Future directions

1. Automation of segmentation

For each case analysed using all the methods available in this study, approximately six hours of operator interaction was required. Of this time, the majority was spent in segmentation, and this is the most difficult part. If larger studies were to be performed, automation of the segmentation process would be important to speed the entire process. Methods of automated segmentation have been discussed (see methods section). In order to use cluster analysis, for example, on high-resolution volumetric data, new sequences would need to be devised allowing the collection of the required data in a reasonable time-frame, minimising costs and patient movement artefact. If non-biological ROI boundaries are used, however, some manual interaction will remain necessary.

2. Régionalisation of surface area derivatives

The surface area measures as currently derived are mean measures across the entire cerebral hemisphere. Régionalisation is likely to increase the utility of the data, as local areas of abnormality will not be averaged out by larger areas of normally-structured brain. This could be performed on the data already available by counting the number of voxels in the surface of each block, the volume of which is already known. The task would be intensive in terms of

operator time because of the way in which the data is acquired currently. There was insufficient time to perform these measurements in the course of this thesis.

The data as currently available are limited to régionalisation in the plane of the original slice data. If the data were available in other planes for similar analysis and régionalisation, its utility might again be increased. Thus, for example, if régionalisation were still performed in blocks, but three mutually-perpendicularly orientated block sets were available, then focal areas of abnormality might be established in the area of the intersection of abnormal blocks.

Perhaps the best régionalisation, however, would be operator-driven and selectable in each case, so that proportionalities in specific areas of interest could be studied. The major problem with this approach would be the determination of comparable areas in control subjects, given that regional cytoarchitectonie variations exist and may be related to cortical anatomy. If in fact general principles are applicable such that a given amount of surface area even when localised is related to a given volume of overlying grey matter (perhaps bearing in mind only whether the region is fundal, crown or wall), then comparison between regions within the same and different subjects may be simply achieved. On the other hand if such principles are not found, then the problem of intersubject comparability again raises its head. Novel methods of shape and gyral analysis would then be required.

3. Grey matter surface area

Currently, definition of the surface area of the grey matter is not possible on the T^-weighted volume acquistion. On T2-weighted images, however, sulcal depths can more easily

be determined. If a rapid sequence with volumetric resolution and T^-weighting could be devised (and acquired perhaps with

a proton-density or T^-weighted data set, for automated segmentation), then the surface area of the grey matter could be more accurately determined. This would be of value for all the methods described in this thesis. In particular, it would allow better definition regionally of cortical thickness, and examination of cortical surfaces other than the dorsolateral fronto-parieto-temporal cortex, where gyral separation is enough to allow clear distinction of the pattern of gyration.

4. Corroborative studies

The direct study of connections in brains affected by dysgenesis would support the postulated hypotheses. This would be a mammoth task, the technology for which is now available. Injection of certain dyes (eg Dil) allows the tracing of axonal connections in the human cortex postmortem. To the author's knowledge, no such study has yet been performed on a brain with dysgenesis. Equally, detailed histological study of an entire brain affected apparently focally by dysgenesis has also not been performed using the Golgi stain that might demonstrate extensive and distributed alterations in neuronal morphology, in support of the suggestion that connectional abnormalities exist in distant parts of the brain.

5. Other cerebral conditions

Extensive connectional and dendritic abnormalities have also been shown and postulated in other diseases: this raises the possibility of using the methods developed in this thesis in other conditions associated with dysgenesis or misconnection from some other cause. Amongst these are dyslexia, mental retardation and schizophrenia.

Dyslexia has been associated with cortical dysgenesis. Galaburda and Kemper (1979) reported a case of a man with developmental dyslexia and easily controlled seizures who had focal polymicrogyria and several areas of focal cortical

dysplasia in the left hemisphere. It is of interest, in the context of the rest of the thesis, that the polymicrogyria was not apparent on macroscopic inspection and that the areas of cortical dysplasia were widespread and noncontiguous. Subtle callosal abnormalities have also been reported in patients with developmental dyslexia (Hynd et al., 1995) . The use of the quantitative methods proposed here may help identify areas of neocortical abnormality in the brain in vivo in such patients.

Schizophrenia is one of a few psychiatric entities that is being increasingly studied using high-resolution MRI in an attempt to find underlying structural abnormalities. The hypothesis tested is often that prefrontal-basal ganglia connections are in some way altered, with changes in volume measures or relationships. On the whole these studies are inadequately performed (see Chapter 2). Using the techniques presented here, changes in brain shape, complexity and regional connectivity might be more rigorously examined.

6. Genes and development

Very little is known about the influence of genetic factors on cerebral development in humans. Most information has been derived from the study of the genetic constitution of patients with abnormal cerebral structure. The Miller-Dieker syndrome consists of microcephaly with peculiar facies, severe developmental delay and intractable seizures, amongst other features. The brain is agyric or pachygyric. A gene deletion associated with this familial lissencephaly syndrome has been identified {LIS-1; Reiner et al.,1993). An important feature is that a marked abnormality of cortical structure is produced by a single genetic defect. Identification of the structural defect by MRI has allowed cases to be identified and families to be studied. Similarly, an X chromosome locus has been suggested for a familial pachygyria and callosal agenesis syndrome (Berry-Kravis and Israel, 1994). In both cases, the identification of cases by MRI facilitated genetic study.

However, such gross cases are relatively rare. In addition, when MRI abnormalities are marked but there are no other

features to suggest a syndromic diagnosis, identification of homologous cases may be difficult. It is possible that some of the methods described here may be of value in this regard.

Firstly, cerebral structural abnormalities may be detected where none are otherwise apparent: thus two patients with abnormalities shown only on three-dimensional reconstruction have a clinical diagnosis of familial frontal lobe epilepsy. Previously, no structural abnormalities were noted on neuroimaging of such patients (Scheffer et al.,1994). Unfortunately, no other members of the families of these patients have been scanned. If, however, it could be shown that the demonstrated gyral abnormalities segregated with the epilepsy, then the identification of genetic defects could be linked more directly to structural changes in the brains of the patients.

Secondly, the surface area derivatives might also be informative in this regard. Disorders of proportion can in some cases be related to laminar histopathology (see above). If either localised or generalised disorders of proportion of cerebral structures could be shown - perhaps even in patients with no apparent structural abnormalities on routine inspection - then genetic study of a larger number of patients might be possible, based on MRI-derived information rather than solely on clinical syndromic diagnosis. Régionalisation of the surface area derivative measures would be most useful for this. In this way, study of the genetic abnormalities associated with specific structural abnormalities might shed further light on normal developmental principles and processes. The importance of MRI - and postprocessing - as a

screening tool in this context should not be overlooked.

Lastly, block analysis suggests that there may be a fundamental structural difference between males and females

with subependymal heterotopia (see Section 5.4) . This suggests that genetic analysis of this condition should be separated for sex and shows how the techniques developed here may allow a purer culture for analysis using other methods.

7. Epileptogenesis in cerebral dysgenesis

The mechanism of epileptogenesis in cerebral dysgenesis is poorly understood. The concurrence of dysgenesis and epilepsy is too frequent to suppose that the two are unconnected. Palmini et al. (1995) suggest that dysgenetic lesions are intrinsically epileptogenic rather than merely irritative to the surrounding cortex, in the way that gliotic scars, for example, might be considered to be. They base this on peculiar electrical activity detected from dysgenetic lesions perioperatively, and from the finding that loss of this activity postexcision is associated with an increased chance of becoming seizure-free. They suggest that dysgenetic lesions may act as seizure pacemakers, entraining other normal cortex in seizure activity. The findings reported here may support this view. If there are other abnormalities of cortical structure, these may also be capable of supporting epileptogenesis and normally be entrained by the clearly abnormal area, but then released to act as pacemaker if the visible lesion is removed, so that seizures may continue with unchanged semiology, perhaps by the mechanism suggested by Gloor (1990). Abnormal connectivity within the cortex may support this pathological activity.

8. The still normal study

There are patients in this report who have focal epilepsy comparable to those in whom structural abnormalities have been demonstrated, yet who are normal on all measures. Equally, there are patients with dysgenesis (eg female patients with SEH) who have refractory epilepsy without extensive structural abnormalities as revealed by the techniques used here. This

may be because any such changes are swamped by an overwhelming majority of normal structure.

Further study is required to define dysgenesis yet more precisely. The use of postprocessing may help in the categorisation of cases more clearly, in the way that patient 2, with an undefined dysgenesis on inspection alone, is likely to have a polymicrogyric pathology on the basis of the surface area derivative findings. Classification including such findings may create a more pure culture for further analyses, whether these be structural, functional or genetic.