Catheter tracking system

Abstract: A catheter tracking system for tracking and monitoring a position of a catheter head within the human or animal body comprises a self referencing catheter. The self referencing catheter comprises an inner core (20), wherein the said outer sheath (1) comprises a plurality of sensors (12) and a fixing means (11) for fixing the said outer sheath (1) at a predetermined position, and wherein the said sensors (12) receive signals generated by a transducer (23), so as to produce measurement signals in accordance with a time of propagation of the said signals for determining the position of the head (21) relative to the said outer sheath (1).

Description: The present invention relates to catheter tracking systems, which operate to determine a position of a catheter within the human or animal body. Furthermore, the invention relates to a method for tracking a catheter within the human or animal body. Electro-cardiography is a process for recording electrical signals created by a heart, using electrodes applied externally and, more particularly, electrodes positioned on tips of catheters inserted within the heart. In known endocardial catheter technology great use is made of in-theatre fluoroscopy to locate and guide the catheters to positions within the heart where measurements are required. The use of fluoroscopy has a disadvantage that inevitably theatre staff and the patient are exposed to X-ray radiation. In order to circumvent this disadvantage, an endocardial catheter tracking system has been recently proposed, in which an AC magnetic field generated by a set of three coils mounted below the patient within the structure of a theatre table is used to track a tip of a catheter provided with a magnetic sensor. The system is complicated by the fact that the patient may move on the table and thus destroy the accuracy in the measurement of the position of the catheter tip relative to the heart. In order to overcome this problem a further catheter is used which also carries a magnetic sensor. This catheter is referred to as a reference catheter. The catheter tracking system is arranged to sense the position of the reference catheter and thereafter to determine the position of the measurement catheter. To this end, the reference catheter must be fixed somewhere in or near the heart in such a way so as to maintain its position relative to the heart. For example, this might be inserted in the coronary sinus or the right ventricular apex. The coronary sinus is a small blood vessel that has its entrance in the auricle close to the inferior vena cava and the entrance to the right ventricle. The coronary sinus is of varying diameter but can be as much as 10 mm. The right ventricular apex is an area at the lower end of the right ventricle in the base of the heart. It is not a well-defined location and does not offer a very good fixing position for the reference catheter. As a result of a possible range of movement of the reference catheter, the known arrangement of reference and measurement catheters provides difficulties in positioning the measurement catheter accurately, in particular when in actual use in the heart. Furthermore, the use of an additional catheter increases a risk of infections and renders the use of this known tracking system cumbersome. US 4,407,294 discloses a pulse-echo ultrasound imaging systems, in which probes for withdrawing tissue samples are inserted into or through body tissue and maneuvered so that a selected portion of that probe is within the imaged area. In view of the drawbacks of the prior art, it is an object of the invention to provide a catheter tracking system for tracking a catheter, which provides an improvement in accuracy and detection of a position of a catheter. The object is solved with a tracking system having the features of Claims 1 or 2. According to a first aspect of the present invention there is provided a catheter tracking system comprising a catheter comprising an inner core having at least one transducer on a head arranged at one end of the core, and an outer sheath adapted to movably receive the inner core, wherein the said outer sheath comprises a plurality of sensors and a fixing means for fixing the outer sheath at a predetermined position, and wherein the sensors receive signals generated by the at least one transducer, so as to produce measurement signals in accordance with a time of propagation of the signals for determining the position of the head relative to the outer sheath. With an arrangement described above, a signal emitted from the transducer is received by at least two sensors provided at slightly different positions. From the time of propagation of the signals, the distances between the source of the signal and the respective sensors, at which the signal is received, can be calculated. Since the positional relationship between the sensors is provided in accordance with a predetermined arrangement of sensors on the outer sheath, a position of the tip of the catheter from which signals were emitted may be determined in accordance with signals received by the sensors. Advantageously, the sensors are arranged in the vicinity of the fixing means or even thereat, so that the determination of the relative position refers to a properly defined fixed point. The fixing means may comprise an extendible cage which is advantageously arranged at a front end of the outer sheath. The cage may be mechanically extendible or may be of an inflatable type. The sensors may be arranged in a form of an array extending along the outer sheath or may be arranged on protruding positions of the cage. With the self referencing catheter, the catheter tracking system according to the invention only requires the introduction of a single catheter into the human or animal body. Thus, the risk of infection inherent with the introduction of a catheter can be reduced as compared to the prior art. Additionally, the use of fluoroscopy (x-ray) can be substantially reduced so that the risks inherent to this technology can also be avoided. Since the reference point, or in other words the fixing point of the outer sheath, is known, the relative position of the catheter head can be transferred into a three dimensional reference to positions on or in the heart, thereby providing a means whereby the catheter head may be returned to a desired point on or within the heart. This transfer can be supported by image processing means and methods which are well known in the art. Furthermore, no modifications of the patient's support apparatus is required, which may be for example a theatre table, since the tracking system is substantially self-contained. According to a second aspect of the present invention there is provided a catheter tracking system comprising a catheter comprising an inner core having a plurality of sensors on a head arranged at one end of the core, and an outer sheath adapted to movably receive the inner core, wherein the said outer sheath comprises at least one transducer and a fixing means for fixing the outer sheath at a predetermined position, and wherein the sensors receive signals generated by the at least one transducer, so as to produce measurement signals in accordance with a time of propagation of the signals for determining the position of the head relative to the outer sheath.Alternatively, the outer sheath can be provided with a plurality of transducers, and the head can be provided with a sensor. In this case, signals can be used which are distinguishable by the at least one sensor of the head. The signals can, for example, either have different frequencies or different timings, so that the respective source of the signal can be recognised. It should be mentioned here that an increasing number of sensors provided at the head or at the outer sheath offers an increasing accuracy of the position since the number of defined points, to which reference can be made, increases. For example, an improvement in the accuracy can be achieved, if the head and the outer sheath are each provided with a plurality of the respective devices. In this case, if an elongated head is used, a plurality of transducers or sensors thereon allows the determination of the angle of inclination of the head relative to the outer sheath. The sensors may be acoustic sensors and the transducer an acoustic transducer and the propagation time may be the time of flight of acoustic signal. The acoustic signals are advantageously ultrasonic signals. The transducers emit ultrasound pulses in sequence and these are received by the sensor in the tip of the catheter. Comparison of the transmitted and received signals gives a time of flight of the signal. By applying alteration techniques in accordance with these measurements the position of the tip in three dimension can be determined. As hereinbefore mentioned, if the angle of the tip is to be measured, then the catheter may be provided with two receivers separated by a distance along the catheter. Alternatively, a number of receivers each with, say, a cardioid response could be used to enable the direction of propagation of the ultrasonic energy to be determined. Alternatively, the signals can be emitted from a transducer arranged on the outher sheath, and can be received by a plurality of sensors arranged on the head. Furthermore, the signals can be emitted by a plurality of transducers arranged on the outer sheath, and can be received by a simple sensor on the head. In this case however, the signals must be distinguishable by the sensor in order to correlate the received signal with the source from which it originates. This distinction of signals can either be achieved with signals having different frequencies or with signals having different timings. Again, the sensors may be acoustic sensors and the transducers may be an acoustic transducer and the propagation time of the said signals may be the time of flight of acoustic signals. For the evaluation of the obtained time of flight values, a correlation for different propagation velocities occurring in homogenous material can advantageously be used to improve the accuracy of the positional relationships obtained. Corrections are applied to reduce the errors resulting from tissue inhomogeneity. This can be carried out by analysing the propagation times and in effect varying the assumed propagation velocity until the greatest degree of consistency is obtained for all the propagation times. Such analysis is useful when the velocity of propagation in the medium is unknown. They can be used to correct for the effects of small-scale inhomogeneity, due to blood vessels, etc. Alternatively, where a distance travelled by the acoustic signals is small, a variation in time of flight due to tissue inhomogeneities may be ignored since an effect thereof introduces negligible errors. Further advantageous modifications for carrying out the invention are depicted in the attached dependent claims. The invention will hereinafter be described in detail by way of example only, with reference being made to the attached drawings in which; FIGURE 1 shows a schematical diagram of an arrangement of a self referencing catheter; FIGURE 2 shows a schematic diagram of a further arrangement of a self referencing catheter shown in Figure 1; FIGURE 3 is a schematic diagram of a self referencing catheter in use with a heart. Figure 1 shows a front end of a self referencing catheter according to a first embodiment of the invention. According to Figure 1, there is provided an outer sheath generally referred to as 1, and a core or inner catheter, generally referred to as 2. The outer sheath 1 has a pipe-shaped outer member or envelope 10, which has appropriate flexibility and stiffness to be inserted as a catheter into a blood vessel or the like. In the vicinity of the front end of the envelope 10, there is an extendible cage 11, which operates to increase the outer diameter of the outer sheath 1 at an end position thereof so as to be used as a fixing means for locking the end position of the outer sheath 1 at predetermined position. The cage 11, can be an inflatable cage or a mechanically operated cage. Such cages are well known in the art so that a detailed description of applicable working principles can be omitted here. It is to be noted that the cage 11, should be selected and arranged such that liquid can still pass the cage 11 in its extended state, in particular, when the outer sheath 1 is arranged and locked in a blood vessel. Further, the end portion of the outer sheath 1 is provided with acoustic sensors 12 which are arranged with a certain distance from each other. The sensors 12, are ultrasonic sensors adapted to receive an ultrasonic signal or pulse. The arrangement of the sensors 12, shown in Figure 1, is merely an example; they can be arranged on the cage 11, or at different positions on the circumference of the envelope 10. As seen in Figure 1, the envelope 10, surrounds an inner catheter or a core 20, which is movably received in the envelope 10. The core 20, is rotatable within the envelope 10, as well as longitudinally movable therein. Advantageously, there is a certain play provided between the outer and the inner element, so as to form a free space 13, in which a wiring, piping or the like which extends along the outer sheath 1, can be received. The core 20, is advantageously a hollow body which provides an inner channel 24, for also receiving wiring or piping. The core 20, has flexibility and stiffness properties which render it suitable to operate as a catheter. An end portion of the core 20, is provided with a head 21, with a measuring tip 22. The measuring tip 22, can be of any type for measuring electric currents, material properties or it can even operate as an optical device for visual inspection. The measuring tip 22, can be replaced by, or combined with a means for providing a further functional tool such as a means for applying electrical current, laser light or the like. The tip can also be provided with a means for taking samples or for applying drugs, glue or the like. According to Figure 1, the head 21 is provided with an acoustic transducer 23. In this case, an ultrasonic transducer 23, is used for emitting ultrasonic signals or pulses to be received by the ultrasonic sensors 12, on the outer sheath 10. The working principle of the device shown in Figure 1 can be briefly described as follows: The outer sheath 1, and the core 2, are introduced into a blood vessel or the like using well known techniques. When the portion of the outer sheath 1, provided with the cage 11, has reached a predetermined reference position, the cage 11 is extended so as to lock the outer sheath 1, at this position. The core 20, carrying the head 21, is then moved relative to the outer sheath 1 by moving within the envelope 10. Acoustic signals are emitted by the acoustic transducer 23, arranged on the head 21. The acoustic signals are received by the acoustic sensors 12 on the outer sheath 1. Since the sensors 12 are arranged at different positions, a difference in the time of flight of a signal emitted by a single source 23, is obtained. From this difference, a position of the transducer 23, relative to the sensors 12, can be determined. Since the sensors 12, are locked to a known predetermined reference position by means of the cage 11, the actual position of the head 21, and, thus of the measuring tip 22, can be determined. It should be noted here, that the number of sensors is not limited to two, with an increased number of sensors an even higher positional accuracy can be achieved. Figure 2 shows an advantageous modification of the embodiment described before in detail with reference to Figure 1. The same reference signs denote the same parts as in Figure 1, so that a repetition of this description can be omitted. According to Figure 2, the head 21, is provided with an additional transducer 25, for emitting acoustic signals. With this arrangement, the angle of inclination of the head 21, relative to the outer sheath 1, can be determined. For this purpose, the signals emitted by the transducers 23 and 25 must be distinguishable by the sensors 12. Distinction between the signals can either be achieved by different signal characteristics, for example, by frequency, or intensity, or by a simple offset in timing for emitting the signals. Figure 3 shows an example for an application of the catheter shown in Figure 1 in a heart. The heart H is connected to a variety of blood vessels B (e.g. vena cava or the auricle) into which the catheter is inserted in order to reach inner portions of the heart H. The reference signs used in Figure 3 denote the same parts as in Figure 1 so that a detailed description is omitted for the sake of brevity. As can be clearly seen in Figure 3, an envelope 10, of an outer sheath of a catheter is inserted into a blood vessel B, and is locked in a predetermined or suitable reference position by means of an extendible cage 11. The head 21 fixed to a core 20 is then moved into the heart H until the measuring tip 22 has reached the desired position(s). A transducer 23, and sensors 12, cooperate in the manner described with reference to Figure 1 so as to monitor or track the actual position of the head 21 and the measuring tip 22, respectively. For this application, a synchronisation of the positional detection (i.e. emitting, receiving and evaluating signals) with the rhythm of the heartbeat is advantageous. However, this modification depends on the individual cases of application of the catheter. It should be clear that the application to a heart is not limiting. Other organs or portions of a human or animal body can be the subject of application of the catheter tracking system. Also non-medical applications are possible, as long as a signal transfer between transducers and sensors is possible.

Claims: A catheter tracking system comprising a catheter comprising an inner core (2, 20) having at least one transducer (23) on a head (21) arranged at one end of the core (2, 20), and an outer sheath (1) adapted to movably receive the inner core (2, 20), wherein the said outer sheath (1) comprises a plurality of sensors (12) and a fixing means (11) for fixing the outer sheath (1) at a predetermined position, and wherein the sensors (12) receive signals generated by the at least one transducer (23), so as to produce measurement signals in accordance with a time of propagation of the signals for determining the position of the head (21) relative to the outer sheath (1). A catheter tracking system comprising a catheter comprising an inner core (2) having a plurality of sensors arranged on a head (21) arranged at one end of the core (2), and an outer sheath (1) adapted to movably receive the inner core (2, 20), wherein the said outer sheath (1) comprises at least one transducer and a fixing means (11) for fixing the outer sheath (1) at a predetermined position, and wherein the sensors receive signals generated by the at least one transducer, so as to produce measurement signals in accordance with a time of propagation of the signals for determining the position of the head (21) relative to the outer sheath (1). A catheter tracking system as claimed in Claims 1 or 2, wherein the head (22) further comprises a measurement sensor adapted for sensing electrical signals in a heart. A catheter tracking system as claimed in any preceding claim, wherein the head (22) further comprises means for treating adjacent tissue in-situ. A catheter tracking system as claimed in Claims 1 or 2, wherein the fixing means comprises an extendible cage (11). A catheter tracking system as claimed in Claim 5, wherein the cage (11) is provided with the transducers (23) or with the sensors (12). A catheter tracking system as claimed in any preceding claim, wherein the transducers (23) and/or the sensors (12) are arranged in an array. A catheter tracking system as claimed in any preceding claim, wherein the head (21) of the catheter (1, 2) is provided with an array of sensors or transducers (23, 12) for determining an inclination angle of the head (21). A catheter tracking system as claimed in any preceding claim, wherein the transducers are acoustic transducers, which operate to generate acoustic signals, and wherein the sensors are acoustic sensors, and wherein the time of propagation between the transducers and the sensors is the time of flight of the acoustic signals. A catheter tracking system as claimed in any preceding claim, wherein the acoustic signals are ultrasonic signals.

Inventors: LLOYD PETER GREGORY, LLOYD, PETER GREGORY
Applicants: ROKE MANOR RESEARCH, ROKE MANOR RESEARCH LIMITED
Application year: 1997
Main classification: A61B8
IPC codes: A61B8, A61B1, A61M25, A61B5, G01S5, A61B19, A61B17