Electricity: measuring and testing – Magnetic – Displacement
Reexamination Certificate
1999-07-29
2001-06-12
Snow, Walter E. (Department: 2862)
Electricity: measuring and testing
Magnetic
Displacement
C324S207230, C128S899000, C600S424000
Reexamination Certificate
active
06246231
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic position measurement system with field containment means. The concept of using transmitting and receiving components with electromagnetic coupling is well known with respect to biomechanics and medical diagnostics, wherein a sensor assembly is mounted on a point of interest and the position of the point is determined relative to a fixed transmitter. This information is then used by computing systems to precisely show the relative motions of the points in question, which, in the medical sense, allows instruments to be precisely located in a human body with respect to the body and each other. This allows new, advanced methods of surgery and diagnostics to be performed.
When conductive materials are present, which is often the case on, below, or near an operating table, they generate eddy current fields, which distort the received magnetic field waveform, which distorts the output of the system unless the system utilizes some distortion reducing or compensating technique. When permeable materials are present, they bend and otherwise distort the magnetic field, with effects similar to conductive materials. In a surgical theater, both conductive and permeable materials are present in substantial quantities. They are a major component of many operating tables, surrounding equipment such as carts and equipment, and are present in the movable spot lamps used to illuminate the surgical field. Many operating tables have many degrees of positional and angular freedom to allow optimal placement of the surgical field relative to the surgeon, and are designed to be extremely stable and sturdy while supporting a heavy human body. As a result of these requirements, the tables contain numerous mechanisms allowing fore, aft, up, down, sideways, roll, and tilt motions. These mechanisms are physically robust and typically fabricated from steel, so that they have substantial field distortion characteristics. Shapes may include screws, rack and pinion gears, or scissors type actuators. The table surface may be one piece, or may be divided into several sections, with each section capable of motion relative to the other sections, to allow a body to be flexed such that various stresses and relative anatomical positions are optimal for a particular surgical or diagnostic procedure. The installed bases of operating tables are extremely diverse in design, and as the tables are often in service for many decades, there are many vendors, with each vendor carrying a number of different operating table designs. This poses a significant problem for magnetic position tracking systems which are used in a critical surgical environment. The operating volume for the tracker is typically within the body which lies on top of the table. This means that the tracking system is operating in close proximity to the metallic structures on, under, and around the table. The magnetic fields are distorted by these structures, which may result in large errors in the reported magnetic sensor position. The large diversity in table designs makes it impossible to predict the severity of distortion experienced on a given table. This is an unacceptable condition for a surgical environment. Attempts to compensate for these degrading effects have been made with varying degrees of effectiveness.
One method already employed is to map the entire operating volume each time the system is used. This is very time consuming and expensive, as potentially thousands of points must be taken in a precise manner if the distortion is severe and the operating volume large. It is also unreliable since during a surgical or diagnostic procedure, the table geometry is often changed which changes the relation of the table metallic structures relative to the tracking system, thereby requiring a new map if errors cannot be tolerated. Instruments and diagnostic equipment are also introduced and removed from the vicinity of the tracking system, thus rendering a map ineffective. Also, for severe distortion, a map may become totally ineffective, as the system may, at two different physical sensor spatial points, determine the sensor to be at the same position. In this case, the output data is of minimal use.
Another known method commonly described in prior art is to use AC fields over a conductive ground plane. The ground plane attenuates the magnetic field below the plane to nearly zero, which has the benefit of making the system insensitive to metallic objects below the plane. In the case of a dipole transmitter, the “method of images” is used to compute the theoretical magnetic field vectors over the plane, which are then used to provide sensor position. This method has drawbacks. One is that near the ground plane, the magnetic field intensity is nearly zero, and the vector crossing angles are degraded, which seriously reduces system performance with respect to accuracy and noise. The net result is that the sensor must be kept a few inches above the plane. Also, the dipole must be located some distance from the ground plane in order to reduce signal losses and degraded vector crossing angles within the operating volume. For a 1 cubic foot volume, the bottom of the transmitter must be about 2 inches above the plane for acceptable performance. To compute the height at which a patient must be elevated if lying on the transmitter, the thickness of the transmitter must be added to this 2 inch figure. Transmitter size is determined by required signal level within the operating volume. Sensor coil size for minimally invasive surgical applications is about 1 mm×5 mm in cross-section, which is very small. The requirement for precise, low noise operation at the extreme edges of the volume requires that a relatively large magnetic field magnitude be present in order to induce sufficient signal in the small coils. Transmitter size is largely dictated by how much field it must output. Since the transmitter is typically a cube, to obtain sufficient signal within a 1 cubic foot volume with a small receiver coil, the practical transmitter dimensions are on the order of 2 inches per side. We can now see that the effective transmitter assembly in this prior art teaching, including the ground plane, is 4 inches thick. In a surgical environment, the patient must be elevated to levels which a surgeon may find uncomfortable. In addition, extra padding may become necessary if the patient must lie flat on the table. Both the transmitter and the padding must be secured to the table. In short, the configuration is cumbersome and may not allow the patient to be positioned in an optimal manner.
Placing the transmitter above the operating volume is not desirable as it will potentially interfere with the surgical field. Also, as the transmitter is placed further from the ground plane, and if the dimensions of the ground plane are fixed to be a square of about 18 inches on a side, the ground plane becomes ineffective at reducing the effects of metallic objects near the operating volume. The metal housings of the surgical lighting equipment will have a greater distorting effect in the upper portions of the operating volume, as they are closer to both the transmitter and receiver. Equipment used during the procedure, including the operating table, will cause potentially life threatening distortion, which is an unacceptable condition.
Position determination depends on relative vector magnitudes from the x, y, and z coils. Distortion effects may again be removed by using a process such as mapping. As the magnitudes of the transmitted magnetic vectors from the x, y, and z coils become more similar, a given fixed amount of error in their determination will result in an increased error in position output. Again, considering the limiting case, if the magnitudes become equal then position determination is not possible. This combined effect of reduced angle of transmitted vector intersection and reduced difference in transmitted vector magnitudes is known to those skilled in the art as vector dilution. Use of a conductive
Ascension Technology Corporation
Snow Walter E.
Spiegel H. Jay
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