WO2010018495A1

WO2010018495A1 - Colour flow imaging in x-ray

Info

Publication number: WO2010018495A1
Application number: PCT/IB2009/053432
Authority: WO
Inventors: Odile Bonnefous; Raoul Florent; Vincent M. A. Auvray
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2008-08-13
Filing date: 2009-08-06
Publication date: 2010-02-18
Anticipated expiration: 2011-02-13

Abstract

In order to provide an improved method in an X-ray imaging system for representing the blood flow, for example for vessel analysis purposes, a method for representing blood flow-related information in X-ray images comprises the steps of generating a digital subtraction angiography DSA image (12) by the means of generating a first image sequence with an X-ray imaging system, generating a second image sequence with the X-ray imaging system at a contrast phase, whereby in the second image sequence a part of the subject has a different contrast than in said first image sequence, determining a corresponding image of the first sequence to each image of the second sequence, using a time registration process and subtracting first corresponding image from second corresponding image generating the DSA image. Further, a vessel registration (14) includes defining a reference vessel image at a reference time after DSA through the detection of sufficient contrast or by the user, registration between the reference vessel image and a current vessel image and warping the current vessel image towards the reference vessel image generating an output registered contrast image. Then the output sequence is time filtered to extract time contrast modulation. Further, vector velocity fields (18) are computed applying motion estimation processing on the time filtered sequence (16) and the velocity data is transformed into a colour flow image (20). Also, an X-ray imaging system is provided for representing blood flow-related information in X-ray images by displaying a coulour flow image superimposed over an X-ray image.

Description

COLOUR FLOW IMAGING IN X-RAY

FIELD OF THE INVENTION

The invention relates to representing blood flow-related information in X- ray images. In particularly, the invention relates to a method for representing blood flow- related information in X-ray images, to an X-ray imaging system for representing blood flow-related information in X-ray images, to a computer program element and to a computer-readable medium.

BACKGROUND OF THE INVENTION Information concerning the blood flow is needed for different reasons.

For example, vessel maps are commonly used for neurological interventions or for the visualization of aneurysm structures. Information on the blood flow quality is needed before an intervention, such as a stent placement for instance, for planning purposes and after the intervention, e.g. the stenting, for an outcome control. But the recovery of satisfactory physiological blood flow cannot be verified. Usually, echography is the preferred imaging technique to get quantitative flow information and to image them. Ultrasound is used for exams of big vessels like carotid artery, abdominal aorta and lower limbs arteries. However, it is difficult to use ultrasound to image neurological vessels, due to the skull bone barrier. It is neither possible to image coronary flows due to the fast motion of the heart in front of the ultrasound probe and the rib cage barrier. With contrast injection, X-ray produces dynamic sequences of flowing contrast agent within vessels. These images allow the detection and the localisation of vessel structures during intervention, useful for stent placement for instance. In the same way, it allows to get vessel maps for neurological intervention and visualization of aneurysm structures. Using contrast agents, X-ray is able to produce images of the anatomy of brain vasculature and coronary arteries during screening and interventional procedures. But it has shown that with common techniques in X-ray procedures, blood flow images are produced in such a way that the qualitative and quantitative information about the blood flow is difficult or even impossible to retrieve. Hence, there is a strong need for vascular functional imaging within an X-ray modality for cardiovascular exams and procedures that can be used for screening, but also for comparing the blood flow quality before and after an intervention. The document JP 2004-321390 A describes a method for indicating a blood flow rate with the means of colours where the pass through of an injected contrast medium is measured to calculate the blood flow rate. For reasons of simplification the arteries are assumed to be cylindrical and further the length of the pathway and the diameter of the blood vessel have to be obtained before X-ray imaging. In US 2007/0098134 Al using a contrast medium the time-concentration curve of an artery is compared with a time-concentration curve of an object region to calculate the delay time of the bloodstream for an object region, which is shown in colour on a blood flow map. Furthermore, WO 2004/008970 Al describes the addition of colours to greyscale images to highlight the tip of a catheter for example. Further also the use of colour for indicating the velocity of a contrast agent without any detailed instructions is mentioned in the latter document.

SUMMARY OF THE INVENTION

The object of the invention is to provide an improved method in an X-ray imaging system for representing the blood flow, for vessel analysis purposes for example. The object is reached with a method for representing blood flow-related information in X-ray images comprising the steps of generating a digital subtraction angiography DSA image by the means of generating a first image sequence, i.e. a pre- contrast image or mask image, with an X-ray imaging system, and generating a second image sequence with the X-ray imaging system at a contrast phase. For example, the contrast phase may include the introduction of a contrast medium into the subject to be examined. Further, a corresponding image of the first sequence to each image of the second sequence is determined, using a time registration process and subtracting first corresponding image from second corresponding image generating the DSA image; vessel registration including defining a reference vessel image at a reference time after DSA through the detection of sufficient contrast or by the user, registration between the reference vessel image and a current vessel image, warping the current vessel image towards the reference vessel image generating an output registered contrast image; time filtering the output sequence to extract time contrast modulation; computing vector velocity fields applying motion estimation processing on the time filtered sequence and transforming the velocity data into a colour flow image. One of the advantages of the invention is that the operator, e.g. a physician in a clinic, is provided with information about the blood flow that is essential or at least very useful for example for further treatments or for analysis reasons. Due to the use of colour it is possible to provide detailed information such that said information can be perceived and used by the user in a very effective and timesaving way. This means that no additional devices such as an echography device are necessary for providing quantitative blood flow information when using an X-ray imaging device. This greatly enlarges the possible field of applications for X-ray imaging devices. The vessel registration is necessary to differentiate global motion of vessels, which is caused by physiological motion, from flow motion within the vessel structures. The step of warping the current vessel image towards the reference image is using a registration process where the shape and the contours of the vessel structure are the elements considered to achieve the matching. The contrast variations within the vessel are then preserved. The registration and warping operations stabilize the artery and remove the global motion of the vessel, this latter being attached to a moving organ. After this step, the observed time/space variations of the contrast are due only to the motion of the contrast within the vessel and not to the moving vessel. A sequence is built showing a flowing liquid in a motionless 'pipe'. This stabilization is necessary prior applying the time filtering and extracting sequence characteristics allowing only fluid motion estimation. While time filtering the sequence, the tube must not move anymore. With the registration and warping step it is possible to get rid of the motion in order to be in a better position to find the fluid motion. Fluid motion is then estimated in applying the time filtering. Computing vector velocity fields generates different aspects of blood flow information. Being able to measure directional local flow fields, it is then possible to derive any kind of parameter describing the flow characteristics. As a result the inventive method enables to extract a full flow field. Hence, the method produces a full flow field. In another embodiment according to the invention the colour flow image is superimposed to the reference vessel X-ray image taken frozen at the reference time. This allows presenting additional information that is included in the X-ray image, which results in a very significant image with high information density. The effect of the superimposition is that coloured pixel representing certain flow characteristics appear within the X-ray image that is traditionally only in black and white, i.e. in greyscale. This superimposition of coloured pixels can be accomplished by a covering overlaying, i.e. an opaque colour application over the greyscale image or by a semitransparent overlaying, i.e. where the greyscale image is still visible to a certain degree. Hence, as an effect the greyscale image partly appears coloured at least in some pixels. Still further preferred is an embodiment wherein the colour flow image reproduces characteristics of ultrasound colour flow imaging, at least concerning colour coding and flow direction. This proposed imaging solution follows the ultrasound imaging example because ultrasound colour flow imaging is a reference for physicians and a very popular visualization technique that is broadly used and relied upon. Emulating this visualization method permits gaining the acceptance of X-ray clinicians for this new imaging technique. Usually ultrasound uses blue/red colour maps. Flow direction is referenced by the ultrasound beam orientation, because ultrasound is limited to the velocity component parallel to the ultrasound beam. Red pixels correspond to blood moving away from the probe and blue pixels correspond to blood moving closer to the probe (or the contrary, this choice being defined by the user). The coded velocity value is then the projection of the velocity vector on the ultrasound beam direction. The colour (red/blue) codes the flow direction and the brightness codes the amplitude of the velocity projection. In theory, knowing the angle between the vessel axis and the ultrasound beam, it is possible to estimate the velocity value. This assumes however that velocity vectors are parallel to the vessel axis, which is not the case for disturbed flows created by stenoses, bifurcations, stents and the like. Moreover, reverse flows produced by these anatomical singularities are naturally displayed in the opposite colour, and then easily detected. As an effect, a coloured image appears that is colour coded just like an ultrasound image. But this coloured image appears in an X-ray environment, where images are commonly greyscale, without the necessity of additional ultrasound devices. In case of a superimposition over an X-ray image, a combination of an X-ray image and an ultrasound colour-coded image appears. The effect is similar to the one described above, i.e. a greyscale image shows coloured pixels at least in certain areas, but the colours are applied according to the ultrasound colour code. This means that they represent the same type of information in the X-ray image. In one preferred embodiment of the invention the vector velocity field is computed by the means of optical flow methods. Detailed information to be evaluated in further processing steps can be achieved with optical flow methods, especially with multiscale optical flow methods. Optical flow processing in particular produces flow sequences, which contain velocity maps in time and space without the necessity to rely on models such as flow models or geometrical models.

In a further preferred embodiment the contrast phase is submitted to the arterial pulsed pressure creating a periodic time modulation of the contrast density and a time filter is adjusted such that the reference vessel image and/or the current vessel image are synchronized with the cardiac period of a subject to be examined. It has been shown that the contrast phase within an artery submitted to the arterial pulsed pressure creates a periodic time modulation of the contrast density. With the blood flow, this modulation is transported in the arterial network, creating a kind of "contrast wave" pattern. By tuning the characteristics of a dedicated time filter to the cardiac period it is possible to enhance the contrast wave pattern. This sequence is an attractive visualization of the flowing contrast and may be the input for the velocity vector field estimation step.

Still further preferred is an embodiment wherein a vessel direction map is created for the colour flow image using the vessel reference image comprising the steps of vessel segmentation computing vessel orientation using a standard oriented ridge filter and extending the vessel orientation on a vessel map. This enables an improved consideration of the actual flow characteristics since the velocity vectors can be set into relation with the vessel direction.

In a preferred embodiment components of the velocity field being longitudinal in respect of the vessel axis are computed, a threshold for the blood flow velocity is determined producing artificial aliasing patterns and the longitudinal flow is colour encoded for the colour flow image using the velocity threshold. These artificial patterns mimic an intrinsic aliasing effect, which occurs for high velocities in Ultrasound CFI keeping a good colour dynamics for normal flows and enhancing the high flows that are typically associated to tight stenoses. To produce this aliasing phenomenon if velocity values are abnormally high, in one embodiment the threshold is corresponding to normal maximum blood flow velocities.

To give more detailed flow information, in one embodiment it is foreseen, that components of the velocity field being transverse corresponding to a projection perpendicular to the vessel axis are computed and the transverse flow is colour encoded for the colour flow image.

In a still further preferred embodiment of the invention flow variance and/or flow vorticity of the velocity field are computed and colour encoded for the colour flow image. This provides the user with very detailed information about the actual blood flow that can be compared to the information density achievable with Ultrasound CFI.

According to the invention, the object s also achieved with an X-ray imaging system for representing blood flow-related information that comprises a source of X-ray radiation, an X-ray image detection module, a data processing unit and a display. According to the invention, the data processing unit is arranged to perform a time registration process to determine a corresponding image of a first sequence to each image of a second sequence and to subtract the first corresponding image from the second corresponding image;. Further, the data processing unit is arranged to perform a registration between a reference vessel image and a current vessel image and to warp the current vessel image towards the reference vessel image to generate an output registered contrast image. Furthermore, the data processing unit is arranged to compute vector velocity fields applying motion estimation processing and to transform the velocity data into a colour flow image. Finally, the display is arranged to display the colour flow image. In a preferred embodiment of the invention the display is provided to display the colour flow image superimposed to an X-ray image.

According to a further exemplary embodiment of the present invention, a computer program element is provided that is characterized by being adapted to perform the steps of the method according to one of the preceding embodiments.

This computer program element might therefore be stored on a computing unit, which might also be part of an embodiment of the present invention. This computing unit may be adapted to perform or induce the performing of the steps of the method described above. Moreover, it may be adapted to operate the components of the above described-X-ray imaging system. The computing unit can be adapted to operate automatically and/or to execute the orders of a user.

This embodiment of the invention covers both a computer program, that right from the beginning uses the invention, and a computer program, that by means of an update turns an existing program into a program that uses the invention.

Further on, the computer program element might be able to provide all necessary steps to fulfil the procedure of representing blood flow-related information in X-ray images as described above.

According to a further embodiment of the present invention, a computer- readable medium is presented wherein the computer-readable medium has a computer program element stored on it which computer program element is described by the preceding section.

According to a further embodiment of the present invention, a medium for making a computer program element available for downloading is provided, which computer program element is arranged to perform the method according to one previously described embodiment of the invention.

An embodiment of the invention will be described hereinafter with reference to the figures. The above-mentioned aspects and other aspects will be apparent from this description. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 schematically describes an X-ray imaging device according to the invention;

Fig. 2 schematically shows the general processing scheme; and

Fig. 3 schematically presents the operations corresponding to the colour flow mapping procedure of figure 2.

DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1 schematically shows an X-ray imaging system 40 for representing blood flow-related information. Note that the example shown is a so-called C-type system. Nevertheless, the invention also relates to other types of X-ray imaging systems. A source of X-ray radiation 42 is provided to generate X-ray radiation. Further, a table 44 is provided to receive a subject to be examined, and an X-ray image detection module 46 is located opposite the source of X-ray radiation 42, i.e. during the radiation procedure, the subject is located between the source of X-ray radiation 42 and the detection module 46. The latter is sending image data to a data processing unit 48, which is connected to both the detection module 46 and the source 42. Furthermore a display 50 is arranged in the vicinity of the table 44 to display information to the person operating the X-ray imaging system, i.e. a clinician. Preferably the display 50 is movably mounted in order for an individual adjustment depending on the examination situation. Also, an interface unit 52 is arranged to input information by the user. Basically the image detection module 46 generates images that are further processed in the data processing unit 48, said procedure being described more detailed in the following. As a result of the process in a preferred embodiment of the invention a colour flow image is displayed on the display 50 to the clinician. As clinicians are usually trained to read images according to specific coding, such as colour coding, the colour flow image displayed is adapted to such a coding. More specifically, the image is adapted to the colour coding used in ultrasound colour flow imaging. In other words, the colour flow image reproduces the characteristics of ultrasound colour flow imaging, at least concerning colour coding and flow direction. Preferably the image displayed on the display 50 is a colour image superimposed to an X-ray image which is commonly in black and white, respectively in greyscale.

For building up a dynamic colour flow imaging sequence according to the invention, the following steps are provided. Figure 2 presents the general processing scheme, corresponding to these steps. Prior to the actual processing steps, a DSA operation 12 is provided. This operation is necessary to extract a contrast signal. Two image sequences are used as an input data: a first sequence / _pre(t) is acquired before a contrast phase, i.e. for example a contrast injection. Then a contrast agent may be injected or introduced in a suitable manner into the vessel to be examined. A second sequence I_ca (t) is acquired during the contrast injection. For each image of the second sequence I_ca (t) , a corresponding image of the first sequence / _pre(t) is determined, using an adequate time registration process.

Then, the subtraction is performed between these two images, producing a DSA image A(t) , which contains only the contrasted image of the vessel.

Further, vessel structures must be registered to cancel the motion of the vessels. This means that it is necessary to differentiate global motion of the vessels, due to physiological motion, from flow motion within the vessel structures. Therefore a vessel registration 14 is performed between a reference vessel image A(t_r) chosen after the DSA procedure and a current vessel image A(t) . The reference image A(t_r ) may be chosen either by the user or through the detection of sufficient contrast filling. The current vessel image A(t) is then warped towards A(t_r ) , using a registration process where the shape and the contours of the vessel structure are the elements considered to achieve the matching. Contrast variations within the vessel structure are then preserved. The output registered contrast image is called A_βow (t) . Further, the output sequence is time filtered to extract time t contrast modulation. It is noted that the motion estimation involved performing these registration and warping operations concerns the tube motion. The inventive method results in the possibility to differentiate vessel motion from fluid motion. With respect to registration and warping, several approaches for a transformation of coordinates may be envisaged like block matching or parametric motion estimation.

Then, in a preferred embodiment, a time filtering process 16 of the sequence is used to enhance the moving components of the contrast. It has been shown that the contrast injection within an artery submitted to the arterial pulsed pressure creates a periodic time modulation of the contrast density. With the blood flow, this modulation is transported in the arterial network, creating a kind of "contrast wave" pattern. It is possible to tune the characteristics of a dedicated time filter to the cardiac period, and enhance the contrast wave pattern, creating the sequence A_βow (t) . This sequence is an attractive visualization of the flowing contrast and is an input for the next step, which is a contrast flow velocity vector field estimation step 18.

In the velocity vector field estimation step 18 velocity vector field representing contrast motion within vessels is extracted with an adequate Optical Flow

(OF) method. The sequences A_βow (t) or A_βow (t) are used to compute the velocity vector field. Preferably, Multiscale Optical Flow (MOF) methods are implemented to estimate the vector flow field. The output of the OF operation is then used to create the colour velocity image. Even though other motion estimations are also imaginable to estimate velocities, such as block matching or local affine motion modelling, it has been found that optical flow can be specifically tailored to the problem of motion estimation. Multiscale optical flow allows to gradually increase the resolution of the flow estimation. The latter is a very important aspect to detect local vorticity of the flow for example (also, see below).

So far, the use of successive image processing steps described above, allows the extraction of the blood velocity vector field. Finally, in order to provide this information to the user, an adequate colour mapping transformation 20 is provided. The colour image encoding 20 is schematically shown in figure 3 and described in more detail below.

For a better understanding it may be beneficial to recall characteristics of ultrasound colour flow imaging (Ultrasound CFI). One aspect to be considered is the flow direction. Flow direction is referenced by the ultrasound beam orientation, because ultrasound is limited to the velocity component parallel to the ultrasound beam. Usually Ultrasound CFI uses Blue/Red colour maps. Red pixels correspond to blood moving away from the probe. Blue pixels correspond to blood moving closer to the probe. Of course this colour coding can be the contrary or altered somehow depending on the needs of the user.

The coded velocity value is then the projection of the velocity vector on the ultrasound beam direction. The colour (red/blue) codes the flow direction and the brightness codes the amplitude of the velocity projection. In theory, knowing the angle between the vessel axis and the ultrasound beam, it is possible to estimate the velocity value. This assumes however that velocity vectors are parallel to the vessel axis, which is not the case for disturbed flows created by stenoses, bifurcations, stent and the like. Moreover, reverse flows produced by these anatomical singularities are naturally displayed in the opposite colour, and then easily detected.

Another aspect is that Ultrasound CFI is submitted to an intrinsic aliasing effect that occurs for high velocities. This aliasing allows keeping a good colour dynamics for normal flows and enhancing the high flows typically associated to tight stenoses. This natural feature is strongly used by physicians.

Further, disturbed flows may be of interest. Therefore other colour images are also produced by Ultrasound CFI. So-called flow variance indexes, measured by colour Doppler techniques, may be displayed. High variance corresponds to disturbed flows, produced by flow jets or other singularities. It may be associated to an aggressive interaction between flow and arterial wall. Usually, velocity variance is coded with a green colour. It may be also mixed to the velocity map, producing a red-yellowish and blue-greenish look up table to image velocity and velocity variance together. For the X-ray CFI coding according to the invention, in the embodiment shown as an example only those characteristics of Ultrasound CFI are reproduced when they are of clinical advantage. Of course, in case there are limitations they can be transformed in a suitable manner.

Hence, flow direction, aliasing thresholds and disturbed flows are aspects described in the following with respect to figure 3. With optical flow processing, it is possible to obtain at least two velocity components. It is then more satisfactory to code flow direction parallel to the vessel axis rather than parallel to a hypothetical ultrasound beam. This vessel direction may vary in the image and the projection direction varies with it. As an example, red colour may be chosen for the most likely flow direction and blue colour for the opposite direction corresponding to reverse flows. The brightness of the colour represents the amplitude of the velocity component parallel to the vessel axis.

Further, artificial aliasing patterns are produced, using a velocity threshold V_th . This threshold may correspond to normal maximum velocity, in order to produce this aliasing phenomenon if velocity values are abnormally high. The aliased velocity V is computed through a modulo operation in the following way:

V = kV_th + V , k e Z

V < V«

'2

The aspect of disturbed flows comprises velocity variance, transverse velocity corresponding to projection perpendicular to the vessel axis and vorticity. According to the invention these aspects are envisaged to be colour coded as well. Figure 3 presents the operations corresponding to these imaging options.

In a vessel geometry determination 22 a vessel direction map D necessary for flow projection vector is computed using the vessel reference image A(t_r ) . After vessel segmentation, the vessel orientation is computed using standard oriented ridge filters, and extended on the vessel map, creating the vessel direction map D . This operation is performed one time for the complete sequence.

Further, longitudinal V_longltudmal (t) and transverse V_transverse (t) components of the velocity field are computed in a velocity projection step 24. V_longltudmal{t) is colour coded using the aliasing threshold V_th . Moreover, velocity variance, i.e. flow variance, is computed in a step 26. Vorticity is computed in a step 28. Transverse velocity variance, and vorticity are classically colour coded with dedicated colour maps in respective steps 30, 32, 34 and 36. Finally an image composition 38 consists in the superimposition of the colour flow image over the grey X-ray image I_ca (t_r ) frozen at time t_r . This superimposed image tends to mimic colour flow imaging sequences produced by ultrasound modality but shows additional information by the means of the X-ray image as well. As the invention provides the creation of a dynamic colour imaging sequence showing blood velocity values or blood flow characteristics within arteries, it shall be noted, that it is not necessary to emulate every characteristic of Ultrasound Colour Flow Imaging, but at least flow direction and its colour coding should be taken into account. Among other aspects, the colour flow imaging according to the invention offers cardiologists and neurologists a way of evaluating vascular lesions, allowing to achieve interventions with better control on quality of resulting blood flow. While the invention has been illustrated and described in details in the drawings and forgoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

Claims

CLAIMS:

1. A method for representing blood flow-related information in X-ray images comprising the steps of generating a digital subtraction angiography DSA image (12) by the means of generating a first image sequence, - generating a second image sequence with the X-ray imaging system at a contrast phase, whereby in the second image sequence a part of the subject has a different contrast than in said first image sequence, determining a corresponding image of the first sequence to each image of the second sequence, using a time registration process and - subtracting first corresponding image from second corresponding image generating the DSA image; vessel registration (14) including defining a reference vessel image at a reference time after DSA through the detection of sufficient contrast or by the user, - registration between the reference vessel image and a current vessel image, warping the current vessel image towards the reference vessel image generating an output registered contrast image; time filtering the output sequence to extract time contrast modulation (16); computing vector velocity fields (18) applying motion estimation processing on the time filtered sequence and transforming the velocity data into a colour flow image (20).

2. A method according to claim 1, wherein the colour flow image is superimposed to the reference vessel X-ray image taken frozen at the reference time.

3. A method according to claim 2, wherein the colour flow image reproduces characteristics of ultrasound colour flow imaging, at least concerning colour coding and flow direction.

4. A method according to claim 1, wherein the vector velocity fields are computed by the means of optical flow methods.

5. A method according to claim 1, wherein

- the contrast phase is submitted to the arterial pulsed pressure creating a periodic time modulation of the contrast density and the time filter (16) response is tuned to the cardiac frequency of a subject to be examined, such that the filtered sequence enhance the time modulation of the contrast image within the vessel.

6. A method according to claim 3, wherein a vessel direction map is created (22) for the colour flow image using the vessel reference image comprising the steps of vessel segmentation - computing vessel orientation using a standard oriented ridge filter and extending the vessel orientation on a vessel map.

7. A method according to claim 6, wherein components of the velocity field being longitudinal in respect of the vessel axis are computed, a threshold for the blood flow velocity is determined producing artificial aliasing patterns and the longitudinal flow is colour encoded for the colour flow image using the velocity threshold.

8. A method according to claim 7, wherein the threshold is corresponding to normal maximum blood flow velocity.

9. A method according to claim 6, wherein components of the velocity field being transverse corresponding to a projection perpendicular to the vessel axis are computed and the transverse flow is colour encoded for the colour flow image.

10. A method according to claim 6, wherein flow variance and/or flow vorticity of the velocity field are computed and colour encoded for the colour flow image.

11. A method for representing blood flow-related information in X-ray images, wherein a digital subtraction angiography DSA image (12) is generated; a vessel registration (14) is performed including

- defining a reference vessel image at a reference time after DSA through the detection of sufficient contrast or by the user, registration between the reference vessel image and a current vessel image, warping the current vessel image towards the reference vessel image generating an output registered contrast image; the output sequence is time filtered to extract time contrast modulation (16); vector velocity fields (18) are computed applying motion estimation processing on the time filtered sequence; the velocity data is transformed into a colour flow image (20) and the colour flow image is displayed superimposed over the reference vessel X- ray image.

12. A method for representing blood flow-related information in X-ray images, wherein a digital subtraction angiography DSA image (12) is generated and a colour flow image (20) representing velocity data is displayed superimposed over a reference vessel X-ray image.

13. An X-ray imaging system for representing blood flow-related information, comprising a source of X-ray radiation (42), an X-ray image detection module (46), a data processing unit (48) and a display (50),

- wherein the data processing unit (48) is arranged to perform a time registration process to determine a corresponding image of a first sequence to each image of a second sequence and to subtract the first corresponding image from the second corresponding image; and

- wherein the data processing unit (48) is arranged to perform a registration between a reference vessel image and a current vessel image, and to warp the current vessel image towards the reference vessel image to generate an output registered contrast image; to compute vector velocity fields applying motion estimation processing; and to transform the velocity data into a colour flow image;

- and wherein the display (50) is arranged to display the colour flow image.

14. An X-ray imaging system according to claim 13, wherein the display (50) is arranged for displaying the colour flow image superimposed to an X-ray image.

15. Computer program element, which, when being executed by a processor, is adapted to carry out the method of claims 1 to 12.

16. Computer readable medium having stored a program element, which, when being executed by a processor, is adapted to carry out the method of claims 1 to 12.