Pulse Wave Velocity Measurement System

Abstract

A system and method are disclosed for measuring pulse wave velocity in a vessel. The intravascular device includes a first marker and a second marker located at different locations along the length of the intravascular device, the first marker and the second marker being locatable by a tracking device. The intravascular device provides a plurality of measurements along the length of the vessel corresponding to a first time and a second time while the intravascular device is moved from the first position to the second position. At the second time, the position of the first marker within the vessel corresponds to the position of the second marker at the first time. A pulse wave velocity value of the vessel is determined based on associated measurements of the plurality of measurements for the first and second times and based on the distance between the locations 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 vascular Pulse Wave Velocity using the medical system. [Background technology]

[0002] Pulse wave velocity (PWV) measurement is a well-established method to measure the distensibility of major blood vessels. Aortic and cardiac PWV are used to assess risk of cardiovascular events, and renal PWV is useful to aid in patient stratification for renal denervation.

[0003] There are two main methods for measuring PWV: 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 wave speed at a single site in humans, Am J Physiol Heart Circ Physiol 290: H878-H885, 2006); b) by timing the onset of a known distance wave or pressure pulse (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). Each method has its advantages and disadvantages. Measurement of PWV by pressure-flow relationship requires intravascular flow-pressure wires. Timing the pressure or flow wave is less technically demanding but works best for long vessels such as the aorta where the delay times are long and relatively easy to measure. However, applications such as patient stratification for renal denervation require measuring the PWV of vessels only a few centimeters long, such as the renal artery. Summary of the Invention [Problem to be solved by the invention]

[0004] It is an object of the present invention to provide a medical system for measuring vascular PWV with improved accuracy and reproducibility. [Means for solving the problem]

[0005] The system comprises an intravascular guidewire / catheter having a single pressure or flow sensor and a processing unit configured to derive the delay time between an electrocardiogram (ECG) signal or an additional pressure signal and the pressure / flow signal measured by the sensor, where the intravascular device is provided with special markings near the sensor that enable imaging or tracking modalities (angiography, ultrasound, electromagnetic tracking) to determine the pullback distance with a high degree of accuracy.

[0006] In one embodiment, a system for measuring vascular PWV comprises an intravascular guidewire / catheter with a single pressure or flow sensor and a processing unit for deriving a delay time between an ECG or additional pressure signal and the pressure / flow signal measured by the sensor, the intravascular device comprising at least two markings on its proximal side and at the sensor, the at least two markings forming a ruler that allows the imaging or tracking modality (angiography, ultrasound, electromagnetic tracking) to determine the pullback distance with high accuracy regardless of the alignment between the imaging plane and the sensor.

[0007] In one aspect of the present invention, a method for measuring Pulse Wave Velocity in a vessel is provided, the method comprising: receiving a plurality of measurements along a length of the tube from an intravascular device, the intravascular device comprising two markers disposed at different locations along the length of the intravascular device, the two markers being locatable by a tracking device; receiving position information of two markers from a tracking device; a first marker at a first location and a second marker at a second location, and associating a measurement of the plurality of measurements with the location of the markers with respect to a first time; detecting from the position information when the first marker reaches a second position as the intravascular device moves and associating a second time; and determining a pulse wave velocity value of the vessel based on associated measurements of the plurality of measurements for the first and second times and based on a distance between the locations of the two markers along the length of the intravascular device.

[0008] Additional aspects and advantages of the present invention will become more apparent from the following detailed description, which can be best understood with reference to and in connection with the accompanying drawings. [Brief description of the drawings]

[0009] [Figure 1] FIG. 2 is an exemplary diagram illustrating a typical pressure signal (pressure versus time for one heart beat). [Diagram 2] FIG. 1 shows a schematic and exemplary diagram of the general concept of a system according to the invention; [Diagram 3] 1 shows a schematic and exemplary measurement procedure according to the present invention; [Figure 4] FIG. 4 shows a schematic and exemplary diagram of an alternative measurement procedure according to the invention; [Diagram 5] FIG. 2 shows a schematic diagram of the functional usage of the system according to the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] By measuring the time delay Δt of the pressure or flow wave as it travels a distance Δx, the Pulse Wave Velocity can be determined as follows:

number

[0011] Thus, it is possible to measure PWV using two (pressure or flow) sensors positioned at a known distance from each other. Figure 1 shows a scheme of using such a dual measurement 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 would also likely cost twice as much as a standard pressure or flow wire with a single sensor, and the need to provide connecting cables for the two sensors would likely compromise the mechanical properties of the device.

[0012] 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 tube and taking successive measurements at different locations. In this case, a reference signal is required from which to time each pulse. 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 usually advantageous as it only requires a single pressure or flow sensor.

[0013] However, the use of a single sensor can significantly reduce accuracy, especially when measuring in short arteries. This is because the need to move the sensor increases the inaccuracy of the distance Δx between the measurement locations, whereas in the case of a two-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 neglected, but in arteries that are only a few centimeters long, the error quickly becomes unacceptable. For example, when measuring the PWV of the aorta over a distance of Δx=40 cm, an error of ±1 mm in the placement of each sensor only results in a relative error of 0.5%. However, when measuring in short arteries such as 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 then results in a relative error of 10%.

[0014] This problem is exacerbated because it is difficult to measure the position of the sensor in the vessel with an accuracy of ±1 mm or better. Harbaoui et al. attempt to overcome this problem by measuring the displacement of the external parts of the device during pullback. Unfortunately, this is not accurate enough to measure PWV in short arteries. The reason for this is that any intravascular device (guidewire, catheter, etc.) will try to follow and pack up against the vessel wall as it is advanced deeper into the vessel. The shaft of the device then tries to straighten out during pullback. Also during pullback, parts of the device may catch or get stuck on the vessel wall, causing the tip of the device to move non-smoothly. As a result, the displacement of the sensor integrated into the tip of the device during pullback is typically less than the displacement of the external shaft. This uncertainty increases with the total length of the device inside the patient, which is most problematic for measuring PWV in short arteries. For example, if a device has a length of 1 m inside the patient and there is a 0.5% deviation between the distal tip displacement and the external shaft displacement, then measuring Δx in a 2 cm long renal artery will have a relative error of 25%, but measuring Δx in a 20 cm long artery will have an error of only 2.5%.

[0015] Alternatively, imaging may be used to attempt to determine sensor displacement. Angiography is typically used to guide the sensor wire or catheter because it allows the physician to directly observe the device and sensor movement within the blood vessel. Unfortunately, the movement seen on a two-dimensional (2D) angiogram is merely a projection onto a 2D plane of the actual three-dimensional (3D) movement. This can lead to significant inaccuracies if the C-arm is not perfectly aligned with the vessel and device, or if the vessel is not perfectly straight.

[0016] For the above reasons, PWV measurements using a single sensor (and manual pullback) are currently not reliable in short arteries. To improve the accuracy of these measurements, there is a need to improve the accuracy of the sensor displacement measurements. The present invention describes an approach to solving this problem.

[0017] For general concept, a schematic and exemplary system 100 is shown in FIG. 2, where the functionality of certain elements can be achieved by various different devices or subsystems for various advantageous embodiments of the system, which will be described in detail later in this specification. The system 100, also referred to as a pulse wave velocity measurement system, is used for patient stratification for treatment purposes. For example, the PWV value of the renal artery can be used to determine whether a patient is suitable for renal artery denervation. Based on the PWV determination, the intravascular system 100 can be used to classify one or more patients into groups, each associated with a different degree of expected therapeutic benefit of renal denervation. Any suitable number of groups or categories are contemplated. For example, the groups include groups for patients with low, medium, and / or high likelihood of therapeutic benefit of renal denervation according to the PWV. Based on the stratification or classification, the system 100 can recommend how suitable one or more patients are as candidates for renal denervation.

[0018] The tube 80 represents a fluid-filled structure, both natural and artificial. The tube 80 is within a patient's body. The walls of the tube 80 define a lumen 81 through which fluid flows within the tube 80. The tube 80 may be a blood vessel, such as an artery or vein, of the patient's vascular system, including the cardiovascular system, peripheral vascular system, cerebrovascular system, renal vascular system, and / or any other suitable lumen within the body. For example, the intravascular device 10 may be used to interrogate any number of anatomical locations within tubes in organs, including the liver, heart, kidneys, gallbladder, pancreas, lungs, intestines, brain, and ureters. In addition to natural structures, the intravascular device 10 may also be used to interrogate artificial structures, such as, but not limited to, grafts, stents, and shunts.

[0019] The tube 80 is positioned within a body part. If the tube 80 is a renal artery, this body part of the patient may include the abdomen, lumbar, and / or thorax. In some examples, the tube 80 is positioned within any part of the patient's body, including the head, neck, chest, abdomen, arm, groin, leg, etc.

[0020] The intravascular device 10 includes an elongated flexible member 11, such as a catheter, guidewire, or guide catheter, or other elongated flexible structure that can be inserted into a vessel 80 of a patient. The vessel 80 can be a renal artery, as shown in FIGS. 3 and 4. The intravascular device 10 of the present disclosure has a cylindrical profile with a circular cross-section that defines the outer diameter of the intravascular device 10, although 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.). The intravascular device 10 may or may not include a lumen extending along its entire length or a portion of its length for receiving and / or guiding 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.

[0021] The intravascular device 10, or various components thereof, may be manufactured from a variety of 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. Additionally, the intravascular devices are manufactured in a variety of lengths, diameters, dimensions, and shapes, including catheters, guidewires, combination catheters and guidewires, and the like. For example, the elongated flexible member 11 is manufactured to have a length ranging from about 115 cm to 195 cm. The elongated flexible member 11 is manufactured to have a length of about 135 cm. The elongated flexible member 11 is manufactured to have an outer transverse dimension or outer diameter ranging from about 0.35 mm to 2.67 mm (1 Fr to 8 Fr). The elongated flexible member 11 is fabricated to have a lateral dimension of 2 mm (6 Fr) or less, which allows the intravascular device 10 to be configured for insertion within a patient's renal vasculature. 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 to allow it to be moved within a patient's vasculature (or other internal lumen) such that at least one of pressure, volumetric flow rate, flow velocity, vessel diameter, and vessel wall thickness can be monitored from within and along the length of the vessel.

[0022] The intravascular device 10 includes a sensor 16 at a distal portion of the elongated flexible member 11. Alternatively or additionally, further sensors may be included along the length of the elongated flexible member. The intravascular device includes two markers 12 and 14 located at different locations along the elongated flexible member, which can be located in the vessel 80 by a tracking device 40. The system further includes an external console, or apparatus 30, for receiving a number of measurements provided by the intravascular device 10, for processing the measurement signals by a processor 31, and for optionally or alternatively storing the received and / or processed measurement signals in a memory unit 32. The measurements provided by the intravascular device 10 are either transmitted directly to the external console 30 or alternatively transmitted via a patient interface module 20 to which the intravascular device is connected. The patient interface module transmits the measurement information to the external console by either a wired or wireless connection. Optionally, the system comprises an electrocardiogram (ECG) unit 50 for measuring an internal electrogram signal or an external electrocardiogram signal, which are transmitted to the apparatus 30 for determining a Pulse Wave Velocity value when required using the ECG signal in combination with the measurement signals received from the intravascular device 10. The internal electrogram signal is measured by electrodes placed on an intravascular device introduced into the blood vessel, while the external electrocardiogram is measured by external leads attached to the patient's body.

[0023] Typically, the sensor 16 provides one or more types of measurements from the group of pressure measurements, flow measurements, pipe diameter measurements, and pipe cross-section measurements. The same sensor may provide multiple types of measurements simultaneously or distributed. An example of such a sensor is a micromachined capacitive sensor, which is capable of emitting and receiving ultrasonic waves and which, by design, comprises a cavity closed by a membrane that can change its capacitance value under pressure, thereby providing a pressure measurement. Alternatively, the pressure value can be determined from the frequency characteristics at which ultrasonic waves are emitted and / or received, which are sensitive to pressure. Operating the micromachined capacitive sensor to emit and receive ultrasonic waves allows ultrasonic imaging of the pipe and measurement of the flow velocity using ultrasonic Doppler principles. From the ultrasonic image, the pipe diameter and / or pipe cross-section can be determined and, if the flow velocity of blood in the vessel is additionally given, the volumetric flow rate can be calculated.

[0024] In a particular embodiment, as shown diagrammatically in FIG. 3, the system comprises an intravascular guidewire with a single pressure sensor at or near its distal tip. A first radiopaque marker is placed in close proximity to the pressure sensor. A second radiopaque marker is placed at a fixed distance Δx proximal to the first marker, where Δx is in the range of 0.5 cm to 4 cm for an intravascular device configured to perform measurements in the renal artery. The system further comprises a device 30 for reading the pressure pulse and for deriving the pulse delay time ΔT using an ECG signal or alternatively a fixed pressure or flow sensor signal as reference. The system further comprises a radiological (x-ray) system or device suitable for angiographic imaging, having the functionality of the tracking device 40 described above and which may additionally provide morphological information (images) of the vascular anatomy. Alternatively, computed tomography (CT) or magnetic resonance imaging (MRI) may be used to obtain three-dimensional morphological information (vascular anatomy) during tracking of the markers. For MRI, tracking coils integrated into the intravascular device may be used.

[0025] To measure the Pulse Wave Velocity, the physician places the intravascular device in the blood vessel under investigation so that the angiogram shows both radiopaque markers inside the vessel. The guidewire between the two markers is sufficiently rigid so that the distance between them is known with high precision, which is Δx. The position of the second marker on the radiographic or angiographic image is recorded or labeled with a marking, 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 several heartbeats. After this first 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 marking indicating the old position of the second radiopaque marker. This indicates a displacement of the sensor 16 a distance Δx from the first measurement location. The physician now measures the pulse delay time ΔT2 at this new position and averages it over the same number of heartbeats as before.

[0026] The processor then calculates the PWV and optionally displays it as:

number

[0027] A second embodiment of the system, shown in Figure 4, is similar to the first embodiment, except that the guidewire / catheter tip includes multiple radiopaque markers proximal to the sensor that are at fixed and known distances from each other, and the guidewire between the markers is sufficiently rigid that the markers form a kind of ruler that is visible in radiometric and / or angiographic imaging.

[0028] To measure the Pulse Wave Velocity, the physician places the intravascular device in the vessel under investigation 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 several heartbeats. During the measurement, the positions of the radiopaque markers on the radiographic or angiographic image are 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 gives a local correction factor for the processing unit, which corrects the local angle between the intravascular device and its projection on the radiographic or angiographic image caused by bending of the tube or shaft. After the first measurement, the physician then gently pulls the intravascular device towards the proximal part of the vessel under investigation. The sensor should still be in the part of the vessel previously covered by the radiopaque marker. The physician now measures the pulse delay time ΔT2 at this new position and averages it over the same number of heartbeats as before. During the measurement, the positions of the radiopaque markers on the angiogram are again automatically recorded and the new locations of the radiopaque markers are compared with their old positions. Using the correction factor derived for the first measurement, the processing unit uses this information to calculate a corrected distance Δx between the positions of the sensor in the first and second measurements.

[0029] The system then calculates and displays the PWV according to Equation 2.

[0030] A third embodiment of the system is either the first or second embodiment of the system, where the radiological device is replaced with an ultrasound imaging system and the radiopaque marker is replaced with a marker that is visible in ultrasound imaging.

[0031] In a fourth embodiment of the system, which is a further alternative to the third embodiment, the marker is an active ultrasound transducer that emits ultrasound to an ultrasound probe, or a passive ultrasound transducer that receives ultrasound from the ultrasound probe, which may be a further ultrasound device configured to be introduced into the subject's body, or an external ultrasound probe, and the location of the active or passive ultrasound marker is tracked on an ultrasound image provided by the further ultrasound device or the external ultrasound probe.

[0032] 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.

[0033] In a sixth embodiment of the system, which may be based on the first or second embodiment of the system, the radiation device is replaced with an electromagnetic tracking system and the radiopaque marker is replaced with an electromagnetic coil that can be tracked using the electromagnetic tracking system.

[0034] In a seventh system embodiment, which may be based on any of the first to sixth system embodiments, the positions of the markers are continuously tracked during measurement to correct for movements during measurement, such as respiratory movements or patient movement.

[0035] In an eighth embodiment of the system, which may be based on any of the first to seventh system embodiments, the system uses a diameter measurement sensor, such as an intravascular ultrasound (IVUS) sensor, instead of a pressure or flow sensor.

[0036] In a ninth embodiment, which is based on the eighth embodiment of the system, an IVUS sensor is used simultaneously with an active ultrasound location marker.

[0037] In any of the related embodiments, the system may be configured to determine a Pulse Wave Velocity value for the vessel based on a plurality of measurements along the length of the vessel obtained as described above, where the plurality of measurements may alternatively be any of the following combinations: pressure and vessel diameter measurements, pressure and flow rate measurements, pressure and volumetric flow rate measurements, flow rate and vessel diameter measurements, pressure and vessel wall thickness measurements. Reference is made to International Application Nos. 2017 / 198800A1, 2017 / 198867A1, 2017 / 198490A1 and 2017 / 198871A1 for a description of the use of particular combinations of measurements for the calculation of Pulse Wave Velocity.

[0038] For any embodiment where the morphology of the vessel is available in addition to the location of the markers within the vessel, the accuracy of the PWV calculation can be further improved by making the apparent distance Δx in Equation 1 or Equation 2 for the PWV measurement the distance from the first marker to the second marker projected onto the vessel axis. For short distances, the apparent distance between the markers (required for PWV measurement) may differ from the physical distance if the markers are not in the same radial position relative to the vessel axis. This is an added benefit compared to dual sensor measurements.

[0039] FIG. 5 shows a schematic of a functional use of any of the above-described embodiments of the system according to the present invention. A method 200 for measuring the Pulse Wave Velocity of a vessel begins with step 202 of receiving a plurality of measurements from an intravascular device along a length of the vessel. As previously described with respect to the system, the intravascular device comprises at least two markers located at different locations along the length of the intravascular device and identifiable 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 of the plurality of measurements with the location of the markers for a first time, the first marker being at a first location and the second marker being at a second location. In step 208, the system detects from the position information when the first marker reaches the second location as the intravascular device moves through the vessel, and then associates a second time. In step 210, a Pulse Wave Velocity value of the vessel is determined based on the associated measurements of the plurality of measurements for the first and second times, and based on the distance between the locations of the two markers along the length of the intravascular device.

[0040] At step 212, the Pulse Wave Velocity value is output to a display. Alternatively or additionally, the processor may automatically compare the Pulse Wave Velocity value to a threshold value to determine whether the patient is suitable for treatment, and an indication of suitability is output to the display. Clinical studies have demonstrated that any value of renal artery Pulse Wave Velocity above a threshold of approximately 9 m / s qualifies a patient for renal denervation treatment to achieve sufficient blood pressure lowering benefits.

[0041] By repeating steps 202 to 212 for successive portions of the vessel, a PWV map along the vessel can be generated and displayed overlaid or next to the morphological information of the vessel. This map is a two-dimensional or three-dimensional image of the vessel obtained by angiography or other imaging modality, such as ultrasound imaging. Patient eligibility for renal denervation therapy with sufficient blood pressure lowering effect can then be assessed based on the more comprehensive set of information provided by the PWV map along the vessel.

[0042] Although the present invention has been primarily described illustratively with respect to measuring Pulse Wave Velocity in renal arteries, the present invention is applicable to measuring Pulse Wave Velocity in any other blood vessel, including cardiovascular and cerebral blood vessels.

[0043] Those skilled in the art in practicing the claimed invention can understand and realize other variations to the disclosed embodiments, from a study of the drawings, the disclosure, and the appended claims.

[0044] 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.

[0045] In the claims, the word "comprising" does not exclude other elements or steps, and the singular does not exclude a plurality.

[0046] Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. 1. A system for measuring Pulse Wave Velocity in a vessel, comprising: The system includes an intravascular device and an apparatus for providing a plurality of measurements along a length of the vessel; the intravascular device comprises two markers at different locations along a length of the intravascular device, the two markers being rigidly spaced apart, and a single sensor at a distal portion thereof proximate one of the two markers for providing a plurality of said measurements along the length of the tube, the two markers being locatable by a tracking device; The apparatus comprises: receiving a plurality of said measurements from said intravascular device; receiving position information of the two markers from the tracking device; the two markers being a first marker and a second marker, the first marker being at a first location and the second marker being at a second location, and associating a measurement of the plurality of measurements with the location of the marker with respect to a first time; detecting from the position information when the first marker reaches the second position as the intravascular device moves, and associating a second time with the position information; determining the Pulse Wave Velocity value of the vessel based on associated measurements of the plurality of measurements for the first and second times and based on a distance between the locations of the two markers along a length of the intravascular device. system.

2. The apparatus further comprises: determining an apparent distance between the first marker and the second marker at the first time from the position information received from the tracking device; comparing the apparent distance to a physical distance between the locations of the two markers on the intravascular device; determining a local correction factor based on the apparent distance and the physical distance; The system of claim 1 , further comprising: determining a corrected Pulse Wave Velocity value based on the Pulse Wave Velocity value and the correction factor.

3. 3. The system of claim 1 or 2, wherein the intravascular device comprises a plurality of markers whose positions can be identified by the tracking device, the plurality of markers being provided at different locations along a length of the intravascular device to form a ruler.

4. The system of claim 1 , further comprising a user interface for displaying the Pulse Wave Velocity value.

5. The system of claim 1 , further comprising a tracking device for determining the location of the marker.

6. The system of claim 5 , wherein the tracking device is a radiological device and the marker comprises a radiopaque material.

7. 7. The system of claim 1, wherein the plurality of measurements along the length of the pipe include at least one of the following types of measurements: pressure measurements, flow measurements, pipe diameter measurements, and pipe cross-section measurements.

8. The system of claim 5 , wherein the tracking device is an ultrasound probe and the marker is an active or passive ultrasound transducer that emits ultrasound to or receives ultrasound from the ultrasound probe.

9. The system of claim 8 , wherein the ultrasound probe is an extracorporeal ultrasound probe.

10. 10. The system of claim 8 or 9, wherein at least one of the markers is used as a sensor to provide a plurality of said measurements along the length of the tube.

11. The system of claim 10 , wherein the sensor provides a plurality of the measurements including a flow measurement.

12. The system of claim 10 , wherein the sensor provides a plurality of the measurements including pipe diameter or cross-sectional measurements.

13. The system of claim 5 , wherein the tracking device is an electromagnetic tracking device and the marker comprises a permanent magnetic material or an electromagnetic coil.

14. a processor of a system for measuring a pulse wave velocity of a vessel, receiving a plurality of measurements along a length of the tube from an intravascular device, the intravascular device comprising two markers at different locations along the length of the intravascular device, the intravascular device being rigid between the two markers and comprising a single sensor at a distal portion thereof proximate one of the two markers for providing the plurality of measurements along the length of the tube, the two markers being locatable by a tracking device; receiving position information of the two markers from the tracking device; the two markers being a first marker and a second marker, the first marker being at a first location and the second marker being at a second location, and associating a measurement of the plurality of measurements with the location of the markers with respect to a first time; detecting from the position information a time when the first marker reaches the second position as the intravascular device moves and associating a second time with the first marker; determining the Pulse Wave Velocity value of the vessel based on associated measurements of the plurality of measurements for the first and second times and based on a distance between locations of the two markers along a length of the intravascular device; A program for executing.

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