WO2002024094A2

WO2002024094A2 - Non-ivasive system and device for locating a surface of an object in a body

Info

Publication number: WO2002024094A2
Application number: PCT/IL2001/000893
Authority: WO
Inventors: Roni Yagel
Original assignee: Insightec-Image Guided Treatment Ltd.
Priority date: 2000-09-25
Filing date: 2001-09-24
Publication date: 2002-03-28
Also published as: AU2001294157A1; WO2002024094A3

Abstract

A system for non-invasively determining the location of a surface of an object, such as a bone, in a body. The system comprises a device, such as a hand-held surgical instrument, carrying an energy transducer for determining linear distance vectors between the energy transducer and a surface of the object. The device further carries a tracker in a known position and orientation relative to the energy transducer. The tracker is adapted to determine a relative position and orientation of the energy transducer in a reference coordinate system. A processor coupled to the energy transducer and tracker is adapted to determine the relative locations in the reference coordinate system of surface points on the object based on linear distance vectors provided by the energy transducer and the relative position and orientation of the energy transducer provided by the tracker for each linear distance vector.

Description

NON-INNASINE SYSTEM AND DEVICE FOR LOCATING A SURFACE OF AN

OBJECT IN A BODY

BACKGROUND OF THE INVENTION

Medical procedures that utilize some form of image guidance are becoming more and more prevalent. For instance, in the area of orthopedic surgery, preoperative tomographic images, such as computerized tomography (CT) images, magnetic resonance images (MRI), and ultrasonic images, are increasingly used as a navigational tool. Other interventional medical procedures also benefit from image guidance. Particularly in orthopedic surgery, it is beneficial to be able to identify, for example, from an ultrasound image, the precise picture elements, or pixels, that represent bone surface points. Such an accurate map of the bone structure is highly beneficial to a surgeon who is relying on these types of tomographic images as a navigational tool.

Traditionally, more invasive registration techniques are employed to determine the exact spatial orientation of a bone in a patient's body. For instance, a physician may surgically expose a bone during an operation and use a three dimensional tracking device to map a large number of points from the bone surface. Algorithms executed by a computer then determine a three dimensional transformation (translation, scale, and rotation) of the bone surface points, and relate this transformation to a pre-operative image of the bone tissue that was acquired using conventional CT or MRI imaging systems. The computed transformation becomes the "registration transform," which maps the pre-operative image onto the intra-operative scenery. The combined image provides a map to a surgeon for navigation during procedures that require image-guided surgery. As can be expected, however, physically pointing, i.e. touching, the bone surface at a large number of locations is time consuming and presents all of the inherent risks associated with subjecting a patient to a surgical procedure.

In order to overcome the need to expose the bone surface and manually plot a series of points for image registration, recent research has suggested the use of tracked intra-operative imaging. This approach is based on scanning a patient's anatomy with a tracked intra-operative imaging device, such as an off-the-shelf ultrasound system, and then transferring the image to a computer. A computer program then extracts the surface of the bone using a combination of automatic and semiautomatic image segmentation algorithms. The resulting surface points serve as the input points to the registration algorithm. Several of these methods are described in T. Ault and M.W. Siegel, "Frameless Patient Registration Using Ultrasonic Imaging," Proceedings of the First International Symposium on Medical Robotics and Computer Assisted Surgery, Volume I, Sessions I-III, pp. 74-81, Pittsburgh, PA., Sep. 22-24, 1995; J. Tonetti, L. Carrat, S. Lavallee, L. Pittet, P. Merloz, and J.P. Chirossel, "Percutaneous Iliosacral Screw Placement Using Image Guided Techniques," Clin. Orthop. 1998 Sep (354): 103- 10; C. Barbe, J. Troccaz, B. Mazier, and S. Lavallee (1993), "Using 2.5D Echography in Computer- Assisted Spine Surgery," IEEE Engineering in Medicine and Biology Society Proceedings, pp. 160-161; G. Champleboux, S. Lavallee, R. Szeliski, and L. Brunie (1992), "From Accurate Range Imaging Sensor Calibration To Accurate Model-Based 3-D Object Localization," IEEE Computer Society Conference on Computer Vision and Pattern Recognition, (CVPR'92), pp. 83-89; J.L. Herring, B.M. Dawant, C.R. Maurer Jr., D.M. Muratore, R.L. Galloway, and J.M. Fitzpatrick, "Surface-Based Registration of CT Images To Physical Space For Image-Guided Surgery of the Spine: A Sensitivity Study," IEEE Trans Med. Imaging, 1998 Oct,

17(5):743-52.

These methods present several disadvantages, however. For instance, automatic segmentation algorithms severely lack robustness requiring a user to maneuver the ultrasound probe and only select images that clearly show the bone surface. A careful application of the ultrasound probe is required in order for the segmentation algorithm to succeed. Furthermore, off-the-shelf diagnostic ultrasound devices are typically set to ignore bone tissue, emphasizing instead, soft-tissue imaging. In order to function properly, the ultrasound device needs to be carefully reset to scan bone, requiring operator intervention and re-calibration in many applications.

Furthermore, off-the-shelf diagnostic ultrasound devices are costly and are built to provide imaging capabilities far beyond what is required for bone detection, i.e., the use of such powerful and sophisticated devices is overkill for mere bone surface detection. Even so, only modern ultrasound devices that support digital image output can provide the image quality required for image segmentation. Most ultrasound equipment in use today is not appropriate for this task.

Floor space in an operating room is a commodity that is not available in large quantities. Since off-the-shelf diagnostic ultrasound devices are bulky, and their footprint too large for the task they perform, they are often not appropriate for these types of procedures.

Even further, existing bone segmentation techniques involve the use of sophisticated algorithms that require time consuming computations. As a result, the process of careful scanning, data transfer, image segmentation, and registration can take a relatively long time, another commodity that is in short supply. SUMMARY OF THE INVENTION

In one embodiment of the invention, a system for non-invasively determining the location of a surface of an object in a body, such as a surface of a bone, comprises a hand-held device carrying an energy transducer and a tracker. The energy transducer is used to provide a plurality of linear distance vectors from point(s) outside the body to the object surface. The tracker, in turn, provides a three-dimensional position and orientation of the energy transducer in a reference coordinate system corresponding to each linear distance vector. A processor, which may be located in the device, determines, based on the respective linear distance vectors from the energy transducer, and the relative position(s) and orientation(s) of the energy transducer from the tracker, locations in the reference coordinate system of points on the object surface. This location information can be displayed to assist, e.g., a physician, as part of a surgical procedure.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate both the design and utility of the preferred embodiments of the present invention, in which similar elements in different embodiments are referred to by the same reference numbers for purposes of ease in illustration, wherein:

Fig. 1 is a perspective view of a bone surface locator device constructed in accordance with one embodiment of the present invention.

Fig. 2 is a magnified view of a sensor array used in the bone surface locator device of Fig. 1. Fig. 3 illustrates the use of a bone surface locator device constructed in accordance with an embodiment of the present invention to determine the distance between the device and a bone surface.

Figs. 4 and 5 illustrate the orientation and position of a bone surface locator device constructed in accordance with an embodiment of the present invention relative to a point on the surface of a patient's body.

Fig. 6 is a flow chart illustrating the operation of a bone surface locator device constructed in accordance with an embodiment the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Fig. 1 shows a bone surface locator device 10 constructed in accordance with one embodiment of the present invention. The device 10 can be used, among other applications, to quickly and easily determine the location of a bone surface without surgically exposing the bone or otherwise subjecting the patient to unnecessary discomfort or physical risk. Generally, the device 10 includes a pointer 20 and a processor 60. The pointer 20 has an elongate body 22, generally in the shape of a stylus, pencil, or a similar type of compact hand held tool. The body 22 has a distal end 24 and a proximal end 26. As used herein, the term distal refers to the portion of the pointer 20 that is in contact with a patient. The pointer 20 is preferably shaped so that it can be securely held in a user's hand and is ergonomically contoured to allow comfortable use for an extended period of time. Various contours may be included in the shape of the body 22 in order to more precisely fit the shape of a user's hand. The body 22 may be made of various materials such as rubber or foam to further increase the comfort to a user. Preferably, a bio-compatible and washable material capable of being sterilized is used to form the body 22. A motion control device 28 and two control buttons 30 and 32 are located on the surface of the body 22 and are positioned so that they can be easily accessed and activated by a user while holding the pointer 20 in a stable position. The motion control device 28 and the buttons 30 and 32 are used to control various features of the device 10 such as power, imaging strength, data transfer, etc. Further embodiments of a bone surface locator device 10 constructed in accordance with the present invention may include additional control buttons or switches that activate various other features of the device. In that regard, the above description is meant to be illustrative and not limiting. For example, one of the buttons may activate a menu selection screen on a computer system. Alternatively, a button may control the transmitted ultrasound intensity, the gain associated with the received signal, or the focus depth.

Located proximate the distal end 24 of the body 22 is a sensor array 50. Preferably, the sensor array 50 is located on the extreme distal tip of the body 22. As shown in Fig. 2, the sensor array 50 is comprised of three energy transducers 52, 54, and 56. While Fig. 2 shows the use of three energy transducers 52, 54, and 56, it will be understood to those skilled in the art that fewer or greater than three energy transducers may be used in the sensor array 50. Preferably, the energy transducers are ultrasonic, however, other types of energy transducers that can both send signals and receive echoed or reflected signals, are also contemplated.

Each of the transducers 52, 54, and 56 send a signal, preferably ultrasonic, from their distal facing surfaces. Each of the energy transducers is also adapted to receive echoed signals. Since solid objects, such as bone, reflect high intensity signals while soft tissue, such as muscle, skin, blood, and fatty tissue, reflect low intensity signals, the device can be configured to distinguish echoed signals reflected by harder materials such as bone from signals echoed by other softer tissue. The transducer is also tailored to emit signals in frequency and intensity such that its echo from a bone will be easily distinguishable from soft tissue echo.

Utilizing the ultrasonic transducers 52, 54, and 56 that are incorporated into the sensor array 50, the device 10 measures the time it takes an ultrasound wave generated by the transducers to echo off of a hard surface (i.e. bone) and return to the transducer face. Since the speed of the ultrasound wave is known, this time measurement can be easily translated, by known methods, into a distance measurement between the transducer distal face and the surface of the bony material located beneath the device. Such methods are further explained in "Essentials of Ultrasound Physics" by James A. Zagzebski, Mosby 1996, the details of which are incorporated by reference into the present application.

The device 10 also includes a tracker 40 located on the body 22 of the pointer 20. The tracker 40 is calibrated to the position of the sensor array 50. The tracker is therefore similarly calibrated to the ultrasound transducers 52, 54, and 56. The tracker 40 is adapted to actively monitor the position and orientation of the sensor array 50. Preferably, the tracker 40 is adapted to monitor the location of the sensor array 50 in five or six degrees of freedom and generates a vector value based on the positioning, location, and orientation of the pointer 20. Such trackers are manufactured by Ascension, Biosense- Webster, and Polhemus. Preferably, a tracker such as those manufactured by Biosense are used due to its small size. The location of the tracker is based on a radio based antenna mounted in proximity to the device, i.e. on or under the patient. The tracker's relative position is thus based on the antenna's known location, the antenna thereby providing a reference coordinate system. Since the relative location and orientation of the tracker is known, and the location and orientation of the sensor array/transducers relative to the tracker is known, the relative position and orientation of the transducer can be calculated. Trackers based on electromagnetic or optical technology can also be utilized in conjunction with the present invention. A data wire 42 couples the tracker 40 to the processor 60 and transfers the location and orientation information generated by the tracker 40 to the processor 60. A data wire 44 exits the pointer 20 through a port 34 and couples the ultrasonic transducers 52, 54, and 56, to the processor 60. The data wire 44 transfers the echo time data received by each of the ultrasonic transducers to the processor 60. Separate data wires 53, 55, and 57 transfer the individual data from each transducer 52, 54, and 56, to the data wire 44. In this regard, the data wire 44 is formed from multiple wires. Alternatively, the information can be transferred through a wireless communication mechanism such as those used in wireless mice and other remote control devices. The processor 60 then takes the vector value generated by the tracker 40, combines it with the distance value generated by the ultrasonic transducers 52, 54, and 56, and computes the precise location of the bone surface, relative to the position of the sensor array 50 and the distal tip of the pointer 20.

While the bone surface locator device 10 is in use, the sensor array 50 is positioned against a patient's skin, proximate the bone surface that is to be mapped. The pointer 20 can then be moved into various positions around the target bone and a number of bone surface point locations can be obtained. It is immaterial how much soft tissue lies between the device and the bone nor does it matter if this amount changes. The resulting set of points will represent the surface location of the underlying bone structure and can then be used by a computer algorithm to perform image registration. Following image registration, the collected data can be superimposed over a real intra-operative situation (e.g., a surgeon's knife during surgery). In other words, the intra-operative surgeon's tool can be displayed over the preoperative CT map.

The processor 60 is preferably a general purpose personal computer (PC) equipped with software that allows it to compute the location of a bone surface from the aggregate information obtained by ultrasonic transducers 52, 54, and 56, and the tracker 40. Alternately, the processor 60 is a specialized computer whose function is limited to calculating the location of a bone surface from the combined inputs of the ultrasonic transducers 52, 54, and 56 and the tracker 40. In a further alternate embodiment, the processor 60 comprises a single integrated circuit that is built into the pointer 20. In this regard, having the processor 60 integrated with the pointer 20 allows the bone surface locator device 10 to be more compact and allows a medical professional who is using the device to maneuver it more freely. Such a compact and integrated device can be used remotely and the bone surface location results can be stored within the device and later downloaded to a computer or viewing system, avoiding the need for a wired connection to the registration device. The information is relayed to a user in the same manner as described above.

Referring to Fig. 3, the pointer 20 is shown positioned relative to a patient 70. For example, the pointer 20 is positioned resting against a patient's arm or leg so that the location of the underlying bone surface can be determined. The pointer 20 is positioned with the sensor array 50 abutting the skin surface 72. A bone 76 is shown, and muscle, fatty, and other soft tissue 74 are positioned intermediate the bone 76 and the skin surface 72. When activated by a user, the pointer 20 sends an ultrasonic signal 82 from the sensor array 50 through the skin surface 72, through the intermediate soft tissue 74, and toward the hard bone surface 78. Once the ultrasonic signal 82 reaches a point 84 on the bone surface 78, it is reflected back at the sensor array 50 as an echoed or reflected signal 83. Since the ultrasonic transducers 52, 54, and 56 are preferably adapted to only send and receive high intensity ultrasound signals, the echoes generated by the intermediate soft tissue 74 and the skin surface 72 are either negligible or are ignored by the ultrasonic transducers. Once the echoed signal 83 is received by the ultrasonic transducers, it is transmitted to the processor

60, where the linear distance between the sensor array 50 and the bone surface point

84 is calculated.

The position and directional orientation of the pointer 20, and thus the sensor array 50, are determined by the tracker 40, which continuously monitors the sensor array, in five or six degrees of freedom and relative to a known antenna location. Figs. 4 and 5 show a diagrammatic top and side view respectively of the pointer 20 as

it is positioned on the skin surface 72 of a patient. In Fig. 4, the angle α can be

determined by the tracker 40, giving the coordinates of the pointer 20 and the sensor

array 50 along the x and y axes. In Fig. 5 the angle β as well as the rotational position

δ can be determined by the tracker 40, giving the coordinates of the pointer 20 and the

sensor array 50 along the z axis and relative to a longitudinal axis w of the pointer 20. The information gathered by these positional coordinates is then transformed into a directional vector by the tracker 40 and transferred to the processor 60, which along with the linear distance vector generated by the ultrasonic transducers, allows the specific position of the bone surface point 84 to be calculated and relayed to an operator.

Fig. 6 is a flowchart depicting a typical procedure 100 that utilizes a bone surface locator device 10 constructed in accordance with an embodiment of the present invention. First, the device 10 is calibrated (110) using standard procedures. The calibration procedure finds the transformation between the tip of the device and the tracker. In the case that the sensor is packaged with the device and cannot be detached, the calibration is a one-time process performed during the manufacturing stage. Otherwise, it has to be repeated every time a sensor is attached to the device. The pointer 20 is then positioned on the skin surface proximate to an underlying bone, the surface of which is desired to be mapped by a user (115). Next, the device is activated and both spatial positing information (120) and distance measurements to the underlying bone surface are obtained (125). Vectors are generated by the tracker

(position and orientation) and a distance vector is generated by the ultrasonic transducers (130). Each of these measurements are conveyed to the processor 60 and the bone surface point is determined accordingly (135). The resulting information is then relayed to a user (140).

Once a user, such as a medical professional, determines the precise bone surface point locations, the information can then be used in conjunction with other known registration techniques to assist in preoperative procedures and during image guided surgery. A bone surface locator device constructed in accordance with the present invention thereby eliminates the need to surgically expose a patient in order to gain an accurate map of a bone surface. The small profile of a bone surface locator device constructed in accordance with the present invention allows it to be easily transported for use in any setting.

Although the invention has been described and illustrated in the above description and drawings, it is understood that this description is by example only and that numerous changes and modifications can be made by those skilled in the art without departing from the scope of the invention, which is not to be restricted, except by the following claims and their equivalents.

Claims

What is claimed:

1. A system for non-invasively determining the location of surface points on an object in a body, comprising: a device carrying an energy transducer and a tracker, the device configured for positioning the energy transducer adjacent an external surface of the body, the tracker in a fixed position and orientation relative to the energy transducer, the tracker adapted to determine a position and orientation of the energy transducer in a reference coordinate system; and a processor coupled to the tracker and the energy transducer, the processor adapted to determine a location in the reference coordinate system of surface points on an object in the body based on information generated by the tracker and the energy transducer.

2. The system of claim 1, wherein the object is a bone.

3. The system of claim 1, wherein the energy transducer is an ultrasound transducer.

4. The system of claim 3, wherein the processor is adapted to detect high intensity echoed ultrasound energy received by the transducer, and to ignore low intensity echoed ultrasound energy received by the transducer.

5. The system of claim 3, wherein the processor is adapted to determine a linear distance vector between the energy transducer and a point on the object surface based on a delay time from when an ultrasound wave is transmitted from the transducer until a portion of the wave reflected by the object surface is received by the transducer.

6. The system of claim 5, wherein the processor is adapted to determine the location and orientation of the surface point on the object in the reference coordinate system based on the location and orientation of the energy transducer provided by the tracker at the times the ultrasound wave is transmitted and received.

7. The system of claim 1, wherein the processor is located in the device.

8. The system of claim 6, wherein the processor is further configured to generate image data based on determined locations of points on the object surface.

9. The system of claim 1, wherein the device carries three energy transducers.