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Definition
In computer vision, sets of data acquired by sampling the same scene or object at different times, or from different perspectives, will be in different coordinate systems. Image registration is the process of transforming the different sets of data into one coordinate system. Registration is necessary in order to be able to compare or integrate the data obtained from different measurements.
In medical imaging (e.g. for data of the same patient taken at different points in time) registration often additionally involves elastic (or nonrigid) registration to cope with elastic deformations of the body parts imaged. Nonrigid registration of medical images can also be used to register a patient's data to an anatomical atlas, such as the Talairach atlas for neuroimaging.
Algorithm Classifications
Area-based vs Feature-based
Image registration algorithms fall within two realms of classification: area based methods and feature based methods. The original image is often referred to as the reference image and the image to be mapped onto the reference image is referred to as the target image. For area based image registration methods, the algorithm looks at the structure of the image via correlation metrics, Fourier properties and other means of structural analysis. However, most feature based methods, instead of looking at the overall structure of images, fine tunes its mapping to the correlation of image features: lines, curves, points, line intersections, boundaries, etc.Transformation model
Image registration algorithms can also be classified according to the transformation model used to relate the reference image space with the target image space. The first broad category of transformation models includes linear transformations, which are a combination of translation, rotation, global scaling and shear components. Linear transformations are global in nature, thus not being able to model local deformations.The second category includes 'elastic' or 'nonrigid' transformations. These transformations allow local warping of image features, thus providing support for local deformations. Nonrigid transformation approaches include polynomial warping, interpolation of smooth basis functions (thin-plate splines and wavelets), and physical continuum models (viscous fluid models and large deformation diffeomorphisms).
Search-based vs direct methods
Image registration methods can also be classified in terms of the type of search that is needed to compute the transformation between the two image domains. In search-based methods the effect of different image deformations is evaluated and compared. In direct methods, such as the Lucas Kanade method, an estimate of the image deformation is computed from local image statistics and is then used for updating the estimated image deformation between the two domains.Image nature
Another useful classification is between single-modality and multi-modality registration algorithms. Single-modality registration algorithms are those intended to register images of the same modality (i.e. acquired using the same kind of imaging device), while multi-modality registration algorithms are those intended to register images acquired using different imaging devices.There are several examples of multi-modality registration algorithms in the medical imaging field. Examples include registration of brain CT/MRI images or whole body PET/ CT images for tumor localization, registration of contrast-enhanced CT images against non-contrast-enhanced CT images for segmentation of specific parts of the anatomy and registration of ultrasound and CT images for prostate localization in radiotherapy.
Other classifications
Further ways of classifying an algorithm consist of the amount of data it is optimized to handle, the algorithm's application, and the central theory the algorithm is based around. Image registration has applications in remote sensing (cartography updating), medical imaging (change detection, tumor monitoring), and computer vision. Due to the vast applications to which image registration can be applied, it's impossible to develop a general algorithm optimized for all uses.Image similarity-based methods
Image similarity-based methods are broadly used in medical imaging. A basic image similarity-based method consists on a transformation model, which is applied to reference image coordinates to locate their corresponding coordinates in the target image space, an image similarity metric, which quantifies the degree of correspondence between features in both image spaces achieved by a given transformation, and an optimization algorithm, which tries to maximize image similarity by changing the transformation parameters.
The choice of an image similarity measure depends on the nature of the images to be registered. Common examples of image similarity measures include Cross-correlation, Mutual information, Mean-square difference and Ratio Image Uniformity. Mutual information and its variant, Normalized Mutual Information, are the most popular image similarity measures for registration of multimodality images. Cross-correlation, Mean-square difference and Ratio Image Uniformity are commonly used for registration of images of the same modality.
See also
"Image registration"