180-degrees from one another, and extend perpendicularly from the base frame 6. Each side frame 8, 10 may contain mounting features to enable the use of both flat tracking disks 22 and spherical markers 24 (only the posts are shown in
The flat disk markers 22 and spherical markers 24 are interspersed with each other. In one embodiment, the pattern and spacing of the markers 22,24 on one side frame 8 is identical to that of the other side frame 10 when viewing from their respective side (i.e., 180 degrees from each other).
The side frames 8,10 are self-aligning and oriented precisely to the base frame 6 through the use of a kinematic mount configuration 12.
Each side frame 8, 10 is designed to be non-interchangeable through the incorporation of a physical keying feature, which prevents users from accidental incorrect installation.
As shown in
As shown in
The first side frame 8 extends laterally on one side of the base frame 6 and its tracking markers 22,24 face away from the base frame 6 in a first direction away from the base frame while the second side frame 10 extends laterally on the other side of the base frame 6 and its tracking markers 22,24 face away from the base frame 6 in a second direction opposite the first direction.
As shown in
In one embodiment, the spherical tracking markers 24 of the side frames 8,10 are adapted to reflect infrared light (NIR) while flat disk markers 22 are adapted to reflect visible light and in some embodiments also the infrared light (NIR).
In order to facilitate the ability to mount to a variety of flat panel C-Arm detector housings, multiple flexible ratchet strap configurations have been designed and implemented.
In one configuration, the first strap 50 of the ratchet strap assembly is composed of an extension component containing multiple thru holes to facilitate adjustability (
A flat panel detector registration fixture 2 as described above provides the following advantages.
Image collimation is possible while still allowing adequate orientation and registration fiducial marker detection for navigation and image processing requirements. Collimation has major advantages with respect to image quality as visualization of patient anatomy in certain scenarios can be extremely challenging without such collimation.
Optical tracking arrays mount to a precision kinematic mount configuration utilizing a clamp that prevents piercing of sterile drape. The clamp is designed in a u-shaped geometry which moves by either a camming handle integrated as actuator or a threaded handle (acting as a leadscrew) for rigid mounting to the base frame 6. Such mounting strategy protects the integrity of the sterile drape.
Non-piercing side frames 8,10 with optical tracking markers facilitate clamping of a separable, sterile, and autoclavable side frame which is unlike the conventional design that uses drape-piercing mount methodology. Side frames 8,10 containing optical tracking markers are separable from the base frame 6, utilize disposable markers, and are machine washable and autoclavable. The separable nature of the side frames 8,10 allows increased optimization for improved accuracy: size, segment length optimization, positioning relative to potential operating room obstructions.
Optical tracking arrays incorporate both passively tracked disks and spheres into single frame assembly to facilitate tracking utilizing NIR (spherical markers) and visible light technology (flat disk markers).
Novel radiopaque fiducial pattern is incorporated into orientation and registration plates to facilitate image processing and navigation workflows.
Ratchet strap mounting configuration utilizing a non-deterministic, compliant strap to facilitate fluoroscopy fixture mounting to a variety of c-arm detector panel geometries. Ratchet strap includes an adjustable extension component fastened in series to a compliant pad with spring-loaded mechanical ratchet, which interfaces with flexible ladder strap belt assembly. Ladder strap may optionally contain a hook or carabiner component for mounting to captive C-Arm handles. Optional design safety elements incorporated on belt assembly include hard stops.
Modular ratchet strap assembly has the ability to be adjusted along the perimeter of fluoroscopy fixture through utilization of self locking clevis pin, incorporating spring loaded wedge or equivalent locating and self-locking feature. Clevis pin incorporation facilitates easy user adjustment of ratchet strap assembly considering a variety of C-Arm detector housings.
Orientation and registration plate spacing has been minimized to between 25 mm to 50 mm in one embodiment to minimize obstruction within surgical work volume for clinical team, as compared to conventional fluoro fixtures with 100 mm or greater plate separation distance. This plate separation may be decreased or increased to improve accuracy, but has been minimized in order to maximize available workspace for clinical surgical team.
Finding True Fluoro Fixture Location
As explained above, one part of the above algorithm for registering the CT image to the tracking cameras is to accurately track the location of the image plane and emitter relative to the patient tracker so that the DRRs are generated with correct corresponding configuration of the virtual emitter and virtual collector relative to the CT volume. One way that software can track the image plane is by tracking a fixture directly attached to the face of the image collector of the C-arm. The emitter location is then accurately determined by using a “pinhole camera” algorithm where the shadows of two planes of BBs passing through the fixture are analyzed to determine where the emitter must have been relative to the collector to create such shadows. One underlying assumption of CT-fluoro registration processes is that the far BB plane of the tracked fluoro fixture, which is up against the physical collector of the C-arm, can be considered to be coincident with the X-ray image plane, even though the BB plane is really several centimeters from the hardware that detects the X-rays.
Note that in the analysis of emitter location based on projections of two planes of BBs, computational approaches are used for which open-source software is available for camera calibration functions. In one computational approach, the geometric locations of the BBs in 3D on the fixture in Cartesian coordinates (x, y, z positions in mm) are called the object points and the projected shadows of the BBs on 2D X-rays in pixel coordinates are called the image points.
Referring to
In treating the optics such that the image plane is considered parallel and coincident with the fluoro fixture, the known location of the tracking array is used to directly determine the position of the image plane (fixture plane) relative to the cameras. Since the center BB of the image plane is considered the origin (0,0,0) of the fluoro fixture, the tracked pose of the sphere/disc optical array represents the tracked pose of the image plane.
With a tilted image plane, however, the actual position of the image plane is not actually at the location in space extrapolated from the markers attached to the fixture. There is instead a rotation about an axis of rotation and through the origin of the image plane (BB plane) that must be considered. That is, having an angled fixture on the C-arm would be the same as having a non-angled fixture on the C-arm with its BB locations and array location shifted by a rotation about an axis that lies on the image plane. To compensate for the rotation, the following algorithm can be used:
Step 1 can be solved knowing the rotation matrix returned by the calibration algorithm. This rotation matrix can be considered a sequence of three planar rotations: (1) a rotation about the fixed Z axis followed by (2) a rotation about the fixed X axis, followed by (3) a second rotation about the fixed Z axis. In other words, rotate the image to dial in the azimuth and then apply the tilt; then rotate the resulting image to the perspective desired. Since rotations about moving axes are the opposite order of rotations about fixed axes, this rotation sequence is the same as (1) a rotation about the fixed Z axis followed by (2) a rotation about the fixed X axis followed by (3) a rotation about the fixed Z axis.
To proceed to the next phase of the algorithm, the object points may be modified by a sequence of three rotations to move them to a location where the image points actually produced would show no tilt angle. This sequence of rotations is similar to the rotation sequence of three rotations previously described, but is slightly different because the z rotation previously used to compensate for screen rotation is now used to compensate for the tilt azimuth. The direction of the tilt to be applied is chosen (rotation about z), the tilt is then applied (rotation about x), and then a compensatory rotation is applied such that there is no twist of the joint. The first and third (z) rotations should sum to zero.
These modified object points are used to compute a modified projection matrix and to compute a modified fixture tracking array location. With modified projection matrix and fixture tracking matrix, a new projection matrix is computed and sent to the registration algorithm in place of the projection matrix from the old algorithm. This new projection matrix represents a projection onto the true image plane instead of onto the fixture plane, giving better accuracy in the final fluoro-CT registration.
Alternative Fixture Tilt Solution
In the above analysis, the fixture tilt is determined using a calibration function and manipulating the resulting projection matrix. If all that is needed is the fixture tilt, which may be used to, for example, provide a warning to the user that the fixture is tilted to indicate that it needs adjustment, another approach can be that is based on the projected appearance of rings. Using rings may be more robust than the above-described method because it is possible that rings are less susceptible to shape changes from pincushion and S-distortion than raw individual BB positions.
This approach can be effective using the 9″ and 12″ fluoro fixtures, e.g., from Globus Medical, Inc., because in both fixtures the patterns of BBs can be interpreted as ring-shaped when processing the image.
More particularly,
Referring to the embodiment of
Referring to the embodiment of
In
In the example embodiment of
Alternately, if anatomy or metal artifact blocks much of the circle 4702 shown in
In the 12″ fixture configuration of
A geometric analysis of the rings in the path of the X-rays is represented schematically (
This z position becomes the z position of the major axis of both ellipses. Therefore, with the far field ellipse scaled and the near field ellipse not scaled, the offset in the centers of the ellipses represents the measurement in image coordinates of the side opposite in a triangle with hypotenuse equal to the distance between rings. At this z position, it is easy to find the hypotenuse in image coordinates: it is the distance between rings in mm times the ratio of near field ellipse major axis over near field ring diameter (or times the ratio of scaled far field ellipse major axis over far field ring diameter, which is by definition of the scaling factor the same).
The arcsine function of Equation 12 described above can be used to solve for α using the the side opposite and hypotenuse. These operations can be applied while each image is processed from the fluoro C_arm, to provide immediate feedback to the user on whether the fixture is tilted.
In a further embodiment, the operations may generate a warning notification to a user when the tilt angle of the fixture satisfies a notification rule, e.g., exceeds a threshold fixture tilt angle. The warning notification may correspond to displaying a warning message informing the user that the fixture is not properly mounted to the flat panel detector of the C_arm and/or may correspond to generate an audible warning and/or activating/deactivating a light. The operations may further disable operation of the C_arm for use in performing imaging until the tilt angle of the fixture is determined to satisfy an imaging operation rule, which may correspond to the notification rule.
Improved Collimation
When capturing a fluoroscopy (fluoro) image for use in a registration algorithm, the surgeon or radiology technician may need to collimate the field to create an image that has even contrast and is not overly dark or bright in certain areas. Collimation is blocking of the X-ray path using a lead curtain and is adjustable on the console of most C-arms. When less than the full-size image is captured, fewer BB shadows are available for the analysis of the emitter location using, e.g., a pinhole camera model.
In order to improve the ability to detect a partially collimated image, multiple rings of orientation BBs may be included on an orientation plate of a flat panel fluoro fixture (FPFF), which is an non-limiting example type of a registration fixture.
A registration fixture for registration of a medical image to a three-dimensional tracking space, can include a base frame, an orientation plate, and a registration plate. The base frame is adapted to be mounted to an x-ray medical imaging device. The orientation plate is attached to the base frame and has a first set of BBs (or other radiopaque markers) arranged along at least four radial lines which extend from a center location, and where the radial lines are angularly spaced apart about the center location, and at least three of the BBs are arranged along each of the radial lines. The registration plate is attached to the base frame and spaced apart from the orientation plate. The registration plate has a second set of BBs (or other radiopaque markers) arranged along a circle.
Although some embodiments are described as using BBs, these and other embodiments may more generally use any type of radiopaque markers. The radiopaque markers are not restricted to being round and are not restricted to being discontinuous spaced apart marker structures. The radiopaque markers can have any shape, such as oblong, square, etc. The radiopaque markers may be spaced apart thicker cross-sectional regions of a continuous material plate, where the thicker cross-sectional regions are configured to be more radiopaque than the plate regions intervening between the thicker cross-sectional regions. The thicker cross-sectional regions are configured to have their locations recognized through image processing of an image of the plate by an x-ray medical imaging device. Accordingly, all references to “BBs” herein can be replaced by the term “radiopaque markers” in accordance with these and other embodiments of the present disclosure.
Referring to
In some other embodiments, the orientation plate is configured with fewer than eight radial lines or with more than eight radial lines. In some embodiments, the first set of radiopaque markers are arranged along at least eight radial lines which extend from the center location, the radial lines are angularly spaced apart about the center location, and at least three of the radiopaque markers of the first set are arranged along each of radial lines. In one embodiment, the first set of radiopaque markers are arranged along six radial lines, such as for a 9″ fixture or a 12″ fixture. In another embodiment, the first set of radiopaque markers are arranged along seven radial lines. The radial lines may be arranged with different unique angles between adjacent pairs of radial lines. The first set of radiopaque markers on each of the radial lines may be arranged with different minimum spacing therebetween and with different radial locations along the radial lines. In another embodiment, the radial lines are uniformly angularly spaced at least 20 degrees apart about the center location.
The orientation of the first set of BBs of the orientation plate illustrated in
As a first consideration, there should not be a sufficient number of BBs in the first set of the orientation plate to allow reliable registration while avoiding an excessive number of BBs in the first set in order to avoid additional unnecessary registration fixture cost, manufacturing complexity, image processing complexity, and obscuring of imaged patient anatomy. Increasing the number of BBs in the first set can generally improve registration accuracy, particularly when the BBs in the first set have a large spread in the image, e.g., corresponding to the area formed by a polygon with the outer BBs as vertices.
As a second consideration, the pattern of BBs in the first set of the orientation plate should be able to be recognized through image processing while subjected to significant collimation. It is desirable to have the pattern of BBs widely dispersed, but with many radial lines (arms) still recognizable through image processing even if the horizontal, vertical, or both portions of the image have been collimated. It is also possible for non-vertical/horizontal collimations to be created on some systems, so at least some BBs in the first set should be near the center location from which the radial lines virtually extend.
As a third consideration, the pattern of BB s in the first set of the orientation plate should provide a uniquely recognizable pattern during processing of the image even with some of the BBs in the first set are not recognizable, e.g., missing from the image, and so the pattern of BBs in the first set can be unambiguously recognized at any rotation and with the pattern of BBs in the first set being potentially flipped, e.g., since some C-arm x-ray medical imaging devices may optionally flip the image during software processing such as along with color inversion and other image processing steps.
As a fourth consideration, the second set of BBs of the registration plate should be distinctly separated geometrically from the first set of BBs of the orientation plate (e.g., along the parallel planes of the orientation and registration plates), even if the emitter is located off-center or the fixture is tilted. The separation allows simpler and more accurate detection of the second set of BBs of the registration plate because they will not intersect either the innermost or the middle ring of BBs in the first set of the orientation plate.
With continued reference to
In some embodiments, an improvement to the ability to accurately recognize the circles of BBs during processing may be obtained by including at least four BBs in the first set of the orientation plate and at least six BBs in the second set of the registration plate. However, in some situations, more than these numbers of BBs will be available, even when collimated in more than one direction and/or when rotated. In
The example pattern shown in
In some other embodiments, the first set of radiopaque markers may be arranged along a plurality of concentric circles. The first set of radiopaque markers may include at least 24 radiopaque markers arranged along at least three concentric circles. The second set of radiopaque markers may include at least four radiopaque markers. The second set of radiopaque markers may include at least 24 radiopaque markers. The orientation plate and the registration plate may be attached parallel to each other on the base frame.
In a further embodiment, the first set of radiopaque markers include at least 24 radiopaque markers arranged along at least three concentric circles, and the second set of radiopaque markers are arranged along the circle with a center location which is aligned, in a direction normal to the orientation plate and the registration plate, with the center location of the at least three concentric circles of the at least 24 radiopaque markers of the first set.
In a further embodiment, the orientation plate and the registration plate are parallel to each other, and none of the radiopaque markers of the first set are aligned with any of the radiopaque markers of the second set in a direction normal to the orientation plate and the registration plate.
In a further embodiment, the registration fixture includes a side frame adapted to detachably mount to a plurality of optical tracking markers and adapted to detachably mount to the base frame.
A further embodiment is directed to a surgical navigation system for registration of a medical image to a three-dimensional tracking space. The surgical navigation system includes at least one processor, and at least one memory storing instructions executable by the at least one processor to perform operations.
While the invention has been disclosed in connection with the preferred embodiments shown and described in detail, various modifications and improvements thereon will become readily apparent to those skilled in the art. Accordingly, the spirit and scope of the present invention is not to be limited by the foregoing examples, but is to be understood in the broadest sense allowable by law.
All documents referenced herein are hereby incorporated by reference.
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US 8,231,638 B2, 07/2012, Swarup et al. (withdrawn) |
Number | Date | Country | |
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20230397957 A1 | Dec 2023 | US |