Methodological Review: Symmetry and asymmetry analysis and its implications to computer-aided diagnosis: A review of the literature <s> Symmetry and its clinical implications <s> We present a completely automated procedure for measuring left and right hemispheric asymmetry in cerebral grey and white matter volumes from MR images using a chain of state-of-the-art image analysis algorithms. After bias correction and tissue classification, left and right hemispheres are separated by non-rigid registration to a template image in which both hemispheres have been carefully segmented. Volume renderings of each hemisphere separately demonstrate the high quality of the resulting segmentations. Because all steps in the procedure are completely automated and do not require user specified parameters, the results are highly reproducible and consistent. We present quantitative results obtained from a database of MR images of 40 schizophrenic patients and 31 normal controls. <s> BIB001 </s> Methodological Review: Symmetry and asymmetry analysis and its implications to computer-aided diagnosis: A review of the literature <s> Symmetry and its clinical implications <s> We present a new method to automatically compute, reorient, and recenter the mid-sagittal plane in anatomical and functional three-dimensional (3-D) brain images. This iterative approach is composed of two steps. At first, given an initial guess of the mid-sagittal plane (generally, the central plane of the image grid), the computation of local similarity measures between the two sides of the head allows to identify homologous anatomical structures or functional areas, by way of a block matching procedure. The output is a set of point-to-point correspondences: the centers of homologous blocks. Subsequently, we define the mid-sagittal plane as the one best superposing the points on one side and their counterparts on the other side by reflective symmetry. Practically, the computation of the parameters characterizing the plane is performed by a least trimmed squares estimation. Then, the estimated plane is aligned with the center of the image grid, and the whole process is iterated until convergence. The robust estimation technique we use allows normal or abnormal asymmetrical structures or areas to be treated as outliers, and the plane to be mainly computed from the underlying gross symmetry of the brain. The algorithm is fast and accurate, even for strongly tilted heads, and even in presence of high acquisition noise and bias field, as shown on a large set of synthetic data. The algorithm has also been visually evaluated on a large set of real magnetic resonance (MR) images. We present a few results on isotropic as well as anisotropic anatomical (MR and computed tomography) and functional (single photon emission computed tomography and positron emission tomography) real images, for normal and pathological subjects. <s> BIB002
Morphologically speaking, a normal human head exhibits a high level of bilateral symmetry, although it is not perfectly symmetrical BIB001 . Corresponding regions of two hemispheres have approximately identical anatomical properties, and also have comparable, if not identical, physiologic (e.g., blood perfusion) properties. The degree of asymmetry has long been thought to be helpful for suggesting a pathological condition and/or providing a diagnostic cue for clinicians. For example, abnormal asymmetry in the brain indicates a wide range of pathologies, such as stroke, bleeding and tumor. Radiologists routinely use symmetry/asymmetry as one of the most discriminating features, in conjunction with other characters such as location, neighborhood relationship, and shape, to assess abnormalities in brain images. (1) is larger than the left one, and the left occipital lobe is larger than the right one  . This illustration is adapted from BIB002 . Functionally speaking, the brain is functionally asymmetrical and each side of the brain assumes distinctive functions. For instance, in most individuals, the left hemisphere is more active in linguistic tasks, while the right hemisphere is specialized for non-verbal tasks such as visual perception  . The degree to which anatomical asymmetry correlates with functional asymmetry remains questionable, but it is evident that morphological differences between hemispheres, however slight, occur systematically in normal brains. One example would be that the right frontal lobe is expected to be bigger than its left counterpart  . In any case, however, one has to be aware of the existence of normal asymmetry might potentially confound the process of extracting abnormal asymmetry.