Evaluation of Subcortical White Matter and Deep White Matter Tracts in Malformations of Cortical Development

Authors


Address correspondence and reprint requests to E. Widjaja, Diagnostic Imaging, Hospital for Sick Children, 555 University Avenue, Toronto, Ontario M5G 1X8, Canada. E-mail: Elysa.Widjaja@sickkids.ca

Abstract

Summary: Aims: Abnormal cortical development will lead to abnormal axons in white matter. The purpose was to investigate (1) the microstructural changes in subcortical white matter adjacent to malformations of cortical development (MCD) and (2) the deep white matter tracts using diffusion tensor imaging (DTI).

Methods: Thirteen children with a variety of MCD were recruited. The fractional anisotropy (FA), trace, and eigenvalues (λmajor, λmedium, λminor) of subcortical white matter of MCD were compared with contralateral normal side. The deep white matter tracts were graded based on the size, color hues and displacement of the tracts as visualized on color vector maps and tractography; grade 1 was normal tract size and color hue, grade 2 was reduced tract size but preserved color hue and grade 3 was loss of color hue or failure of tracking on tractography.

Results: The subcortical white matter adjacent to abnormal cortex demonstrated reduced FA (p < 0.05) and tendency to increase trace (p = 0.06). There was a significant elevation in λmedium and λminor (p < 0.05), but no significant change in λmajor (p > 0.05). Twelve cases demonstrated alteration in white matter tracts. Seven cases of focal cortical dysplasia and two cases of transmantle MCD demonstrated grade 3 pattern of white matter tract.

Conclusion: Reduced FA is a sensitive but nonspecific marker of alteration in microstructure of white matter. The elevated λmedium and λminor may reflect a dominant effect of abnormal myelin. Alteration in white matter tracts was observed in most cases of MCD.

Malformations of cortical development (MCD) affect the formation of gray matter at different stages of development. The underlying white matter may be secondarily affected by the abnormal gray matter. Abnormal signal has been reported in the white matter of hemimegalencephaly (Barkovich, 2000). The signal abnormality is thought to be secondary to gliosis and/or abnormal myelination. Previously, knowledge of the white matter in MCD has been limited to alteration in signal intensity and/or volume of white matter (Hayashi et al., 2002). MCD may affect the white matter without necessarily changing its signal, by altering the microstructure of the subcortical white matter and of the white matter tracts of the affected hemisphere. Diffusion tensor imaging (DTI) provides information on the microstructure of tissues and the white matter tracts in vivo. The aims of this study are to investigate (1) the microstructural changes of subcortical white matter adjacent to MCD, and (2) the deep white matter tracts of the affected hemisphere.

METHODS

This study has the approval of institutional research ethics board. 13 children, mean age 10.2 years (age range 2–18 years) diagnosed with MCD, were recruited into the study (Table 1). Nine children were referred with a variety of seizures, including generalized tonic–clonic seizures, complex partial seizures and simple partial seizures. The duration of seizures at the time of imaging varies from two to thirteen years (mean of seven years). Four children had developmental delay but no seizures and of these, two had hemiparesis. Eight children had focal cortical dysplasia, two had subcortical heterotopia, one had subependymal gray matter heterotopia, one had unilateral schizencephaly and another had unilateral polymicrogyria. All eight cases of focal cortical dysplasia demonstrated high T2 signal in the subcortical white matter associated with blurring of the gray white matter junction. The remaining five cases of MCD demonstrated normal signal in the white matter on T2 and FLAIR images.

Table 1. Clinical presentation and grading of white matter tracts in children with malformations of cortical development
Case/Gender/Age (years)Clinical presentation (seizure duration in years)MR findingsAssociation tractsGradeProjection tractsGradeCommissural tractsGradeOthers
1/M/11Generalized tonic–clonic seizures (9)R parietal FCD with extension to the R temporal lobeR superior longitudinal fasciculus3R CS/CP tract (posteriorly)3 
R inferior longitudinal fasciculus2a 
R inferior frontooccipital fasciculus2a 
2/M/10L clonic seizures (3)R mesial parietal FCD R CS/CP tract (posteriorly)3 
3/F/17Complex partial seizures (9)L frontal basal FCDL inferior frontooccipital fasciculus (anteriorly)3 
4/M/18Generalized tonic–clonic seizures (12)L frontal FCD L anterior thalamic radiation3 
5/M/5Partial seizures with tonic component (13)R occipital FCD R optic radiation3R forceps major3 
6/F/18Partial seizures with ocular symptoms (5)L mesial occipital FCD L forceps major3 
7/M/15Partial seizures (5)L middle temporal gyrus FCD 
8/F/5Complex partial seizures (3)L frontal FCDL superior occipito frontal fasciculus3 
9/F/16L partial motor seizures (2), L hemiparesisR sylvian schizencephalyR superior longitudinal fasciculus2bR CS/CP tract3 Transvesely oriented fibers posterior to cleft, in continuity with superior longitudinal fasciculus
R optic radiation3 
10/M/2Developmental delay, oculomotor apraxia, ataxiaR subependymal heterotopia (also has cerebellar cortical malformations) R optic radiation2a Thinning of the middle cerebellar peduncles
11/F/5Speech delay, bifid tongue, cleft palateL mesial parietooccipital subcortical heterotopiaL superior longitudinal fasciculus2bL CS/CP tract2bL forceps major2b 
12/M/2Developmental delay, bilateral sensorineural deafness, strabismusL frontal transmantle subcortical heterotopiaL superior frontooccipital fasciculus3L CS/CP tract (anterior limb of internal capsule)1b 
L anterior thalamic radiation3 
13/M/9Developmental delay & L hemiparesisR perisylvian polymicrogyriaR superior longitudinal fasciculus2aR CS/CP tract2a ↑ R subcortical U fibers

MR imaging

Magnetic resonance imaging was done on 1.5T GE Signa LX (General Electric, Milwaukee, WI, U.S.A.) using quadrature head coil. Imaging consisted of a variety of sequences including sagittal T1 (TR/TE =566/14 ms, slice thickness =5 mm, field of view = 21 cm, matrix = 256 × 192), axial T2/dual echo (TR/TE =4000/50/100 ms, slice thickness = 5 mm, field of view = 22 cm, matrix = 384 × 224), coronal T2/dual echo (TR/TE=3000/50/100 ms, slice thickness = 5 mm, field of view = 22 cm, matrix = 512 × 224), axial and coronal FLAIR (TR/TE=9000/160 ms, slice thickness = 5 mm, field of view = 22 cm, matrix = 256 × 192), as well as axial and/or coronal 3D SPGR (TR/TE = 11/4.2 ms, slice thickness = 2 mm, field of view = 22 cm, matrix = 256 × 192).

Diffusion tensor imaging

Diffusion tensor imaging was performed on the same scanner using single shot diffusion-weighted echo planar imaging. Twenty-five axial contiguous slices were obtained aligned to the anterior commissure—posterior commissure line to cover the whole brain, giving a total imaging time of 4 min 40 s. At each slice position, in addition to b = 0 images, a single b-value of 1,000 s/mm2 was applied in 25 spatially isotropically arranged noncollinear directions (Jones et al., 1999), with the following parameters TR = 10,000 ms; TE = 113 ms; slice thickness = 5 mm; FOV= 26 cm; matrix of 128 × 128.

Postprocessing of diffusion tensor metrics and white matter fiber tracking was carried out on the GE workstation (General Electric), processed with Functool version 12.0 M4HD research mode (General Electric Medical System). Echo planar image distortion was corrected automatically. The DTI raw datasets were fitted to the diffusion tensor equations to yield six independent tensor elements using a non-linear least-squares fitting routine (Bruder et al., 1992). Diagonalization of the tensor yielded three eigenvalues (major (λ1), medium (λ2), and minor (λ3) eigenvalues) and three eigenvectors (Basser et al., 1994a, 1994b). From these elements, maps of trace, fractional anisotropy (FA) and tensor eigenvalues were calculated by using the following equations:

image

The major, medium, and minor eigenvalues specify the rate of diffusion along each of the three orthogonal axes of the diffusion ellipsoid (λmajor, λmedium, λminor). Trace provides an overall evaluation of the magnitude of diffusional motion in a voxel or region. Trace does not include the anisotropic diffusion effects and limits the result to an invariant scalar metric, which is independent of the orientation of the reference frame (Papadakis et al., 1999). Trace has the unit square millimeters per second. FA, which represents the ratio of the anisotropic component of the diffusion tensor to the whole diffusion tensor, was used as FA has been reported to be the best rotationally invariant scalar metric for measuring diffusion anisotropy (Basser et al., 1994b; Basser and Pierpaoli, 1996; Pierpaoli et al., 1996). Rotationally invariant anisotropy metrics are advantageous because they are independent of the frame of reference, of the direction of the applied diffusion gradients and of the orientation of the studied structures within the voxels. FA metrics are scalar indices and are unitless, and FA values range from 0 to 1, where 0 represents maximal isotropic diffusion as in a perfect sphere and 1 represent maximal anisotropic diffusion.

Quantitative analysis

The anatomical images were reviewed to identify the location of the malformation prior to placing region of interest (ROI). Two pediatric neuroradiologists placed the ROI independently. An elliptical ROI was placed in the subcortical white matter adjacent to the abnormal cortex on an axial image (Fig. 1B), except in schizencephaly and subependymal heterotopia. Identical ROI was placed in the subcortical white matter, on the same slice and at the same anatomical location on the contralateral normal appearing side in all cases. In the case of schizencephaly, the ROI was placed in the white matter subjacent to the abnormal cortex bordering the schizencephalic cleft, and close to the cortex overlying the brain surface. In this case, identical ROI was placed at the same anatomical location on the contralateral side. In the case of subependymal heterotopia, the ROI was placed in the white matter subjacent to the gray matter heterotopia and identical ROI was placed in the white matter of the same anatomical location on the contralateral side. The ROI was placed on the trace map and were then transposed into anatomically coregistered positions on the FA and eigenvalue maps. The mean values of FA, trace, λmajor, λmedium, λminor values were measured. The size of the ROIs varied from 10 to 20 mm2.

Figure 1.

Case 1.(A) Axial T2 image shows thickening of the cortex, blurring of gray white matter junction and high T2 signal in the white matter of the right parietal lobe. There is also volume loss in the right temporal lobe (not demonstrated here). The findings are in keeping with focal cortical dysplasia. (B) Axial trace map demonstrating ROIs placement in the subcortical white matter adjacent to the abnormal cortex, and in the subcortical white matter of the same anatomical location on the contralateral normal appearing side for quantitative analysis. (C and D) Color vector map and tractography show grade 3 pattern of white matter tract changes, i.e., loss of color hue and non-tracking of the right superior longitudinal fasciculus (arrowhead) and posterior aspect of right corticopontine tract (blue). The ROIs placement for seed point of fiber tracking of the superior longitudinal fasciculus is shown on color vector map. (E and F) Color vector map and tractography show grade 2 pattern or reduced caliber of the right inferior longitudinal fasciculus (arrow).

Semiqualitative analysis

The FA and eigenvectors were used to calculate vector orientation maps, which were generated by mapping the major eigenvector directional components in x, y, and z into RGB color channels and weighting the color brightness by fractional anisotropy (Pajevic and Pierpaoli, 1999). The convention used for directional RGB mapping is red for left-right, green for anteroposterior, and blue for superior-inferior. Color vector map was used to guide placement of ROI for seed point of fiber tracking. Fiber tracking was generated by using the fiber assignment by continuous tracking (FACT) algorithm (Mori et al., 1999). Tracking was performed from all pixels inside the ROI using a “brute force” approach, according to the direction of the principal eigenvector in each voxel. The tracts in the dataset were computed by seeding each voxel that had FA greater than 0.3 and the tract length was set at a threshold of 160 mm. The FA threshold was set at 0.3 to avoid generating spurious tracts and the tract length was set at 160 mm to avoid premature termination of tracts.

The association, projection and commissural tracts were assessed from the directional color vector maps and tractography. The association tracts evaluated included the superior longitudinal fasciculus, inferior longitudinal fasciculus, inferior frontooccipital fasciculus and superior frontooccipital fasciculus. The projection tracts assessed include the optic radiations, anterior thalamic radiations and corticospinal/corticopontine tracts. The commissural tracts assessed include the forceps major and minor of corpus callosum. Identification of the white matter tracts was guided by published data (Mori et al., 2005; Wakana et al., 2004). Based on anatomical knowledge of fiber projection, tracts belonging to each fiber bundle were selected by placing a single elliptical ROI in the middle third of the expected projection of the tract on the axial view of color vector map, distal from the subcortical white matter (Fig. 1C). The ROIs range from 30 to 60 mm2 in size. Identical ROI was placed on the same tract on the contralateral normal hemisphere as on the affected hemisphere, except for the forceps major and minor of corpus callosum, where the ROI was placed in the splenium or genu of the corpus callosum respectively.

These white matter tracts were graded based on the size, color hue and displacement of the white matter tracts on all the axial color vector maps and tractography. These tracts were visually inspected and graded by two pediatric neuroradiologists by consensus. Grade 1 was characterized by normal tract size and normal color hue on the color vector map compared to the contralateral side and normal tract size on tractography. Grade 2 was characterized by reduced tract size but preserved color hue on the color vector map and reduced tract size on fiber tractography compared to contralateral side. Both grade 1 and 2 were further subdivided to “a” or “b” depending on the absence or presence of displacement of the white matter tracts respectively. Grade 3 was characterized by loss of the directional color hue on color vector map and failure of tracking on tractography along all or part of the tract.

Statistical analysis

Data were analyzed using SPSS version 12 (SPSS, Chicago, Ill). The inter-rater agreement on FA, trace, λmajor, λmedium and λminor of the subcortical white matter adjacent to MCD and the contralateral normal side were evaluated using intraclass correlation coefficient. Intraclass correlation coefficient of 0–0.2 indicated poor agreement, 0.21–0.40 fair agreement, 0.41–0.60 moderate agreement, 0.61–0.8 substantial agreement, and 0.81–1.0 nearly perfect agreement. The average of the measurements acquired from ROIs was used for further analysis. The FA, trace, λmajor, λmedium and λminor of the subcortical white matter adjacent to the abnormal cortex were compared to the contralateral side using nonparametric test Wilcoxon signed ranks. The MCD group were further subdivided into those with focal cortical dysplasia and those without focal cortical dysplasia. The white matter of the subgroups was then compared with contralateral side. The FA, trace, λmajor, λmedium and λminor of the subcortical white matter adjacent to the abnormal cortex in those with and without epilepsy were compared using Mann-Whitney U test. The presence or absence of epilepsy was compared in those with grade 3 versus grade 1 and 2 white matter tract changes using chi-square analysis. P-values <0.05 were considered statistically significant.

RESULTS

Quantitative analysis

The intraclass correlation coefficients of the subcortical white matter adjacent to MCD and contralateral normal side for FA were 0.41 and 0.36, respectively; for trace were 0.74 and 0.78, respectively; for λmajor were 0.73 and 0.66, respectively; for λmedium were 0.69 and 0.83, respectively; and for λminor were 0.69 and 0.42, respectively. The FA, trace, λmajor, λmedium and λminor of the subcortical white matter were summarized in Table 2. The FA in the subcortical white matter adjacent to the abnormal cortex was lower compared to contralateral side (p < 0.05). Subset analysis demonstrated significantly reduced FA for both focal cortical dysplasia and other MCD compared to contralateral side (p < 0.05). The trace in the subcortical white matter adjacent to the abnormal cortex was higher compared to the contralateral side, but this did not reach statistical significance. The λmedium and λminor of the subcortical white matter adjacent to the abnormal cortex were significantly higher compared to the contralateral side (p < 0.05), but the λmajor were not significantly different compared to the contralateral side (p > 0.05).

Table 2. Subcortical white matter of MCD versus contralateral side
 Contralateral side (n = 13) (SD)WM of MCD (n = 13) (SD)WM of FCD (n = 8) (SD)WM of non FCD (n = 5) (SD)
  1. SD, standard deviation; WM, white matter; MCD, malformations of cortical development; FCD, focal cortical dysplasia.

FA0.38 (0.06)0.26 (0.08)p < 0.050.25 (0.09)p < 0.050.29 (0.06)p < 0.05
Trace× 10−32.51 (0.44)2.88 (0.73)p = 0.063.04 (0.84)p > 0.052.63 (0.49)p > 0.05
λmajor × 10−31.18 (0.20)1.21 (0.25)p > 0.051.25 (0.28)p > 0.051.14 (0.22)p > 0.05
λmedium × 10−30.78 (0.17)0.94 (0.24)p < 0.051.00 (0.28)p < 0.050.85 (0.15)p < 0.05
λminor × 10−30.54 (0.11)0.73 (0.25)p < 0.050.79 (0.30)p < 0.050.63 (0.15)p = 0.08

The FA, trace, λmajor, λmedium and λminor of the subcortical white matter adjacent to MCD in those with epilepsy were not significantly different compared to those without epilepsy (p > 0.05) (Table 3).

Table 3. Subcortical white matter of malformations of cortical development in those with and without epilepsy
 WM of MCD in those with epilepsy (n = 9) (SD)WM of MCD in those without epilepsy (n = 4) (SD)p-Value
  1. SD, standard deviation; WM, white matter; MCD, malformations of cortical development.

FA0.25 (0.08)0.31 (0.05)p > 0.05
Trace × 10−32.96 (0.82)2.72 (0.52)p > 0.05
λmajor × 10−31.22 (0.28)1.19 (0.22)p > 0.05
λmedium × 10−30.97 (0.28)0.88 (0.15)p > 0.05
λminor × 10−30.77 (0.29)0.64 (0.17)p > 0.05

Semiqualitative analysis

Out of the eight cases that had focal cortical dysplasia, seven demonstrated grade 3 pattern of white matter tracts that project to/from the focal cortical dysplasia (Figs. 1 and 2) and one (case 7) did not demonstrate any perceivable changes in the white matter tracts (Table 1). Of the remaining MCD, schizencephaly and transmantle subcortical heterotopia demonstrated grade 3 pattern of some of the white matter tracts. The two cases that demonstrated displacement and reduction in size of some of the white matter tracts (grade 2b) were nontransmantle subcortical heterotopia (Fig. 3) and schizencephaly (Fig. 4). Two cases had aberrant or modified tracts; in the case of schizencephaly (Fig. 4), transversely oriented fibers were localized posterior to the schizencephaly cleft and in the case of polymicrogyria (Fig. 5), increased numbers of transversely oriented fibers in the subcortical white matter were identified extending to the polymicrogyric cortex. Those with epilepsy were more likely to have grade 3 pattern of white matter changes compared to those with grades 1 and 2 white matter changes (p < 0.05).

Figure 2.

Case 6.(A) Axial T2 demonstrating blurring of gray white matter junction and high T2 signal in the white matter of the left occipital lobe, in keeping with focal cortical dysplasia. (B) Color vector map and (C) tractography show thinning of the splenium of corpus callosum and grade 3 pattern of the left forceps major (arrow).

Figure 3.

Case 11.(A) Axial T2 demonstrates gray matter heterotopia involving the left mesial parietal lobe. The lesion indents on the medial aspect of the left lateral ventricle. On the (B) color vector map and (C) tractography the left superior longitudinal fasciculus (green) is of reduced caliber and is displaced laterally (grade 2b) (black arrowhead). (D) Sagittal oblique view of the corticospinal/corticopontine tract shows distortion and displacement of the tract (white arrows) by the mass effect of subcortical heterotopia. (E) The left forceps major (black arrow) is of reduced caliber and displaced anteriorly by the subcortical heterotopia (grade 2b).

Figure 4.

Case 9.(A) Axial T2 shows a cleft extending from the surface of the brain to the ventricular margin in the right hemisphere, with abnormal cortex bordering the cleft, consistent with schizencephaly. (B) Color vector map shows grade 3 pattern of the posterior aspect of the corticospinal/corticopontine tract and transversely oriented fibers posterior to the schizencephaly cleft (red) (small arrow). (C) On lateral projection of fiber tractography, the superior longitudinal fasciculus is displaced superiorly (large white arrow) as compared to the contralateral normal side (white arrowhead). (D) On axial projection of fiber tractography, the right superior longitudinal fasciculus is in continuity with the transversely oriented fibers posterior to the schizencephaly cleft (small arrow).

Figure 5.

Case 13.(A) Axial T2 shows thickening of the cortex with irregular margin involving most of the right hemisphere, consistent with polymicrogyria. (B) Color vector map and (C) tractography show reduce caliber or grade 2 pattern of the right superior longitudinal fasciculus as compared to the contralateral side. (B) Color vector map and (D) tractography demonstrate increase number of transversely oriented tracts in the subcortical white matter.

DISCUSSION

Current classification system of MCD incorporates knowledge of the imaging appearance of the malformations, embryology and genetics (Barkovich et al., 2005). With advances in imaging techniques, our knowledge of the imaging appearance of MCD has increased tremendously in the last two decades. However, most of the current concepts of MCD derived from imaging have been based on the appearances of the gray matter. While it is accepted that the underlying white matter is abnormal, very little attention has been paid to the white matter.

Diffusion tensor imaging can be used to indirectly evaluate the integrity of the axonal microenvironment by assessing the diffusion of water molecules and its directionality in a three-dimensional space. The three eigenvalues fully determine the size and shape of the diffusion ellipsoid corresponding to the diffusion tensor. λmajor represent water diffusivity parallel to the axonal fibers, also referred to as axial diffusivity, whilst λmedium and λminor represent water diffusion perpendicular to the axonal fibers, that is, radial or transverse diffusivities. Animal models have shown tensor eigenvalues were more specific markers of myelination and axonal morphology (Ono et al., 1995; Kinoshita et al., 1999; Song et al., 2002). Changes in λmajor reflect axonal integrity and have been observed in cases of Wallerian degeneration (Thomalla et al., 2004). In contrast, transverse or radial diffusivities reflect alteration in myelination (Song et al., 2002, 2003).

We have found significantly elevated λmedium and λminor, but no significant difference in λmajor in the subcortical white matter adjacent to MCD compared to contralateral side. The elevated transverse or radial diffusion suggested abnormality in myelination of the subcortical white matter of MCD. Gross et al. (2005) have also found increased perpendicular water diffusivity that was greater than the relative increase in the parallel diffusivity in three cases of focal cortical dysplasia. Studies of histological specimens from MCD have demonstrated the presence of demyelination, remyelination, and myelin pallor of the white matter (Adamsbaum et al., 1998; Gomez-Anson et al., 2000; Kakita et al., 2005).

Several studies have found reduced FA (Wieshmann et al., 1999; Eriksson et al., 2001; Lee et al., 2004; Schoth and Krings, 2004; Trivedi et al., 2006) in the subcortical white matter adjacent to MCD. Whilst FA is a sensitive marker of alteration in tissue microstructure, it lacks specificity. Reduced FA may indicate one of the three processes: (i) degradation of both axonal membranes and myelin (Beaulieu et al., 1996; Werring et al., 2000; Pierpaoli et al., 2001), (ii) abnormalities of myelin with sparing of the axons (Gulani et al., 2001; Song et al., 2002), or (iii) reduced density of myelinated axons (Takahashi et al., 2002). In vitro model of Wallerian degeneration in frog sciatic nerve demonstrated a decrease in diffusion anisotropy due to reduced axial and increased radial diffusivity (Beaulieu et al., 1996). Concha et al. (2006) have demonstrated that measures of eigenvalues provide additional information regarding reduced anisotropy in epilepsy patients who have had corpus callosotomy. These investigators have found a reduction in parallel diffusivity, in keeping with axonal fragmentation, a week following corpus callosotomy. In contrast, the decreased anisotropy 2–4 months following corpus callosotomy was due to an increase in perpendicular diffusivity, in keeping with myelin degradation. The observed reduced FA in the subcortical white matter adjacent to MCD in our cohort of patients can be explained by the increased λmedium and λminor. Although myelin played a large role in determining the observed reduced anisotropy, Song et al. (2002) have found that absence of myelin resulted in increased radial diffusivity of only about 20% relative to controls. We have found 28% increased λmedium and 35% increased λminor in the subcortical white matter adjacent to MCD. If the reduced FA was purely due to abnormal myelination, a greater increase in λmedium and λminor would be expected. This indicates that additional factors are responsible for the reduced FA, which remains to be elucidated.

In our cohort of patients, we have found a tendency to increase trace in the subcortical white matter adjacent to MCD. Trace and mean diffusivity are considered to reflect cellular density and extracellular volume (Gass et al., 2001). The increase in trace may reflect reduced cellular and therefore fiber density and increased intercellular space in the white matter. Our patient cohort consisted of children with MCD, some of whom presented with intractable seizures, in particular, those with focal cortical dysplasia. The increased trace could be attributed to changes in cellular density or in permeability of the axonal membrane, which alter fluid shift between the intra and extracellular compartment, as indicated by the increased λmedium and λminor (Song et al., 2002). Eriksson et al. (2001) have found increased diffusivity in MCD. The authors have suggested that the increase in diffusivity was related to reduce cell density and increased extracellular space due to failure of neurogenesis or later cell loss. In contrast, Wieshmann et al. (1999) have found no change in diffusivity in two of three patients with MCD. Trivedi et al. (2006) demonstrated increased diffusivity in the polymicrogyric cortex but no significant difference in diffusivity of the subcortical white matter subjacent to the polymicrogyric cortex compared to controls. They have attributed the lack of altered diffusivity in the subcortical white matter possibly to the presence of ectopic neurons.

Eriksson et al. (2001) have used a combined approach of DTI and statistical parametric mapping to compare tissue organization in 22 patients with MCD and 30 controls. In that study, the images were normalized to MNI space (http://imaging.mrc-cbu.cam.ac.uk/imaging/MniTalairach) using linear transformations to minimize loss of focal individual difference. A voxel-by-voxel comparison using SPM was then made between each individual patient and the control group. Such an approach is statistically rigorous. However, voxel-by-voxel comparisons are only possible if the brains are normalized to a common space, such as the MNI space. The white matter fiber tracts are unique to each individual and there is some variability between individuals due to variability in head shape. By fitting the brains to a common space, distortions of the white matter may occur, thereby reducing technique sensitivity. Wallis et al. (2006) have demonstrated that linear affine transformations resulted in geometric distortion of pediatric brain malformations. Some MCD such as lissencephaly are associated with thickened cortex and reduced white matter. Therefore, voxel-by-voxel comparison may not necessarily compare abnormal gray matter with normal gray matter in controls because of the thickened cortex. In addition, there is lack of pediatric normal templates for children less than six years of age. Hence, we have used the ROI approach to evaluate the subcortical white matter in MCD.

Focal cortical dysplasia accounted for 8 of the 13 cases of MCD. The MR features of FCD include thick cortex, blurring of the cortical-subcortical junction, abnormal signal in the white matter and focal alteration of the gyration and sulcation pattern (Colombo et al., 2003; Widdess-Walsh et al., 2006). Some cases of focal cortical dysplasia are subtle and difficult to detect on structural imaging. Colombo et al. (2003) have found that structural MR did not detect focal cortical dysplasia in 39% of their cases. All eight cases of focal cortical dysplasia in our cohort of patients have abnormalities detected on structural MR. We have found reduced fractional anisotropy in the subcortical white matter of focal cortical dysplasia. Gross et al. (2005) have also found reduced anisotropy in three cases of focal cortical dysplasia that demonstrated abnormal underlying white matter hyperintensities on T2-weighted imaging, but did not detect DTI abnormalities in two patients with FCD but without white matter hyperintensities on T2-weighted imaging. The absence of DTI changes in these two patients could be technique related or due to differences in the underlying pathology of the lesions. Eriksson et al. (2001) have found reduced FA that extended beyond the margins of visible MCD in 4 of the 11 patients. However, the authors have not specified the type of malformations that demonstrated reduced FA beyond the visible malformation.

Current application of DTI in MCD have focused on measurements of FA and/or MD (Eriksson et al., 2001; Trivedi et al., 2006). The full potential of DTI along with information on the directional organization of white matter tracts has yet to be exploited for characterizing the structural alterations of specific tracts in MCD. Grade 3 was characterized by loss of the directional color hue on color vector map and failure of tracking on tractography along all or part of the tract. The loss of normal hue on color vector map suggested more severe disturbance of the white matter tracts. We do not have pathologic data to explain the loss of normal hue on color vector map. Since directional color map was generated using information from FA and major eigenvector, significant reduction in FA or loss of directional organization of the white matter tracts may lead to failure of tracking on tractography. The elevated λmedium and λminor in the subcortical white matter adjacent to MCD suggested that abnormal myelin might play a dominant role for the reduced FA. Grade 2 was defined as reduced tract size on color vector map with preserved color hue or reduced tract size on tractography. Grade 2 pattern of white matter tract changes suggested a less severe abnormality of the white matter tracts compared to grade 3 white matter tract changes. The presence of intact color vector hue of the white matter tract despite a change in the size of the tract could be secondary to reduce axonal diameter or axonal density, or less severe abnormality of myelin, and preserved orientation of the white matter tracts. We have found no significant change in λmajor in the subcortical white matter adjacent to MCD, which suggested that there was no significant abnormality of axons. Less severe abnormality of myelin or additional factors that remain to be elucidated, may contribute to reduce white matter tract size.

We have found that 7 of the 8 cases with focal cortical dysplasia demonstrated grade 3 pattern of white matter tracts. The white matter tracts evaluated consisted of major tracts projecting to/from the focal cortical dysplasia and contain many more axons than just those arising from the area of the cortex affected by the focal cortical dysplasia visible on MR. Of these seven cases, one also had grade 2 pattern involving the white matter tract, that is, thinning of the white matter tract but preserved orientation and geometry of the tract. Lee et al. (2004) have found a reduction of the subcortical fibers and interruption of the connection between the subcortical and deep white matter in all cases of focal cortical dysplasia. These authors have selected cases of focal cortical dysplasia based on the location of the lesion, where seed point of tractography could be placed either along the superior longitudinal fasciculus or posterior corona radiata. We have not applied such selection criteria to our patients. However, we have also found loss of color hues on color vector maps and failure of the white matter tracts to connect to the cortex affected by focal cortical dysplasia. In one case of focal cortical dysplasia, no abnormality was visualized in the white matter tracts. This may be related to the location of the lesion in the middle temporal gyrus, which may lead to reduced conspicuity of white matter tract changes.

Despite the lack of significant difference in the FA, trace, λmajor, λmedium and λminor in the subcortical white matter of those with and without epilepsy, those who presented with epilepsy were more likely to be associated with grade 3 pattern of white matter tract changes in the major white matter tracts. However, this may be biased as over half of our patient population has focal cortical dysplasia and all the cases of focal cortical dysplasia presented with epilepsy. The loss of color hue on color vector map and failure of tracking on tractography could be the direct effect of focal cortical dysplasia and/or secondary effects of seizures. Serial quantitative measures of cerebral gray and white matter, CSF and intracranial volumes in cases with chronic active epilepsy have demonstrated volume changes (Lemieux et al., 2000; Liu et al., 2001). Volume loss in chronic epilepsy has been attributed to neuronal loss (Mathern et al., 2002). Sankar et al. (1995) have found that alteration in the MR signal characteristics of microscopic cortical lamination defects was attributed to alteration in myelination secondary to seizures. Even though the difference in FA, λmedium and λminor in the subcortical white matter of those with and without epilepsy was not significant, the FA was reduced, λmedium and λminor were elevated in those with epilepsy compared to those without epilepsy. There were several confounders in this study. The number of cases evaluated, particularly those without epilepsy, was small. A variety of MCDs have been evaluated in this study; some MCD such as focal cortical dysplasia were more epileptogenic compared to others such as heterotopia. Those cases that did not present with epilepsy were younger than those that presented with epilepsy. It is possible that those that did not present with epilepsy at the time of the study may develop epilepsy later in life. Longitudinal study is required to clarify the effects of chronic epilepsy on the white matter tracts.

Two of the remaining five cases of MCD had malformations that extended across the cerebral mantle. These two cases of transmantle MCD had grade 3 pattern of white matter tract changes located adjacent to the malformations. Any malformation that traverses the extent of the cerebral mantle is likely to disrupt the white matter tracts. A second case of subcortical heterotopia, but nontransmantle, did not demonstrate grade 3 white matter tract changes. In this case, the adjacent white matter tracts showed grade 2 changes, that is, smaller caliber but preserved directionality of the tracts. Some of the white matter tracts in the case of subcortical heterotopia were displaced by the physical effect of the malformation. Aberrant fiber tracts were identified in two cases, polymicrogyria and schizencephaly. In the case of polymicrogyria, the white matter tracts demonstrated grade 2 pattern, with reduced caliber but preserved directional color hues. Increased number of transversely oriented fibers was identified in the subcortical white matter of the affected hemisphere. The transversely oriented tracts in the subcortical white matter may represent aberrant or modified tracts related to abnormal organization of the cortex. However, we do not have pathological correlation for the aberrant or modified white matter tracts. In the case of schizencephaly, a bundle of transversely oriented white matter tract was identified posterior to the cleft. This transversely oriented tract appeared to be continuous with the superior longitudinal fasciculus that was partly displaced by the schizencephaly.

In summary, we have found an increase in λmedium and λminor (transverse or radial diffusivities), but no significant change in λmajor (parallel or axial diffusivity) in the subcortical white matter adjacent to MCD. The increase in transverse or radial diffusivities implied that abnormal myelin played a more dominant role in the observed decrease FA. We have also found that alterations in major white matter tracts were observed in twelve of the thirteen cases of MCD. Grade 3 pattern of major white matter tract changes were more likely to be visualized in focal cortical dysplasia and transmantle MCD. Although there was no significant difference in FA, trace, λmajor, λmedium and λminor in the subcortical white matter of those with and without epilepsy, those with epilepsy were more likely to be associated with grade 3 pattern of the major white matter tracts. Longitudinal study with larger number of cases is required to clarify the effects of epilepsy on the white matter.

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