KEYWORDS: Image registration, 3D image processing, Image segmentation, 3D acquisition, Heart, Medical imaging, Astatine, Single photon emission computed tomography, Image restoration, Control systems
In this paper we present a new approach for the registration of cardiac 4D image sequences of different subjects,
where we assume that a temporal association between the sequences is given. Moreover, we allow for one (or two)
selected pair(s) of associated points in time of both sequences, which we call the bridging points in time, the use
of additional information such as the semi-automatic segmentation of the investigated structure. We establish
the 3D inter-subject registration for all other pairs of points in time exploiting (1) the inter-subject registration
for the bridging pair of points in time, (2) the intra-subject motion calculation in both sequences with respect
to the bridging pair, and (3) the concatenation of the obtained transformations. We formulate a cost functional
integrating the similarity measures comparing the images of the bridging pair(s) of points in time and of the
current pair of points in time, respectively. We evaluated our algorithm on 8 healthy volunteers leading to 28
inter-subject combinations and we analyze the behaviour for different parameter settings weighting differently the involved pairs of points in time. The approach based on the bridging pairs outperforms a direct 3D registration of corresponding points in time, in particular in the right ventricle we gain up to 33% in registration accuracy. Starting with a cost functional taking into account the similarity at the first bridging point in time, the results improve stepwise by integrating, firstly, information from the current pair of points in time and secondly, from a second bridging point in time. Our results also show a steep rise of the importance of regularization on the registration accuracy when registering the current point in time with our procedure (17% gain in accuracy) with respect to a direct registration in the bridging point (less than 1%). However, regularization during intra-sequence registration had only minor effects on the accuracy of our registration procedure.
The endovascular repair of an abdominal aortic aneurysm is a minimal
invasive therapy which has been established during the past 15 years. A stent-graft is placed inside the aorta in order to cover the weakened regions of its wall. During a time interval of one or
more years the stent-graft can migrate and deform with the risk of
the occlusion of one of its limbs or of the rupture of the aneurysm.
In this work we developed several strategies to quantify the
migration and deformation in order to assess the risk coming with
these movements and especially to characterize appearing
complications by them. We calculated the rigid movement of the
stent-graft and the aorta relative to the spinal canal. For this
purpose, firstly, we rigidly registered the spinal canals, extracted
for the different points in time, in order to establish a fixed
reference system. All objects have been segmented first and surface
points have been determined before applying a rigid and non-rigid
point set registration algorithm. The change in the residual error
after registration of the stent-graft with an increasing number
of degrees of freedom indicates the amount of change in the
stent-graft's morphology. We investigated a sample of 9. Two cases could be clearly
distinguished by the quantified parameters: a high global migration
and a strong reduction of the residual error after non-rigid
registration. In both cases, strong complications have been
detected by the examination of clinical experts but only by means of the
images acquired one year later.
This paper focuses on the problem of ill-posedness of deformable point set registration and we propose a new approach to restrict the solution space using shape information. The basic elements of the investigated kind of registration algorithm are a cost functional, an optimization strategy and a motion model. The motion model determines the kind of motions and deformations that are allowed and how they are restricted. The motion model itself is mainly determined by the kind of parameterized transformation used to express the motion/deformation. Here, we observe that matching with more degrees of freedom (the parameters of the transformation) than necessary can introduce mismatches due to a higher sensitivity to noise or by destroying local shape information. In this paper we propose a cost functional which is robust to noise and we introduce a new method to specify a shape adapted deformation model based on thin-plate splines and initial control point placing using point clustering. We show that these initial positions have a strong impact on the match and we define them as cluster centers where we cluster on one of the point sets (weighting each point of this set with its distance to the other point set). Our experiments with known ground truth show that the shape adapted model recovers constantly very accurately corresponding points. In our evaluation with more than 1200 single experiments we showed that, compared to a conventional octree based scheme, we could save more than 60% of degrees of freedom while preserving matching quality.
The aim of this paper is to introduce a structural dissimilarity measure which allows to detect outliers in automatically extracted landmark pairs in two images. In previous work, to extract landmarks automatically, candidate points have been defined using invariance criteria coming from differential geometry such as maximum curvature; or they are statistical entities such as gravity centers of confiners, where the confiners are defined as the connected components of the level sets. After a first estimation of the semi-rigid transformation (representing translation, rotation, and scaling) relating the candidate point sets, outliers are detected applying the euclidian distance between corresponding points. However, this approach does not allow to distinguish between real deformations and outliers coming from noise or additional features in one of the images. In this paper, we define a structural dissimilarity measure which we use to decide if two associated candidate points come from two corresponding confiners. We select landmarks pairs with a dissimilarity value smaller than a given threshold and we calculate the affine transformation relating best all selected landmark pairs. We evaluated our technique on successive slices of a MRI image of the human brain and show that we obtain a significantly sharper error diminution using the new dissimilarity measure instead of the euclidian distance for outlier rejection.
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