An MRI-Based Probabilistic Atlas of Neuroanatomy

Abstract

Three-dimensional MRI imaging techniques offer new possibilities for qualitative and quantitative studies of gross neuroanatomy, functional neuroanatomy and for neurosurgical planning. The digital nature of the data allows for the reconstruction of realistic three-dimensional models of an individual brain which can be sliced at arbitrary orientations for optimal visual inspection of often complex neuroanatomy and pathology. This is particularly relevant in the assessment of potential neuroanatomical correlates of temporal lobe epilepsy. Re-formatting of contiguous thinly sliced (1–2 mm thick) volumetric MRI data along planes parallel and perpendicular to the temporal plane allow finer visual discrimination and greater standardisation in qualitative procedures than previously possible. Perhaps more exciting are the applications of quantitative analysis where, for instance, accurate measurements of hippocampus and/or amygdala volumes provide important indicators of unilateral mesial temporal sclerosis which compare favourably with EEG and more invasive methods of lateralising the epileptogenic focus (Jack et al., 1990; Cascino et al., 1991; Watson et al, 1992; Cendes et al., 1993 a,b). For instance, by combining volumetric measurements of both hippocampus and amygdala, Cendes et al., (Chapter 9) quote correct lateralisation of focus in 93 of 100 temporal lobe epilepsy cases. The study of epilepsy arising from cortical abnormalities has been limited in the past by the difficulties of visualising the cortical surface from a set of conventional two-dimensional MRI slices. New acquisition techniques with gradient echo as opposed to spin echo techniques allow for an improved signal-to-noise ratio in thin slices in times compatible with clinical examinations. Whole brain coverage with thin slice data is now possible, such that partial volume effects are minimised with consequent improvements in fine detail. Numerous authors have reported dramatic improvements in the assessment of cortical dysplasia and grey matter heterotopias, particularly for more subtle abnormalities (Palmini et al., 1991a,b,c; Barkovich and Kjos, 1992a,b,c). The impact of this improved raw data when combined with new techniques for generating three-dimensional surface renderings in reasonably interactive circumstances is yet to be fully realised but initial experience is promising. At present, most studies have relied upon visual inspection to identify abnormalities in gyration on three-dimensional surface-rendered MRI. Such methods are quite acceptable for gross pathologies but, in a manner similar to mesial temporal volumetrics, the identification of more subtle distortions may require quantitative analysis of left/right differences and comparison of individual gyral surface area or gyral/sulcal locations with previously established population norms. Cook et al., (Chapter 47) have approached the problem by application of fractal analysis to two-dimensional MRI images from normal and frontal lobe epilepsy (FLE) patients. The grey-white matter interface was extracted by image processing procedures as a continuous contour and the fractal dimension, an index of contour complexity, derived. Results indicate that 10 of 16 FLE patients had a fractal index more than 3 standard deviations (3SD) below normal. In its present form, the method provides a non-specific indicator of cortical abnormality, yielding an overall index of complexity rather than identifying specific abnormalities, and is implemented in two dimensions rather than three dimensions. Nevertheless, it illustrates the potential of quantitative analysis for detecting aberrant cortical morphology. For a more directed analysis of cortical folding, a model of normal neuroanatomical variability, expressed in three-dimensional coordinates, is necessary. Keyserlingk and co-workers have developed methods for digitising sulcal patterns from post-mortem brains and constructed a map, with cuboid elements of 4 mm or 8 mm edge length, of major sulcal anatomy from 30 such brains (Keyserlingk et al., 1983, 1985, 1988; Niemann et al, 1988). The advent of high resolution MRI scanning offers finer spatial and contrast resolution in normal brain in vivo. At the Montreal Neurological Institute, MRI and PET imaging techniques are combined with three-dimensional graphics and computational analysis in the study of functional neuroanatomy of cognitive and sensorimotor processing. As part of this “brain mapping” programme, we have collected a database of over 300 MRI volumes from young normal subjects and are presently engaged in a series of projects whose long-term goal is the construction of a probabilistic description of normal neuroanatomy derived from high-resolution (1 mm thick slices) MRI data. In this chapter we briefly describe the current brain mapping environment at our institute and the current development of the MRI atlas project in both volumetric and surface domains.