GB2460082A

GB2460082A - Navigation equipment and methods for determining the position and orientation of a wireless probe

Info

Publication number: GB2460082A
Application number: GB0808932A
Authority: GB
Inventors: Marcel Jan Marie Kruip
Original assignee: Siemens Magnet Technology Ltd; Siemens PLC
Current assignee: Siemens PLC
Priority date: 2008-05-16
Filing date: 2008-05-16
Publication date: 2009-11-18
Also published as: GB0808932D0; WO2009138766A1

Abstract

The equipment includes at least three coils in known mutual orientations located distant from a probe. The wireless probe comprises one or more magnetic sensors and a means for communicating measurements from the sensor(s) to a receiver. In use, time-varying currents of differing frequency are applied to each of the coils, resulting in a magnetic field consisting of a constant flux density and a gradient of flux density. By comparing the magnetic field measured by the probe with that predicted from a knowledge of the coils, the position and orientation of the probe can be calculated. This calculation may involve an iterative approach, and may also involve deriving a rotational matrix. The device can preferably be used in medical endoscopy, but could also find application in investigating hollow vessels in general, for example storage tanks, or in plumbing.

Description

NAVIGATION EQUIPMENT AND METHODS FOR DETERMINING THE

POSITION AND ORIENTATION OF A SELF-CONTAINED PROBE

Endoscopy by a capsule style probe is highly preferable to traditional endoscopy where an optical endoscope is inserted through the oesophagus. The latter is a highly unpleasant experience for the patient and awkward for both practitioner and patient.

Currently, a system is being developed where a probe of <35 mm length, <12 mm diameter, with hemispherical end caps, is swallowed by a patient. The probe contains a battery, camera(s) and an RF or other wireless transmitter and transmits images to the outside world. The probe would normally float on the surface of water in the patient's stomach.

Moving and manipulating the probe inside the patient's body may be accomplished as follows. The probe contains a small permanent magnet. A system of externally positioned coils provides a magnetic field gradient which will result in a force on the probe and a magnetic field, which will result in a force urging the magnet in the probe to align with

the magnetic field.

Whilst the probe can be manipulated and pointed and moved in any desired direction in such an arrangement, the position and orientation in space of the probe is not known. The present invention provides an improved method and apparatus for navigation of such a probe.

US patent 6,226,547 describes a catheter tracking system.

The present invention accordingly provides methods and apparatus as defined in the appended claims.

The above, and further, objects, advantages and characteristics of the present invention will become more apparent from consideration of the following description of certain embodiments thereof, in conjunction with the accompanying drawings wherein: Fig. 1 shows an embodiment of the present invention comprising three orthogonal coil pairs; arid Fig. 2 illutrates the variation of the magnitude of magnetic flux density BJ in the field of view for a single Y coil in an embodiment of the invention.

The present invention provides improved navigation equipment for a self-contained probe, which comprises at least three coils located outside the patient. The transmitting coils are preferably, although not necessarily, planar and are preferably, although not necessarily, arranged in a mutually orthogonal orientation. Instead of three single coils, three coil pairs may be used.

The coils of the navigation equipment are preferably also used for steering the orientation and trajectory of the probe, and may also be used as transmitting coils.

Figure 1 shows a preferred embodiment comprising three orthogonal pairs of planar coils. For the sake of clarity we will assume that one coil pair has its normal aligned parallel to the X-axis, and substantially lies in a plane parallel to the YZ plane. This will be referred to as the X coil pair. Similarly, one coil pair has its normal aligned parallel to the Y-axis, substantially lies in a plane parallel to the XZ plane, and will be referred to as the Y coil pair. Finally, one coil pair has its normal aligned parallel to the Z-axis, substantially lies in a plane parallel to the XY plane, and will be referred to as the Z coil pair.

Each coil is powered by a time varying current, with a frequency selected so as to avoid any substantial attenuation by the human body and/or water present near the probe; for example radio frequencies up to about 1MHz. Each coil pair will produce a magnetic flux density which consists of a constant magnetic flux density plus a gradient which is approximately linear across the region of interest, such that the field is nowhere zero across the region of interest. The field will be described by B(x,y,z,t) = sin(2it ft).(Bo+x.dB/dx +y.dBldy+ z.dB/dz + higher order terms).

In the case of the X coil we have dR/dy and dR/dz <<dB/dx Figure 2 shows a representation of the relative magnitude of magnetic flux density IRI in the field of view for a single Y coil. In the case of fields produced by pairs of coils, and equivalent relative magnitude of magnetic flux density IBI in the field of view may be obtained by the following method, assuming that the coils are equal in size and are symmetrically disposed. Let the first coil, have a current Ta and the other coil a current Tb. The constant part of the field is then proportional to (Ia+Ib) and the gradient is proportional to (Ta-Tb).

The frequency of the driving signal for each coil pair is different.

The respective frequencies are selected such that the signals can be separated during decoding without significant cross-over. For instance 21 kHz for the X coil, 22 kHz for the Y coil and 23 kHz for the Z coil. The signal strength is the same for the three coil pairs in the preferred embodiment.

The probe contains three orthogonal magnetic field sensors, each with a directional sensitivity, for example a cosine distribution, Examples of such sensors are Hall sensors, or planar coils. The signals of each of these sensors is suitably decoded and added to the data stream of the camera(s), which is then transmitted to a receiver outside the patient. The normals of the orthogonal magnetic field sensors of the probe coincide with a local coordinate frame which may be referred to as X'Y'Z'. The local magnetic field acting on the orthogonal magnetic field sensors is a vector which may be represented as B'.

The signals of each of the coils are decoded by the receiver. The method of derivation of the position of the probe will be explained by reference to an example calculation of the x position of the probe. The X coil pair generates a magnetic field and a magnetic field gradient which varies at 21kHz, in the described example. By taking the 21 kHz content of the signal from each of the orthogonal magnetic field sensors of the probe, and by calculating the square root of the sum of the squares of the corresponding signal magnitudes, an initial guess of the x position of the probe may be calculated. Initial guesses of the y and z positions of the probe may be established by similar methods, using 22kHz and 23kHz signals, respectively. The positional variation of the magnitude of the magnetic field IBI produced by the X coil pair as a function of x,y,z may be known from calculation and/or from mapping. Using this known variation, a refined approximation of the x position of the probe may be obtained by iteration. Similarly, improved approximation of the y and z positions may be obtained. Such iteration may not be necessary, however, if the linearity of the field gradient of the respective X, Y and Z coil pairs is adequate.

The preceding paragraph describes a method for deriving a position of the probe. The method and apparatus of the present invention also allows the orientation of the probe to be calculated.

Euler's angles (or Eulerian angles) are a set of three angles, commonly represented as i which are useful in describing the orientation or motion of a body about a known point, combining local reference axes X'Y'Z' with reference axes XYZ. The Euler angles of the probe, at the approximate x,y,z position derived as described above and referred to the reference axes X,Y,Z, may be calculated by comparing the strength of the magnetic field generated by each of the X, Y and Z coil pairs detected in each of the orthogonal magnetic field sensors of the probe. The local field B' is given by B'=R B where R is the rotation matrix.

While the present invention has been described with particular reference to embodiments having mutually orthogonal pairs of coils, other embodiments having single coils in orthogonal planes may also be used.

While the present invention has been particularly described with reference to the determination of the position and orientation of a probe within a human patient, many other applications of the present invention are possible. For example, corresponding veterinary applications may be envisaged in which the "patient" is non-human. Similarly, non-medical applications such as the remote investigation of plumbing, machinery, liquid storage tanks and so on may benefit from the apparatus of the present invention.

Claims

CLAIMS1. A wireless probe and apparatus for determining the position and/or orientation of the wireless probe, said apparatus comprising at least three coils located distant from the probe in known mutual orientations; each of the coils being arranged to receive a time varying current, of known frequency, the frequency of the time-varying current applied to each coil differing from the frequency of the time-varying current applied to each of the other coils, such that each coil produces, in use, a magnetic flux density which consists of a constant magnetic flux density plus a gradient of magnetic flux density, such that the magnetic flux density from each coil is nowhere zero across a region of interest; the wireless probe comprising one or more sensors, sensitive to magnetic fields oriented in known mutual orientations, and further comprising means for communicating measurements from the sensor(s) to a receiver.
2. A wireless probe and apparatus according to claim 1 wherein each coil is composed of two parallel coils.
3. A method for calculating the position of a wireless probe, comprising the steps of: -providing a wireless probe and apparatus for determining the position of the wireless probe according to claim 1 or claim 2; -operating the coils so as to generate a magnetic flux density which consists of a constant magnetic flux density plus a gradient of magnetic flux density, such that the magnetic flux density from each coil is nowhere zero across a region of interest; -communicating measurements from the sensor(s) to a receiver; decoding the measurements from the sensor(s) to obtain an initial guess of the position of the probe.
4. A method according to claim 3 further comprising the method of iteratively refining the initial guesses of the position of the probe according to a known positional variation of the magnitude of themagnetic field RI produced by each of the coils.
5. A method for calculating the orientation of a wireless probe, comprising the steps of: calculating the position of the wireless probe according to the method of claim 3 or claim 4; -calculating the magnetic flux density B at the calculated position with reference to a set of reference axes XYZ; -calculating the magnetic flux density B' experienced by the probe at the calculated position with respect to a set of local axes X'Y'Z'; and -deriving a rotational matrix R by comparison of the flux densities B'=R B using the relationship B'=R B.