Data processing: measuring – calibrating – or testing – Measurement system – Orientation or position
Reexamination Certificate
2000-07-20
2002-11-19
Hilten, John S. (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system
Orientation or position
C702S094000
Reexamination Certificate
active
06484118
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to object tracking systems, and specifically to non-contact, electromagnetic medical systems and methods for tracking the position and orientation of an object. The present invention is also directed to a novel calibration method for electromagnetic-based medical tracking systems that can account for the effects of interference from nonmoving metallic objects.
BACKGROUND OF THE INVENTION
Non-contact methods of determining the position of an object based on generating a magnetic field and measuring its strength at the object are well known in the art. For example, U.S. Pat. No. 5,391,199, and PCT patent application publication WO 96/05768, which are incorporated herein by reference, describe such systems for determining the coordinates of a medical probe or catheter inside the body. These systems typically include one or more coils within the probe, generally adjacent to the distal end thereof, connected by wires to signal processing circuitry coupled to the proximal end of the probe.
U.S. Pat. No. 4,710,708, which is incorporated herein by reference, describes a location determining system using a single axis solenoid with a ferromagnetic core as a radiating coil. There are a plurality of magnetic coil receivers. The position of the solenoid is determined assuming that it radiates as a dipole.
PCT patent application publication WO 94/04938, which is incorporated herein by reference, describes a position-finding system using a single sensing coil and an array of three, three-coil radiators. The radiator coils are wound on non-ferromagnetic forms. The position of the sensing coil is determined based on a dipole approximation to the magnetic fields of the coils where an estimate of the orientation of the sensor coil is first utilized in order to determine the position of the sensor coil in that order. Additionally, the radiator coils of each array are energized sequentially using a time multiplexing approach. Interestingly, although this reference discloses that frequency multiplexing can be utilized in order to significantly increase the operating speed of the position system, it clearly indicates that there are disadvantages to this type of approach due to its complexity. It is also important to note that although this reference teaches a single axis sensor position and orientation tracking system, it does not address any specific method for calibrating the system.
Accordingly, to date, there is no known system or method that provides for a electromagnetic position sensor single axis system and method that is capable of being simultaneously driven through frequency multiplexing utilizing a novel exact solution technique and a novel calibration method.
SUMMARY OF THE INVENTION
The present invention is a novel system and method used for determining the position and orientation of a medical device having a single sensor arranged along the longitudinal axis of the device. The system comprises a plurality of field radiators wherein each field radiator has a plurality of radiator elements. Each radiator element generates a magnetic field that is distinct from the others through its frequency which is sometimes referred to as “frequency multiplexing”. A signal processor is operatively connected to the field radiators and the sensor of the medical device for receiving a sensing signal from the sensor indicative of the magnetic field sensed at the sensor. The sensing signal defines a measured magnetic field at the sensor. The signal processor also has a desired range of accuracy for the system which is stored therein. The signal processor includes an initial position estimator for establishing an initial position estimate for the sensor (which is based on the dipole approximation); a magnetic field calculator for calculating the magnetic field at the initial position estimate; a steepest descent calculator for calculating a steepest descent of the calculated magnetic field to the measured magnetic field; and a new position estimate calculator for calculating a new position estimate of the sensor based on the steepest descent. The magnetic field estimator and the steepest descent calculator use the exact theoretical field expressions and pre-stored calibration data which are unique to the system. The signal processor determines the position of the sensor when the new position estimate of the sensor is within the desired range of accuracy for the system.
The system also includes pre-stored calibration information for use with the magnetic field calculator and the steepest descent calculator for calculation of the magnetic field and the steepest descent step respectively. This calibration data is uniquely derived for each system using a novel calibration system and method along with its unique algorithm. The system also has a predetermined and stored desired range of accuracy of <0.1 cm (the accuracy of the system). However, the incremental steps (iterations) for the algorithm are stopped as soon as the change from a previous step is less than 0.001 cm which is necessary in order to get better than 1 mm accuracy for the system.
One embodiment for the plurality of field radiators are arranged in a fixed arrangement and are contained in a fixed plane on a location pad. Other field radiator embodiments as described later do not necessarily have to lie in the same plane. In the first embodiment, the radiator elements of the field radiators are mutually orthogonal. In this embodiment, the system has three fixed radiators wherein each radiator has three generator elements or coils mutually orthogonal to each other.
Additionally, the signal processor determines both the position and orientation of the sensor such that the position of the sensor is derived in three different directions (X, Y, Z) and at least two orientations (pitch and yaw) which is generally known as 5 degrees of freedom (DOF). However, the restriction to 5 DOF is due to the coil sensor symmetry as shown. Thus, it is contemplated by the present invention to also provide for 6 DOF (X, Y, Z directions and three orientations roll, pitch and yaw) by changing the configuration of the sensor coil to an asymmetrical shape.
The system further comprises a display operatively connected to the signal processor for displaying the position and orientation of the sensor. Moreover, the display displays the position and the orientation of the sensor with respect to an anatomical feature of a patient. This is particularly useful for navigating a surgical instrument within a patient's anatomy for performing a surgical procedure. The system further utilizes a reference device, which can be an external removable patch, for establishing a frame of reference. One particular use of the system is to map the heart thereby creating a 3D model of the heart. The sensor can be used together with a physiological sensor, such as an electrode in order to map a physiological condition, for instance, a local activation time (LAT).
The present invention also includes a novel method of determining the position and orientation of a sensor relative to a plurality of field radiators of known location wherein each of the field radiators comprises a plurality of co-located radiator elements. Each radiator element produces a differentiable field from all other field generating elements through frequency multiplexing. The sensor produces sensing signals indicative of the magnetic field at the sensor and from which the field at said sensor may be calculated. The method comprises the steps of:
(a) establishing a desired range of accuracy;
(b) determining an initial estimate of sensor position and orientation;
(c) calculating the magnetic field at the estimated sensor position and orientation;
(d) calculating the steepest descent from the calculated magnetic field at the estimated sensor position and orientation to the measured field at the sensor;
(e) calculating a new estimate for said sensor position and orientation from the steepest descent;
(f) iterating steps (c)-(e) based on the newly calculated s
Biosense Inc.
Capezzuto Louis J.
Herman Frederick L.
Hilten John S.
Sun Xiuqin
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