Fig. 21.1
Axial FLAIR (a) demonstrates enlargement of the left lateral ventricle temporal horn and the left hippocampus is relatively smaller and hyperintense compared to the contralateral side. Note that the lateral aspect of the left hippocampal body is abnormally smooth, and hippocampal head digitations are reduced (arrow). Companion coronal FLAIR (b) and T2-weighted MRI (c) demonstrate volume loss, hyperintensity, and subtle laminar blurring. These are classic MRI findings for left hippocampal sclerosis. If the amygdala also is involved, this can be classified as left mesial temporal sclerosis
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Fig. 21.2
Coronal T2-weighted images demonstrating globular left hippocampus (arrow, panel a) more vertical left collateral sulcus and low-lying left body of the fornix (arrow, panel b) consistent with incomplete hippocampal inversion (IHI). This patient also had left hippocampal sclerosis, but it remains controversial whether IHI predisposes to sclerosis or is just an incidental association
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Fig. 21.3
Coronal FLAIR (a), T2-weighted (b) and 3D T1-weighted demonstrate bilateral hippocampal body hyperintensity, laminar blurring, and volume loss, respectively. Findings are consistent with bilateral hippocampal sclerosis
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Fig. 21.4
Oblique coronal T2 demonstrates obvious volume loss and laminar blurring of the right hippocampal head (arrow, panel a) consistent with right hippocampal sclerosis and right fornix atrophy (b). There is a small gray matter heterotopia in the lateral wall of the right lateral ventricle, best seen on the coronal double-inversion recovery image (arrow, panel c) compared to companion T2 and T1-weighted MRI (b and d)
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Fig. 21.5
Axial and coronal T2-weighted MRI, and coronal FLAIR demonstrate a small encephalocele that involves a focal portion of the fusiform gyrus cortex extending into the right foramen ovale (arrows). The MRI abnormality is not always associated with seizures, but should be considered suspicious. In this case, the finding was concordant with semiology and EEG
Neuronal migrational disorders: Heterotopias are neuronal migrational disorders (NMDs) where gray matter gets arrested as neurons migrate from periventricular regions toward pia during embryonic stages. High-resolution 3D T1-weighted volumetric imaging provides superior gray–white contrast that is critical to identify subtle cortical malformations in patients with epilepsy (Fig. 21.6). Higher magnetic strengths (3- or 7-Tesla) can detect very subtle cortical dysplasias.
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Fig. 21.6
Coronal FLAIR MRI (a) and serial axial FLAIR MRI of the frontal lobes (b) demonstrating subtle gray–white blurring and FLAIR hyperintensity in the left anterior cingulate gyrus and adjacent left medial frontal gyrus (arrows) from a pathologically proven cortical dysplasia
Heterotopias can either be focal, nodular, or multifocal (as in TS) or preferentially involve one hemisphere as in hemimegalencephaly. Subcortical band heterotopias (SBH) are typically periventricular, bilateral nodular collections of gray matter with relatively smooth margins, which gives the appearance of a double cortex. Pachygyria is abnormal tissue in the right location with abnormal sulcation and gyration of the mantle which is typically > 8 mm thick (Fig. 21.7a). Polymicrogyria (PMG) is either two- or four-layered cortex, which is less than 5–7 mm (Fig. 21.7b). PMG is commonly associated with hypoxic-ischemic injury, or prenatal cytomegalovirus (CMV) infection.
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Fig. 21.7
a Coronal 3D T1-weighted (A) and axial FLAIR MRI (B) demonstrate broad, simplified gyri with relatively shallow sulci in the bifrontal regions compared to the temporal and parietal regions consistent with pachygyria. b Coronal and axial FLAIR (A and B), sagittal, and axial post-contrast 3D T1 (C and D) MRI show abnormal cortex that appears thickened extending from the posterior right Sylvian fissure (arrow). The fissure is lengthened with a vertical orientation toward the vertex (arrow, panel C). Findings consistent with unilateral polymicrogyria
FCDs are classified into three categories (Type I, II, and III) and further divided into various subtypes (Table 21.1). In a fully myelinated brain, FCD type I may be characterized by subtle blurring of the gray–white junction with typically normal cortical thickness, moderately increased white matter signal hyperintensities on T2/FLAIR images and decreased signal intensity on T1-weighted images. FCD Type IIA cortical dysplasias are characterized by marked blurring of the gray–white junction on T1 and T2-FLAIR images due to hypomyelination or dysmyelination of the subcortical white matter with or without cortical thickening. Here, the increased white matter signal changes on T2, WI, and FLAIR images frequently tapers toward the ventricles (aka the “transmantle sign”) which marks the involvement of radial glial neuronal bands. This radiological feature differentiates FCD from low-grade tumors. Type II lesions are more commonly seen outside the temporal lobe with predilection for the frontal lobes. Type III FCD is typically associated with another principal lesion such as hippocampal sclerosis, tumor, a vascular malformation, or other acquired pathology during early life.
Table 21.1
Classification of focal cortical dysplasias
Types | Features |
---|---|
Type I | Ia: abnormal vertical alignment of neurons Ib: abnormal horizontal alignment Ic: horizontal and vertical malalignment |
Type II | IIa: dysmorphic neurons without balloon cells IIb: dysmorphic neurons with balloon cells |
Type III | IIIa: mesial temporal sclerosis IIIb: glioneural tumors e.g., ganglioglioma, DNET IIIc: vascular malformations (CCMs, AVMs, telangiectasias, and meningoangiomatosis) IIId: prenatal or perinatal ischemic injury, TBI, and scars due to inflammatory or infectious lesions |
Other important NMDs include lissencephaly, which is characterized by smooth brain surface and abnormal gyration, which varies between agyria and pachygyria. Lissencephaly with posteriorly predominantly gyral abnormalities is caused by mutation in LIS1 gene (Fig. 21.8). Anteriorly predominant lissencephaly in heterozygous males and subcortical band heterotopia (SBH) in heterozygous females are caused by mutations of the XLIS (double cortex gene on chromosome X). Schizencephaly is another rare form of MCD which is characterized by the presence of a transcortical cleft that can extend from ventricles to the pia with open or fused lips, and often polymicrogyria is seen on the lips of the schizencephaly (Fig. 21.9). Hemimegalencephaly is the unilateral hamartomatous excessive growth of all or part of one cerebral hemisphere at different phases of embryologic development. MRI in these cases reveals an enlarged hemisphere with increased white matter volume, cortical thickening, agyria, pachygyria, polymicrogyria or lissencephaly, and blurring of the gray–white matter junction. Often, a large, ipsilateral irregularly shaped ventricle may be seen.
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Fig. 21.8
MRI brain axial T2-weighted image shows lissencephaly agyria and smooth brain. Especially posteriorly in a patient with LIS1 mutation
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Fig. 21.9
CT brain axial image shows a cleft in the left hemisphere consistent with schizencephaly
Brain tumors: Approximately 20–40% of the adults with primary brain tumors experience one seizure prior to the tumor diagnosis, and another 20–45% will suffer from seizures during the course of the illness [1]. This incidence rate varies depending on the tumor type, the grade of the tumor, and its location. Seizures are more common in slow growing tumors such as meningiomas, gangliogliomas (GGs), dysembyoplastic neuroepithelial tumors (DNETs), or diffuse low-grade tumors such as Grade II astrocytomas, oligodendrogliomas, and oligoastrocytomas (Table 21.2). Typically, the low-grade tumors do not enhance on Gd-administration. The most common location is temporal lobe, followed by the parietal, frontal, and occipital lobes. Gangliogliomas typically present with temporal lobe epilepsy, presumably due to the temporal lobes being a favored location (Fig. 21.10). Gangliogliomas are closely related to gangliocytomas, which contain essentially only mature neural ganglion cells, and ganglioneurocytoma, which in addition have small mature neoplastic neurons. On MRI, these tumors may show cystic changes or calcifications.
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Table 21.2
Common epilepsy associated tumors
Tumors | Radiological features |
---|---|
Meningioma | Isointense on T1 and T2; homogeneous enhancement with Gd, extra-axial, dural tail, and CSF cleft sign |
Ganglioglioma | Cyst with enhancing mural nodule/solid; calcifications in ~50% |
Dysembryoplastic neuroepithelial tumors (DNET) | Bubbly cystic appearance with small cysts within the tumor that are hyperintense on T2WI, wedge shaped mass which expands the affected gyri and point toward the ventricle, swollen gyrus, may be associated with focal cortical dysplasia |
Pleiomorphic xanthoastrocytoma (PXA) | Supratentorial cyst with enhancing mural nodule which abuts the peripheral meninges, peritumoral edema, mild meningeal enhancement |
Oligodendroglioma | Hypointense on T1, hyperintense on T2, calcification seen as areas of blooming, 50% enhance heterogeneously, minimal peritumoral edema |
Hypothalamic hamartomas | Nonenhancing non-neoplastic congenital gray matter heterotopia in the region of tuber cinereum of the hypothalamus which can be sessile or pedunculated |
Subependymal giant cell astrocytomas (SEGA) | Heterogeneous mass near the Foramen of Monro, usually >1 cm; hypo or isointense on T1 and hyperintense on T2, marked enhancement; other findings of Tuberous Sclerosis such as cortical tubers and subependymal nodules, “transmantle sign” in some tubers; nodular, ill-defined, cystic and band-like lesions seen in the white matter and radial bands |
Glioblastoma multiforme (GBM) | Hypo or isointense on T1, hyperintense on T2, vasogenic edema, susceptibility artifact on T2 from intratumoral lesions due to hemorrhage or rarely calcification, “butterfly glioma” when bilateral and cross the corpus callosum, necrosis may be present, peripheral or irregular nodular enhancement; no diffusion restriction but lower ADC than low-grade tumors |
Metastases | Hypointense on T1 (except melanomas can be hyperintense), hyperintense on T2 and FLAIR, intense enhancement (ring-enhancing, punctate or uniform), often multiple lesions present at diagnosis, vasogenic edema out of proportion to the size of the lesion, hemorrhage, and necrosis may be seen |
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Fig. 21.10
Axial FLAIR (a), T2-weighted (b), pre-contrast (c), and post-contrast T1-weighted MRI (d) demonstrating solid and cystic mass in the left posteromedial temporal lobe with focal areas of contrast enhancement most consistent with ganglioglioma. CT also often demonstrates focal calcification
DNETs are cortically based benign neoplastic cortical malformations, which may show subcortical extension in approximately 30% of tumors giving them a triangular appearance (Fig. 21.11). These tumors appear as well-defined lobulated, and solid tumors that are hyperintense on T2WI and may erode the overlying calvarial bone or show microcystic changes. The most common location is temporal (60%) followed by temporal lobes (30%). Meningiomas are the most common extra-axial tumors of the central nervous system. They are nonglial neoplasms that originate from the arachnoid cap cells of the meninges and have characteristic imaging findings, although there are many variants (Fig. 21.12). GBMs are aggressive malignant tumors that are associated with significant vasogenic edema and heterogeneous enhancement. The overall incidence of seizures in Grade IV glioblastoma multiforme (GBM) patients, without considering the location, has been reported between 25 and 50% at presentation and another 20–30% during the course of the disease [2]. Metastatic lesions tend to have a smaller risk for seizures, one exception being metastatic melanoma.
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Fig. 21.11
Axial T2- and T1-weighted MRI (a and b) demonstrate bubbly T2 bright lesion in the left superior frontal gyrus with minimal mass effect that does not enhance (contrast not shown). In a patient with seizures, these findings are most consistent with dysembyroblastic neuroepithelial tumor (DNET)
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Fig. 21.12
Axial pre-contrast (a) and post-contrast T1-weighted MRI (b) demonstrate an extra-axial mass overlying the right frontal operculum that is isointense to gray matter and shows homogeneous enhancement with dural tail consistent with meningioma
Perfusion-weighted imaging is a useful tool, which involves several image acquisitions during the first pass of a bolus of contrast agent. This method allows the radiologist to determine the relative cerebral blood volume (rCBV). In general, the underlying principle is the greater the rCBV, the higher the grade of tumor. Lack of notable flow indicates a nonneoplastic etiology with abnormal signal intensity, such as demyelination. Of note, mixed oligodendrogliomas can have low rCBV. Besides the prognostic information it provides, perfusion-weighted imaging can increase the yield of brain biopsy and help in differentiating recurrent neoplasm from radiation necrosis. On perfusion MRI, GBMs typically show increased regional blood flow (Fig. 21.13).
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Fig. 21.13
MRI brain post-Gd T1-weighted image (a) showing ring enhancement in the right thalamus and deep gray structures with associated mass effect, midline shift, and obstructive hydrocephalus. Pathology was consistent with glioblastoma multiforme. Perfusion MRI (b) shows increased regional cerebral blood flow at the tumor margins
Another interesting but rare kind of focal congenital tumor is hypothalamic hamartoma (HH). These tumors are typically associated with ictal spells of laughter without mirth or gelastic seizures (Fig. 21.14). HH are composed of cytologically normal, small, and large neurons, which are organized in poorly demarcated clusters of variable size and density. These tumors are categorized by the Delalande classifications I–IV. Type I has a horizontal orientation and may be lateralized on one side; Type II has a vertical orientation and an intraventricular location; Type III is a combination of types I and II; Type IV is a giant hamartoma.