ANGIOGRAPHIC EXAMINATION METHOD

A method is provided for angiographic examination of an organ, vascular system or other body regions as the examination object of a patient by means of 4D rotational angiography. A step S1 of the method involves acquisition of projection images in different cardiac phases. A further step S2 involves reconstruction of 3D volume images in the different cardiac phases. A further step S3 involves calculation of a motion map. A further step S4 includes image combination of the 3D volume images with the motion map to produce resulting, corrected 3D volume images in the different cardiac phases. A further step S5 involves presentation of the resulting, corrected 3D volume images.

DETAILED DESCRIPTION OF INVENTION

FIG. 2shows the relationships for EKG-correlated acquisition with a C-arm device according toFIG. 1during a rotation, as performed at a heart rate of 90 to 131 bpm for a duration of 10 s to 15 s and with or without cardiac phase control (pacing). If pacing does not take place, a known manual sorting of the phases from the EKG is brought about.

This figure shows a first EKG13, which has different cardiac phases c0to cN. Assigned to these cardiac phases c0to cNare different projection angles θ0 to θ0+n*Δθ. Thus for a first image14of a first cardiac phase c0 a value P(θ0, c0) results, for a first image15of a second cardiac phase P(θ0+Δθ, c1), for a first image16of a third cardiac phase P(θ0+2Δθ, c2) and for a first image17of an Nth cardiac phase P(θ0+NΔθ, cN) P(θ0+NΔθ, cN).

This continues as symbolized by the arrow18until a second EKG19is reached.

Different projection angles θ0+n*Δθ to θ0+(n+N)*Δθ are again assigned to these cardiac phases c0to CN. Thus for a second image20of a first cardiac phase c0a value P(θ0+nΔθ, c0) results, for a second image21of a second cardiac phase P(θ0+(n+1)Δθ, c1), for a second image22of a third cardiac phase P(θ0+(n+2)Δθ, c2) and for a second image23of an Nth cardiac phase P(θ0+(n+N)Δθ,cN).

FIG. 3shows the series of projection images24produced according to a standard method with approx. 30 projections per cardiac phase at 120 bpm and 13 s scan time with interfering streak artifacts. The indices c0to CNdesignate the projection images24of the current cardiac phases.

FIG. 4shows a sequence of reconstructed 3D volume images26, produced with approx. 30 projections per cardiac phase, from which a calculation27is performed of an image-based motion map28according to the formula

The indices fc0to fcNof the 3D volume images26designate the reconstructed 3D volume for the corresponding cardiac phase (c0to CN) and contain the image information.

As the motion map28also features interfering streak artifacts25, postprocessing is performed on the motion map28, as described in more detail with reference toFIGS. 5 to 8.

One method is analysis in the frequency domain. InFIG. 5in a 3D volume image26and the motion map28two representatively selected pixels29and30are considered, of which the first pixel29features significant motion at low frequency and the second pixel30features little motion at high frequency.

FIG. 6shows the signal profiles of the pixels29and30, the signal profile31of the first pixel29having a lower frequency than the signal profile32of the second pixel30.

InFIG. 7data relating to the modulation of heart motion and streak artifacts25is plotted over spatial frequency u, showing a modulated signal profile33of the first pixel29and a modulated signal profile34of the second pixel30, which have a modulation direction35.

FIG. 8shows data after demodulation of heart motion and streak artifacts25plotted over spatial frequency u with a demodulated signal profile36of the first pixel29and a demodulated signal profile37of the second pixel30.

The principle of modulation and demodulation essentially means that at some points, for example at the second pixel30, the pixel values only change quasi-periodically due to the streak artifacts25. These quasi-periodic changes of the streak artifacts25are based on the so-called windmill effect. They are sampling artifacts as a function of time. At other points, for example at the first pixel29, the change to said pixel30can be traced back as a function of time to the windmill effect and heart motion artifacts. This type of change should be identified to process such selective diffusion with filters, for example demodulation.

The principles of modulation and demodulation are generally known from signal theory or signal processing; Fourier analysis or band filtering can be used here.

Modulation is defined by the recording itself; demodulation is used to isolate the “carrier” signal from the “true” signal. With the type of recording specified here this is relatively simple, as the windmill artifacts have quite a defined frequency, which is only a function of the recording geometry and can therefore be calculated easily beforehand.

Morphological operations such as for example erosion and/or dilatation of the motion map28can be used as further methods for postprocessing the motion map28.

The for example bilinear or spline subsampling and interpolation method can also be used for postprocessing the motion map28.

As a result of postprocessing the motion map28using one of these methods, a corrected motion map is obtained, which is almost free of streak artifacts25.

One example of an image combination shown inFIG. 9is a linear combination with linear interpolation. However other types of combination are also possible, for example polynomial or quadratic image combinations. Image combinations with a convolution operator are also conceivable.

One of the possible image combinations, which results generally from the following equation, is now described with reference toFIG. 9:

where cnrepresents the respective cardiac phase c0to cN.

The pixels of the reconstructed 3D volume images26f(x, y, z, cn) are multiplied by the pixels of the corrected motion map38MM(x, y, z). Added to this is the product of one minus corrected motion map38MM(x, y, z) and the mean value image39f(x, y, z) over all phase images. The result F(x,y, z, cn) is the resulting, corrected 3D volume images40.

This multiplication represents the simplest instance of an image combination, in which a pixel or voxel-based multiplication (weighting) of the two images (or volumes) is always performed per phase, with the motion map remaining constant after postprocessing.

In other words the result for the example of the first cardiac phase c0would appear as follows:

This is shown thus by way of example for a linear interpolation. In the case of a non-linear combination a corresponding function f(MM(x,y,z)) would have to be defined, e.g. polynomially. In the present instance it is mainly a matter of weighting the individual volumes according to the motion map.

The result of postprocessing can also be described in more detail and illustrated symbolically based onFIGS. 10 to 13, which show the time sequence of image production. The starting point is the image series “before motion map postprocessing” of the reconstructed 3D volume images26. The motion map28is calculated therefrom. This motion map28is then corrected based on the processing described inFIGS. 5 to 8to produce a “motion map postprocessing” of the corrected motion map38. Finally the resulting, corrected 3D volume images40“after motion map postprocessing” are calculated according to the above equation.

The method proposed above operates on the basis of the reconstructed layers, the 3D volume images26.

One type of acquisition is rotation with effective angle sampling, for example a sampling time of 13 s, 0.5° angle increment and 2×2 binning. This produces around 380 projections over all phases. Available redundant information is utilized as only some of the voxels in the image change. The change to the voxels is calculated by means of the motion map28per layer. The motion map28shows the content of the motion or the change to the voxel values over time. A voxel has a different motion function, in other words change function or gradient, in the heart, from when it is present in a different body part.

The motion map28is also influenced by streak artifacts25in the first step. To reduce this, three postprocessing methods arte proposed, to isolate changes due to streak artifacts25and changes due to pure heart motion. This results in a reduction of the streak artifacts25in the motion map28.

The motion map28is utilized as a combination weighting between the reconstruction of an individual phase (e.g. c0) and the mean value image from all phases. It is assumed here that the voxel values in the motion map28with a small value contribute less to heart motion.

The image combination can be produced by linear interpolation but other types of combination are also possible.

The resulting corrected 3D volume images40have significantly fewer streak artifacts25.

The inventive method can be used for monoplanar and biplanar systems. Unlike many other known methods it is a purely image-based method. It does not require raw data, geometry or other information.

The inventive method eliminates streak artifacts25from 4D rotational angiography, so-called 4D DynaCT® images, almost completely with limited loss of spatial and temporal resolution.

The generation and postprocessing of the motion map28further reduces interfering streak artifacts25.

The inventive method can also be used for other protocols with changes in the time direction, for example perfusion.

The available reconstruction chain is utilized effectively for the calculations.