Utility of diffusion tensor imaging parameters for diagnosis of hemimegalencephaly

Abstract

Background: Hemimegalencephaly is a rare hamartomatous entity characterised by enlargement of all or part of the cerebral hemisphere ipsilaterally with cortical dysgenesis, large lateral ventricle and white matter hypertrophy with or without advanced myelination. Although conventional magnetic resonance imaging (MRI) is useful for detecting these diagnostic features, hemimegalencephaly is not always easily distinguished from other entities, especially when hemimegalencephaly shows blurring between the grey and white matter. Diffusion tensor imaging (DTI) is a functional MRI technique commonly used to assess the integrity of white matter. The usefulness of DTI in assessing hemimegalencephaly has not been fully elucidated. In this study, we clarified the characteristics of hemimegalencephaly with regard to DTI and its parameters including fractional anisotropy and apparent diffusion coefficient.

Methods: Three patients with hemimegalencephaly underwent MRI including DTI. We first visually compared fractional anisotropy mapping and conventional MRI. Next, we quantitatively measured the fractional anisotropy and apparent diffusion coefficient values in the subcortical white matter of the hemisphere with hemimegalencephaly and corresponding normal-appearing contralateral regions and analysed the values using the Mann-Whitney U test.

Results: On fractional anisotropy mapping, we could clearly distinguish the junction of grey and white matter and observed thicker white matter in the hemisphere with hemimegalencephaly, which was unclear on conventional MRI. The white matter in the hemisphere with hemimegalencephaly showed significantly higher fractional anisotropy (P<0.0001) and lower apparent diffusion coefficient (P=0.0022) values than the normal contralateral side.

Conclusion: DTI parameters showed salient hemimegalencephaly features and could be useful in its assessment.

Keywords: Hemimegalencephaly; apparent diffusion coefficient; diffusion tensor imaging; fractional anisotropy; magnetic resonance imaging.

Figure 1.

Representative locations of regions of interest. ROIs are placed symmetrically at subcortical regions beneath the cortex in the affected hemisphere and the corresponding normal-appearing contralateral regions on (a) FA and (b) ADC mapping. 128 × 76 mm (300 × 300 DPI). ROI: region of interest; FA: fractional anisotropy; ADC: apparent diffusion coefficient.

Figure 2.

Patient 1. A 1-month-old boy with right-sided HME. (a) On T2 and (b) T1-weighted images, the junction of cortical grey and white matter is inconspicuous as a result of the decreased signal of white matter on the T2-weighted image and increased signal on the T1-weighted image, suggesting advanced myelination. (c) FA and (d) colour FA maps clearly show the junction of grey and white matter as well as increased white matter volume in the right hemisphere. The colour FA map shows fibre directions; anteroposterior (green), right and left (red) and craniocaudal (blue). HME: hemimegalencephaly; FA: fractional anisotropy.

Figure 3.

Patient 3. An 18-year-old man with right-sided HME. The diagnosis was uncertain when the patient visited our hospital with a tentative diagnosis of brain tumour. (a) On T2 and (b) T1-weighted images, the junction of cortical grey and white matter is inconspicuous in the right frontal lobe. (a) On the T2-weighted image, the right frontal white matter shows slight hyperintensity compared with the contralateral normal side. (c) FA and (d) colour FA maps clearly show the junction of grey and white matter as well as increased white matter volume in the right frontal lobe, indicating a diagnosis of HME, not a brain tumour. The colour FA map shows fibre directions; anteroposterior (green), right and left (red) and craniocaudal (blue). HME: hemimegalencephaly; FA: fractional anisotropy.

Figure 4.

Box plot of FA and ADC values of all HME patients. The white matter in the hemimegalencephalic hemisphere (affected side) showed (a) significantly higher FA values (P < 0.0001) and (b) lower ADC values (P = 0.0022) than the normal contralateral side. FA: fractional anisotropy; ADC: apparent diffusion coefficient.