Fluid flow analysis

1. A method of determining a measure of wave speed in a fluid flowing through a fluid conduit, the method comprising:

using ultrasound measurements to determine the conduit diameter or conduit cross-sectional area, as a function of time, at a longitudinal position of the conduit;

using ultrasound measurements to determine fluid velocity, as a function of time, in a volume element at said longitudinal position of the conduit, the ultrasound measurement to determine fluid velocity being effected by tracking objects within the fluid flow in successive frames sampling the volume element, and obtaining displacement vectors for the objects, in which the objects being tracked comprise red blood cells, white blood cells, or platelets;

determining a wave speed from a function of (i) the change in fluid velocity at the longitudinal position as a function of time and (ii) the change in the conduit diameter or conduit cross-sectional area, as a function of time.

2. The method of claim 1 in which the wave speed is determined from a ratio of the change in fluid velocity at the longitudinal position as a function of time and the change in a logarithmic function of the conduit diameter as a function of time.

3. The method of claim 2 in which the wave speed is determined from a ratio of a change in fluid velocity at the longitudinal position within a time interval and a corresponding change in a logarithmic function of the conduit diameter within the same time interval.

4. The method of claim 1 in which the wave speed is determined from the change in fluid velocity at the longitudinal position within a time interval and a corresponding change in the conduit diameter within the same time interval.

5. The method of claim 1 in which the fluid velocity is determined using imaging velocimetry.

6. The method of claim 1 in which the ultrasound measurements for determining fluid velocity comprise a series of B-mode images or B-mode RF data, and obtaining displacement vectors comprises correlating the locations of the tracked objects in successive frames of the B-mode images or B-mode RF data.

7. The method of claim 1 in which the ultrasound measurements for determining conduit diameter and fluid velocity are both derived from a common ultrasound transducer head.

8. The method of claim 7 in which the common transducer head is directed orthogonally to a fluid flow axis of the fluid flow conduit.

9. The method of claim 7 in which the ultrasound measurements for determining conduit diameter or conduit cross-sectional area, and fluid velocity are both derived from common ultrasound excitation and response signals.

10. The method of claim 1 in which the wave speed, c, is determined according to the equation c=0.5(dU/dInD), where dU is a change in the fluid velocity as a function of time and dInD is a change in the natural logarithm of D as a function of time.

11. The method of claim 1 in which the fluid conduit comprises a part of the human or animal circulatory system.

12. The method of claim 11 further including repeating the ultrasound measurements and determining a wave speed at multiple times during a cardiac cycle and correlating the measurement of wave speed with time within the cardiac cycle.

13. The method of claim 1 further including determining a measure of fluid conduit wall elasticity or distensibility at the volume element, from the wave speed.

14. The method of claim 1 in which the wave speed, c, is determined according to the equation c=AdU/dA or c=dU/dInA, where A is the cross-sectional area, dA is the change in cross-sectional area as a function of time, dU is the change in the fluid velocity as a function of time, and dInA is the change in the natural logarithm of A as a function of time.

15. A method of determining a measure of wave speed in a fluid flowing through a fluid conduit, the method comprising:

using ultrasound measurements to determine the conduit diameter, as a function of time, at a longitudinal position of the conduit;

using ultrasound measurements to determine fluid velocity, as a function of time, in a volume element at said longitudinal position of the conduit, the ultrasound measurement to determine fluid velocity being effected by tracking objects within the fluid flow in successive frames sampling the volume element, and obtaining displacement vectors for the objects, in which the objects being tracked comprise red blood cells, white blood cells, or platelets;

determining a change in flow rate through the conduit based on the fluid velocity measurements and the conduit diameter measurements;

determining a change in cross-sectional area of the conduit based on the conduit diameter measurements; and

determining a wave speed from a ratio of the change in flow rate at the longitudinal position as a function of time and the change in conduit cross-sectional area as a function of time.

16. The method of claim 15 in which the fluid velocity is determined using imaging velocimetry.

17. The method of claim 15 in which the ultrasound measurements for determining fluid velocity comprise a series of B-mode images or B-mode RF data, and obtaining displacement vectors comprises correlating the locations of the tracked objects in successive frames of the B-mode images or B-mode RF data.

18. The method of claim 15 in which the ultrasound measurements for determining conduit diameter and fluid velocity are both derived from a common ultrasound transducer head.

19. The method of claim 18 in which the common transducer head is directed orthogonally to the fluid flow axis of the fluid flow conduit.

20. The method of claim 19 in which the ultrasound measurements for determining conduit diameter and fluid velocity are both derived from common ultrasound excitation and response signals.

21. The method of claim 15 in which the wave speed, c, is determined according to the equation c=dQ/dA), where dQ is a change in the fluid flow rate and dA is a change in the cross-sectional area of the conduit.

22. The method of claim 15 in which the fluid conduit comprises a part of the human or animal circulatory system.

23. The method of claim 22 further including repeating the ultrasound measurements and determining a wave speed at multiple times during a cardiac cycle and correlating the measurement of wave speed with time within the cardiac cycle.

24. The method of claim 15 further including determining a measure of fluid conduit wall elasticity or distensibility at the volume element, from the wave speed.

25. An apparatus for determining a measure of wave speed in a fluid conduit, comprising:

an analysis module configured to perform the steps of claim 1 .

26. An apparatus for determining a measure of wave speed in a fluid conduit, comprising:

an analysis module configured to perform the steps of claim 15 .