Digital Subtraction Angiography (DSA) is a powerful technique for
the visualization of blood vessels in the human body. In DSA a
sequence of X-ray projection images is taken to show the passage of a
bolus of injected contrast material through one or more vessels of
interest. The following images show, respectively, an image taken
prior to injection (the mask image), and an image containing
contrasted vessels (the live or contrast image):
| | |
| Mask Image | Live Image |
The background structures are largely removed by subtracting the
mask image from the live image:
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| Original DSA Image |
The resulting image is usually displayed using contrast
enhancement:
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| Contrast Enhanced DSA Image |
The subtraction technique is based on the assumption that during
exposure, tissues do not change in position or density. As can be
observed from the DSA images shown above, this is not always the case.
In practice, patient motion frequently occurs, which causes the
subtraction images to show artifacts that may hamper proper diagnosis.
In order to reduce these so called
motion artifacts, the misalignment of the successive images in
the sequence needs to be determined and corrected for; operations
often referred to as registration and motion correction
. In clinical practice, this is partially accomplished by
pixel-shifting, a rather cumbersome manual technique, which
allows for correction of translational motion only.
An example of the limitations of the manual pixel-
shifting technique is given in the image below, from which it is clear
that if patient motion is more complex (in this case a rotation of the
patient's head), a correction in one part of the image implies the
introduction of new artifacts in other parts (see yellow arrows):
 |
| Motion Correction by Pixel-Shifting |
This explains the need for a fully automatic elastic registration method. Although many studies have been carried out on this subject, they have not led to algorithms which are sufficiently fast so as to be acceptable for integration in a clinical setting. We have developed a new approach for registration of digital angiographic image sequences which is both effective, and computationally very efficient.
In general,
the registration of successive images in a sequence involves two
operations: (i) Computation of the correspondence between successive
pixels; (ii) Applying the correction based on this correspondence.
Since it is computationally too expensive to compute the
correspondence explicitly for every pixel, we only consider a reduced
set of so called control points. In our approach, the control
points are selected in regions containing strong object edges,
resulting in an irregular grid. This has three advantages over
approaches that use regular grids of control points:
The following image shows the regions containing strong object
edges in the original mask image (see above). Note that this image
gives a very good indication of the regions containing motion
artifacts in the subtraction image:
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Based on this information the control
points are selected, using a two-parameter algorithm which
constrains the minimum and maximum distance between the points
(white dots):
|
The local displacements at the control
points are computed using a template matching approach:
using windows in the mask image of about 50x50 pixels, the optimally
corresponding window in the live image is searched using a hill-
climbing optimization algorithm. In our algorithm we use the
energy of the histogram of grey-value differences as a
measure of similarity.
 |
In order to correct for motion
in the entire image, the computed local displacement vectors need to
be interpolated so as to obtain a complete displacement vector
field. In our algorithm we use the computationally cheapest approach
to do this: linear interpolation. This requires a triangulation of
the set of control points:
 |
The nice thing about this approach is that it can be mapped very
efficiently on modern graphics hardware (programmed using OpenGL),
resulting in a real-time implementation of the final warp operation.
By repeating the above described technique of
displacement computation, interpolation, and warping of the mask image
according to the computed displacement vector field, entire sequences
can be corrected at a rate of about one image per second, even on
relatively low-cost graphics workstations such as Silicon Graphics'
O2.
The following two images show, respectively, an original DSA image,
and the corrected image after applying the proposed approach:
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| Original DSA Sequence | Corrected DSA Sequence |
The following two animations show the complete sequence:
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| Original DSA Sequence | Corrected DSA Sequence |
This page describes work by Erik Meijering, Karel Zuiderveld, Wiro Niessen and others. More information can be found in the following publications: