WO2008129510A2

WO2008129510A2 - Localization system for interventional instruments

Info

Publication number: WO2008129510A2
Application number: PCT/IB2008/051539
Authority: WO
Inventors: Joachim Kahlert; Michael Perkuhn
Original assignee: Koninklijke Philips Electronics N.V.; Philips Intellectual Property & Standards Gmbh
Priority date: 2007-04-24
Filing date: 2008-04-22
Publication date: 2008-10-30
Also published as: WO2008129510A3

Abstract

The invention relates to a localization system that can be used for example in invasive interventions for determining the position of a catheter (2) such that this position can be mapped onto a previously acquired image (22) of the examined body region. The system comprises a transmitter (TX) that can be attached to the instrument to be located and that emits electromagnetic localization signals (S). Moreover, it comprises receivers (RX1, RX2, RX3) disposed at different known locations that pickup the localization signals outside the patient body. An evaluation unit (10) can then infer the spatial position of the transmitter (TX) from the measured arrival times of the localization signal at the different receivers (RX1, RX2, RX3).

Description

Localization system for interventional instruments

The invention relates to a localization system for interventional instruments and to medical systems comprising such a localization system.

Today many diagnostic and therapeutic procedures are done by minimal invasive interventions to reduce the costs and the risks for the patient. A typical example of such interventions is the navigation of a catheter through the veins into the heart. In general the tracking and the steering of the catheter can be monitored in real time by X-ray fluoroscopy. The exact position of the catheter tip cannot be determined by the 2D fluoroscopy images. However, there is a need to map and represent the position of the catheter in a 3D model of the heart as it is typically generated in advance by a Computed Tomography (CT) or

Magnetic Resonance Imaging (MRI) system. Another aspect is that fluoroscopy times of more than one hour are typical for cardiac interventions. This leads to a high X-ray exposure time for the patient, and furthermore due to safety reasons the radiation exposure for the clinicians cannot be neglected.

The US 7 152 608 B2 describes in this respect a localization system comprising external magnetic field generators and a sensor probe attached to a catheter which is inserted into the body of a patient. By sensing the magnitude of the externally generated magnetic fields, the sensor can infer its own spatial position and thus localize the catheter within the body.

Based on this background it was an object of the present invention to provide alternative means for the localization of interventional instruments, wherein it is desired that these means are cost effective in an everyday clinical use and/or robust with respect to their measurement results.

This object is achieved by a localization system according to claim 1, a receiver system according to claim 12, a transmitter according to claim 13, a catheter system according to claim 14, and a medical system according to claim 15. Preferred embodiments are disclosed in the dependent claims.

The localization system according to the present invention serves for the localization of interventional instruments, i.e. for the determination of the coordinates of such instruments (or at least one dedicated point on such instruments) with respect to a given spatial coordinate system. The interventional instruments may particularly comprise catheters, endoscopes, needles, or other surgical devices that need to be located and that are typically navigated without direct visual control by the surgeon. The localization system comprises the following components:

A transmitter that can be attached to the instrument to be localized and that is able to emit electromagnetic "localization signals", for example radio frequency (RF) signals. The transmitter comprises usually at least an antenna for emitting the localization signals from the position that shall be localized. - At least one receiver for receiving the aforementioned localization signals emitted by the transmitter. The receiver comprises usually at least an antenna for picking up the localization signals at a position that can by definition be considered as "the spatial position of the receiver".

An evaluation unit for determining the location of the transmitter relative to the receiver based on time-of- flight information about the localization signal. In this context, the "determination of the location" of the transmitter shall in a broad sense mean any restriction of the possible whereabouts of the transmitter. By determining the distance between the transmitter and a receiver, the whereabouts of the transmitter can for example be restricted to lie (anywhere) on a sphere around the receiver. Preferably, the transmitter is localized with no remaining degree of freedom or uncertainty, i.e. at a particular point in space.

The time-of- flight (i.e. the time a localization signal needs to travel from the transmitter to a receiver) depends on the propagation velocity, which is given by the characteristics of the material in which this propagation takes place, and the distance between transmitter and receiver. As the material constants are usually known or at least measurable in advance, the time-of- flight will therefore carry information about the distance between transmitter and receiver, which can be exploited to localize the transmitter relative to the receiver. The evaluation unit can for example be realized by dedicated electronic hardware and/or a digital data processing unit with appropriate software.

The proposed localization system has the advantage that it provides a cost-effective solution as the transmitter can be designed as a low-cost disposable device that can be used together with disposable catheters and that does not have to withstand straining sterilization procedures. Another advantage of the localization system results from the exploitation of time-of- flight information because this is less prone to disturbances by e.g. other fields or ferromagnetic or conductive materials in the pathway than measurements which are based on spatially varying field strengths.

In a preferred embodiment of the invention, the transmitter comprises a pulse generator for generating a sequence of binary pulses, preferably binary pulses of the same duration. Binary pulses have the advantage to allow a robust detection in spite of signal deformations that may take place during the propagation of the localization signals through a body. Moreover, counting the pulses may provide a simple way of determining times-of- flight or differences in times-of- flight.

In a further development of the aforementioned approach, the transmitter additionally comprises a frequency generator for generating an alternating electrical carrier signal, and a modulator for modulating said carrier signal with the sequence of binary pulses. Thus the localization signal can be generated in the form of the carrier signal modulated with the sequence of binary pulses, wherein the carrier signal can be chosen at a radio frequency that is optimally suited for a wireless transmission.

In another embodiment of the invention, the transmitter comprises a timer for controlling the emission of localization signals at regular intervals. Thus there will be pauses between two consecutive localization signals that can be used by the external receivers to detect the start, the end, or some other characteristic feature of the localization signals. Moreover, the transmitter can operate autonomously in this way, i.e. without a wired connection to the external receivers.

The transmitter may already be firmly attached to the instrument to be localized, for example by producing it integrally together with said instrument. In a preferred embodiment, the transmitter is however a component of its own that is adapted to fit around the tip of a catheter. Thus it can be provided as a separate product that is attached to catheters as necessary.

In a preferred approach to exploit the time-of- flight information, the localization system comprises at least two, most preferably at least three receivers that are disposed at different positions in space, wherein these positions are known to the evaluation unit (e.g. in coordinates with respect to a given spatial coordinate system). The time-of- flight information that can be measured by one receiver can be used to eliminate one degree of freedom with respect to the possible whereabouts of the transmitter. Using two receivers, the location of the transmitter can therefore be restricted to a one-dimensional line in space, which may sometimes be sufficient for a user. Using three receivers allows in principle to eliminate all three degrees of freedom and thus to localize the transmitter at a definite point in space. More than three receivers can favorably be used to increase the accuracy of the localization by providing additional data for error-correction procedures and for resolving possible ambiguities in the data of three receivers.

In a further development of the aforementioned approach, the receivers comprise timers for measuring the corresponding arrival times of a localization signal with respect to a common time base. The receivers shall apply in this context the same definition of the "arrival time" for a temporarily extended localization signal (defined e.g. as the time at which the sensed localization signal exceeds a given threshold for the first time). Using a common time base is necessary because it would otherwise be impossible to infer exact relative positions from the measurements.

In a further development of the aforementioned approach, the evaluation unit is adapted to determine the location of the transmitter relative to the receivers based on the differences in the measured arrival times. Using only the differences of arrival times measured by the receivers has the advantage that no information about the emission time of the localization signals is necessary. The transmitter inside the body can therefore operate completely autonomously, i.e. without complicated wired connections to the evaluation unit or the receivers.

In another embodiment of the invention, the localization system comprises a timer for determining the time-of- flight of the localization signal on its way from the transmitter to the at least one receiver. In this case the timer has to know the exact point in time at which the considered localization signal was emitted by the receiver. This can for example be achieved by a wired connection between the transmitter and the timer or by providing the transmitter with a precisely synchronized and stable internal clock.

The evaluation unit may optionally comprise a phase-detection unit for determining a phase- shift in the measurements of the localization signal by different receivers. As will be explained in more detail with reference to the Figures, the measurement of a phase-shift can be used for a refined determination of time-of- flight differences.

According to a further development of the invention, the evaluation unit may optionally comprise an "orientation-detection unit" for determining the orientation of the instrument to be localized. Like every solid object, the instrument will usually have five degrees of freedom with respect to its location in space, which can be expressed as "position" (of a dedicated point) and "orientation". In cases like the navigation of a catheter, information about the orientation of the instrument is of great value, too. A convenient method to determine the orientation of the instrument is to attach a second transmitter to it and to determine the spatial position of this second transmitter, which can be done by the same time- of- flight principle and with the same receivers as the localization of the first transmitter. The orientation can then be expressed as the vector pointing from the position of the first transmitter to the position of the second transmitter. It should be noted that other principles to determine the orientation of the instrument can be used, too.

As the components of the localization system can be produced and sold as separate products, the invention also comprises a "receiver system" with at least one receiver and with an evaluation unit of the kind described above. Moreover, the invention separately comprises a transmitter for a localization system of the kind described above.

The invention further relates to the catheter system comprising - a catheter for invasive examinations, and a localization system of the kind described above, wherein the transmitter of this localization system is attached to the catheter.

Finally, the invention relates to a medical system for invasive interventions in a body volume with an instrument, wherein said system comprises the following components: a localization system of the kind described above for determining the location of its transmitter with respect to a given spatial coordinate system; a data processing unit, e.g. realized by a microcomputer with appropriate software, with a registration module for mapping said spatial coordinate system onto image coordinates of a model of the body volume. The model of the body volume is typically a two- or three-dimensional data structure describing the anatomy of the body volume that was obtained in advance in e.g. a CT or MRI procedure.

The medical system allows to track the position of an interventional instrument in a model of the body volume without a need for a continuous fluoroscopy.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. These embodiments will be described by way of example with the help of the accompanying drawings in which:

Figure 1 schematically shows an examination system according to the present invention;

Figure 2 illustrates the determination of the difference in the times-of- flight of a binary localization signal measured by two receivers;

Figure 3 illustrates the refined determination of the difference in the times-of- flight via a measurement of the phase-shift of the carrier frequency of the localization signal measured by two receivers.

In the following, a localization system according to the present invention will be described with respect to a medical application, though the invention is not restricted to this area.

Many known localization systems for medical applications are based on measurements of the signal strengths, i.e. a receiver (integrated e.g. in the tip of a catheter) measures the signal strengths of electromagnetic fields sent by different (at least three) transmitters located outside the human body. Based on the known locations of the transmitters and the known attenuation of the magnetic field in the human body, the position of the catheter can then be determined mathematically. However, the amount of data processing is high and has to be done in the receiver. From an economical point of view it is a drawback to do the data processing in the receiver as catheters (with integrated receiver) are disposable devices and very expensive.

It is therefore proposed here to integrate in the catheter tip a low-cost transmitter that sends in regular time intervals an electromagnetic pulse pattern (called "beacon"), and to dispose receivers outside the body. By measuring the time-of- flight and the phase shift of the beacon sent by the transmitter at the different receivers, the position of the transmitter (resp. the catheter tip) can be determined mathematically. This allows to determine in real-time the position and orientation of a catheter in a reference coordinate system and to realize all movements of the catheter in respect to said coordinate system.

Figure 1 shows schematically a possible setup of an examination system that realizes the aforementioned ideas. A patient 1 lies on a table 3 while a catheter 2 is navigated through the vessel system in e.g. a cardiac examination procedure. A fluoroscopic X-ray apparatus 30 coupled to a data processing unit, e.g. a workstation 20, can be used to monitor the intervention when necessary. Moreover, a three-dimensional model of the body volume of interest is stored in the workstation 20. This 3D model has typically been generated preoperatively with CT or MRI images. Representations of the model can be depicted on the screen of a monitor 21 that is connected to the workstation 20. The model is associated with image coordinates X₁, yi, Z₁.

In order to determine the actual position of the catheter tip in the 3D model, a localization system is used that comprises following components:

A transmitter TX that is attached to the tip of the catheter 2 and that isotropically emits electromagnetic "localization signals" S. The transmitter TX is preferably an autonomous, e.g. battery-powered device. It can be realized in different forms, for example as a ring that can be pulled over catheters of various vendors. The transmitter can optionally be switched on manually when it is fixed to the catheter 2 or alternatively in a wireless way by a remote control (not shown).

Three (or more) receivers RXl, RX2, RX3 disposed at different positions in space. At least some of the receivers (RXl, RX2) can be integrated into the patient table 3. Receivers may also be attached to the body of the patient. The receivers comprise antennas for picking up the localization signal S, wherein the pulse pattern of these signals S is fixed and known to the receivers. The receivers are further equipped with a timer and are thus able to measure the arrival times of the localization signal S with respect to an internal time base, wherein this time base is the same for all receivers.

An evaluation unit 10 to which all receivers RXl, RX2, RX3 are coupled (by wire - as shown - or wirelessly). Optionally, also the transmitter TX may be coupled to the evaluation unit 10, though this is for reasons of simplicity a less preferred embodiment. The evaluation unit 10 knows the x,y,z-coordinates of all receivers with respect to a given reference coordinate system and provides the common time base for them. The receivers can send via a defined protocol the measured arrival times of the received beacons S to the evaluation unit 10.

The arrival time (the time-of- flight) of a beacon S depends on the distance between transmitter TX and receiver RXi and the transmission velocity of an electromagnetic signal in the human body. Knowing the transmission velocity V and the positions of the receivers, the evaluation unit is thus able to calculate the position of the transmitter TX based on the measured arrival times at the receivers. The resolution of the calculated position is typically in the range of a millimeter.

An advantage of the localization system is that it may fulfill economical constraints because the transmitter can be realized as a low-cost disposable device that can be pulled over any existing catheter while the receivers are external parts that can be reused.

The localization system can be calibrated in advance by placing the catheter 2 with the transmitter TX at predefined positions, e.g. on the operating table or the human body. During the calibration the real position of the transmitter will be matched with the position calculated by the localization system.

Figure 1 further shows a link between the evaluation unit 10 and the workstation 20 via which the determined spatial x,y,z-coordinates of the transmitter TX can be communicated to the workstation. With the help of a predetermined registration between the spatial x,y,z- coordinates and the image coordinates xi,yi,zi, the computer system can then map in real-time the determined actual position of the transmitter TX (i.e. of the catheter tip) onto the 3D model.

Figure 2 shows in more detail exemplary measurements of the localization signal S at two different receivers RXl, RX2, respectively, wherein the time axes refer to the common time base. The pulse pattern representing the localization signal S is a digital pattern consisting of a predefined binary sequence of LOW and HIGH states, wherein the pulses have a fixed duration T. This binary sequence is used to modulate a high frequency sinusoidal signal (not shown) that is transmitted via the wireless interface of the transmitter. Each receiver demodulates the received signal, wherein all receivers use the same time base and are synchronized.

The arrival time (time-of- flight) measurement may be achieved in a two-step approach. First, the receivers measure the LOW-HIGH transitions and store the detected transitions in respect to the given time base. Due to the given location of all receivers and the known transmission velocity V in a human body, the evaluation unit 10 can calculate the number of pulse periods T the signal needs to travel from the transmitter to the receivers. This allows a rough positioning of the catheter with a resolution that is limited to a spatial grid of a grid point distance of D, wherein D is determined by the multiplication of time period T and the transmission velocity V: D = V-T.

To achieve a higher resolution within a spatial grid cubicle, the phases of the received sinusoidal signals are compared. This allows the determination of sub-grid points within one cubicle.

Figure 2 illustrates this with respect to the determination of the time difference Δ in the arrival times t_al and t_a2 at the receivers RXl and RX2. In this Figure, only the binary shape of the localization signal S is shown, and the composition of this shape by a sinusoidal carrier signal of a carrier frequency is only indicated in one pulse. In a first approximation, the difference Δ comprises an integral number n of time periods T. The first approximation is done in the time domain by counting the number of periods T. The term (n-T) determines the delay between the received signal at RXl and the signal received at RX2. The calculation is done in the evaluation unit 10. A refinement of the first approximation can be achieved if the additional shift x-T (0 < x < 1) of the binary pulses with respect to a single period T is determined, too, yielding Δ = n-T+x-T. Figure 3 illustrates that for such a further refinement to increase the spatial accuracy the received signals S at the receivers RXl and RX2 can be analyzed in the frequency domain. In this case the phase of the carrier frequency received at RXl and RX2 is calculated in the evaluation unit 10. By knowing the transmission velocity V in the material, the period of the carrier frequency inside the material tp can be calculated. The phase angle CC then relates to a time-of- flight t_fi via the formula: t_fl = tp • α / 2π If the orientation of the catheter 2 is of interest, too, this can readily be determined by attaching a second transmitter (not shown) to the tip of the catheter a distance away from the first transmitter. By determining the position vectors of both transmitters, the orientation of the catheter is given as the difference of these vectors.

In summary, the invention allows to calculate the exact position of a catheter and to represent it in x,y,z-coordinates. The position of the catheter can be mapped and visualized in real-time into an already existing 3D dataset of the patient's body by use of the x,y,z-coordinates determined by the localization system. A navigation of the catheter or any interventional tool can thus be done in respect to the 3D model of the patient's body. The electrophysiologist can do such a navigation of the catheter manually in the 3D model representing the human heart, but the x,y,z-coordinates can also be used by a robotic system to steer the catheter automatically to predefined target points that are planned by the electrophysiologist (EP).

The fluoroscopy time will be significantly reduced as the X-ray fluoroscopy system is no longer needed for the navigation of the system rather than to validate the target position of the catheter. The overall procedure time will typically also be reduced.

As described, the invention can be used for any minimal invasive intervention. It can be part of the infrastructure of an EP-Lab. In combination with imaging systems it gives the clinicians opportunities in pre-interventional treatment planning and imaging supported navigating of interventional tools.

The transmitter can have a universal design in which it can be pulled over existing catheters and surgical tools. Typical applications of such a catheter are: positioning of the ablation tip electrode in Atrial Fibrillation ablation; positioning of a multielectrode catheter for sensing and ablation; positioning of endocardial and epicardial sensing electrodes; positioning of stimulation leads in cardiac pacemaker, defibrillators and CRT devices; - positioning of surgical tools; positioning of drug delivery system for location-dependent medication.

Finally it is pointed out that in the present application the term "comprising" does not exclude other elements or steps, that "a" or "an" does not exclude a plurality, and that a single processor or other unit may fulfill the functions of several means. The invention resides in each and every novel characteristic feature and each and every combination of characteristic features. Moreover, reference signs in the claims shall not be construed as limiting their scope.

Claims

CLAIMS:

1. A localization system for interventional instruments (2), comprising a transmitter (TX) that can be attached to the instrument for emitting electromagnetic localization signals (S); at least one receiver (RXl, RX2, RX3) for receiving said localization signals; - an evaluation unit (10) for determining the location of the transmitter (TX) relative to the receiver (RXl, RX2, RX3) based on time-of- flight information about the localization signal.

2. The localization system according to claim 1, characterized in that the transmitter (TX) comprises a pulse generator for generating a sequence of binary pulses.

3. The localization system according to claim 2, characterized in that the transmitter (TX) further comprises a frequency generator for generating an AC carrier signal and a modulator for modulating said carrier signal with the sequence of binary pulses.

4. The localization system according to claim 1, characterized in that the transmitter (TX) comprises a timer for controlling the emission of localization signals (S) at regular time intervals.

5. The localization system according to claim 1, characterized in that transmitter (TX) is adapted to fit around the tip of a catheter (2).

6. The localization system according to claim 1, characterized in that it comprises at least two, preferably at least three receivers (RXl, RX2, RX3) disposed at different locations that are known to the evaluation unit (10).

7. The localization system according to claim 1, characterized in that the receivers (RXl, RX2, RX3) comprise timers for measuring the corresponding arrival times (t_al, t_a2) of the localization signals (S) with respect to a common time base.

8. The localization system according to claim 7, characterized in that the evaluation unit (10) is adapted to determine the location of the transmitter (TX) relative to the receivers (RXl, RX2, RX3) based on the differences in the measured arrival times (t_al, t_a2).

9. The localization system according to claim 1, characterized in that it comprises a timer for determining the time-of- flight of the localization signal (S) on its way from the transmitter (TX) to the receiver (RXl, RX2, RX3).

10. The localization system according to claim 1, characterized in that the evaluation unit (10) comprises a phase-detection unit for determining a phase-shift in the measurements of the localization signal (S) by different receivers (RXl, RX2, RX3).

11. The localization system according to claim 1 , characterized in that the evaluation unit (10) comprises an orientation-detection unit for determining the orientation of the instrument (2), preferably via the localization of a second transmitter attached to the instrument.

12. A receiver system comprising at least one receiver (RXl, RX2, RX3) and an evaluation unit (10) for a localization system according to claim 1.

13. A transmitter (TX) for a localization system according to claim 1.

14. A catheter system comprising: - a catheter (2) for invasive interventions; a localization system according to claim 1 with its transmitter (TX) being attached to the catheter.

15. A medical system for invasive interventions in a body volume with an instrument (2), comprising: a localization system according to claim 1 for determining the location of its transmitter (TX) with respect to a given spatial coordinate system (x,y,z); a data processing unit (20) with a registration module for mapping the spatial coordinate system (x,y,z) onto image coordinates (xi, yi) of a model of the body volume.