Specific Imaging Findings

Focal cortical dysplasia (FCD) Type I shows only localized blurring of the gray–white matter junction and sometimes decreased volume of the subcortical white matter and cortex that may be detected with dedicated high spatial resolution heavily T1-weighted inversion recovery spin echo and 3D gradient echo images. The lesions are preferentially located at the bottom of an abnormally deep sulcus. The subcortical white matter may show hyperintense T2 signal, best depicted on high-resolution FLAIR images. These findings can be very subtle, typically not seen on CT and routine MRI scans, and in a significant number of cases not even on dedicated MR imaging. Functional studies (PET, SPECT and MEG) may be able to localize the seizure focus and tailored MRI of the suspicious area with a surface coil may then depict the lesion. Co-registration of PET and MR images substantially increases sensitivity. FCD Type II shows localized cortical thickening and T2 hyperintensity, which can characteristically extend in a tapered linear fashion towards the ventricle, known as the transmantle sign. Gray–white matter junction blurring and subtle white matter T1 hyointensity may be present. The gyral pattern may be abnormal with broad gyri and irregular sulci. A lesion detected on imaging is not necessarily the seizure focus, and FCD may occur in a multifocal and multilobar distribution.