WO2009138940A1

WO2009138940A1 - 4d volume imaging

Info

Publication number: WO2009138940A1
Application number: PCT/IB2009/051923
Authority: WO
Inventors: Nicolaas J. Noordhoek
Original assignee: Koninklijke Philips Electronics N. V.
Priority date: 2008-05-14
Filing date: 2009-05-11
Publication date: 2009-11-19

Abstract

4D (=3D+time) soft tissue reconstructions of the heart using c-arm systems, is becomingan important topic. This invention disclosure addresses a simple and robust technique that provides significantly better 4D (time resolved 3D) image quality than conventional techniques. Forward projection of a non gated reconstruction is used to create a4D reconstruction in a second reconstruction pass that suffers very much less from streak artefacts than conventional (gated) 4D reconstruction. It also has much less noise, since more of the projections are used for each reconstruction. The improved image quality has significant benefits when it comes to segmentation or visual identification of the anatomy of the heart. Since recently, extremely fast forward and backprojection is available on cheap graphics cards.

Description

4D VOLUME IMAGING

FIELD OF THE INVENTION

The invention relates to 4D (3D + time) volume imaging. Particularly, the invention relates to a method and device for processing data of a tomography system for 4D volume imaging of a periodically moving object.

BACKGROUND OF THE INVENTION

CT like imaging has recently become "state of the art". The next clinical area that is being conquered is the cardiac domain. Since a heart is moving fast (1 beat/s) in comparison to the rotational speed of a c-arm (5 seconds minimum), new techniques are required to reconstruct a heart with sufficient image quality in 4D (time resolved 3D). The electro cardiogram (ECG) of the patient is recorded for each projection. To reconstruct each cardiac ECG phase, only projections that correspond to that ECG phase are use for backprojection. Typically, a scan now takes about 20 seconds, so images are available for each ECG phase from about 20 directions. This is much less than might be suitable to reconstruct a non-moving target like the head, which might need 400 projections to achieve a 256 cube volume.

One big disadvantage of 3D reconstruction using ECG gating is that a lot of information of non-moving parts is thrown away, by discarding the images with the wrong ECG phase. Classical gated reconstruction results in much streaks and in loss of resolution, especially further away from the centre. Motion is still visible when this is played as a movie, but the streaks are very dominant. An example of a reconstruction of a classical gated reconstruction is shown in figure 3b.

SUMMARY OF THE INVENTION The following concepts are used for the explanation of the embodiments according to the invention: '4D' has the meaning of four dimensions', these dimensions are three spatial dimensions and one time dimension, i.e. a temporal sequence of pictures of 3D representations.

'Sinogram' describes a representation of single slices in a temporal sequence lined up. For example the first slice of the slices in figure 2 might be recorded repeatedly and from different directions and the sequence of these records is represented directly besides each other. The wording sinogram is deduced from sinusoidal curves of single structures which arise, when the said records are shown side by side. In other words, a 'sinogram is a projection over time of ID detector, wherein horizontally the time or projection angle 0-360 degrees is plotted, and vertically the detector pixels are plotted.

'back projection' describes the aspect of the reconstruction of a slice due to signals which were recorded in one of the rows of the sensors 300. Circulation of the radiation source together with the detector around an object of interest, results in a plurality of raw 2D data. A reconstruction of a 3D volume image consisting of several slices, from 2D data is, hereinafter, named back projection.

'forwards projection' describes the aspect that a representation of a three-dimensional reconstruction is virtually radiated to generate virtual 2D data.

It might be an object of the invention to provide a better resolution of the reconstructed 4D volume images of a periodically moving object together with surrounding areas.

This is achieved by the subject matter of each independent claim. Further exemplary embodiments are described in the respective dependent claims.

Generally and in accordance with an exemplary embodiment of the invention, a method of processing data of a tomography system for 4D volume imaging of a periodically moving object, comprises the steps of back-projecting raw 2D data, to generate a basic 3D image of the object, forward-projecting the basic 3D image to generate virtual 2D data, subtracting the virtual 2D data from the raw 2D data, to achieve 2D data of the moving parts of the object, back-projecting 2D data of the moving parts, to generate a 3D image of the moving parts, and adding the 3D image of the moving parts to the basic 3D image. An aspect of the method is that same phases of the periodical movement are identified to generate a gate signal. With the gate signal, the back-projecting of the 2D data of the moving parts might be gated, to generate 3D image of the moving parts for each identified phase. According to another aspect of the invention, the method further comprises the step of generating a 3D movie by displaying the 3D image of the moving parts, added to the basic 3D image, successively as periodically moving.

According to an embodiment of the invention, each 3D image includes a plurality of slices generated from the respective 2D data. An apparatus according to an exemplary embodiment of the invention, for examination and visualization of an object of interest, comprises a tomography system, a monitor, and a calculation unit adapted to perform the steps of the above mentioned method.

It should be noted that the tomography system might be a computer tomography system (CT), a magnet resonance tomography system (MRT), a photon emission tomography system (PET), a single photon emission computer tomography system (SPECT), or a system applying electro magnetic radiation like X-ray or ultrasound.

The invention relates also to a computer program for an image processing device, such that the method according to the invention might be executed on an appropriate system. The computer program is preferably loaded into a working memory of a data processor. The data processor is thus equipped to carry out the method of the invention. The computer program may be stored an a computer readable medium, such as a CD-Rom. The computer program may also be presented over a network like the worldwide web and can be downloaded into the working memory of a data processor from such a network.

In other words, the invention provides a solution to use the non-moving part of all projections for 4D reconstruction and separate gating of the information on the moving parts in the projections. This results in much better image quality than with conventional gating. It has much less streaks from non-moving parts of the anatomy. Since it uses more information from the projections, the noise level is also much lower. Advantage is also that this technique is very robust, in the sense that is does not depend on user interaction or segmentation of the moving anatomy.

The separate gating of the moving parts in the projections, while retaining al information on non-moving parts is achieved by forward projection of a conventional non-gated reconstruction that is made first. In a second pass, this reconstruction is "corrected" for deviations over time, that depicts the motion of the heart. The recent availability of back/forward-projection capable cheap GPU' s (graphic processor units) allows for doing this forward/backprojection extremely fast (10Ox faster than CPU), making is fast enough for a commercial product. These and other aspects of the present invention are apparent and will be elucidated with reference to the embodiment described hereinafter and with reference to the following drawing.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows components of a system which might be used to perform the method according to the invention. Figure 2 shows a detector array together with slices corresponding to the respective rows of the array.

Figure 3 a is a schematic illustration of an image reconstructed according to a method of the prior art.

Figure 3b is an illustration of an image actually reconstructed according to a method of the prior art. Figure 4 shows a flow chart of the method according to an exemplary embodiment of the invention. Figure 5 a is a schematic illustration of an image reconstructed by way of a method according to an exemplary embodiment of the invention. Figure 5b is an illustration of an image actually reconstructed by way of a method according to an exemplary embodiment of the invention.

Figure 6 shows an illustration of a non gated sinogram. Figure 7 is an illustration of a sinogram showing a forward projected non gated reconstruction minus an original sinogram. Figure 8 is an illustration of a gated sinogram of heart segmented out of surrounding tissue/structures. Figure 9 is an illustration of a smoothed gated sinogram of heart segmented out of surrounding tissue/structures. Figure 10 is an illustration of a gated reconstruction of the heart is superposed to a non-gated reconstruction of the chest.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 shows an exemplary embodiment of a system according to the invention, suitable to perform the method of the invention. The system comprises a CT ring 100, a radiation source 200 and a radiation detector 300. An object of interest might be placed in the ring, to be investigated by means of the system. The ring 100 allows that the radiation source 200 can be moved continuously around the object of interest together with the detector 300 so that an examination of the object of interest is possible from many different directions. Further, the system comprises an computing device 400 having a processing unit, and a monitor 500 for illustrating 4D volume images.

The detector 300 is also shown in figure 2 wherein the detector 300 includes an array of sensors. This array of sensors consists of 1 to n lines of sensors in a first direction, and of 1 to m rows of sensors in a second direction substantially vertical to the first direction. Usually the detector 300 is bent or curved as shown in figure 1. The detector represents a plane in which the sensor array is located. As soon as beams from the radiation source 200 hit the detector 300, signals might be generated which will be named in the following text, as raw 2D data.

Further, in figure 2, there are depict 1 to m planes representing slices of a 3D volume image which might be generated by back-projecting the raw 2D data. Every one of these slices corresponds to the respective row of sensors.

Figure 3 shows an example of one of the slices in figure 2, wherein figure 3 a is a schematic illustration of an image reconstructed, and figure 3b is an illustration of an image actually reconstructed. The lines going at an angle through the representation in figure 3, symbolize different X-ray examination directions. Due to the plurality of 2D raw data, the slice can be reconstructed. The numbers at the vertical axis and the horizontal axis describe the number of individual pixels in vertical and horizontal directions. A resolution of approximately 250 x 250 pixels is used only exemplarily here. It is noted that higher resolutions or also lower resolutions are possible. A 4D cardiac X-ray scan is typically made with a long slow rotation (20 seconds), or several consecutive forward rotational scans around the patient. During the scan, the heart chambers are enhanced with intravenous contrast agent. Since the heart is beating/moving during the rotation, gating has to be applied on the images: to reconstruct 1 cardiac phase of the heart, only the images that correspond to that phase can be used for reconstruction.

When however cardiac scans are made, the heart moves (beats) during the soft tissue acquisition. A heart typically beats at 60 beats per second, and a cardiac soft tissue scan takes about 20 seconds. The heart can be set still by recording the ECG of the patient and only use the images that correspond to a single ECG phase for a volume reconstruction. This way for each cardiac phase (for example 16 different cardiac phases between two ECG peaks) a separate reconstruction can be made. When these 15 volumes are played as a movie, a 4D (x,y,z,t) reconstruction results, showing the motion of the heart over time.

However, since the scan lasts 20 seconds maximum, each cardiac phase is 'seen' only from about 20 direction, not enough for a good reconstruction. Severe streaks though occur using a conventional "gating" method: any high contrast transition in the patient (tissue/air or contrast agent/tissue) produces a line through the volume, also the non moving high contrast transitions. Normally the lines are compensated by projections from all directions (180 degrees), but since the gating throws away data, projections are missing.

The invention proposes a method to make better use of the information in the sinograms by splitting them up in a moving and a non-moving contribution. This way the non-moving parts are reconstructed almost as good as when a normal 3D scan of a static object was made. Only the moving contribution to the sinogram is gated. Since the moving and non-moving parts of the patient are projected over each other in the x-ray images, they are hard to separate, by simply "segmenting" out the moving parts in the projections. This invention provides a simple robust and quite fast (reconstruction becomes less than two times slower) method to separate the moving and non-moving contributions to the projections, even though they are projected on top of each other in the x-ray images. The method is robust, since it does not depends on segmentation, which always involves setting of arbitrary thresholds, or any user interaction. The image quality is very significantly better than that of conventionally gated reconstructions.

Figure 4 shows an exemplary embodiment of the method according to the invention, which may consist of the following steps:

Step S 1 : recording a rotational x-ray scan with intravenous contrast agent, while recording the ECG which will be used for gating the projections according to cardiac phase

Step S2: non-gated 3D reconstructing of the volume by straightforward backprojection of all images, resulting in blurred moving parts and sharp non-moving parts (S3) Step S4: forward projecting of the non-gated 3D reconstruction to produce "non gated" projections (S5), these are different from the original projections, since motion is blurred out.

Step S6: subtracting of the non gated projections (S5) from the original projections (Sl) resulting in a "moving part only" of the projections (S 7) Step S8: making gated reconstructions of the "moving part only" projections (possibly with interpolation between images to compensate for missing data, reducing streaks), to generate gated 3D moving part only volumes (S9)

Step SlO: adding the reconstruction of the moving parts (S9) to the non gated reconstruction (S3) to form a sharp 3D reconstruction of a single cardiac phase Step SI l . doing this for a number of ECG phases to provide a 4D reconstruction (3D movie)

In the following an exemplary embodiment of the method according to the invention will be described with reference to slice images which illustrate intermediate steps of the method. As indicated in figure 5a, the following illustrations are slices reconstructed from an X-ray scan of the chest of a patient. Generally, each illustration shows an area 10 representing the heart, an area 20 representing the lungs (black) with alveoles (little white points), and an oval ring of the chest 30, with black background. The numbers of the vertical axis 40 and the horizontal axis 50 show the number of pixels used for the reconstruction of the slice.

Figure 5b shows a slice of the heart as it is beating in the chest, used as input data in the simulations. This is actually one frame of a simulated movie of a beating heart. This can be used to simulate a 3D rotational scan of a beating heart.

Figure 6 shows a sinogram that results from scanning a patient with beating heart over 360 degrees in 37 seconds, 60 beats/m, 30 projection frames/s. The sine shapes comes from the non-moving chest that is projected while rotating. The oscillation is the beating heart in the projections. As can be seen, the heart and static chest are not easily separable.

Figure 7 shows a non-gated reconstruction from the sinogram of figure 6, i.e., the difference between the original projections and the forward projection of the non-gated reconstruction. The result only shows the moving parts of the body (the heart borders) over time The non-moving parts are reconstructed well, the moving parts are blurred. The next steps will also make the heart appear sharp as a function of time.

When this non-gated reconstruction is forward projected and subtracted from the original images, a sinogram results that only shows the moving parts. This way we haves separated the moving and the non moving parts in the sinogram. Now we can very targeted only gated the moving parts. This way optimal use can be made of the information on the sinograms to both reconstruct the non moving parts of the body and the moving parts of the body in an optimal way, resulting in the beat possible image quality.

This "heart only:" sinogram of figure 7 can now be used to very specifically gated the heart and not any other non-moving part. This is shown in figure 8. To compensate for missing data, the gated heart only can be interpolated on the time axis, which is shown in figure 9, wherein the interpolation on the time axis is used to reduce motion streaks in the sinogram.

This sinogram can be reconstructed. The result is the difference between the non gated reconstruction of the chest-and-heart and the reconstruction of the patient in a specific cardiac phase. By simply adding this result to the non gated reconstruction a reconstruction of the patient in that cardiac phase results. Figure 10 shows the result, wherein a non-gated reconstruction and a gated heart reconstruction results in a reconstruction of the patient in a specific cardiac phase, with minimal streaks from high-contrast parts outside the heart.

Nowadays, very fast back and forward projections can be performed on off-the shelf graphics cards (GPU, graphic processor unit), that cost as little as 250 euros. These cards are 100 times faster than the existing CPU implementations. This offers a wonderful opportunity to implement a method like this.

The main application of this invention is 4D cardiac reconstruction using an x-ray c-arm and intravenous contrast agent. Otherwise, this invention is applicable to any projection reconstruction problem, which involves motion that is periodical and can be detected for use in gating of projections. One can for example think of breathing motion during a (typically long) Positron Emission Tomography scan. The breathing motion can be detected and used to create a 4D reconstruction with a minimum of streaks. While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and. not restrictive; the invention is not limited to the disclosed embodiments.

It has to be noted that embodiments of the invention have been described with reference to different subject matters. In particular, some embodiments have been described with reference to apparatus type claims whereas other embodiments have been described with reference to method type claims. However, a person skilled in the art will gather from the above description that, unless other notified, in addition to any combination of features belonging to one type of subject matter also any combination between features relating to different subject matters, in particular between features of the apparatus type claims and features of the method type claims is considered to be disclosed with this application.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. A method of processing data of an X-ray system for 4D volume imaging of a periodically moving object, the method comprising the steps of: back-projecting raw 2D data, to generate a basic 3D image of the object, forward-projecting the basic 3D image to generate virtual 2D data, subtracting the virtual 2D data from the raw 2D data, to achieve 2D data of the moving parts of the object, back-projecting 2D data of the moving parts, to generate a 3D image of the moving parts, and adding the 3D image of the moving parts to the basic 3D image.

2. The method of claim 1 , further comprising the step of: identifying same phases of the periodical movement to generate a gate signal, wherein the back-projecting of the 2D data of the moving parts is gated, to generate 3D image of the moving parts for each identified phase.

3. The method of claim 1 or 2, further comprising the step of: generating a 3D movie by displaying the 3D image of the moving parts, added to the basic 3D image, successively as periodically moving.

4. The method of any of claims 1 to 3, wherein each 3D image includes a plurality of slices generated from the respective 2D data.

5. An apparatus for examination and visualization of an object of interest, wherein the apparatus comprises a tomography system, a monitor, and a calculation unit adapted to perform the steps of the method according to any of claims 1 to 4.

6. A computer program product adapted for controlling an apparatus for examination and visualization of an object of interest, according to claim 5, wherein data, acquired from an tomography system, are processed to generate an 4D volume image of the object of interest.

7. A computer-readable medium, in which a computer program is stored, for controlling an apparatus for examination and visualization of an object of interest; according to claim 5.