Pulse Wave Velocity Measurement System

Abstract

Systems and methods for measuring the pulse wave velocity of a vessel are disclosed. An intravascular device includes first and second markers provided at different locations along the length of the intravascular device. When the intravascular device is moved from a first position to a second position, the first position and the second position corresponding to first and second times, the intravascular device provides a plurality of measurements along the length of the vessel. At the second time, the position of the first marker in the vessel corresponds to the position of the second marker at the first time. The pulse wave velocity value of the vessel is determined based on measurements associated according to the plurality of measurements for the first time and the second time and based on a distance between the positions of the two markers along the length of the intravascular device.

Description

Technical Field

[0001] The present invention relates to a medical system and a method for measuring the pulse wave velocity of a blood vessel using the medical system. Background Art

[0002] Pulse wave velocity (PWV) measurement is a well-established way to measure compliance of the great vessels. Aortic and cardiac PWV are used to assess the risk of cardiovascular events, whereas renal PWV can help stratify patients for renal denervation.

[0003] There are two main ways to measure PWV:

[0004] a) by measuring flow and pressure at (approximately) the same location and deriving PWV from the relationship between changes in pressure and changes in flow velocity (e.g. Khir et al., Determination of wave speed and wave separation in the arteries, Journal of Biomechanics 34, 2001, 1145–1155; and Davies et al., Use of simultaneous pressure and velocity measurements to estimate arterial wavespeed at a single site in humans, Am J Physiol Heart Circ Physiol 290: H878–H885, 2006);

[0005] b) by timing the rise of the wave or pressure pulsation over a known distance (Millasseau et al., Evaluation of Carotid-Femoral Pulse Wave Velocity: Influence of Timing Algorithm and Heart Rate, Hypertension, 2005; 45: 222-226; and Harbaoui et al., Development of Coronary Pulse Wave Velocity: New Pathophysiological Insight Into Coronary Artery Disease, Journal of the American Heart Association 2017; 6: e004981). Both methods have their advantages and disadvantages. Measuring PWV via a pressure-flow relationship generally requires an intravascular flow-pressure line. Timing the pressure or flow wave is technically undemanding, but is best suited for long vessels like the aorta, where the delay time is long and is easier to measure. However, for applications like patient stratification for renal denervation, PWV must be measured in vessels that are only a few centimeters long, such as the renal artery. Summary of the invention

[0006] It is an object of the present invention to provide a medical system for measuring PWV in a blood vessel with improved accuracy and reproducibility.

[0007] The system comprises an intravascular guidewire / catheter having a single pressure or flow sensor and a processing unit configured to derive a delay time between an electrocardiogram (ECG) signal or another pressure signal and the pressure / flow signal measured by the sensor, whereby the intravascular device comprises special markers near the sensor that allow an imaging or tracking modality (angiography, ultrasound, electromagnetic tracking) to determine the pullback distance with a high degree of accuracy.

[0008] In one embodiment, a system for measuring PWV in a vessel comprises an intravascular guidewire / catheter having a single pressure or flow sensor to derive a delay time between an ECG or other pressure signal and a pressure / flow signal measured by the sensor, wherein the intravascular device comprises at least two markers at and proximal to the sensor, wherein the at least two markers form a scale allowing an imaging or tracking modality (angiography, ultrasound, electromagnetic tracking) to determine the pullback distance with a high degree of accuracy independent of the alignment between the imaging plane and the sensor.

[0009] In an aspect of the present invention, a method of measuring the pulse wave velocity of a vessel is proposed, the method comprising:

[0010] receiving a plurality of measurements along the length of the vessel from an intravascular device, the intravascular device comprising two markers provided at different locations along the length of the intravascular device, the two markers being locatable by a tracking device;

[0011] receiving position information of the two markers from the tracking device;

[0012] associating a measurement of the plurality of measurements with the position of the marker for a first time, wherein the first marker is at a first position and the second marker is at a second position;

[0013] detecting when the first marker reaches a second position based on the position information after the intravascular device moves, and associating a second time;

[0014] The pulse wave velocity value of the vessel is determined based on measurement results from the plurality of measurement correlations for the first time and the second time and based on a distance between the locations of the two markers along the length of the intravascular device.

[0015] Other aspects and advantages of the present invention will become more apparent from the following detailed description, which may be best understood by reference to and in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In the attached picture:

[0017] Figure 1 A typical pressure signal (pressure versus time within one heartbeat) is shown by way of example.

[0018] Figure 2 The general concept of the system according to the invention is illustrated schematically and exemplarily.

[0019] Figure 3 The measuring procedure according to the invention is shown schematically and exemplarily.

[0020] Figure 4 An alternative measuring procedure according to the invention is shown schematically and exemplarily.

[0021] Figure 5 The functional use of the system according to the invention is schematically shown. DETAILED DESCRIPTION

[0022] The pulse wave velocity can be determined by measuring the time delay Δt of the pressure or flow wave while traveling a distance Δx as;

[0023]

[0024] It is therefore possible to measure PWV using two (pressure or flow) sensors positioned at a known distance from each other. Figure 1 It is shown how such a dual measurement can be used to determine Δt. However, this requires an intravascular device with two pressure sensors. Such a device is not currently commercially available for human use. It will also likely cost twice as much as a standard pressure or flow line with a single sensor, and the need to include connectivity cables for both sensors will likely compromise the mechanical properties of the device.

[0025] It is also possible to measure the time delay Δt of the pressure or flow wave with a single sensor (pressure or flow), by pulling the sensor back along the vessel to measure sequentially at different positions. In this case, a reference signal is needed according to which each pulse is timed. Typically, an ECG signal or a signal from a central pressure sensor is used. Harbaoui et al. describe this method in detail. This method is generally advantageous because it only requires a single pressure or flow sensor.

[0026] However, using a single sensor can significantly reduce accuracy, especially for measurements in short arteries. This is because the need to move the sensor increases the inaccuracy in the distance Δx between the measurement locations, whereas with a dual sensor probe the distance between the sensors is mechanically fixed with very small errors. For measurements in long arteries, the relative error in distance can usually be ignored, but in arteries that are only a few centimeters long, the error quickly becomes unacceptably large. For example, if PWV is measured in the aorta over a distance of Δx=40 cm, an error of ±1 mm in the placement of each sensor only causes a relative error of 0.5%. But if measurements are made in short arteries like the renal arteries, where the distance between the measurement points is only Δx=2 mm, the same absolute error of ±1 mm in the placement of each sensor already causes a relative error of 10%.

[0027] This problem is exacerbated by the difficulty of measuring the position of the sensor in the vessel with an accuracy of ±1 mm or better. Harbaoui et al. attempted to overcome this problem by measuring the displacement of the outer portion of the device during pullback. Unfortunately, this is not accurate enough for measuring PWV in short arteries. The reason for this is that any intravascular device (guidewire, catheter, etc.) will follow the vessel wall and bunch up when being pushed deeper into the vessel. Subsequently, during pullback, the axis of the device will straighten. Also during pullback, parts of the device may get stuck or hooked at the vessel wall, resulting in uneven movement of the top of the device. Therefore, the displacement of the sensor integrated at the top portion of the device will generally be less than the displacement of the external shaft during pullback. This uncertainty is proportional to the total length of the device in the patient and is most problematic for measuring PWV in short arteries. For example, if the device has a length of 1 m inside the patient and if there is a 0.5% deviation between the displacement of the distal tip and the displacement of the external axis, then there will be a relative error of 25% when measuring Δx inside a 2 cm long renal artery, but the error will only be 2.5% when measuring Δx inside a 20 cm long artery.

[0028] Alternatively, one can attempt to use imaging to determine sensor displacement. Angiography is often used to navigate sensor wires or catheters because the physician can directly observe the movement of the device and sensor inside the blood vessel. Unfortunately, the movement seen on a two-dimensional (2D) angiogram is only a projection of the actual three-dimensional (3D) movement on a 2D plane. This can introduce significant inaccuracies if the C-arm is not perfectly aligned with the vessel and device, or if the vessel is not perfectly straight.

[0029] For the reasons given above, PWV measurements using a single sensor (and manual pullback) are currently unreliable in short arteries. In order to increase the accuracy of these measurements, it is necessary to increase the accuracy of the sensor displacement measurements. The present invention describes a way to solve this problem.

[0030] For general ideas, Figure 2A schematic and exemplary system 100 is shown in the drawings, wherein, for various advantageous embodiments of the system, the functionality of the elements may be implemented by various different devices or subsystems, as will be explained in the specification. The system 100 (which may be referred to as a pulse wave velocity measurement system) may be used to stratify patients for treatment purposes. For example, a PWV value in a renal artery may be used to determine whether a patient is suitable for renal artery denervation. Based on the PWV determination, the intravascular system 100 may be used to divide one or more patients into groups associated with different degrees of expected therapeutic benefit from renal denervation. Any suitable number of groups or categories is contemplated. For example, based on PWV, a group may include those patients with a low, medium and / or high likelihood of having a therapeutic benefit from renal denervation, respectively. Based on the stratification or classification, the system 100 may be able to make recommendations on the extent to which one or more patients are suitable candidates for renal denervation.

[0031] Vessel 80 can represent a fluid-filled structure, both natural and artificial. Vessel 80 can be within the body of a patient. The wall of vessel 80 defines a lumen 81 through which fluid flows within vessel 80. Vessel 80 can be a blood vessel, such as an artery or vein of the patient's vascular system, including cardiac vasculature, peripheral vasculature, cerebral vasculature, renal vasculature, and / or any other suitable lumen within the body. For example, intravascular device 10 can be used to examine any number of anatomical locations in the vasculature of an organ, including the liver, heart, kidneys, gallbladder, pancreas, lungs, intestines, brain, urinary tract. In addition to natural structures, intravascular device 10 can be used to examine artificial structures, such as but not limited to grafts, stents, shunts.

[0032] Vessel 80 may be located within a body part. When vessel 80 is a renal artery, the patient's body part may include the abdomen, lumbar region, and / or chest region. In some embodiments, vessel 80 may be located within any part of the patient's body, including the head, neck, chest, abdomen, arm, groin, leg, etc.

[0033] The intravascular device 10 may include a flexible elongated member 11, such as a catheter, guidewire, or guide catheter, or other long, thin, elongated flexible structure that can be inserted into a patient's vessel 80. The vessel 80 may be, for example, Figure 3 and 4. The intravascular device 10 of the present disclosure has a cylindrical profile of a circular cross-section defining the outer diameter of the intravascular device 10, and in other examples, all or a portion of the intravascular device may have other geometric cross-sectional profiles (e.g., oval, rectangular, square, elliptical, etc.). In some embodiments, the intravascular device 10 may or may not include a lumen extending along all or a portion of its length to receive and / or guide other instruments. If the intravascular device 10 includes a lumen, the lumen may be centered or offset relative to the cross-sectional profile of the intravascular device 10.

[0034] The intravascular device 10 or its various components can be made of various materials, including, by way of non-limiting example, plastics, polytetrafluoroethylene (PTFE), polyether block amide (PEBAX), thermoplastics, polyimides, silicones, elastomers, metals (such as stainless steel, titanium, shape memory alloys such as nitinol), and / or other biocompatible materials. In addition, the intravascular device can be manufactured in various lengths, diameters, sizes, and shapes, including catheters, guidewires, combinations of catheters and guidewires, etc. For example, the flexible elongated member 11 can be manufactured to have a length ranging from about 115cm-195cm. The flexible elongated member 11 can be manufactured to have a length of about 135cm. The flexible elongated member 11 can be manufactured to have an outer transverse dimension or diameter ranging from 0.35mm-2.67mm (1Fr-8Fr). The flexible elongated member 11 can be manufactured to have a transverse dimension of 2mm (6Fr) or less, thereby allowing the intravascular device 10 to be configured for insertion into the renal vasculature of a patient. These examples are provided for illustrative purposes only and are not intended to be limiting. In some examples, the intravascular device is sized and shaped so that it can be moved within the vasculature (or other internal lumen(s)) of a patient such that at least one of the pressure, volume flow, flow velocity, vessel diameter, and wall thickness of the vessel can be monitored from within the vessel or along the length of the vessel.

[0035] The intravascular device 10 includes a sensor 16 at a distal portion of the flexible, elongated member 11. Additionally or alternatively, further sensors may be included along the length of the flexible, elongated member. The intravascular device includes two markers 12 and 14 provided at different locations along the flexible, elongated member and whose positions within the vessel 80 can be located by a tracking device 40. The system also includes an external console or device 30 to receive a plurality of measurements provided by the intravascular device 10, process the measurement signals by a processor 31, and optionally or alternatively store the received measurement signals and / or the processed measurement signals in a memory unit 32. The measurements provided by the intravascular device 10 may be transmitted directly to the external console 30, or alternatively they may be transmitted via a patient interface module 20 to which the intravascular device is connected. The patient interface module may transmit the measurement information to the external console via a wired or wireless connection. Optionally, the system includes an electrocardiogram (ECG) unit 50 for measurement of internal electrogram or external electrocardiogram signals, which are transmitted to the apparatus 30 for use in conjunction with multiple measurement signals received from the intravascular device 10 when necessary for determining the pulse wave velocity using the ECG signal. The internal electrogram signal can be measured by electrodes placed on the intravascular device introduced in the vessel, while the external electrocardiogram can be measured by external wires attached to the patient's body.

[0036] In general, the sensor 16 can provide a type of measurement or multiple types of measurement results from the group of pressure measurement, flow measurement, vessel diameter measurement and vessel cross-section measurement. Multiple types of measurements using the same sensor can be performed simultaneously or interspersed. An example of such a sensor is a capacitive micromachined sensor capable of transmitting and receiving ultrasound, and according to its design, includes a chamber closed by a film that can change the capacitance value after being compressed and can therefore provide pressure measurement results. Alternatively, the pressure value can be determined based on the frequency characteristics at which ultrasound emission and / or reception occur, and the frequency characteristics are pressure sensitive. Once the capacitive micromachined sensor is operated to transmit and receive ultrasound, ultrasonic imaging of the vessel can be performed, and the flow rate can be measured by the ultrasonic Doppler principle. The vessel diameter and / or vessel cross-section can be determined based on the ultrasound image, and in addition, with the flow rate of the blood in the vessel, the volume flow rate can be calculated.

[0037] exist Figure 3In a specific embodiment schematically illustrated in , the system includes an intravascular guidewire with a single pressure sensor at or near the distal tip. A first radiopaque marker is placed near the pressure sensor. A second radiopaque marker is placed proximal to the first marker at a fixed distance Δx, wherein Δx is in the range of 0.5 cm to 4 cm for a measurement intravascular device configured for use in a renal artery. The system also includes a device 30 for reading the pressure pulse and deriving a pulse delay time ΔT using an ECG signal or alternatively a static pressure or flow sensor signal as a reference. The system also includes a radiology (X-ray) system or an angiographic imaging device suitable for utilizing the functions of the tracking device 40 mentioned above and can additionally provide morphological information (images) of the vascular anatomical structure. Alternatively, computed tomography (CT) or magnetic resonance imaging (MRI) can be used to collect three-dimensional morphological information (vascular anatomical structure) when tracking the marker. For MRI tracking, a coil integrated in the intravascular device can be used.

[0038] To measure pulse wave velocity, the physician places the intravascular device into the vessel being investigated so that the angiogram shows two radiopaque markers inside the vessel. Because the guidewire between the two markers is stiff enough, the distance between the two markers is known with a high degree of accuracy; it is Δx. The position of the second marker on the radiological or angiographic image is recorded or marked with a marker, either automatically by the system or manually by the physician. The physician then measures the pulse delay time ΔT1 at this position, typically by averaging over a number of heartbeats. After this first time measurement, the physician then gently pulls back the intravascular device until the new position of the first radiopaque marker on the image coincides with the marker showing the old position of the second radiopaque marker. This indicates that the sensor 16 has been displaced a distance Δx from the first measurement position. The physician will now measure the pulse delay time ΔT2 at this new position, averaging over the same number of heartbeats as before.

[0039] The processor will then calculate and optionally display the PWV as;

[0040]

[0041] Figure 4 The second embodiment of the system illustrated in is similar to the first embodiment, except that the guidewire / catheter tip includes a plurality of radiopaque markers proximal to the sensor at fixed known distances from each other. Because the guidewire between the markers is sufficiently rigid, these markers form a kind of ruler visible under radiological and / or angiographic imaging.

[0042] To measure pulse wave velocity, the physician places the intravascular device into the vessel being investigated so that the pressure sensor is positioned as distally as possible. The physician then measures the pulse delay time ΔT1 at this position, typically by averaging over multiple heartbeats. During the measurement, the position of the radiopaque markers on the radiological or angiographic image is automatically recorded, and the distance between the markers on the image is compared with the true distance of the markers. The ratio of the apparent distance to the true (or physical) distance provides the processing unit with a local correction factor for correcting the local angle between the intravascular device and its projection on the radiological or angiographic image caused by the curvature of the vessel or axis. After the first time measurement, the physician then gently pulls back the intravascular device toward the proximal portion of the vessel being investigated. The sensor must still be within the portion of the vessel that was previously covered by the radiopaque marker. The physician will now measure the pulse delay time ΔT2 at this new position, averaging over the same number of heartbeats as before. During the measurement, the position of the radiopaque marker on the angiogram is again automatically recorded and the new position of the radiopaque marker is compared with its old position. Using the correction factor derived for the first time measurement, the processing unit uses this information to calculate the corrected distance Δx between the positions of the sensors in the first and second time measurement cases.

[0043] The system will then calculate and display the PWV according to Equation 2.

[0044] A third embodiment of the system is any of the first and second embodiments of the system, wherein the radiology device is replaced with an ultrasound imaging system and the radiopaque markers are replaced with markers visible under ultrasound imaging.

[0045] In a fourth embodiment of the system (which is yet another alternative to the third embodiment), the marker is an active or passive ultrasonic transducer that transmits or receives ultrasonic waves to or from an ultrasound probe (which can be yet another ultrasound device configured to be introduced into the body of a subject) or an external ultrasound probe, respectively, wherein the position of the active or passive ultrasonic marker is tracked on an ultrasound image provided by yet another ultrasound device or the external ultrasound probe.

[0046] In a fifth embodiment of the system, which is a further alternative to the fourth embodiment of the system, the sensor is an ultrasonic flow sensor and is simultaneously used as an active ultrasonic position marker.

[0047] In a sixth embodiment of the system (which may be based on the first or second embodiment of the system), the radiological device is replaced with an electromagnetic tracking system and the radiopaque markers consist of electromagnetic coils that can be tracked using the electromagnetic tracking system.

[0048] In a seventh embodiment of the system (which may be based on any of the first to sixth embodiments of the system), the position of the markers is continuously tracked during measurement to correct for movement during measurement, such as respiratory movement or patient movement.

[0049] In an eighth embodiment of the system (which may be based on any of the first to seventh embodiments of the system), the system uses a vessel diameter measurement sensor (like an intravascular ultrasound (IVUS) sensor) instead of a pressure or flow sensor.

[0050] In a ninth embodiment of the system, which may be based on the eighth embodiment of the system, the IVUS sensor is simultaneously used as an active ultrasound position marker.

[0051] In any of the related embodiments, the system can be configured to determine a pulse wave velocity value for the vessel based on a plurality of measurements along the length of the vessel acquired as described above, wherein the plurality of measurements may alternatively be any of the following combinations: pressure and vessel diameter measurement, pressure and flow rate measurement, pressure and volume flow measurement, flow rate and vessel diameter measurement, pressure and vessel wall thickness measurement. For a description of the use of specific measurement combinations for the calculation of pulse wave velocity, reference is made to WO2017 / 198800, WO2017 / 198867A1, WO2017 / 198490A1, WO2017 / 198871A1.

[0052] For any of the embodiments where the morphology of the vessel is available in addition to the position of the markers within the vessel, the accuracy of the PWV calculation can be further improved by using the apparent distance Δx in Equation 1 or Equation 2 for PWV measurement to be the distance from the first marker to the second marker projected on the axis of the vessel. For small distances, the apparent distance between the markers (which is necessary for PWV measurement) will be different from the physical distance if the markers are not in the same radial position relative to the axis of the vessel. This is an additional benefit compared to dual sensor measurements.

[0053] Figure 5The functional use of any embodiment of the above system according to the present invention is schematically illustrated. The method 200 of measuring the pulse wave velocity of a vessel begins with a step 202 of receiving a plurality of measurements along the length of the vessel from an intravascular device. As already described for the system, the intravascular device comprises at least two markers provided at different positions along the length of the intravascular device, the positions of which can be located by a tracking device. In step 204, the system receives position information of the two markers from the tracking device, and in step 206, the system associates a measurement in the plurality of measurements with the positions of the markers for a first time, wherein the first marker is at a first position and the second marker is at a second position. In step 208, the system detects when the first marker reaches a second position according to the position information after the intravascular device has moved within the vessel, and associates a second time. In step 210, the pulse wave velocity value of the vessel is determined based on the measurements associated according to the plurality of measurements for the first time and the second time and based on the distance between the positions of the two markers along the length of the intravascular device.

[0054] In step 212, the pulse wave velocity value is output to a display. Alternatively or additionally, the pulse wave velocity value may be automatically compared to a threshold by the processor to determine whether the patient is suitable for treatment, and an indication of suitability output to the display. Clinical studies provide evidence that any value of the pulse wave velocity of the renal artery above a threshold of approximately 9 m / s will make the patient suitable for renal denervation treatment with sufficient benefit in terms of blood pressure reduction.

[0055] By repeating steps 202 to 212 for successive portions of the vessel, a PWV map along the vessel can be generated and displayed in an overlay on or near morphological information of the vessel, which can be a two- or three-dimensional image of the vessel acquired by angiography or other imaging modalities (e.g., ultrasound imaging). The suitability of a patient for renal denervation treatment with sufficient benefit in blood pressure reduction can be evaluated based on a more comprehensive set of information enhanced by the PWV map along the vessel.

[0056] Although the present invention is exemplarily described mainly with respect to the measurement of pulse wave velocity in the renal artery, it is applicable to the measurement of pulse wave velocity in any other vessel, including cardiovascular and cerebrovascular vessels.

[0057] Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

[0058] A single unit or device may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0059] In the claims, the word "comprising" does not exclude other elements or steps, and the word "a" or "an" does not exclude a plurality.

[0060] Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A system (100) for measuring the pulse wave velocity of a vessel (80), comprising: An intravascular device (10) configured to provide a plurality of measurements along the length of the vessel, wherein the intravascular device comprises two markers (12, 14) provided at different positions along the length of the intravascular device, the positions of the two markers being capable of being located by a tracking device (40); An apparatus (30) configured to: receiving the plurality of measurements from the intravascular device; receiving position information of the two markers from the tracking device; associating a measurement of the plurality of measurements with the position of the marker for a first time, wherein a first marker (12) is at a first position and a second marker (14) is at a second position; After the intravascular device moves, detecting when the first marker (12) reaches the second position based on the position information and associating a second time; The pulse wave velocity value of the vessel is determined based on measurements associated for the first time and the second time from the plurality of measurements and based on a distance between the locations of the two markers along the length of the intravascular device.

2. The system according to claim 1, wherein: The intravascular device includes a sensor (16) at a distal portion thereof adjacent one of the two markers, the sensor being configured to provide the plurality of measurements along the length of the vessel.

3. The system according to claim 1, wherein: The device is also configured to: determining an apparent distance between the first marker and the second marker at the first time based on the position information received from the tracking device; comparing the apparent distance to a physical distance between the locations of the markers on the intravascular device; determining a local correction factor based on the apparent distance and the physical distance; A corrected pulse wave velocity value is determined based on the pulse wave velocity value and the correction factor.

4. The system according to any one of claims 1 to 3, wherein: The intravascular device comprises a plurality of markers which are locatable by the tracking arrangement and which are provided at different positions along the length of the intravascular device, wherein the plurality of markers form a ruler.

5. The system according to any one of claims 1-3, further comprising a user interface configured to display the pulse wave velocity value.

6. The system of any one of claims 1-3, further comprising a tracking device configured to locate the position of the marker.

7. The system according to claim 6, wherein: The tracking device is a radiological device and the marker comprises a radiopaque material.

8. The system according to any one of claims 1 to 3, wherein: The plurality of measurements along the length of the vessel include at least one of the types of pressure measurements, flow measurements, vessel diameter measurements, and vessel cross-section measurements.

9. The system according to claim 6, wherein: The tracking device is an ultrasound probe, and wherein the marker is an active or passive ultrasound transducer that transmits or receives ultrasound waves from or by the ultrasound probe, respectively.

10. The system according to claim 9, wherein: The ultrasound probe is an extracorporeal ultrasound probe.

11. The system according to claim 9 or 10, wherein: At least one of the markers is used as a sensor to provide the plurality of measurements along the length of the vessel.

12. The system according to claim 11, wherein: The sensor is configured to provide the plurality of measurements including flow measurements.

13. The system according to claim 11, wherein: The sensor is configured to provide the plurality of measurements including vessel diameter or cross-section measurements.

14. The system according to claim 6, wherein: The tracking device is an electromagnetic tracking device, and wherein the marker comprises a permanent magnetic material or an electromagnetic coil.

15. A computer program product comprising a computer program for measuring a pulse wave velocity of a vessel, the computer program comprising computer readable instructions which, when executed by a processor, cause the processor to: receiving a plurality of measurements (202) along the length of the vessel from an intravascular device, the intravascular device comprising two markers (12, 14) provided at different locations along the length of the intravascular device, the positions of the two markers being locatable by a tracking device; receiving position information of the two markers from the tracking device (204); associating a measurement of the plurality of measurements with the position of the marker for a first time (206), wherein a first marker (12) is at a first position and a second marker (14) is at a second position; detecting when the first marker (12) reaches a second position based on the position information after the intravascular device moves and associating a second time (208); The pulse wave velocity value (210) of the vessel is determined based on measurements associated from the plurality of measurements for the first time and the second time and based on a distance between the locations of the two markers along the length of the intravascular device.

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