Registration of challenging image pairs: initialization, estimation, and decision.

Our goal is an automated 2D-image-pair registration algorithm capable of aligning images taken of a wide variety of natural and man-made scenes as well as many medical images. The algorithm should handle low overlap, substantial orientation and scale differences, large illumination variations, and physical changes in the scene. An important component of this is the ability to automatically reject pairs that have no overlap or have too many differences to be aligned well. We propose a complete algorithm, including techniques for initialization, for estimating transformation parameters, and for automatically deciding if an estimate is correct. Keypoints extracted and matched between images are used to generate initial similarity transform estimates, each accurate over a small region. These initial estimates are rank-ordered and tested individually in succession. Each estimate is refined using the Dual-Bootstrap ICP algorithm, driven by matching of multiscale features. A three-part decision criteria, combining measurements of alignment accuracy, stability in the estimate, and consistency in the constraints, determines whether the refined transformation estimate is accepted as correct. Experimental results on a data set of 22 challenging image pairs show that the algorithm effectively aligns 19 of the 22 pairs and rejects 99.8% of the misalignments that occur when all possible pairs are tried. The algorithm substantially out-performs algorithms based on keypoint matching alone.

Predictive scheduling algorithms for real-time feature extraction and spatial referencing: application to retinal image sequences.

Real-time spatial referencing is an important alternative to tracking for designing spatially aware ophthalmic instrumentation for procedures such as laser photocoagulation and perimetry. It requires independent, fast registration of each image frame from a digital video stream (1024 x 1024 pixels) to a spatial map of the retina. Recently, we have introduced a spatial referencing algorithm that works in three primary steps: 1) tracing the retinal vasculature to extract image feature (landmarks); 2) invariant indexing to generate hypothesized landmark correspondences and initial transformations; and 3) alignment and verification steps to robustly estimate a 12-parameter quadratic spatial transformation between the image frame and the map. The goal of this paper is to introduce techniques to minimize the amount of computation for successful spatial referencing. The fundamental driving idea is to make feature extraction subservient to registration and, therefore, only produce the information needed for verified, accurate transformations. To this end, the image is analyzed along one-dimensional, vertical and horizontal grid lines to produce a regular sampling of the vasculature, needed for step 3) and to initiate step 1). Tracing of the vascular is then prioritized hierarchically to quickly extract landmarks and groups (constellations) of landmarks for indexing. Finally, the tracing and spatial referencing computations are integrated so that landmark constellations found by tracing are tested immediately. The resulting implementation is an order-of-magnitude faster with the same success rate. The average total computation time is 31.2 ms per image on a 2.2-GHz Pentium Xeon processor.

The edge-driven dual-bootstrap iterative closest point algorithm for registration of multimodal fluorescein angiogram sequence.

Motivated by the need for multimodal image registration in ophthalmology, this paper introduces an algorithm which is tailored to jointly align in a common reference space all the images in a complete fluorescein angiogram (FA) sequence, which contains both red-free (RF) and FA images. Our work is inspired by Generalized Dual-Bootstrap Iterative Closest Point (GDB-ICP), which rank-orders Lowe keypoint matches and refines the transformation, going from local and low-order to global and higher-order model, computed from each keypoint match in succession. Albeit GDB-ICP has been shown to be robust in registering images taken under different lighting conditions, the performance is not satisfactory for image pairs with substantial, nonlinear intensity differences. Our algorithm, named Edge-Driven DB-ICP, targeting the least reliable component of GDB-ICP, modifies generation of keypoint matches for initialization by extracting the Lowe keypoints from the gradient magnitude image and enriching the keypoint descriptor with global-shape context using the edge points. Our dataset consists of 60 randomly-selected pathological sequences, each on average having up to two RF and 13 FA images. Edge-Driven DB-ICP successfully registered 92.4% of all pairs, and 81.1% multimodal pairs, whereas GDB-ICP registered 80.1% and 40.1%, respectively. For the joint registration of all images in a sequence, Edge-Driven DB-ICP succeeded in 59 sequences, which is a 23% improvement over GDB-ICP.

Automated retinal image analysis over the internet.

Retinal clinicians and researchers make extensive use of images, and the current emphasis is on digital imaging of the retinal fundus. The goal of this paper is to introduce a system, known as retinal image vessel extraction and registration system, which provides the community of retinal clinicians, researchers, and study directors an integrated suite of advanced digital retinal image analysis tools over the Internet. The capabilities include vasculature tracing and morphometry, joint (simultaneous) montaging of multiple retinal fields, cross-modality registration (color/red-free fundus photographs and fluorescein angiograms), and generation of flicker animations for visualization of changes from longitudinal image sequences. Each capability has been carefully validated in our previous research work. The integrated Internet-based system can enable significant advances in retina-related clinical diagnosis, visualization of the complete fundus at full resolution from multiple low-angle views, analysis of longitudinal changes, research on the retinal vasculature, and objective, quantitative computer-assisted scoring of clinical trials imagery. It could pave the way for future screening services from optometry facilities.