WO2016012572A1

WO2016012572A1 - Flow estimation

Info

Publication number: WO2016012572A1
Application number: PCT/EP2015/066947
Authority: WO
Inventors: Lasse HENZE
Original assignee: Hze Aps
Priority date: 2014-07-24
Filing date: 2015-07-23
Publication date: 2016-01-28

Abstract

Disclosed is a method for estimating the distribution of a first movement characteristic for a part of a fluid positioned within a combined measurement region (317, 318). The method comprises the step of: generating a first velocity signal (330), the frequencies of the first velocity signal (330) being dependent on the distribution (331) of velocity components along a first axis (308) for the part of the fluid positioned within a first measurement region (318) centred at a first spatial point (319a); generating a second velocity signal (340), the frequencies of the second velocity signal (340) being dependent on the distribution (341) of velocity components along a second axis (309) for the part of the fluid positioned within a second measurement region (317) centred at a second spatial point (319b); and processing said first velocity signal (330) together with said second velocity signal (340) to estimate the distribution of the first movement characteristic.

Description

Title Flow estimation Field The present invention relates to a method for estimating the distribution of at least one movement characteristic for a fluid using transmissions of ultrasound, and to an ultrasound imaging system.

Background Ultrasound based estimation of movement characteristics of body fluids is an important clinical tool. Especially estimation of blood flow is important, as it may be used to diagnose a wide range of cardio vascular diseases.

In traditional ultrasound based flow estimation methods such as the pulsed spectral Doppler method, only the velocity component of the blood along the ultrasonic beam is estimated. An estimate of the true velocities may be found by correcting for the beam to flow angle. In commercial available systems, the operator typically manually attempts to estimate the flow angle based on the b-mode image obtained of the blood vessel. This may however be a difficult task. WO0197704 discloses a method where the flow angle is automatically estimated and used to correct for the beam to flow angle.

However, the estimated velocity magnitudes are highly dependent on the estimated flow angle, especially when the beam to flow angle is close to 90 degrees. If the flow angle is estimated with a high standard deviation, the estimated velocities becomes correspondingly imprecise. Furthermore, it may be difficult for the operator to know the precision of the angle estimate in a particular situation.

US5409010 discloses a method where a first receiving transducers is used to measure the velocity component along a first axis, and a second receiving transducer is used to measure the velocity component along a second axis.

This allows two spectrums to be estimated at the same time.

It may however be difficult to perceive the measurement results for an operator.

US201 10196237 discloses a method where several values of a scalar blood flow characteristic determined in a time interval are plotted individually at a single location along a timescale that corresponds to the time interval in which the several values were determined. However, it may be difficult to assess the precession of the individual values. Furthermore the temporal and / the spatial resolution may be limited as a plurality of individual estimates are grouped together.

Consequently, it remains a problem to provide a method and / or system that can estimate movement characteristics of body fluids and present the measurement results in an intelligible manner to an operator.

Additionally, it remains a problem to provide a robust method and / or system that can estimate movement characteristics of body fluids in the present of turbulence and / or at deep depths.

Summary

According to a first aspect, the invention relates a method for estimating the distribution of a first movement characteristic for a part of a fluid positioned within a combined measurement region, said first movement characteristic is selected from the group of movement characteristics consisting of: the velocity magnitude in a two dimensional plane, the velocity magnitude in three dimensional space and the velocity angle in a two dimensional plane, wherein the distribution of the first movement characteristic is a 1 dimensional distribution, the method comprising the steps of:

• transmitting at least one ultrasound signal;

· receiving at least one ultrasound echo signal;

• generating a first velocity signal, the frequencies of the first velocity signal being dependent on the distribution of velocity components along a first axis for the part of the fluid positioned within a first measurement region centred at a first spatial point;

wherein the method further comprises the steps of:

· generating a second velocity signal, the frequencies of the second velocity signal being

dependent on the distribution of velocity components along a second axis for the part of the fluid positioned within a second measurement region centred at a second spatial point; and

• processing said first velocity signal together with said second velocity signal to estimate the

distribution of the first movement characteristic wherein the distribution of the first movement characteristic is estimated without combining individual estimates of velocity magnitudes or velocity angles made at different points in time or at different spatial locations.

Consequently, an estimate of the distribution of a velocity magnitude or velocity angle may in an effective and robust manner be determined. The measurement of a distribution instead of a single value further enables the operator to determine the type of flow present more precisely.

The method is furthermore more robust, especially in the present of turbulence, as the method is not based on the erroneous assumption of a single velocity angle within the measurement region. This further enables the method to functions well at deeper depth, where the point spread function (measurement region) get larger. Thus, the method may be used for making precise angle independent velocity estimation in the heart without having to introduce an ultrasound transducer inside the body of the patient.

In some embodiments, the first velocity signal is processed together with said second velocity signal by:

· determining the frequency spectrum for the first velocity signal and the frequency spectrum for the second velocity signal ;

• selecting a plurality of frequency sets, each frequency set comprising a frequency from both the frequency spectrum of the first velocity signal and the frequency spectrum of the second velocity signal;

· processing said plurality of frequency sets to estimate the distribution of the first movement

characteristic

wherein it is predetermined which frequency sets are to be selected before the at least one ultrasound signal is transmitted, the plurality of frequency sets are processed together by mapping using a mapping function each frequency set into one or more target array element(s) of a first array, wherein a frequency set value is added to the stored value(s) of said one or more target array element(s), said frequency set value being dependent on the amplitude of the frequencies in the frequency set.

The method may both be used in continuous wave systems (CW) (see [1] page 1 1 1-154) and pulsed wave systems (see [1] page 155-194). In the following the invention will be disclosed with focus on pulsed wave systems, however the skilled person will easily be able to modify the teachings and apply them to a CW system.

In some embodiments,

• the step of transmitting at least one ultrasound signal comprises: transmitting a plurality of

ultrasound signals at different points in time using an array transducer;

• the step of receiving at least one ultrasound echo signal comprises: receiving a plurality of

ultrasound echo signals at different points in time using the array transducer;

• the first velocity signal is generated by beamforming a first group of the plurality of received

ultrasound echo signals at the first spatial point;

• the second velocity signal is generated by beamforming a second group of the plurality of

received ultrasound echo signals at the second spatial point.

The array transducer may be any type of array transducer such as 1 D 1.5D or 2D array transducer comprising a plurality of transducer elements.

The transducer may further have any shape such as linear, concave or convex. The first axis is not the same axis as the second axis. This first and the second axis are further not parallel. The transmitted ultrasound signals is preferably transmitted with a sub aperture, i.e. not using all the elements of the array transducer. Each received ultrasound echo signal is sampled by some or all of the elements of the array transducer. Thus, each received ultrasound echo signal comprises a plurality of element signals.

Consequently, the step of beamforming the first group of the plurality of received ultrasound echo signals to obtain the first velocity signal comprise for each ultrasound echo signal of the first group the step of: electronically beamforming some or all of its element signals at the first spatial point. Correspondingly, the step of beamforming the second group of the plurality of received ultrasound echo signals to obtain the second velocity signal comprise for each ultrasound echo signal of the second group the step of:

electronically beamforming some or all of its element signals at the second spatial point. The first spatial point and the second spatial point is preferably the same point. However, the method will still work when there is a slight spatial offset between the first spatial point and the second spatial point as may happens when imprecise electronically or mechanical focusing is used. In some embodiments, the first, second, third and fourth spatial point are substantially the same point e.g. the maximum distance between any two of the four points are less than 20% of the maximum width of the array transducer e.g. less 15%, 10%, 5% or 2% of the maximum width of the array transducer. The size and orientation of the first

measurement region and the second measurement region are determined by the point spread function of the used imaging setup. The combined measurement region is influenced by the first measurement region and the second measurement region. If no more than two velocity signals are generated, the combined measurement region is directly given by the first measurement region and the second measurement region. Even thou individual estimates of velocity angles or velocity magnitudes made at different points in time or at different spatial locations at are not combined to estimate the distribution of the first movement characteristic, estimates of distributions made at different points in time or at different spatial locations may be combined (averaged) to estimate the distribution of the first movement characteristic e.g. a plurality of estimated distributions of 2 dimensional velocity vectors as shown in Fig. 3d made at different spatial locations may be combined before the resulting spatially averaged estimated distributions of 2 dimensional velocity vectors are processed as shown in Fig. 3e or Fig. 3f to determine the distribution of the first movement characteristic.

The first group and the second group of the plurality of received ultrasound echo signals may be completely identical, i.e. the first group and the second group may originate from the same ultrasound emissions. This may be beneficial when measurements are performed on shallow depth as will be explained in more detail latter. Alternatively, the first group and the second group of the plurality of received ultrasound echo signals may be completely different, i.e. the first group and the second group may originate from different ultrasound emissions. This may be beneficial when measurements are performed at deeper depth such as below 4 cm, 6 cm, or 9 cm.

The following may be done to secure that the frequencies of the first velocity signal are dependent on the distribution of velocity components of the fluid along the first axis, and the frequencies of the second velocity signal are dependent on the distribution of velocity components of the fluid along the second axis: Each point of the first velocity signal is obtained by beamforming at the first spatial point element signals obtained from a single emission. Correspondingly, each point of the second velocity signal is obtained by beamforming at the second spatial point element signals obtained from a single emission. Thus, the first velocity signal and the second velocity signal may be what are known in the art as a "slow time signal".

The points of the first velocity signal may be beamformed using a first receive sub-array and the points of the second velocity signal may be beamformed using a second receive sub-array, wherein there is an offset between the centre of the first receive sub-array and the centre of second receive sub-array. This offset may partially of fully determines the angle between the first axis and the second axis, as explained in relation to Figs. 4a-b.

The beamformed signals are preferably quadrature signals containing directional information, see [1] page 174 - 179 for details regarding generation of quadrature signals. Thus, the first and second velocity signals may correspond to traditional pulsed spectral Doppler signals angled with a first and second angle respectively.

Alternatively, the points of the first and second velocity signal may be beamformed with the same receive sub-array, but the points of the second velocity signal is beamformed using a special receive apodization as discussed in [2]. This results in that the second axis will become perpendicular to the first axis.

The distribution of the first movement characteristic is a one dimensional distribution, however, a plurality of distributions may be estimated at consecutive points in time, stacked together and displayed using color coding as a two dimensional image e.g. similar to manner in which traditional pulsed spectral Doppler is displayed.

In some embodiments, the method further comprises the step of generating a third velocity signals, the frequencies of the third velocity signal being dependent on the distribution of velocity components along a third axis for the part of the fluid positioned within a third measurement region centred at a third spatial point.

The third velocity signal may be generated in a similar manner as the first and second velocity signal e.g. the third velocity signal may be generated by beamforming a third group of the plurality of received ultrasound echo signals at the third spatial point.. The third axis may be arranged outside the 2 dimensional plane spanned by the first axis and the second axis. This allows the distribution of the velocity magnitude in three dimensional space to be determined.

In some embodiments, the third axis is arranged outside the 2 dimensional plane spanned by the first axis and the second axis, wherein said distribution of the first movement characteristic is estimated by processing said first velocity signal together with said second velocity signal and said third velocity signal. In some embodiments, said first velocity signal is processed together with said second velocity signal by:

• determining the frequency spectrum for the first velocity signal and the frequency spectrum for the second velocity signal;

• processing said plurality of frequency sets to estimate the distribution of the first movement characteristic.

In some embodiments, said first velocity signal is processed together with said second velocity signal and said third velocity signal by:

• determining the frequency spectrum for the first velocity signal, the frequency spectrum for the second velocity signal and the frequency spectrum for the third velocity signal;

• selecting a plurality of frequency sets, each frequency set comprising a frequency from both the frequency spectrum of the first velocity signal, the frequency spectrum of the second velocity signal and the frequency spectrum of the third velocity signal; and

In some embodiments, the plurality of frequency sets are processed together by mapping using a mapping function each frequency set into one or more target array element(s) of a first array, wherein a frequency set value is added to the stored value(s) of said one or more target array element(s), said frequency set value being dependent on the amplitude of the frequencies in the frequency set.

Consequently, a precise and computational efficient way of estimating the distribution is provided.

The first array may preferably be a one dimensional array and directly represent the distribution of the at least one movement characteristic. This is a computational efficient implementation.

Alternatively, the distribution may be found by processing the first array e.g. by mapping the first array into a second array. This allows filtering processes to be performed on the first array before it is mapped to the second array. The first array may be a one dimensional array, a two dimensional array or a three dimensional array. The first array is preferably initialized to contain all zeros. If a frequency set is mapped into a plurality of array elements the frequency set value may be divided between the plurality of array indices by a weight determined by the mapping function e.g. if a particular frequency set is mapped to the array elements having indices 67 and 68 and the frequency set value is 0.03, the mapping function may determine that a value of 0.02 is added to the array element having index 67 and a value of 0.01 is added to array element having index 68. In some embodiments, for each of the plurality of frequency sets, the frequency from the frequency spectrum of the first velocity signal corresponds to a specific velocity component along said first axis, the frequency from said frequency spectrum of said second velocity signal corresponds to a specific velocity component along said second axis, and wherein the mapping function for each frequency set project the frequency sets at least two 1 dimensional velocity components into a resulting 2 dimensional or 3 dimensional velocity vector, the array indices of said one or more target array element(s) being derived from said resulting velocity vector. The array indices of said one or more target array element(s) may be derived from the magnitude of the resulting velocity vector, a direction (angle) in a two dimensional plane of the resulting velocity vector, or the (orthogonal) components of the resulting velocity vector. As an example if the first array comprises 50 elements and represent a velocity magnitude from 0 to 0.5 m/s, and a resulting velocity vector of a frequency set has a magnitude of 0.257 m/s, the frequency set may be mapped to index 26. Alternatively, the frequency set may be mapped to both index 25 and 26 to reduce sampling artefacts as explained in relation to FIG. 3h. The mapping function is preferably predetermined, i.e. determined before data is measured since the 'resulting 2 dimensional or 3 dimensional velocity vector' only is dependent on the measurement setup (the orientation of the first, second and third axis; the emitted centre frequency, the pulse repetition frequency etc.) and not the measured data.

In some embodiments, the frequency set value is determined by multiplying a first value with a second value wherein the first value is dependent on the amplitude of the frequency from the frequency spectrum of the first velocity signal and the second value is dependent on the amplitude of the frequency from the frequency spectrum of the second velocity signal.

Consequently, an effective way of estimating the frequentness of the resulting velocity vector in the measurement region for a particular frequency set is provided.

In some embodiments, the first value and the second value is the amplitude or the amplitude multiplied by itself one or more times.

In some embodiments, the frequency spectrum of the first velocity signal and the frequency spectrum of the second velocity signal are normalized before the first value and the second value are found. Consequently, it is secured that differences in energy between the two or spectra does not influence the result. This may further secure that the first array is directly scaled correct, i.e. the sum of all values of the first array equals one.

In some embodiments, the frequency spectrum of the first velocity signal and the frequency spectrum of the second velocity signal are normalized so that for each frequency spectrum: • where N is the number of frequency components for the spectrum, A_n is the n'th frequency component and k is a positive constant.

In some embodiments, it is predetermined which frequency sets are to be selected before the at least one ultrasound signal is transmitted.

Preferably it is predetermined which frequency sets are to be selected i.e. independent of the first and second velocity signal (and possibly the third velocity signal). More preferably at least 40%, 50%, 70%, 80%, 90% off all possible frequencies sets are selected. In some embodiments, the only frequency sets that are not selected are the ones having a 'resulting 2 dimensional or 3 dimensional velocity vector' (see below) with a 'non-possible' magnitude e.g . a magnitude being larger than the largest possible velocity magnitude in the vessel where measurements are being made. However, since the 'resulting 2 dimensional or 3 dimensional velocity vector' only is dependent on the measurement setup (the orientation of the first, second and third axis; the emitted centre frequency, the pulse repetition frequency etc.) and not the measured data, the frequency sets not having a 'resulting 2 dimensional or 3 dimensional velocity vector' (see below) with a 'non-possible' magnitudes may be identified before any data is measured.

Consequently, a simpler and more robust method is provided since there is no need for attempting to identify individual flow components in the first and second velocity signal.

In some embodiments, the method further comprises the step of generating a filtering velocity signal, the frequencies of the filtering velocity signal being dependent on the distribution of velocity components along a fourth axis for the part of the fluid positioned within a fourth measurement region centred a fourth spatial point, wherein the frequency spectrum for the filtering velocity signal is determined and for each frequency set the frequency set value is filtered with a filtering value derived from the frequency spectrum of the filtering velocity signal.

Consequently, by filtering the frequency set values, unwanted artefact may be reduced. Unwanted artefacts may be a significant problem when a plurality of distinct velocity vectors are present within the combined measurement region. The frequency set value may be filtered with the filtering value by multiplying the frequency set value with the filtering value. The filtering velocity signal may be generated in a similar manner as the first and second velocity signal e.g. the filtering velocity signal may be generated by beamforming a fourth group of the plurality of received ultrasound echo signals at the fourth spatial point. The first, second, third, and fourth spatial point are preferably the same point. If the method is in 2D mode , i.e. the third velocity signal is not generated, then the fourth axis should preferably be arranged in the plane spanned by the first and second axis so that the normal vector of the fourth axis can be generated from a linear combination of the normal vectors of the first and second axis.

In some embodiments, the filtering value derived from the frequency spectrum of the filtering velocity signal for each frequency set is a non-Boolean value e.g. the filtering value derived may be a particular value our of at least 3 possible value, preferably out of at least 4, 8 , 16, 32, 64, 128 or 256 possible values.

Consequently, a more robust method is provided since the method does not attempt to guess whether a particular frequency set value is a result of an artefact or is a result of an actual velocity vector in the combined measurement region, but instead simply may attempt to estimate the likelihood that the particular frequency set value is a result of an actual velocity vector and use that estimated likelihood to proportionally enhance or reduce the contribution of the particular frequency set value to the finished estimated distribution of the first movement characteristic.

In some embodiments, the frequencies from the frequency spectrum of said filtering velocity signal corresponds to specific velocity components along said fourth axis, wherein the resulting 2 dimensional or 3 dimensional velocity vector (of the frequency set) is projected onto said fourth axis thereby resulting a filtering velocity component, wherein the filtering value is derived from the filtering velocity component and the frequency spectrum of the filtering velocity signal.

The filtering value may be derived from the amplitude of the frequency of the filtering velocity signal having a corresponding velocity component along said fourth axis being closest to the filtering velocity component

e.g. it may simply be the amplitude it self or an average (possibly weighted) of the amplitude and the amplitudes of the neighbouring frequencies.

In some embodiments, the filtering value is determined without comparing the amplitudes of the two or three frequencies in the frequency set with each other or with an amplitude of a frequency of the filtering velocity signal e.g. the amplitudes of the two or three frequencies in the frequency set are not compared with the amplitude of the frequency of the filtering velocity signal having a corresponding velocity component along said fourth axis being closest to the filtering velocity component to see if they all match as they theoretically should (under some assumptions). Consequently, a simpler and more robust method is provided e.g. if stationary echo cancelling is poorly performed for one of the velocity signals, the impact on the estimated distribution may be relative minimal since the method of filtering specified above still will function and will remove most of the resulting artefacts.

In some embodiments, the method uses spatial averaging by: • beamforming a plurality of points in spatial proximity to said first spatial point when creating said first velocity signal, and beamforming a plurality of points in spatial proximity to said second spatial point when creating said second velocity signal; and / or

• estimating the distribution of the first movement characteristic at plurality of closely positioned points.

Consequently, the SNR may be improved.

In some embodiments, the angle between said first axis and said second axis is below 40 degrees e.g. below 35, 30, 25, or 20 degrees.

By keeping the angle low between the first axis and the second axis, it can be prevented that one of the axis are position along the main flow direction. This allows a lower pulse repetition frequency to be used without aliasing. It should be noted that in some situations it may be beneficial to have a higher angling e.g. above 20, 25, 30, 35, or 45 degrees.

In some embodiments, both the distribution of a first movement characteristic and a second movement characteristic are estimated, both the first and the second movement characteristic are selected from the group of movement characteristics consisting of: an velocity magnitude in a two dimensional plane, a three dimensional velocity magnitude and a velocity angle in a two dimensional plane.

Consequently, a plurality of features of the flow may simultaneously be determined and displayd to the operator. This allows the operator to get a more detailed understanding of the flow. In some embodiments, said first movement characteristic is a two or three dimensional velocity magnitude and said second movement characteristic is a velocity angle in a 2 dimensional plane.

In some embodiments, the first and the second movement characteristic is displayed on a display simultaneously.

In some embodiments, a plurality of distributions of said at least one movement characteristic are estimated at consecutive points in time and together displayed.

Consequently, the estimated distributions may be shown in a manner similar to traditional spectral Doppler systems, allowing the operator to study temporal variations in the flow.

The distributions may be shown in an image, where each vertical line correspond to a distribution, each point on each vertical line correspond to a particular value, and the colour of each point illustrates the estimated frequentness of that particular value at the particular point in time e.g. a bright colour shows a high frequentness and dark colour shows a low frequentness or vice versa. In some embodiments, the first movement characteristic is a velocity magnitude, a plurality of distributions of said velocity magnitude are determined at a plurality of consecutive points in time and displayed, and for at least some of said plurality of consecutive points in time a velocity angle is determined and displayed as a vector, each vector being arranged in connection with the distribution of said velocity magnitude determined for the same point in time, whereby a user may in an easy manner both see the distribution of the velocity magnitude and the primary flow direction as a function of time.

Consequently, an operator may in an easy manner study temporal variations in the velocity magnitude and the main flow angle. By presenting the main flow angle as a vector, the operator does need to perform the relative cognitive complicated task of transforming an angle value into a spatial direction. This will make it easier for the operator to use the method in real time as is the common practice with ultrasound. In some embodiments, the first group of said plurality of received ultrasound echo signals are beamformed using element signals from a first sub-array of said array transducer, and the second group of said plurality of received ultrasound echo signals are beamformed using element signals from a second sub-array of said array transducer. In some embodiments, the first group of said plurality of received ultrasound echo signals are originating from a first group of said plurality of transmitted ultrasound signal, the second group of said plurality of received ultrasound echo signals are originating from a second group of said plurality of transmitted ultrasound signal, and wherein said first group of said plurality of transmitted ultrasound signals are transmitted with a third sub-array of said array transducer and said second group of said plurality of transmitted ultrasound signals are transmitted with fourth a sub-array of said array transducer.

Consequently, measurements at greater depth may be performed as sufficient angling may be obtained by angling in both transmit and receive. The centre of the first sub-array and the centre of third sub-array may be positioned closely together e.g. with a distance between them lower than 50%, 25%, or 10% of the width of the first sub-array.

Correspondingly, the centre of the second sub-array and the centre of the fourth sub-array may be positioned closely together e.g. with a distance between them lower than 50%, 25%, or 10% of the width of the second sub-array.

In some embodiments, it is selected, dependent on the distance from the array transducer to the first and second measurement region, whether:

(A) the first group of said plurality of received ultrasound echo signals are originating from a first group of said plurality of transmitted ultrasound signal, the second group of said plurality of received ultrasound echo signals are originating from a second group of said plurality of transmitted ultrasound signal, and wherein said first group of said plurality of transmitted ultrasound signals are transmitted with a third sub-array of said array transducer and said second group of said plurality of transmitted ultrasound signals are transmitted with fourth sub-array of said array transducer; or

(B) the first group of said plurality of received ultrasound echo signals and said second group of said plurality of received ultrasound echo signals are originating from the same transmitted ultrasound signals.

Consequently, both a high frame rate may be obtained for shallow depths, where angling in receive is enough, and a high max depth.

Preferably option B is selected for shallow depth and option A is selected for deeper depths.

According to a second aspect the invention relates to an ultrasound imaging system configured to estimate the distribution of a first movement characteristic for a part of a fluid positioned within a combined measurement region, said first movement characteristic is selected from the group of movement characteristics consisting of: the velocity magnitude in a two dimensional plane, the velocity magnitude in three dimensional space and the velocity angle in a two dimensional plane, wherein the distribution of the first movement characteristic is a 1 dimensional distribution said ultrasound imaging system comprising:

• an transducer;

• transmit circuitry configured to transmit at least one ultrasound signal using the transducer;

• receive circuitry configured to receive at least one ultrasound echo signal using the transducer;

• a beamformer configured to: generate a first velocity signal, the frequencies of the first velocity signal being dependent on the distribution of velocity components along a first axis for the part of the fluid positioned within a first measurement region centred at a first spatial point; and generate a second velocity signal, the frequencies of the second velocity signal being dependent on the distribution of velocity components along a second axis for the part of the fluid positioned within a second measurement region centred at a second spatial point;

• a processing unit configured to process said first velocity signal together with said second velocity signal to estimate the distribution of the first movement characteristic wherein the distribution of the first movement characteristic is estimated without combining individual estimates of velocity magnitudes or velocity angles made at different points in time or at different spatial locations. In some embodiments, the processing unit is configured to:

o determine the frequency spectrum for the first velocity signal and the frequency spectrum for the second velocity signal;

o selecting a plurality of frequency sets, each frequency set comprising a frequency from both the frequency spectrum of the first velocity signal and the frequency spectrum of the second velocity signal; o processing said plurality of frequency sets to estimate the distribution of the first movement characteristic;

In some embodiments,

• the transducer is an array transducer;

• the transmit circuitry is configured to a transmit a plurality of ultrasound signals at different points in time using the array transducer;

• the receive circuitry configured to receive a plurality of ultrasound echo signals at different points in time using the array transducer;

• the beamformer is configured to generate the first velocity signal by beamforming a first group of the plurality of received ultrasound echo signals at the first spatial point; and

• the beamformer is configured to generate the second velocity signal by beamforming a second group of the plurality of received ultrasound echo signals at the second spatial point. In some embodiments, the beamformer is further configured to generate a third velocity signals, the frequencies of the third velocity signal being dependent on the distribution of velocity components along a third axis for the part of the fluid positioned within a third measurement region.

In some embodiments, the processing unit process said first velocity signal together with said second velocity signal by:

In some embodiments, the processing unit process said first velocity signal together with said second velocity signal and said third velocity signal by:

• determining the frequency spectrum for the first velocity signal, the frequency spectrum for the second velocity signal and the frequency spectrum for the third velocity signal; selecting a plurality of frequency sets, each frequency set comprising a frequency from both the frequency spectrum of the first velocity signal, the frequency spectrum of the second velocity signal and the frequency spectrum of the third velocity signal; and

processing said plurality of frequency sets to estimate the distribution of the first movement characteristic.

In some embodiments, for each of the plurality of frequency sets, the frequency from the frequency spectrum of the first velocity signal corresponds to a specific velocity component along said first axis, the frequency from said frequency spectrum of said second velocity signal corresponds to a specific velocity component along said second axis, and wherein the mapping function for each frequency set project the frequency sets at least two 1 dimensional velocity components into a resulting 2 dimensional or 3 dimensional velocity vector, the array indices of said one or more target array element(s) being derived from said resulting velocity vector. In some embodiments, the frequency set value is determined by multiplying a first value with a second value, wherein the first value is dependent on the amplitude of the frequency from the frequency spectrum of the first velocity signal and the second value is dependent on the amplitude of the frequency from the frequency spectrum of the second velocity signal. In some embodiments, the first value and the second value is the amplitude or the amplitude multiplied by itself one or more times.

In some embodiments, the processing unit is configured to normalize the frequency spectrum of the first velocity signal and the frequency spectrum of the second velocity signal before the first value and the second value are found.

In some embodiments, the processing unit is configured to normalize the frequency spectrum of the first velocity signal and the frequency spectrum of the second velocity signal so that for each frequency spectrum: In some embodiments, the beamformer is further configured to generate a filtering velocity signal, the frequencies of the filtering velocity signal being dependent on the distribution of velocity components along a fourth axis for the part of the fluid positioned within a fourth measurement region centred a fourth spatial point, wherein processing unit is further configured to determine the frequency spectrum for the filtering velocity signal and for each frequency set filter the frequency set value with a filtering value derived from the frequency spectrum of the filtering velocity signal

In some embodiments, the filtering value derived from the frequency spectrum of the filtering velocity signal for each frequency set is a non-Boolean value e.g. the filtering value derived may be a particular value our of at least 3 possible value, preferably out of at least 4, 8 ,16, 32, 64, 128 or 256 possible values.

In some embodiments, the ultrasound imaging system is configured to use spatial averaging by:

· beamforming using the beamformer a plurality of points in spatial proximity to said first spatial point when creating said first velocity signal, and beamforming a plurality of points in spatial proximity to said second spatial point when creating said second velocity signal; and / or • estimating the distribution of the first movement characteristic at plurality of closely positioned points.

In some embodiments, the ultrasound imaging system is configured to estimate both the distribution of a first movement characteristic and a second movement characteristic, both the first and the second movement characteristic are selected from the group of movement characteristics consisting of: an velocity magnitude in a two dimensional plane, a three dimensional velocity magnitude and a velocity angle in a two dimensional plane. In some embodiments, said first movement characteristic is a two or three dimensional velocity magnitude and said second movement characteristic is a velocity angle in a 2 dimensional plane.

In some embodiments, the ultrasound imaging system further comprises a display, wherein the first and the second movement characteristic is displayed on the display simultaneously.

In some embodiments, the first movement characteristic is a velocity magnitude, a plurality of distributions of said velocity magnitude are determined at a plurality of consecutive points in time and displayed, and for at least some of said plurality of consecutive points in time a velocity angle is determined and displayed as a vector, each vector being arranged in connection with the distribution of said velocity magnitude determined for the same point in time, whereby a user may in an easy manner both see the distribution of the velocity magnitude and the primary flow direction as a function of time.

In some embodiments, the first group of said plurality of received ultrasound echo signals are beamformed using element signals from a first sub-array of said array transducer, and the second group of said plurality of received ultrasound echo signals are beamformed using element signals from a second sub-array of said array transducer.

In some embodiments, the first group of said plurality of received ultrasound echo signals are originating from a first group of said plurality of transmitted ultrasound signal, the second group of said plurality of received ultrasound echo signals are originating from a second group of said plurality of transmitted ultrasound signal, and wherein said first group of said plurality of transmitted ultrasound signals are transmitted with a third sub-array of said array transducer and said second group of said plurality of transmitted ultrasound signals are transmitted with fourth a sub-array of said array transducer.

According to a third aspect the invention relates to a computer program product comprising program code means adapted to cause a ultrasound imaging system to perform the steps of the method according to any one of claims 1 through 14 , when said program code means are executed.

According to a fourth aspect the invention relates to a computer-readable medium having stored thereon program code means adapted to cause a ultrasound imaging system to perform the steps of the method according to any one of claims 1 through 14 , when said program code means are executed.

The computer readable medium may be a non-transitory computer readable medium.

According to a fifth aspect the invention relates to a method for estimating the distribution of a first movement characteristic for a part of a fluid positioned within a combined measurement region, the method comprising the steps of:

• transmitting at least one ultrasound signal;

• receiving at least one ultrasound echo signal;

• generating a second velocity signal, the frequencies of the second velocity signal being

dependent on the distribution of velocity components along a second axis for the part of the fluid positioned within a second measurement region centred at a second spatial point;

• determining the frequency spectrum for the first velocity signal and the frequency spectrum for the second velocity signal ; • selecting a plurality of frequency sets, each frequency set comprising a frequency from both the frequency spectrum of the first velocity signal and the frequency spectrum of the second velocity signal;

• processing said plurality of frequency sets to estimate the distribution of the first movement

characteristic

Examples of movement characteristics are the velocity magnitude in a two dimensional plane, the velocity magnitude in three dimensional space, the velocity angle in a two dimensional plane, the two dimensional velocity vectors in a two dimensional plane, or the three dimensional velocity vectors in three dimensional space e.g. the first movement characteristic may be selected from the group of movement characteristics consisting of: the velocity magnitude in a two dimensional plane, the velocity magnitude in three dimensional space, the velocity angle in a two dimensional plane, the two dimensional velocity vectors in a two dimensional plane, or the three dimensional velocity vectors in three dimensional space

According to a sixth aspect the invention relates to an ultrasound imaging system configured to estimate the distribution of a first movement characteristic for a part of a fluid positioned within a combined measurement region, said ultrasound imaging system comprising:

• an transducer;

· transmit circuitry configured to transmit at least one ultrasound signal using the transducer;

• a beamformer configured to: generate a first velocity signal, the frequencies of the first velocity signal being dependent on the distribution of velocity components along a first axis for the part of the fluid positioned within a first measurement region centred at a first spatial point; and generate a second velocity signal, the frequencies of the second velocity signal being dependent on the distribution of velocity components along a second axis for the part of the fluid positioned within a second measurement region centred at a second spatial point;;

• a processing unit configured to:

The first array is preferably a one dimensional array when the first movement characteristic is the velocity magnitude in a two dimensional plane, the velocity magnitude in three dimensional space or the velocity angle in a two dimensional plane; a two dimensional array when the first movement characteristic is the two dimensional velocity vectors in a two dimensional plane; and a three dimensional array when the first movement characteristic is the three dimensional velocity vectors in three dimensional space. Here and in the following, the terms 'processing means' and 'processing unit' are intended to comprise any circuit and/or device suitably adapted to perform the functions described herein. In particular, the above term comprises general purpose or proprietary programmable microprocessors, Digital Signal Processors (DSP), Application Specific Integrated Circuits (ASIC), Programmable Logic Arrays (PLA), Field Programmable Gate Arrays (FPGA), special-purpose electronic circuits, etc., or a combination thereof.

The different aspects of the present invention can be implemented in different ways including as a method for estimating the distribution of at least one movement characteristic for a fluid and an ultrasound imaging system, described above and in the following, each yielding one or more of the benefits and advantages described in connection with at least one of the aspects described above, and each having one or more preferred embodiments corresponding to the preferred embodiments described in connection with at least one of the aspects described above and/or disclosed in the dependant claims. Furthermore, it will be appreciated that embodiments described in connection with one of the aspects described herein may equally be applied to the other aspects, in particular embodiments disclosed in relation to the first aspect may be applied to the fifth aspect, and embodiments disclosed in relation to the second aspect may be applied to the sixth aspect.

Brief description of the drawings The above and/or additional objects, features and advantages of the present invention, will be further elucidated by the following illustrative and non-limiting detailed description of embodiments of the present invention, with reference to the appended drawings, wherein:

Figs. 1a-d illustrates how the distribution of a single velocity component for blood flowing within a blood vessel may be determined using the traditional pulsed spectral Doppler technique. Fig. 2 illustrates schematically a method a method for estimating within a combined measurement region the distribution of at least one movement characteristic for a fluid, according to an embodiment.

Figs. 3a-h show schematically how two velocity signals may be processed together to determine an estimate of a movement characteristic, according to an embodiment of the present invention.

Fig. 4a-f shows schematically the resulting axis for different imaging setups.

Fig. 5a show schematically an ultrasound imaging system 500 configured to estimate the distribution of a first movement characteristic for a part of a fluid positioned within a combined measurement region according to an embodiment of the present invention.

Fig. 5b show a flow chart for a method for estimating the distribution of a first movement characteristic for a part of a fluid positioned within a combined measurement region according to an embodiment of the present invention.

Fig. 6 shows simulation results.

Fig. 7 shows a flow chart for a method for estimating the distribution of a first movement characteristic for a part of a fluid positioned within a combined measurement region according to an embodiment of the present invention.

Detailed description

In the following description, reference is made to the accompanying figures, which show by way of illustration how the invention may be practiced.

Figs. 1a-d illustrates how the distribution of a velocity component for blood flowing within a blood vessel, may be estimated using the traditional pulsed spectral Doppler technique. The distribution specifies the velocity components present in a first measurement region 103 centred at a first spatial point 105. The velocity component is the velocity component along the ultrasonic beam 106. Fig. 1a shows schematically the measurement setup. Shown is a blood vessel 104, having blood flowing within it with velocities illustrated by the arrows 1 10-1 16. From the arrows it can be seen that the velocity of the blood is highest in the centre of the vessel. To estimate the distribution, a plurality of ultrasound signals 102 are transmitted at different points in time using an array transducer 101. The plurality of ultrasound signals 102 are preferably focused in transmit at the first spatial point 105. Typically, each transmitted ultrasound signal comprises between 2 and 8 oscillations at the centre frequency of the array transducer 101. Each transmitted ultrasound signal 102 result in an ultrasound echo signal which is received by the array transducer 101. Thus, the array transducer 101 receives a plurality of ultrasound signals at different points in time. Each received ultrasound echo signal is sampled by some or all of the elements of the array transducer 101. Thus, each received ultrasound echo signal comprises a plurality of element signals. For each received ultrasound echo signal, its element signals are electronically beamformed at the first spatial point 105. As is well known to the skilled person, the size, shape, orientation etc. of the first measurement region 103 is given by the point spread function of the chosen measurement setup. By combining all the beamformed points, the slow time signal 130 shown in Fig. 1 b results. Thus, each point 120-128 on the slow time signal 130 originate from a specific emission. For simplicity only every third point on the slow time signal are provided with a reference number. Thus, the point 120 result from electronically beamforming the element signals of the first received ultrasound echo signal at the first spatial point 105, the point 121 result from electronically beamforming the element signals of the fourth received ultrasound echo signal at the first spatial point 105, the point 122 result from electronically beamforming the element signals of the seventh received ultrasound echo signal at the first spatial point 105 and so forth. As is well known to the skilled person, the frequencies of the slow time signal 130, are given by the velocity components present in the first measurement region 103. The specific relationship is given by:

2v 2|v[ cos0

where fp is an specific frequency, fO is the emitted ultrasound frequency, c is the speed of sound, Vz is the blood velocity in the z direction, and ^;® , is angle between the ultrasound beam and the 2 dimensional velocity vector in the XZ plane. Thus, by determining the frequency spectrum (typically using the FFT), and then using the frequency spectrum to determine the amplitude spectrum (or power spectrum or the like) an estimate of the distribution of the velocity components along the ultrasonic beam is found. Fig. 1 c shows the amplitude spectrum 131 for the slow time signal 130. As can be seen the amplitude spectrum is symmetric and comprises a true component 133 and a "mirror" component 134. The presence of the mirror component 134 makes it impossible to determine whether the blood is flowing towards the array transducer 101 or away from the array transducer 101 . However, using well known techniques, the mirror component 134 may be removed. This may be done by for creating quadrature signals. The steps, described in relation to Fig. 1a-b are preferably continuously performed, whereby a plurality of amplitude spectra 132 133 may be determined. These amplitude spectra are then typically combined in a two dimensional image using colour coding, whereby temporal variations of the flow may be studied, as shown in Fig. 1d An estimate of the true velocity magnitudes may be found by correcting for the beam to flow angle. In commercial available systems, the operator typically manually attempts to estimate the flow angle based on the b-mode image obtained of the blood vessel. This may however be a difficult task. Furthermore if there are more than one flow angle present within the first measurement region 103 (as is often the situation under pathological condition where turbulence is present), this task becomes impossible. The risk of having a plurality of flow angles present within the measurement region furthermore increases with depth, as the lateral size of the point spread function increases as a function of depth.

Fig. 2 illustrates schematically a method for estimating within a combined measurement region the distribution of at least one movement characteristic for a fluid, according to an embodiment. The at least one movement characteristic is selected from the group of movement characteristics consisting of: the velocity magnitude in a two dimensional plane, the velocity magnitude in three dimensional space and the velocity angle in a two dimensional plane. The method comprises the steps of: transmitting a plurality of ultrasound signals 210-213 at different points in time using an array transducer 201 ;

receiving a plurality of ultrasound echo signals 214-217 at different points in time using the array transducer 201 ;

beamforming a first group of the plurality of received ultrasound echo signals at a first spatial point to generate a first velocity signal 230, the frequencies of the first velocity signal being dependent on the distribution of velocity components along a first axis for the part of the fluid positioned within a first measurement region centred at the first spatial point;

beamforming a second group of the plurality of received ultrasound echo signals at a second spatial point to obtain a second velocity signal 240, the frequencies of the second velocity signal being dependent on the distribution of velocity components along a second axis for the part of the fluid positioned within a second measurement region centred at the second spatial point; and

processing 250 the first velocity signal 230 together with the second velocity signal 240 to estimate the distribution 260 of the at least one movement characteristic.

Shown in the top of Fig. 2 is the first 210, the second 21 1 , the third 212, and the N'th 213 transmitted ultrasound signal. Each transmitted ultrasound signal 210-213 results in a particular received ultrasound echo signal 214-217. Each received ultrasound echo signal 214-217 is sampled by some or all of the elements of the array transducer 201. Thus, each received ultrasound echo signal 214-217 comprises a plurality of element signals. In this schematic drawing only seven element signals are shown. However, preferably each received ultrasound echo signal 214-217 is sampled by at least 32, 64, 128, 256, 1024 or more elements. The element signals are inputted to a beamformer 218 such as a delay and sum beamformer. The beamformer 218 generates the first velocity signal 230 by for each ultrasound echo signal of the first group, electronically beamforming some or all of its element signals at the first spatial point. Correspondingly, the beamformer 218 generates the second velocity signal 240 by for each ultrasound echo signal of the second group, electronically beamforming some or all of its element signals at the second spatial point. The first and second velocity signals 230 240 are for simplicity shown as real signals. However preferably, the first and the second velocity signals are quadrature signals whereby the direction of flow along the first or second axis may be determined. The first group and the second group of the plurality of received ultrasound echo signals may be the same group. Thus, the beamformer 218 may beamform for each received ultrasound echo signal, a point on both the first velocity signal 230 and the second velocity signal 240 e.g. the beamformer may generate both the first point 222 of the first velocity signal 230 and the first point 227 of the second velocity signal 240 by electronically beamforming the element signals of the first received ultrasound echo signal 214 and so forth. Thus, the beamformer 218 may be capable of beamforming a plurality of lines in parallel. Alternatively, the first group and the second group may differ. As an example the members of first group may be all the odd numbered received ultrasound echo signals 214 216 and the members of the second group may be all the even numbered received ultrasound echo signals 215 217. Thus, the beamformer 218 may generate the first point 222 of the first velocity signal 230 by beamforming the element signals of the first received ultrasound echo signal 214 and generate the first point 227 of the second velocity signal 240 by beamforming the element signals of the second received ultrasound echo signal 215 and so forth. The sampling frequency of the first velocity signal (also know as the pulse repetition frequency) may be the same as the sampling frequency of the second velocity signal or it may differ. Figs. 3a-h show schematically how two velocity signals 330 340 may be processed together to estimate the distribution of a movement characteristic, according to an embodiment of the present invention. Shown in Fig. 3a is a blood vessel 304, having blood flowing within it with velocities illustrated by the arrows 310-316. From the arrows it can be seen that the velocity of the blood is highest in the centre of the vessel. The blood vessel is angled with an angle of 90 degrees relative to the centre axis 306 of the array transducer 301. The first velocity signal 330 (only shown schematically) has frequencies being dependent on the distribution of velocity components along a first axis 308 for the part of the fluid positioned within a first measurement region 318 centred at a first spatial point 319a. The second velocity signal 340 (only shown schematically) has frequencies being dependent on the distribution of velocity components along a second axis 309 for the part of the fluid positioned within a second measurement region 317 centred at a second spatial point 319b. In this embodiments the first spatial point 319a and the second spatial point 319b is the same spatial point. The first and second velocity signal 330 340 are processed to determine their respective frequency spectrum. Shown are the amplitude spectrum 331 of the first velocity signal 330 and the amplitude spectrum 341 of the second velocity signal 340. It can be seen that the first and the second velocity signal 330 340 are complex signals as their amplitude spectra are single sided i.e. the mirror component is removed. The amplitude spectrum 331 may be perceived as an estimate of the distribution of velocity components along the first axis 308 for the part of the blood positioned within the first measurement region 318. Correspondingly, the amplitude spectrum 341 may be perceived as an estimate of the distribution of velocity components along the second axis 309 for the part of the blood positioned within the second measurement region 317. Each frequency 332-336 of the amplitude spectrum 331 corresponds to a specific velocity component along the first axis 308 and the amplitude is an estimate of the specific velocity components frequentness in the first measurement region 318. Correspondingly, each frequency 342-346 of the amplitude spectrum 341 corresponds to a specific velocity component along the second axis 309 and the amplitude is an estimate of the specific velocity components frequentness in the second measurement region 317. In this explanatory embodiment, each frequency spectrum is determined with a resolution of 13 frequencies. In other embodiments the frequency spectra may be determined with a higher resolution such as at least 16, 32, 64, 128, or 256 frequencies. To determine an estimate of the distribution of a movement characteristic, a plurality of frequency sets are selected, each frequency set comprising a frequency from both the frequency spectrum of the first velocity signal 330 and the frequency spectrum of the second velocity signal 340. All possible frequency sets may be selected (169 in this example).. Each frequency set are then mapped using a mapping function into a target array element of a first array 351 (se Fig. 3b), and a frequency set value is added to the stored value of said target array element, wherein the frequency set value is dependent on the amplitude of the frequencies in the frequency set. In this embodiment, the frequency set value is determined by multiplying a first value being the amplitude of the frequency from the frequency spectrum of the first velocity signal 330 with a second value being the amplitude of the frequency from the frequency spectrum of the second velocity signal 340.

The first array is only shown schematically. In this embodiment the first array is a two dimensional array. The individual elements of the first array 351 are for simplicity not shown. The first array may comprise any number of elements such as (25,25) elements, (50,50) elements, (200,200) elements, (2000,2000) elements, or (1000,2000) elements.

As mentioned above each frequency from the frequency spectrum of the first velocity signal 330 corresponds to a specific velocity component along the first axis 308 and each frequency from the frequency spectrum of the second velocity signal 340 corresponds to a specific velocity component along the second axis 309. Thus, each frequency set corresponds to a unique combination of two 1 dimensional velocity components. For each frequency set, the mapping function, project the frequency sets two 1 dimensional velocity components into a resulting two dimensional velocity vector and the array index of the target array element are derived from the X,Z components of the resulting two dimensional velocity vector. As an example if the first array 351 comprises (200, 200) elements and represent Vx values in the range -1 m/s to + 1 m/s, and Vz values in the range -1 m/s to +1 m/s and the resulting two dimensional velocity vector for a particular frequency set has a Vx component of 0.6 m/s and a Vz component of 0.0 m/s the mapping function may derive the array index of the target element for the particular frequency set to be (160, 100). Thus, the frequency set value of said particular frequency set is added to the value stored in array element of the first array 351 having index (160, 100).

Fig. 3c illustrates as an example for all frequency sets where the amplitude of both frequencies in the set is non-zero, how the mapping function projects the two 1 dimensional velocity components into a resulting two dimensional vector. The frequency set 332 342 is projected into the two dimensional velocity vector terminating in point 361 , the frequency set 332 343 is projected into the two dimensional velocity vector terminating in point 365, the frequency set 332 344 is projected into the two dimensional velocity vector terminating in point 369, the frequency set 332 345 is projected into the two dimensional velocity vector terminating in point 373, the frequency set 333 342 is projected into the two dimensional velocity vector terminating in point 362, the frequency set 333 343 is projected into the two dimensional velocity vector terminating in point 366, the frequency set 333 344 is projected into the two dimensional velocity vector terminating in point 370, the frequency set 333 345 is projected into the two dimensional velocity vector terminating in point 374, the frequency set 334 342 is projected into the two dimensional velocity vector terminating in point 363, the frequency set 334 343 is projected into the two dimensional velocity vector terminating in point 367, the frequency set 334 344 is projected into the two dimensional velocity vector terminating in point 371 , the frequency set 334 345 is projected into the two dimensional velocity vector terminating in point 375, the frequency set 335 342 is projected into the two dimensional velocity vector terminating in point 364, the frequency set 335 343 is projected into the two dimensional velocity vector terminating in point 368, the frequency set 335 344 is projected into the two dimensional velocity vector terminating in point 372, and the frequency set 335 345 is projected into the two dimensional velocity vector terminating in point 376. For simplicity only the two dimensional velocity vector 361a terminating in point 361 is shown. As explained above, for each frequency set the array index of the frequency sets target array element is derived from the X,Z components of the resulting two dimensional velocity vector, and the frequency set value is added to the target array element.

Fig. 3d shows the first array 351 after the mapping function has mapped all frequency sets into it. The crossed part 351a illustrates the position of the elements in the first array 351 having a value above zero stored (assuming the first array 351 was initialized to contain only zeros). The first array 351 can be seen as an estimate of the distribution of two dimensional velocity vectors (in the XZ plane)for the blood positioned within the combined measurement region. It may however be difficult to deduct the clinical relevant information from the first array by directly displaying it as an image (using colour coding to illustrate the values stored in the elements). Thus, to determine the estimate of the distribution of the movement characteristic the first array 351 is processed .

Fig. 3e shows how the first array 351 may be processed, when the movement characteristic is the velocity magnitude in a two dimensional plane for the part of the blood positioned within the combined measurement region, according to an embodiment. In this embodiment, the two dimensional plane is the XZ plane. The first array 351 is processed by 'integrating it along 19 circles 377a-385a' having an increasing radius and all being centred at vz = 0 and vx=0. Thus, each circle 377a-385a represents a particular interval of velocity magnitudes. To make the figure more intelligible only every second circle has been provided with a reference number. Each array element of the first array 351 are mapped to the circle being closest the centre of the element, and the values of the array elements mapped to a particular circle are summed to determine the distribution 352. Thus, the value 377b results from summing the values of all array elements being closest to the circle 377a, the value 378b results from summing the values of all array elements being closest to the circle 378a, the value 379b results from summing the values of all array elements being closest to the circle 379a, the value 380b results from summing the values of all array elements being closest to the circle 380a, the value 381 b results from summing the values of all array elements being closest to the circle 381 a, the value 382b results from summing the values of all array elements being closest to the circle 382a, the value 383b results from summing the values of all array elements being closest to the circle 383a, the value 384b results from summing the values of all array elements being closest to the circle 384a, and the value 385b results from summing the values of all array elements being closest to the circle 385a. It can be seen that the distribution 352 peaks around the point 383b, this is partly because the circle 383a intersect the non-zero part of the first array 351 close to where it is broadest, but also because the non-zero part of the first array 351 has highest values in its centre as can be deducted from Fig. 3c. In this embodiment, the first function 351 is integrated along 19 circles, however in other embodiments this number may be larger or smaller.

Furthermore, to take account of sampling problems the first array 351 may be up sampled before it is being integrated. Alternatively / additionally, each array element may be mapped to two or more of the closest circles.

Fig. 3e shows how the first array 351 may be processed, when the movement characteristic is the velocity angle in a two dimensional plane for the part of the blood positioned within the combined measurement region, according to an embodiment. In this embodiment, the two dimensional plane is the XZ plane. The first array 351 is processed by 'integrating it along 19 lines 386a-395a' centred at vz = 0 and vx=0. Thus, each line 386a-395a represents an interval of velocity angles. To make the figure more intelligible only every second line has been provided with a reference number. Each array element of the first array 351 are mapped to the line being closest the centre of the element, and the values of the array elements mapped to particular line are summed to determine the distribution 353. Thus, the value 386b results from summing the values of a illll array elements being closest to the line 386a, the value 387b results from summing the values of a illll array elements being closest to the line 387a, the value 388b results from summing the values of a illll array elements being closest to the line 388a, the value 389b results from summing the values of a illll array elements being closest to the line 389a, the value 390b results from summing the values of a illll array elements being closest to the line 390a, the value 391 b results from summing the values of a illll array elements being closest to the line 391a, the value 392b results from summing the values of a illll array elements being closest to the line 392a, the value 393b results from summing the values of a illll array elements being closest to the line 393a, and the value 394b results from summing the values of a illll array elements being closest to the line 394a. It can be seen that the distribution 353 peaks around the point 388b, corresponding to a flow angle of 90 degrees. In this embodiment, the first array 351 is integrated along 20 lines however in other embodiments this number may be larger or smaller. Furthermore, to take account of sampling problems the first function 351 may be up sampled before it is being integrated. Alternatively / additionally, each array element may be mapped to two or more of the closest lines.

Thus, in the embodiments described in relation to Figs. 3d-f the first array 351 does not directly represent the distribution of the movement characteristic, but the distribution is found by processing the first array 351 . Both distributions 352 353 may simultaneously be determined and displayed to an operator.

Fig. 3g shows how the plurality of selected frequency sets may be mapped directly into an array representing the first movement characteristic, according to an embodiment. In this embodiment the first movement characteristic is the velocity magnitude in the two dimensional XZ plane for the part of the blood positioned within the combined measurement region. The first array 351a is a one dimensional array having 19 array elements. For each frequency set, the mapping function, project the frequency sets two 1 dimensional velocity components into a resulting two dimensional velocity vector and the array index of the target array element are derived from the magnitude of the of the resulting two dimensional velocity vector. Thus, each array element in the first array 351a represents an interval of velocity magnitudes. As an example if the first array 351a represents velocity magnitudes from 0 m/s to 1.9 m/s and the resulting two dimensional velocity vector for a particular frequency set has a magnitude of 0.575 m/s the mapping function will derive the array index of the target element of the particular frequency set to be the sixth array element and the frequency set value of the particular frequency set is added to the sixth array element. The numbers 361a-376a in the array elements of the first array 351a refers to the numbers 361-376 in Fig. 3c and illustrates how the frequency sets are mapped to the array elements. Thus the number 361a refers to the number 361 and shows that the frequency set 332 342 is mapped to the 10^th array element and its frequency set value is added to that element, and so forth. 352a shows a plot of the first array 351a after all frequency sets have been mapped. It can be seen that there is a sampling error in that no frequency sets are mapped to the 14^th array element. Fig. 3h shows how sampling errors may be reduced according to an embodiment. In this embodiment, each frequency set is mapped into two target array elements and the frequency set value is divided between the two target array elements. The frequency set value is preferably divided with a weight determined by the mapping function. The mapping function may determine the weight dependent on the 'distance' between the magnitude of the resulting velocity vector and the 'centre' of each of the target array elements. As an example if the first array 351 b represents velocity magnitudes from 0 m/s to 1.9 m/s and the resulting two dimensional velocity vector for a particular frequency set has a magnitude of 0.575 m/s the mapping function will derive the array indices of the target elements of the particular frequency set to be the sixth and seventh array element and the frequency set value of the particular frequency set is divided between the sixth and seventh array element with a weight determined by the mapping function dependent on the 'distance' between the magnitude of the resulting velocity vector and the 'centre' of each of the target array elements e.g. 75% of the frequency set value may be added to the sixth array element and 25% of the frequency set value may be added to the seventh array element. The numbers 361a-376b in the array elements of the first array 351 b refers to the numbers 361-376 in Fig. 3c and illustrates how the frequency sets are mapped to the array elements. Thus the numbers 361a 361 b refers to the number 361 and shows that the frequency set 332 342 is mapped to the 10^th array element and the 1 1^th array element and its frequency set value is divided between them and so forth. 352b shows a plot of the first array 351 b after all frequency sets have been mapped. The first array 351 b may alternatively represent an velocity angle. This can be achieved by deriving the array indices of the target elements from the direction (angle) of the resulting 2 dimensional velocity vectors.

Fig. 4a shows schematically the resulting first and second axis when the first group of said plurality of received ultrasound echo signals are beamformed using signals from a first sub-array 401 a of the array transducer, and the second group of said plurality of received ultrasound echo signals are beamformed using signals from a second sub-array 401 b of the array transducer, and the first group of said plurality of received ultrasound echo signals are originating from a first group of the plurality of transmitted ultrasound signal, the second group of the plurality of received ultrasound echo signals are originating from a second group of said plurality of transmitted ultrasound signal, and wherein the first group of said plurality of transmitted ultrasound signals are transmitted with the first sub-array 401a of the array transducer and the second group of the plurality of transmitted ultrasound signals are transmitted with fourth the second sub-array 401 b of the array transducer. Fig. 4b shows schematically the resulting first and second axis when the same sub array 401 e are used in transmit and differing sub arrays 401 c 401d are used in receive. The orientation of the first and second axis 408b 409b are given by the centre of their respective effective aperture (the convolution between the transmit aperture and the receive aperture) as is well-know to the skilled person.

Fig. 4c is similar to Fig. 4b but here but here is a fourth group of the plurality of received ultrasound signals beamformed to generate a filtering velocity signal, where the sub array 401 e is used both in transmit and receive to generate the filtering velocity signal. The line 410a shows the resulting fourth axis.

Fig. 4d-f shows an imaging setup enabling 3D estimation. Shown is a 2D array, where fig. 4d shows a top view, Fig. 4e shows a side view (seen from direction 440), and Fig. 4f shows a front view (seen from the direction 441 ). A first sub array 420 is used in both transmit and receive to generate a first velocity signal, a second sub array 421 is used in both transmit and receive to generate a second velocity signal, a third sub array 423 is used in both transmit and receive to generate a third velocity signal, and a fourth sub array 423 is used in both transmit and receive to generate a filtering velocity signal. The line 430 shows the resulting first axis, the line 431 shows the resulting second axis, the line 432 shows the resulting third axis, and the line 433 shows the resulting fourth axis. Here angling is used both in transmit and receive, however the fourth sub array 423 may be used in transmit for generating all velocity signals similar to the measurement setup disclosed in relation to Fig 4b.

Fig. 5a show schematically an ultrasound imaging system 500 configured to estimate the distribution of a first movement characteristic for a part of a fluid positioned within a combined measurement region, the first movement characteristic is selected from the group of movement characteristics consisting of: the velocity magnitude in a two dimensional plane, the velocity magnitude in three dimensional space and the velocity angle in a two dimensional plane. The ultrasound imaging system comprises:

• an transducer 501 ;

• transmit circuitry 503 configured to transmit at least one ultrasound signal using the transducer 501 ;

• receive circuitry 504 configured to receive at least one ultrasound echo signal using the

transducer 501 ;

• a beamformer 50 configured to: generate a first velocity signal, the frequencies of the first velocity signal being dependent on the distribution of velocity components along a first axis for the part of the fluid positioned within a first measurement region centred at a first spatial point; and generate a second velocity signal, the frequencies of the second velocity signal being dependent on the distribution of velocity components along a second axis for the part of the fluid positioned within a second measurement region centred at a second spatial point;

• a processing unit 506 configured to process said first velocity signal together with said second velocity signal to estimate the distribution of the first movement characteristic. The entire functionality or a part of the functionality of the beamformer 505 may be implemented in the processing unit 506 or another processing unit.

Fig. 5b show a flow chart for a method for estimating the distribution of a first movement characteristic for a part of a fluid positioned within a combined measurement region according to an embodiment of the present invention. The first movement characteristic is selected from the group of movement

characteristics consisting of: the velocity magnitude in a two dimensional plane, the velocity magnitude in three dimensional space and the velocity angle in a two dimensional plane, the method comprising the steps of:

transmitting at least one ultrasound signal 510; receiving at least one ultrasound echo signal 51 1 ;

generating a first velocity signal 512, the frequencies of the first velocity signal being dependent on the distribution of velocity components along a first axis for the part of the fluid positioned within a first measurement region centred at a first spatial point; generating a second velocity signal 513, the frequencies of the second velocity signal being dependent on the distribution of velocity components along a second axis for the part of the fluid positioned within a second measurement region centred at a second spatial point; and processing 514 said first velocity signal together with said second velocity signal to estimate the distribution of the first movement characteristic.

Simulations have been performed using Field II [3] [4]. A blood vessel with a vessel radius r=0.5cm and a centre positioned in a depth of d=2.5cm has been simulated. The flow in the vessel has been simulated using the Womersley-Evans pulsatile flow model [5] [6]. The pulsatile flow in the human femoral artery has been imitated. The method disclosed in relation Fig. 3g is used for estimating the distributions i.e. the first array directly represents the distribution. The following imaging parameters have been used: 1 D linear array having 96 elements; centre frequency f0= 3Mhz; width of transducer elements w=c/f0; kerf K=0.035 mm; the central 32 elements are used in transmit; 8 oscillations at the centre frequency are emitted; a transmit focus is placed in a depth of 2.5cm; the first group and the second group of the plurality of received ultrasound echo signals are identical and comprises 150 ultrasound echo signals; pulse repetition frequency fprf= 8979; the left most 32 elements are used for beamforming points of the first velocity signal; the right most 32 elements are used for beamforming points of the second velocity signal; the first spatial point is equal to the second spatial point both being arranged in the centre of the vessel [x=0,y=0,z=2.5cm]; sound velocity c= 1540 m/s. Simulations have been performed with a beam to flow angle of 90 degrees and 75 degrees. Fig. 6 shows the results. The horizontal axis in all the plots / images 601-606 represents time measured in seconds, the vertical axis in plots / images 601 603 605 represent velocity magnitude in the XZ plane measured in m/s, and the vertical axis in plots / images 602 604 606 represent the velocity angle in the XZ plane. The simulated flow is laminar. The plot 605 shows the mean velocity in the central part of the blood vessel through a simulated heart cycle. The plot 606 shows the mean velocity angle through the simulated heart cycle, where the line 607 is for the simulation performed for a beam to flow angle of 75 degrees and the line 608 is for the simulation performed for a beam to flow angle of 90 degrees. It can be seen that there is reverse flow present just after the systolic phase of the simulated heart cycle. A plurality of distributions of said at least one movement characteristic are estimated at consecutive points in time and together displayed. The estimated distributions are shown as images 601-604, where each vertical line correspond to a distribution, each point on each vertical line correspond to a particular value, and the colour of each point illustrates the estimated frequentness of that particular value at the particular point in time i.e. a dark colour shows a high frequentness and light colour shows a low frequentness. Image 601 show estimated distributions of the velocity magnitude in the XZ plane for a beam to flow angle of 90 degrees, image 602 show estimated distributions of the velocity angle in the XZ plane for a beam to flow angle of 90 degrees, image 603 show estimated distributions of the velocity magnitude in the XZ plane for a beam to flow angle of 75 degrees, image 604 show estimated distributions of the velocity angle in the XZ plane for a beam to flow angle of 75 degrees.

A plurality of vectors 607 are shown above image 601 . The vectors 607 show velocity angles determined at particular points in time, each vector being arranged in connection with the distribution of the velocity magnitude determined for the same point in time, whereby a user may in an easy manner both see the distribution of the velocity magnitude and the primary flow direction as a function of time.

It can be seen from image 601-606 that both the distribution of the velocity magnitude in the XZ plane and the distribution of the velocity angle in the XZ plane may precisely be estimated for both a flow angle of 75 degrees and 90 degrees. Furthermore, it is interesting to observer that the distributions of the velocity angle in the XZ plane have an approximate constant width throughout the simulated heart cycles (as it should have since the simulated flow completely laminar). Thus, the width is apparently relatively independent of the absolute velocity of the blood, the acceleration of the blood, and the uniformity of the velocity profile in the blood vessel. Consequently, the distributions of the velocity angle, estimated using embodiments of the present invention, looks like a promising candidate for estimating the amount of turbulence present in blood vessels.

Fig. 7 shows a flow chart for a method for estimating the distribution of a first movement characteristic for a part of a fluid positioned within a combined measurement region according to an embodiment of the present invention. The method starts in step 701 with determining a mapping function for all possible frequency sets. As explained previously the mapping function is only dependent on the measurement setup (the orientation of the first, second and possibly third axis; the emitted centre frequency, the pulse repetition frequency etc.). Next, the method in step 702 determines which frequency sets are to be selected and processed to estimate the distribution of the first movement characteristic. In this embodiment, the only frequency sets that are not selected are the ones having a 'resulting 2 dimensional or 3 dimensional velocity vector' with a 'non-possible' magnitude e.g. a magnitude being larger than the largest possible velocity magnitude in the vessel where measurements are being made. However, in other embodiments all possible frequency sets may be selected. Next, in step 703 for each frequency set a filtering velocity component is determined by projecting the frequency sets resulting 2 dimensional or 3 dimensional velocity vector onto the fourth axis. If filtering is not to be used, step 703 may be skipped whereby the method continues directly to step 704. In step 704, an output array (first array) is initialized so that the value of each array elements is zero. The output array will represent the first movement characteristic. Next in step 705 a counter n is set to the value 1. Then in step 706 data is measured as explained previously thereby resulting in a first velocity signal, a second velocity signal and if filtering is used a filtering velocity signal. Furthermore, if the measurement system is a 3D system a third velocity signal is also generated. Next in step 707, all the velocity signals a fed to an FFT algorithm and their amplitude spectra are found. The method then continues to step 708 where the n'th frequency set (being the first frequency set since n is equal to 1 ) is selected. The mapping functions then maps the frequency set value of the n'th frequency into one or more predetermined (in step 701 ) target array element(s) of the output array, wherein the frequency set value is added to the stored value(s) of said one or more target array element(s). If filtering is used the frequency set value is filtered with a filtering value derived from the filtering velocity component of the n'th frequency set and the frequency spectrum of the filtering velocity signal. Then the method continues to step 709 where it is checked whether the counter n has reached N, where N is the total number of frequency sets selected. If n is below N, the method returns to step 708 increases n by 1 and selects the next frequency set. Alternatively, if n is equal to N, meaning that all selected frequency sets have been mapped to the output array, the output array now represents a finished estimate of the distribution of the first movement characteristic and the method continues to step 710, where the estimate of the distribution of the first movement characteristic is displayed. This may preferably be as a single vertical line in an image as shown in Fig. 6. If previous estimated distributions are being displayed the previous estimated distributions may be moved one position to the right in the image, as is known from standard spectral Doppler displays. The method then returns to step 704, where the output array is re-initialized, continues to step 705 where the counter n is set to 1 , and measures a new set of data in step 706 etc. It is important to note that the selected frequency sets will always be the same (unless the measurements setup is changed) i.e. the selection is not influenced by measured data only the frequency set values of the frequency sets are influenced by the measured data. The above method is computational simple and very robust since it only relies on simple mathematical operations and not complex noise vulnerable and error prone computational methods such as peak detection, pattern recognition, mean frequency detection etc. Although some embodiments have been described and shown in detail, the invention is not restricted to them, but may also be embodied in other ways within the scope of the subject matter defined in the following claims. In particular, it is to be understood that other embodiments may be utilised and structural and functional modifications may be made without departing from the scope of the present invention. In device claims enumerating several means, several of these means can be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims or described in different embodiments does not indicate that a combination of these measures cannot be used to advantage. It should be emphasized that the term "comprises/comprising" when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof. REFERENCES

[1] Estimation of Blood Velocities Using Ultrasound, A Signal Processing Approach, J0rgen Arendt Jensen, Cambridge University Press, New York, 1996, ISBN 0-521-46484-6.

[2] Spectral Velocity Estimation in the Transverse Direction, J0rgen Arendt Jensen, IEEE International Ultrasonics Symposium, Prague, Czech Republic, 2013.

[3] J. A. Jensen and N. B. Svendsen, "Calculation of Pressure Fields from

Arbitrarily Shaped, Apodized, and Excited Ultrasound Transducers,"

IEEE Trans. Ultrason., Ferroelec, Freq. Contr., vol. 39, pp. 262-267,

1992.

[4] J. A. Jensen, "Field: A program for simulating ultrasound systems,"

Med. Biol. Eng. Comp., vol. 10th Nordic-Baltic Conference on Biomedical Imaging, Vol. 4, Supplement 1 , Part 1 , pp. 351-353, 1996.

[5] J. R. Womersley, "Oscillatory motion of a viscous liquid in a thin-walled

elastic tube. I: The linear approximation for long waves," Phil. Mag., vol. 46, pp. 199-221 , 1955.

[6] W. W. Nichols and M. F. O'Rourke, McDonald's Blood Flow in Arteries, Theoretical, Experimental and Clinical Principles. Philadelphia: Lea & Febiger, 1990.

Claims

1. A method for estimating the distribution of a first movement characteristic for a part of a fluid positioned within a combined measurement region, said first movement characteristic is selected from the group of movement characteristics consisting of: the velocity magnitude in a two dimensional plane, the velocity magnitude in three dimensional space and the velocity angle in a two dimensional plane, wherein the distribution of the first movement characteristic is a 1 dimensional distribution, the method comprising the steps of:

• transmitting at least one ultrasound signal;

· receiving at least one ultrasound echo signal;

wherein the method further comprises the steps of:

• processing said first velocity signal together with said second velocity signal to estimate the distribution of the first movement characteristic wherein the distribution of the first movement characteristic is estimated without combining individual estimates of velocity magnitudes or velocity angles made at different points in time or at different spatial locations.

2. A method according to claim 1 , wherein the first velocity signal is processed together with the second velocity signal by:

· determining the frequency spectrum for the first velocity signal and the frequency spectrum for the second velocity signal;

· processing said plurality of frequency sets to estimate the distribution of the first movement characteristic

3. A method according to any one of claims 1 or 2, wherein

ultrasound signals at different points in time using an array transducer;

ultrasound echo signals at different points in time using the array transducer;

• the first velocity signal is generated by beamforming a first group of the plurality of received ultrasound echo signals at the first spatial point;

received ultrasound echo signals at the second spatial point.

4. A method according to any one of claims 1 to 3, wherein the method further comprises the step of generating a third velocity signals, the frequencies of the third velocity signal being dependent on the distribution of velocity components along a third axis for the part of the fluid positioned within a third measurement region centred at a third spatial point.

5. A method according to any one of claims 1 to 4, wherein said first velocity signal is processed together with said second velocity signal by:

· selecting a plurality of frequency sets, each frequency set comprising a frequency from both the frequency spectrum of the first velocity signal and the frequency spectrum of the second velocity signal;

6. A method according to claim 4, wherein said first velocity signal is processed together with said second velocity signal and said third velocity signal by:

· selecting a plurality of frequency sets, each frequency set comprising a frequency from both the frequency spectrum of the first velocity signal, the frequency spectrum of the second velocity signal and the frequency spectrum of the third velocity signal; and

7. A method according to any one of claims 5 to 6, the plurality of frequency sets are processed together by mapping using a mapping function each frequency set into one or more target array element(s) of a first array, wherein a frequency set value is added to the stored value(s) of said one or more target array element(s), said frequency set value being dependent on the amplitude of the frequencies in the frequency set.

8. A method according to any one of claims 2 and 5 to 7, wherein for each of the plurality of frequency sets, the frequency from the frequency spectrum of the first velocity signal corresponds to a specific velocity component along said first axis, the frequency from said frequency spectrum of said second velocity signal corresponds to a specific velocity component along said second axis, and wherein the mapping function for each frequency set project the frequency sets at least two 1 dimensional velocity components into a resulting 2 dimensional or 3 dimensional velocity vector, the array indices of said one or more target array element(s) being derived from said resulting velocity vector.

9. A method according to any one of claims 7 to 8, wherein the frequency set value is determined by multiplying a first value with a second value wherein the first value is dependent on the amplitude of the frequency from the frequency spectrum of the first velocity signal and the second value is dependent on the amplitude of the frequency from the frequency spectrum of the second velocity signal.

10. A method according to claim 9, wherein the first value and the second value is the amplitude or the amplitude multiplied by itself one or more times.

1 1 . A method according to claim 10, wherein the frequency spectrum of the first velocity signal and the frequency spectrum of the second velocity signal are normalized before the first value and the second value are found.

12. A method according to claim 1 1 , wherein the frequency spectrum of the first velocity signal and the frequency spectrum of the second velocity signal are normalized so that for each frequency spectrum:

• where N is the number of frequency components for the spectrum, A_n is the n'th frequency component and k is a positive constant.

13. A method according to any one of claims 5 to 12, wherein it is predetermined which frequency sets are to be selected before the at least one ultrasound signal is transmitted.

14. A method according to any one of claims 5 to 13, wherein the method further comprises the step of generating a filtering velocity signal, the frequencies of the filtering velocity signal being dependent on the distribution of velocity components along a fourth axis for the part of the fluid positioned within a fourth measurement region centred a fourth spatial point, wherein the frequency spectrum for the filtering velocity signal is determined and for each frequency set the frequency set value is filtered with a filtering value derived from the frequency spectrum of the filtering velocity signal.

15. A method according claim 14, wherein the filtering value derived from the frequency spectrum of the filtering velocity signal for each frequency set is a non-Boolean value e.g. the filtering value derived may be a particular value our of at least 3 possible values, preferably out of at least 4, 8 , 16, 32, 64, 128 or 256 possible values.

16. A method according to any one of claims 14 to 15, wherein the frequencies from the frequency spectrum of said filtering velocity signal corresponds to specific velocity components along said fourth axis, wherein the resulting 2 dimensional or 3 dimensional velocity vector (of the frequency set) is projected onto said fourth axis thereby resulting a filtering velocity component, wherein the filtering value is derived from the filtering velocity component and the frequency spectrum of the filtering velocity signal.

17. A method according to any one of claim 14 to 16, wherein the filtering value is determined without comparing the amplitudes of the two or three frequencies in the frequency set with each other or with an amplitude of a frequency of the filtering velocity signal.

18. A method according to any one of claims 1 to 17, wherein the angle between said first axis and said second axis is below 40 degrees e.g. below 35, 30, 25, or 20 degrees.

19. A method according to any one of claims 1 to 18, wherein both the distribution of a first movement characteristic and a second movement characteristic are estimated, both the first and the second movement characteristic are selected from the group of movement characteristics consisting of: an velocity magnitude in a two dimensional plane, a three dimensional velocity magnitude and a velocity angle in a two dimensional plane.

20. A method according to claim 19, wherein said first movement characteristic is a two or three dimensional velocity magnitude and said second movement characteristic is a velocity angle in a 2 dimensional plane.

21. A method according to any one of claims 20 to 21 , wherein the first and the second movement characteristic is displayed on a display simultaneously.

22. A method according to any one of claims 1 to 21 , wherein a plurality of distributions of said at least one movement characteristic are estimated at consecutive points in time and together displayed.

23. A method according to claim 22, wherein the first movement characteristic is a velocity magnitude, a plurality of distributions of said velocity magnitude are determined at a plurality of consecutive points in time and displayed, and for at least some of said plurality of consecutive points in time a velocity angle is determined and displayed as a vector, each vector being arranged in connection with the distribution of said velocity magnitude determined for the same point in time, whereby a user may in an easy manner both see the distribution of the velocity magnitude and the primary flow direction as a function of time.

24. A method according to any one of claims 3 to 23, wherein the first group of said plurality of received ultrasound echo signals are beamformed using element signals from a first sub-array of said array transducer, and the second group of said plurality of received ultrasound echo signals are beamformed using element signals from a second sub-array of said array transducer.

25. A method according to any one of claims 3 to 24, wherein the first group of said plurality of received ultrasound echo signals are originating from a first group of said plurality of transmitted ultrasound signal, the second group of said plurality of received ultrasound echo signals are originating from a second group of said plurality of transmitted ultrasound signal, and wherein said first group of said plurality of transmitted ultrasound signals are transmitted with a third sub-array of said array transducer and said second group of said plurality of transmitted ultrasound signals are transmitted with fourth a sub-array of said array transducer.

26. A method according to any one of claims 3 to 25, wherein it is selected, dependent on the distance from the array transducer to the first and second measurement region, whether:

27. An ultrasound imaging system configured to estimate the distribution of a first movement characteristic for a part of a fluid positioned within a combined measurement region, said first movement characteristic is selected from the group of movement characteristics consisting of: the velocity magnitude in a two dimensional plane, the velocity magnitude in three dimensional space and the velocity angle in a two dimensional plane, wherein the distribution of the first movement characteristic is a 1 dimensional distribution said ultrasound imaging system comprising:

• an transducer;

a processing unit configured to process said first velocity signal together with said second velocity signal to estimate the distribution of the first movement characteristic wherein the distribution of the first movement characteristic is estimated without combining individual estimates of velocity magnitudes or velocity angles made at different points in time or at different spatial locations.

28. An ultrasound imaging system according to claim 27, wherein the processing unit is configured to:

• determine the frequency spectrum for the first velocity signal and the frequency spectrum for the second velocity signal;

• processing said plurality of frequency sets to estimate the distribution of the first movement characteristic;

29. An ultrasound imaging system according to any one of claims 27 to 28, wherein

• the transducer is an array transducer;

• the beamformer is configured to generate the second velocity signal by beamforming a second group of the plurality of received ultrasound echo signals at the second spatial point.

30. An ultrasound imaging system according to any one of claims 27 to 29, wherein the beamformer is further configured to generate a third velocity signals, the frequencies of the third velocity signal being dependent on the distribution of velocity components along a third axis for the part of the fluid positioned within a third measurement region.

31. An ultrasound imaging system according to any one of claims 27 to 30, wherein the processing unit process said first velocity signal together with said second velocity signal by: • determining the frequency spectrum for the first velocity signal and the frequency spectrum for the second velocity signal;

32. An ultrasound imaging system according to claim 30, wherein the processing unit process said first velocity signal together with said second velocity signal and said third velocity signal by:

33. An ultrasound imaging system according to any one of claims 31 to 32, wherein the plurality of frequency sets are processed together by mapping using a mapping function each frequency set into one or more target array element(s) of a first array, wherein a frequency set value is added to the stored value(s) of said one or more target array element(s), said frequency set value being dependent on the amplitude of the frequencies in the frequency set.

34. An ultrasound system according to any one of claims 31 to 33, wheerin for each of the plurality of frequency sets, the frequency from the frequency spectrum of the first velocity signal corresponds to a specific velocity component along said first axis, the frequency from said frequency spectrum of said second velocity signal corresponds to a specific velocity component along said second axis, and wherein the mapping function for each frequency set project the frequency sets at least two 1 dimensional velocity components into a resulting 2 dimensional or 3 dimensional velocity vector, the array indices of said one or more target array element(s) being derived from said resulting velocity vector.

35. An ultrasound imaging system according to any one of claims 31 to 34, wherein the frequency set value is determined by multiplying a first value with a second value, wherein the first value is dependent on the amplitude of the frequency from the frequency spectrum of the first velocity signal and the second value is dependent on the amplitude of the frequency from the frequency spectrum of the second velocity signal.

36. An ultrasound imaging system according to claim 35, wherein the first value and the second value is the amplitude or the amplitude multiplied by itself one or more times.

37. An ultrasound imaging system according to any of claims 35 to 36, wherein the processing unit is configured to normalize the frequency spectrum of the first velocity signal and the frequency spectrum of the second velocity signal before the first value and the second value are found.

38. An ultrasound imaging system according to claim 37, wherein the processing unit is configured to normalize the frequency spectrum of the first velocity signal and the frequency spectrum of the second velocity signal so that for each frequency spectrum:

39. An ultrasound imaging system according to any one of claims 27 to 39, wherein the beamformer is further configured to generate a filtering velocity signal, the frequencies of the filtering velocity signal being dependent on the distribution of velocity components along a fourth axis for the part of the fluid positioned within a fourth measurement region centred a fourth spatial point, wherein processing unit is further configured to determine the frequency spectrum for the filtering velocity signal and for each frequency set filter the frequency set value with a filtering value derived from the frequency spectrum of the filtering velocity signal.

40. An ultrasound imaging system according to claim 38, wherein the filtering value derived from the frequency spectrum of the filtering velocity signal for each frequency set is a non-Boolean value e.g. the filtering value derived may be a particular value our of at least 3 possible value, preferably out of at least 4, 8 , 16, 32, 64, 128 or 256 possible values.

41 . An ultrasound imaging system according to any one of claims 27 to 40, wherein the angle between said first axis and said second axis is below 40 degrees e.g. below 35, 30, 25, or 20 degrees.

42. An ultrasound imaging system according to any one of claims 27 to 41 , wherein the ultrasound imaging system is configured to estimate both the distribution of a first movement characteristic and a second movement characteristic, both the first and the second movement characteristic are selected from the group of movement characteristics consisting of: an velocity magnitude in a two dimensional plane, a three dimensional velocity magnitude and a velocity angle in a two dimensional plane.

43. An ultrasound imaging system according to claim 42, wherein said first movement characteristic is a two or three dimensional velocity magnitude and said second movement characteristic is a velocity angle in a 2 dimensional plane.

44. An ultrasound imaging system according to claim 43, wherein the ultrasound imaging system further comprises a display, wherein the first and the second movement characteristic is displayed on the display simultaneously.

45. A computer program product comprising program code means adapted to cause an ultrasound imaging system to perform the steps of the method according to any one of claims 1 to 26 , when said program code means are executed.

46. A computer-readable medium having stored thereon program code means adapted to cause an ultrasound imaging system to perform the steps of the method according to any one of claims 1 to 26, when said program code means are executed.

47. A method for estimating the distribution of a first movement characteristic for a part of a fluid positioned within a combined measurement region, the method comprising the steps of:

• transmitting at least one ultrasound signal;

• receiving at least one ultrasound echo signal;