Abstracts

The folding or gyrification of the cerebral cortex is a developmental process, which allows for an increase in cortical surface area and inter-neuron connectivity without a disproportionate increase in cranial size relative to body size. Cortical developmental malformations (CDM) may give rise to disruption in the folding process. The isoperimetric ratio (IR), a measure of the degree of cortical folding, is defined as the ratio of the total cortical surface area (SA) to the total volume (V) of the cerebrum to the power of two thirds. With the advent of MRI, CDMs are increasingly recognised as a cause of epilepsy. An abnormal IR may be a surrogate for subtle CDMs. This measurement has not previously been reported for an epilepsy population.
Stereology is a tool used to estimate geometric information of a three-dimensional object based on two-dimensional information of that object. Stereological estimates are based on the rules of geometric probability and are considered to be mathematically unbiased, in addition they are routinely robust and time-efficient. Stereological estimates are generated by counting the number of intersections a test probe has with the geometric structure of interest. For isoptropic, uniform, randomly orientated test probes with respect to the object, the number of intersections generated is proportional to the geometric property of interest. High resolution 3D SPGR MRI brain image data was acquired from five normal control subjects and five temporal lobe epilepsy (TLE) patients. For each individual the IR was derived from stereological estimates of cerebral volume and surface area. The mean and standard deviation of cerebral surface areas for the control group was 1595cm² (32.14), and for the TLE group was 1686cm² (192.22). Mean (SD) cerebral volumes were 1143cm³ (53.27) and 1146cm³ (66.29) for the control and TLE groups respectively. For the control group the mean (SD) IR was 15.03 (0.59) and for the TLE group was 16.11 (1.45). Three out of five TLE patients demonstrated increased IR measuring greater than 2 standard deviations above the mean of the control group. This preliminary study demonstrates the application of stereological techniques for the [italic]in-vivo[/italic] quantification of the degree of gyrification of the human cerebrum based on the IR. Volume, surface area and IR measurements in this study are in agreement with previously reported studies in literature. Results support the concept of IR as a surrogate for subtle CDMs, which may not be readily identifiable under routine visual inspections of MRI data. The identification of CDMs may facilitate a fuller understanding of the influencing factors of the epileptic brain. A larger study is planned to apply the IR to a homogeneous group of epileptic patients as well as a group of age, handed and sex-matched controls. (Supported by the Brain Research Foundation and the Irish Institute of Clinical Neuroscience.)