WO2013001437A1

WO2013001437A1 - A tracking system for tracking interventional tools in ultrasound guided interventions and an ultrasound diagnostic system comprising such a tracking system

Info

Publication number: WO2013001437A1
Application number: PCT/IB2012/053195
Authority: WO
Inventors: Peter Dirksen; Wouter Anthon Soer; Antonia Cornelia Van Rens; Bout Marcelis; Ronald Dekker
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2011-06-29
Filing date: 2012-06-25
Publication date: 2013-01-03

Abstract

An interventional tooltracking system is disclosed which utilises the response ofa CMUT-based tracking sensor located on the interventional tool to insonification, from a remoteultrasoundsource, in order to determine the position of the interventional tool during an interventional procedure.

Description

A TRACKING SYSTEM FOR TRACKING INTERVENTIONAL TOOLS IN ULTRASOUND GUIDED INTERVENTIONS AND AN ULTRASOUND DIAGNOSTIC SYSTEM COMPRISING SUCH A TRACKING

SYSTEM

FIELD OF THE INVENTION

The present invention lies in the field of tracking systems for tracking interventional tools in ultrasound guided interventions. Such tracking systems are used to locate and direct probes introduced into an anatomy, the probes thereby being rendered invisible to an operator without use of further imaging techniques. The invention further relates to an ultrasound diagnostic system comprising such a tracking system and to a method of tracking an interventional tool.

BACKGROUND OF THE INVENTION

Interventional techniques are well known in the art. These procedures utilize devices which are small enough to be introduced into the anatomy of a patient without the need for major surgery. Examples include catheters, guidewires, biopsy needles, etc. The devices can be used to obtain quantitative physical data, samples, or may deliver substances to a specific anatomical region.

Image-guided surgical interventions allow less invasive, less traumatic procedures. Intra-operative ultrasound enables real-time imaging of surgical tools. However, visualisation of tools is often difficult or incomplete.

Procedures are often guided to avoid unnecessary disruption or damage to anatomical regions and to make the interventional procedure as accurate and effective as possible. The determination of location and orientation of an interventional tool is an important part of the interventional procedure. This is often facilitated by tracking such as X- ray imaging or optical fibre or EM (electromagnetic) guidance systems, which facilitates tracking display to an operator, the displayed image containing a view of the interventional tool in the anatomical region during the intervention or overlaid position information. Both X-ray imaging and EM tracking have drawbacks. During the interventional procedures both patient and operator are subjected to the possible risk of radiation exposure in the course of the procedure. The patient may also be exposed to injection of contrast agent e.g. to enhance the Xray imaging capability. EM data is subject to distortions which severely affect the accuracy of the positioning information. Optical fibre guidance and EM both lack a volumetric reference for the position information.

An example of an interventional procedure is the stent placement procedure, in which measurements of blood pressure in an artery close to the heart is measured. This procedure may be effected using a pressure sensor comprising a CMUT (capacitive micromachined ultrasound transducer) detector for pressure sensing. CMUTs are also applied in the ultrasound field for imaging purposes. The transducers are capable of transmitting and receiving ultrasound over a range of frequencies (e.g. lMHz to 15MHz).

It is often desirable to obtain extra information during a procedure. Patent application US2004/0236223 Al describes the use of transducer arrays, utilising both CMUT and more conventional piezoelectric crystal devices as active ultrasound elements, as applied in transducer probes. The document describes hand held probes and probes for use as a catheter, the primary function of which is to provide imaging information. The document further describes the provision of an additional sensor, which is mounted on the probe housings adjacent to the imaging transducers. Such sensors may comprise sensors designed for temperature measurement, pressure measurement, detection of chemicals etc. The sensors work independently of the imaging transducers and provide additional information to the operator.

A problem with the current art is that the interventional procedures subject both patient and operator to added safety risks. Further, the systems needed to obtain the imaging information, necessary to safely perform the procedures, are complex and expensive.

SUMMARY OF THE INVENTION

It is an object of the present invention to solve the problems of the prior art by provision of :

A tracking system for tracking an interventional tool in ultrasound guided interventions, comprising

- A CMUT-based tracking sensor located on the interventional tool,

- An ultrasound source arranged remote from the CMUT-based tracking sensor and capable of insonifying a region comprising the interventional tool to produce an ultrasound image,

- A tracking device arranged to cooperate with the ultrasound source, further arranged to process electronic response data resulting from the CMUT-based tracking sensor due to reception of said insonification, to obtain a position of the interventional tool, - The tracking device being further arranged to report the position of the interventional tool, compatible with direct display on the ultrasound image of the insonified region.

Such a system has the advantage that no additional exposure to radiation such as Xray radiation is necessary in order to locate the position of an interventional tool during a procedure. The remote source of ultrasound is harmless to the body and is localized in the region of insonification.

The CMUT-based tracking sensor responds to the insonification by the remote ultrasound source in a characteristic manner. The electrical response of the device is transported by the associated wiring or circuitry from the sensor to the tracking device, where the signals are decoded. An algorithm processes the available information to determine a position of the device in terms of the image being produced by means of the ultrasound source. Thus the position of the interventional tool is directly related to the ultrasound image without the need to translate measured coordinates to an image reference frame. For example, an automatic overlay between ultrasound image and position pressure wire

(guidewire device) can provide position information of the guidewire even for a beating heart. Thus there is provided a (potentially continuous) real-time reporting of the relative position of the interventional tool and the, sometimes moving, anatomy of the insonified region.

A further advantage can be derived from the positioning of the sensor, which reacts to the presence of the ultrasound, on the interventional tool itself. This location gives an accurate positioning of the interventional device at the point of operation.

Examples of CMUTs which can be used in the present invention include, but are not limited to, CMUTs comprising an ONO (oxide-nitride-oxide) layer, temperature independent CMUTs which are capable of being compensated for temperature effects e.g. by membrane design, etc. The CMUTs may be operated in various modes e.g. conventional, collapsed or pre-collapsed.

The remote ultrasound source used in the invention is required to transmit ultrasound pulses to an area comprising the part of the interventional tool in which the CMUT-based tracking sensor is located. Usually an ultrasound transmitter is also an ultrasound receiver, the delays and changes in the initial ultrasound insonification give information back to the ultrasound device so that an image can be formed. In addition, the ultrasound source also provides a trigger pulse related to the insonification, which is used to characterise the ultrasound produced. In the present invention, such an image of a target region can be, and is, formed. A CMUT device is in principle capable of being employed as an imaging device by ultrasound - and the remote ultrasound source may indeed comprise CMUTs. The insonification of the CMUT-based tracking sensor is very important but it does not take part in image formation. All reactions of the CMUT-based tracking sensor to the ultrasound pulses are carried by the electronics to the tracking device. Thus the CMUT- based tracking sensor is not employed for imaging purposes. It is independent, electrically and physically, from the remote ultrasound source used for insonification. A further distinction is that, when in active use, the CMUT-based tacking sensor is internal to the patient anatomy whereas the ultrasound source is external to the anatomy.

In a further embodiment of the invention, the CMUT-based tracking sensor is located on a body of the interventional tool adjacent to a distal end of the interventional tool.

The closer the CMUT-based tracking sensor can be placed on or near the end of the interventional tool, the more accurate the determination of the end of the interventional tool. This may be important for guidance purposes while the tool is being navigated into a target region.

In a further embodiment of the invention, an electrical connection is provided between CMUT-based tracking sensor and a power supply.

The CMUT-based tracking sensor requires electrical power to operate. This can be arranged by providing at least two wires to conduct the power to the CMUT-based tracking sensor which are incorporated in the general wiring scheme to supply power to the interventional tool. It is possible therefore to add a CMUT-based tracking sensor to an existing design of an interventional tool and provide a means of power to the CMUT-based tracking sensor without a complete redesign of the interventional tool.

In a further embodiment of the invention, the CMUT-based tracking sensor is provided with a dedicated ASIC

CMUT devices can be manufactured in a manner compatible with semiconductor processing. This has been described in the art, particularly with emphasis on the processing temperatures and materials required. It is common to use a CMUT which is processed on top of an ASIC (application specific integrated circuit), either in a standard orientation or by means of flip-chip technology. This is referred to in the art as monolithic integration or hybrid integration, respectively. Such a CMUT is advantageous for use as part of the present invention due to its compact size, suitable for inclusion on a device so miniaturized as an interventional tool, and for the possibility of provision of reduced scale electronics which have advantages in the operation of the sensor. In a further embodiment of the invention, the CMUT-based tracking sensor is arranged on a substrate shared by a second interventional device.

Provision of the CMUT-based tracking sensor on a single substrate (or ASIC chip) such that a second device is present and shares the space, helps to reduce the overall footprint of these active devices. The manufacturing of the devices is simplified. The electrical connections are rendered common and thereby the complexity of wiring to the active device is reduced.

In a further embodiment of the invention, the second interventional device is a CMUT-based pressure sensor.

It is especially advantageous that a further CMUT-based device is present.

The processing for these devices is common to both, the circuitry and electronics can be more conveniently and efficiently designed.

In a further embodiment of the invention, the CMUT-based tracking sensor is a CMUT-based pressure sensor.

A CMUT comprises a membrane suspended over a cavity. On each side of the cavity electrodes are provided, one located in the membrane. As the membrane moves or vibrates in response to pressure, or as a reaction to insonification by ultrasound, the capacitance between the electrodes alters. This change can be detected electronically and related to a measurement of a particular parameter.

The characteristics of the CMUT behavior in response to different physical conditions, related to e.g. pressure sensing or insonification, vary. Thus the arrangement or design of the device for different uses can be tailored for optimum performance in the different situations. The device can be set up in a pressure sensing mode and switched to an ultrasound receiving mode (in this present context to a tracking mode using the response to insonification from a remote device as a means of positioning) when required. A particularly strong advantage of the present invention is that it is made possible to simultaneously operate the CMUT-based sensor for both pressure sensing and tracking.

The pressure sensor is equipped with additional functionality to receive and pre-amplify a signal from an external ultrasound transducer. The membrane parameters are chosen such that the sensor is capable of receiving ultrasound within the frequency band of the transducer of a remote ultrasound source. The signal is subsequently fed back into a tracking algorithm to determine the location of the pressure sensor.

Thus only one CMUT-based sensor is required on the interventional tool, which reduces complexity of tool design, manufacturing materials and processing. In a further embodiment of the invention, the CMUT -based tracking sensor is operated in conventional mode.

CMUTs can be operated in different operating modes e.g. conventional, collapsed and pre-collapsed. Collapse can be achieved by applying a bias or by bringing the transducer in a pre-collapsed state by manufacturing and/or design.

All CMUT modes can be applied to the present invention. However, a preferred embodiment utilizes the conventional mode in combination with a substantial bias close to the collapse voltage. This gives a high pressure sensitivity with a good receive ultrasound sensitivity. The CMUT-based tracking sensor is hereby optimized for receipt of ultrasound, there is no requirement for this sensor to send ultrasound.

In a further embodiment of the invention, the electrical connection between tracking device and CMUT-based tracking sensor comprises three or four wires to provide supply voltage, signal out and signal in functionality to the CMUT-based tracking sensor.

An interventional tool usually comprises an "in- vivo" part, designed to be placed inside a body (and on which the CMUT-based tracking sensor is mounted), and an "ex- vivo" part outside the body. Physical and electrical connection can be established between the two parts by means of e.g. guidewires.

The CMUT-based tracking sensor requires electrical power and input and output signals in order to function. Working as an ultrasound detection device, the CMUT- based tracking sensor has a minimum requirement of two wires to connect it to the outside world. In a preferred embodiment, the CMUT-based tracking sensor typically uses three wires . However, if the same sensor is required to act as a pressure sensor in addition to providing tracking functionality then a further, fourth, wire is preferably used in a particular embodiment for operation. The extra functionality with an extra wire facilitates the additional electronics required for simultaneous operation in the two functions.

In a further embodiment of the invention, the CMUT-based tracking sensor is provided with a bias voltage.

The CMUT-based tracking sensor according to one embodiment of the invention is operated in collapse mode and is provided with a bias voltage. The bias voltage is used to optimize performance. The number of electrical wires running to the interventional tool is minimized to enable integration in tools with a very small diameter, such as guidewires. Preferably the driving voltage of the ASIC and the bias voltage for the CMUT- based tracking sensor are supplied over the same wire (or line). As many ASIC technologies are not compatible with the high bias voltage a solution is to superimpose a high-frequency ripple on the bias, which can be converted to an acceptable DC voltage by a rectifying bridge. This enables a wire count of three. The AC-ripple with a rectifying bridge is an

implementation choice of a preferred embodiment. Using a DC-DC converter would be another possibility.

In a further embodiment of the invention, the interventional tool is provided with a diffractive structure above a substrate of the interventional tool.

In a particular embodiment of the invention a diffractive structure is located above the substrate and CMUT-based tracking sensor. The diffractive structure is designed to redistribute the incident ultrasound signal by diffractive reflection and transmission, such that it reaches the substrate (and thus also the CMUT-based tracking sensor) regardless of the insonification angle. The structure can be provided as part of the body of the interventional tool and is designed to fit within the same diameter as the rest of the tool.

In an alternative embodiment CMUT-based tracking sensors may be positioned on multiple sides of the interventional tool using a flexible substrate or multiple rigid substrates connected by wires or flexible connectors. For example, the sensor may be implemented double-sided with substrates back-to-back so that the membranes are facing opposite directions.

In a further embodiment of the invention, the tracking device comprises a tracking algorithm comprising means to process electrical response signals from a plurality of CMUT-based tracking sensors.

The position of the CMUT-based tracking sensor is derived from the electrical response via the CMUT membrane due to insonification. Use of two or more CMUT-based tracking sensors on the same interventional tool, each of which provides electrical response data which can be distinguished by the tracking device to obtain individual positions for each CMUT-based tracking sensor, permits orientation of the tool to be established.

In another aspect of the invention, an ultrasound diagnostic system comprises a tracking system according to any of the embodiment of the invention.

A particular strength of the many embodiments of the present invention is that the invention may be applied to many types of interventional tools and conventional ultrasound systems. In many examples, e.g. guidewire technology for pressure sensing using a CMUT-based pressure sensor, the ASIC and electrical leads needed to implement the functionality of the CMUT-based tracking sensor are already present. The present invention is also suitable to be incorporated into a therapeutic ultrasound system, the therapeutic system often operating at higher intensities of ultrasound and different frequencies (or frequency ranges) as compared to diagnostic systems.

In another aspect of the invention, there is provided,

a method of tracking an interventional tool in ultrasound guided interventions comprising the steps of :

- Providing a CMUT-based tracking sensor on the interventional tool

- Insonifying a region comprising the interventional tool using an ultrasound source located remote from the interventional tool

- Receiving electronic response data, resulting from reception of said insonification, from the CMUT-based sensor

- Processing the electronic response data using a tracking device arranged to cooperate with the ultrasound source to obtain a position of the CMUT-based tracking sensor and hence the interventional tool

- Reporting the position of the interventional tool

In a further embodiment of the invention, the method comprises the further step of:

- Providing one or more additional CMUT-based tracking sensors on the interventional tool

- Using an algorithm in the tracking device for obtaining the position and orientation of the intervention tool

In another aspect of the invention there is provided, a computer program comprising program code for causing a computer to process the electronic response data from a CMUT-based tracking sensor located on an interventional tool as utilised in the methods outlined above.

The arrangement for processing of the signal comprising electronic response data from the CMUT-based tracking sensor device, especially as adapted for use in the embodiment of the invention comprising the CMUT-based tracking sensor employed as combined pressure and tracking sensor, allows efficient determination of sensor location.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further elucidated with respect to the drawings. Fig 1 illustrates a CMUT transducer device in pressure sensing mode. Fig 2, parts a and b, illustrates a diffractive structure provided on an interventional tracking tool according to an embodiment of the present invention.

Fig 3 is a schematic diagram of an embodiment of the present invention comprising a single CMUT-based tracking sensor.

Fig 4 provides a schematic diagram of the concept associated with the invention.

Fig 5 illustrates an embodiment of the invention for combined pressure and tracking functionality, comprising four wires.

Fig 6 illustrates an embodiment of the invention for combined pressure and tracking functionality, comprising three wires.

Fig 7 illustrates an in vivo circuit for an embodiment of the invention comprising a CMUT-based tracking sensor comprising two CMUT transducer devices.

Fig 8 shows simulated output data for the embodiment of the invention shown in Fig 7.

Fig 9 illustrates another embodiment of the in vivo circuit 90 of the present invention where the system comprises a CMUT-based tracking sensor based on one CMUT transducer device.

Fig 10 illustrates the waveform associated with the output of the embodiment of Fig 9.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Fig 1 illustrates a simplified example of a known CMUT device. In this example the CMUT is acting as a pressure sensor, but it should be noted that a CMUT device is not limited to this application. The drawing has been simplified to explain the features most relevant to the understanding of the invention. If should be noted that the CMUT device may comprise other features or layers or layer stacks, as necessary for the processing and electrical operation of the device. The external connections to a possible associated ASIC or any other connections to the outside environment are not shown here.

The basic CMUT, shown schematically in Fig 1, is founded on a silicon substrate 10. This silicon substrate 10 is provided with a first electrode 11, commonly known as the bottom electrode, which may be directly in contact with the silicon substrate 10 or may be arranged close to the silicon substrate 10 but separated by some other base processing layers. A cavity 12 is provided - this cavity is normally kept at low pressure close to vacuum and provides a space between the silicon substrate 10 and a silicon nitride membrane 13. The silicon nitride membrane 13 may also be biased into a so-called "collapsed" mode, in which instance the membrane may be (partly) in contact with the silicon substrate 10 due to the application of an applied voltage. The silicon nitride membrane 13 has a second electrode 14 embedded in it. This second electrode 14 forms an electrode pair with the first electrode 11 and is commonly known as the "top" electrode. The capacitive effect of the CMUT comes from the provision of these two electrodes 11 and 14. A common length value of the electrodes 11 and 14 as indicated by arrow 15 is 25 to 250μιη diameter of a circular device. The electrodes 11 and 14 are generally manufactured to be of similar length. A typical height of the vacuum cavity 12 is around 0.5μιη, as indicated by arrow 16. A typical height of the silicon nitride membrane 13 is around Ιμιη, as indicated by arrow 17. In operation, the silicon nitride membrane 13 experiences a pressure, as indicated by arrow 18, which causes the silicon nitride membrane 13 to flex. The change in position of the silicon nitride membrane 13 causes a change in distance between the first and second electrodes 11 and 14, thereby changing a capacitance established between them. This change in capacitance is detected and converted into a pressure measurement change and indicates the pressure change.

Fig 2 shows section 20 of an interventional tracking tool from above (Fig 2a) and in cross section (Fig 2b). The section 20 comprises a view of a schematically illustrated CMUT-based tracking sensor 21 located on a guidewire 22. The purpose of the diffractive structure 23 is to redistribute an incident ultrasound signal or insonification by diffractive reflection and/or transmission. This effect ensures the incoming ultrasound reaches the CMUT-based tracking sensor 21 regardless of the insonification angle. As the CMUT-based tracking sensor responds to the incoming ultrasound signal to locate the position of the sensor itself and thereby also locate the position of the interventional tool, this feature enables the tracking system to consistently and continuously monitor the location of the interventional tool. In this example the CMUT device is acting as an ultrasound transducer.

Fig 3 shows a schematic diagram of an embodiment of the present invention comprising a single CMUT-based tracking sensor 30. The CMUT transducer 31 is operable in pressure sensing mode 32 and ultrasound mode 33. This embodiment is also referred to as the "three wire solution". The term comes from the wiring connecting the CMUT-based tracking sensor 30 wherein a first wire 34 provides the bias voltage and provides the base for a on-chip supply voltage to the device. Here low voltage means typically a few volts, preferably around 3V. The supply voltage is typically much lower than the bias voltage, which is usually of the order of around 50V, and often in a range of 10 to 150V (depending on application and device design including integrated circuit technology). A superimposed ripple may also be provided depending on specific implementation of the invention, but use of a DC-DC converter on chip is also possible. A second wire 35 facilitates the transport of an output signal and a third wire 36 connects the device to a ground. The bias voltage is required to allow the CMUT-based tracking sensor 30 to be receptive to ultrasound. The bias voltage is not required for pressure sensing but may be used to operate the sensor efficiently. The ripple supplied on top of the bias voltage is rectified by a recifying bridge, e.g. a diode 37 as shown in the figure, and serves as a power supply to an ASIC circuit (not shown) on which the CMUT-based tracking sensor is mounted (during manufacture of the device).

Fig 4 provides a schematic diagram of the concept associated with the invention. A tracking system according to the present invention is employed in a clinical setting 40 as part of an ultrasound guided interventional procedure 40A on a patient 41. The procedure utilizes an interventional tool 42 which is provided with a CMUT-based tracking sensor 43 according to the invention. The CMUT-based tracking sensor 43 is located at a tip of the interventional tool 42. The interventional tool 42 is associated with a tracking device 44 which interprets electrical response data from the CMUT-based tracking sensor 43 and, in cooperation with the ultrasound source 47, processes said data to obtain position information. Depending on the particular embodiment of the invention, other parameters, such as pressure information may also be obtained during the processing. The tracking device cooperates with a display 45 to show the position of the interventional tool 42 in relation to the ultrasound image e.g. the measured position of the interventional tool is superimposed on the image. This facilitates guidance of the interventional tool during the procedure.

Electrical response data is obtained from the CMUT-based tracking sensor in response to insonification by ultrasound 46 provided by an ultrasound source 47 remote from the interventional tool 42.

The ultrasound source 47 may be comprised in a diagnostic ultrasound machine, which could be further modified to accommodate the tracking device of the present invention.

The ultrasound source 47 emits an ultrasound pulse at a specific time and with a specific beam angle orientation. As a normal part of the interventional procedure, received reflections are used to build up an ultrasound image of the insonified region or volume, which may be directly displayed on the display 45. As part of the insonification the CMUT- based tracking sensor 43 receives part of the ultrasound beam. The membrane makes a displacement in response and produces an electrical signal due to the biasing arrangement of the membrane. This electrical signal is transferred to the tracking device 44. The tracking device 44 separates out the electrical signal from other noise or other data signals. The pulse information from the ultrasound source 47 is available, via cooperation, to the tracking device 44. This enables quantification of the electrical response signal of the CMUT-based tracking sensor 43, by means of timing and orientation information, in terms of its position. The position of the CMUT-based tracking sensor 43 can then be displayed in relation to the ultrasound image of the target region, thereby indicating the position of the interventional tool.

The use of CMUT-based devices in various applications, e.g. as pressure sensor or as ultrasound transducer, have been described in the art and can be utilised in the present invention in a conventional manner. Embodiments of the present invention, however, go even further in the possible use of these devices by combining two functionalities of pressure sensing and ultrasound detection in one device by appropriate circuitry and response data processing. Various embodiments of this combined functionality can be envisaged, comprising use of single or multiple CMUT-based tracking sensors and different wiring connections and circuits applied to different approaches to data processing.

The following figures illustrate a number of possible embodiments to enable the combined functionality option of the present invention using CMUT devices. Combined functionality reduces the overall device footprint, essential in an interventional tool. It should be noted, however, that the invention can be applied to all operating modes of the CMUT. If should also be noted that a CMUT-based tracking sensor can comprise one or more CMUT membrane elements on the same sensor element or substrate, the same elements being shared between the two applications. Such an arrangement is highly space -saving for the design of the device : the CMUT membrane is usually larger than an associated ASIC (application specific integrated circuit).

The basic functionality of the CMUT-based tracking sensor employed for combined determination of both pressure and location will now be discussed.

For pressure sensing use is made of the change in capacitance of the CMUT device due to pressure exerted on the membrane of the CMUT transducer. The CMUT is brought and kept in a position close to collapse. For this the electrical capacitance of the CMUT is measured by means of an oscillator circuit which converts it into a frequency. The oscillator circuit may make use of different waveforms of oscillation : triangular (preferable), sawtooth, sinusoidal are all examples applicable here but the choice is not limited to these examples. The generated frequency is detected and measured in a frequency detection circuit, remote from the point of application of the interventional tool comprising the pressure detecting CMUT, and compared with a reference clock frequency. The resulting frequency is a measure for the in- vivo pressure level. Note that internal to patient during interventional procedure is here referred to as "in vivo" and external to the patient as "ex- vivo".

In this collapse mode state the CMUT-based tracking sensor is also capable of detecting local ultrasound energy. A pulse, generated by a remote (external) source will cause a small deflection of the CMUT membrane which in turn results in a temporary change in capacitance. As the CMUTs are used in a high impedence circuit environment, this capacitive change will result in a voltage change. Such a voltage change is visible as small "spikes" (without DC contribution) on a overall voltage plot. This is due to the pressure sensor frequency signal having a much lower frequency than that of the ultrasound response. Thus it is advantageous to design the system so that the two relevant frequency ranges are sufficiently different.

Fig 5 illustrates an embodiment of the invention for combined pressure and tracking functionality, comprising four wires connecting the in- vivo part of the system with the ex- vivo part. A CMUT-based tracking sensor comprises a single CMUT 51 which is provided with a bias voltage52 (from a bias control 52A) and is subject to voltage driving by an oscillator 53. The readout from the CMUT is obtained via an output voltage 54. A positive supply voltage (VDD) 55 and ground or negative supply voltage (VSS) 56 are also supplied. The output voltage 54, equivalent to the electrical response data from the CMUT- based tracking sensor, is received in the ex vivo part of the system 50B and is processed. Two detections are facilitated, one for ultrasound detection 57, related to tracking capability, and the other for frequency detection 58, related to the pressure measurement capability.

Fig 6 also illustrates an embodiment of the invention for combined pressure and tracking functionality, this embodiment comprising only three wires due to the provision of a DC-DC converter 61 in the in vivo 60A part of the system. The ex- vivo 60B device remains unaltered. References are shown numbered as per Figure 5 for items which are consistent between Fig 5 and Fig 6.

Fig 7 illustrates an in vivo circuit 70, for an embodiment of the invention comprising a CMUT-based tracking sensor, the sensor comprising two CMUTs 71 and 72 embedded into an oscillator circuit. The two CMUTs 71, 72 are driven alternately by means of switches 73 and 74 which are related to threshold voltages across the individual CMUTs 71, 72. Implementation of this combined pressure sensor and ultrasound tracking circuitry produces an output signal which is of sawtooth shape for each CMUT and which is illustrated in Fig 8 by means of simulation data. The voltage response is shown against time for CMUT 71 81, CMUT 72 82. Graph 83 shows the combined response from both CMUTs 71 and 72 when combined to form an output voltage 75, which is transferred to the ex vivo part of the system (not shown) as the electrical response data output of the devices. Graph 84 illustrates the response data after signal processing. The sawtooth shape of the graphs is illustrative of the pressure sensing functionality of the system, and from the periodicity of the sawtooth a frequency can be derived to which a pressure value can be related (through use of known calibration). The small spike 85 is the ultrasound detection pike. This signal is the basis for the position and tracking functionality of the system. Such a spike 85 tends to occur randomly when compared with the periodic sawtooth pattern, and is dependent on the timing of insonification produced by the remote ultrasound source.

Fig 9 illustrates another embodiment of the in vivo circuit 90 of the present invention where the CMUT-based tracking sensor comprises one CMUT 91. The output signal has a triangular shape as shown in Fig 10. The triangular output voltage 101 has ultrasound detection spikes 102 superimposed. This response data is then signal processed as before in order to obtain the pressure and location information from the CMUT-based tracking sensor.

List of reference numerals

1 CMUT device

10 silicon substrate

11 First electrode

12 cavity

13 silicon nitride membrane

14 second electrode

1 arrow

16 arrow

17 arrow

18 arrow

20 section of an interventional tracking tool

21 CMUT-based tracking sensor

22 guidewire

23 diffractive structure

30 single MCUT-based tracking sensor

31 CMUT transducer

32 pressure sensing mode

33 ultrasound mode

34 first wire

35 second wire

36 third wire

37 diode

40 clinical setting in which a tracking system according to the present invention is used 40 A ultrasound guided interventional procedure

41 patient

42 interventional tool

43 CMUT-based tracking sensor

44 tracking device

45 display 46 insonification by ultrasound

47 ultrasound source

50A in vivo part of system

50B ex vivo part of system

51 CMUT

52 bias voltage

52A bias control

53 oscillator

54 output voltage

55 positive supply voltage

56 ground or negative supply voltage

57 ultrasound detection

58 frequency detection

60A in vivo part of system

60B ex vivo part of system

61 DC-DC converter 70 in vivo part of system

71 CMUT

72 CMUT

73 switch

74 switch

75 output voltage

81 voltage response CMUT 71

82 voltage response CMUT 72

83 Combined CMUT voltage response 84 response data after signal processing

85 ultrasound detection spike

90 in vivo circuit

91 CMUT triangular output voltage ultrasound detection spike

Claims

CLAIMS:

1. A tracking system for tracking an interventional tool (42) in ultrasound guided interventions (40 A), comprising

- A CMUT-based tracking sensor (43) located on the interventional tool (42),

- An ultrasound source (47) arranged remote from the CMUT-based tracking sensor (43) and capable of insonifying a region comprising the interventional tool (46) to produce an ultrasound image,

- A tracking device (44) arranged to cooperate with the ultrasound source (47), further arranged to process electronic response data resulting from the CMUT-based tracking sensor (43) due to reception of said insonification (46), to obtain a position of the

interventional tool (42),

- The tracking device (44) being further arranged to report the position of the interventional tool compatible with direct display (45) on the ultrasound image of the insonified region.

2. A tracking system as claimed in claim 1 wherein the CMUT-based tracking sensor is located on a body of the interventional tool adjacent to a distal end of the interventional tool.

3. A tracking system as claimed in claims 1 or 2 further comprising an electrical connection between CMUT-based tracking sensor and a power supply.

4. A tracking system as claimed in any of the above claims wherein the CMUT- based tracking sensor is provided with a dedicated ASIC.

5. A tracking system as claimed in any of the above claims wherein the CMUT- based tracking sensor is arranged on a substrate shared by a second interventional device.

6. A tracking system as claimed in claim 5 wherein the second interventional device is a CMUT-based pressure sensor.

7. A tracking system as claimed in any of the above claims wherein the CMUT- based tracking sensor is a CMUT-based pressure sensor.

8. A tracking system as claimed in any of the above claims where the CMUT - based tracking sensor is operated in conventional mode.

9. A tracking system as claimed in claim 8 wherein the electrical connection between tracking device and CMUT-based tracking sensor comprises three or four wires to provide supply voltage, signal out and signal in functionality to the CMUT-based tracking sensor.

10. A tracking system as claimed in claims 7, 8 or 9 wherein the CMUT-based tracking sensor is provided with a bias voltage.

11. A tracking system as claimed in any of the above claims wherein the interventional tool is provided with a diffractive structure above a substrate of the interventional tool.

12. A tracking system as claimed in any of the above claims wherein the tracking device comprises a tracking algorithm comprising a means to process electrical response signals from a plurality of CMUT-based tracking sensors.

13. An ultrasound diagnostic system comprising a tracking system according to any of the above claims.

14. A method of tracking an interventional tool in ultrasound guided interventions comprising the steps of :

- Providing a CMUT-based tracking sensor on the interventional tool - Insonifying a region comprising the interventional tool using an ultrasound source located remote from the interventional tool

- Reporting the position of the interventional tool.

15. A method of tracking an interventional tool as claimed in claim 14, further comprising the step of:

- Using an algorithm in the tracking device for obtaining the position and orientation of the intervention tool.

16. A computer program comprising program code for causing a computer to process the electronic response data from a CMUT-based tracking sensor located on an interventional tool as utilised in the method of claims 14 or 15.