Epilepsy
Figure 1: High Resolution Magnetic Resonance Imaging Showing Mesial Temporal Sclerosis
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Figure 2: Ultrahigh-field-strength Magnetic Resonance Imaging in Evaluation of Cryptogenic Epilepsy
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A: Coronal fast multiplanar inversion recovery (FMPIR) imaging demonstrating prominent atrophy of the right hippocampus in a 41-year-old woman with medically refractory temporal lobe epilepsy; B: Magnification of area in the left panel shows a smaller right hippocampus and subcortical signal abnormalities (arrowhead); C: Coronal FMPIR demonstrating subtle hippocampal changes in a 22-year-old woman with medically refractory epilepsy arising from the left temporal lobe; D: Two sequential slices of the hippocampus showing blurring of the CA1 region and other internal structures (arrowheads) within the left hippocampus compared with the right. Pathologic evaluation was consistent with mesial temporal sclerosis.
hippocampus rather than overt atrophy or T2 signal prolongation (see Figure 1B). Numerous studies have demonstrated that MTS is a positive prognostic indicator of good surgical outcome, with seizure-free rates ranging from 70 to 90 %.8–12
By contrast, the post-operative seizure-free
rate in TLE patients with unremarkable MRI is significantly lower, ranging from 50 to 60 %.13–15
To increase the sensitivity and specificity for detecting subtle MTS in patients with TLE, several automated analysis methods have been employed with some degree of success. The technique of voxel-based morphometry (VBM) was developed to detect subtle regional changes in tissue volume or density throughout the whole brain.16
Interesting
comparisons between patients with TLE and normal subjects using VBM revealed a widespread network of changes, including a reduction in gray matter volume in the cingulate, thalamus and frontal lobes.17–19 A meta-analysis of VBM studies in TLE noted consistent findings of volume loss in the ipsilateral hippocampus (82 % of all studies), parahippocampal gyrus (47 %), entorhinal cortex (23 %), contralateral hippocampus (17 %), thalamus (ipsilateral 61 %, contralateral 50 %), and parietal lobes (ipsilateral 47 %, contralateral 52 %).20
VBM studies are
prone to low specificity given the nature of the analysis; by warping the subjects’ MRI to a standardised space before comparing them, the differences may be an artifact from the warping process itself. Despite these limitations, using large control populations and other voxel-based analysis with different imaging sequences may reduce the false-positives and yield clinically relevant information regarding MTS.21
Recently, 3-Tesla (3T) and 7-Tesla (7T) MRI machines have been developed to provide even higher-resolution images. A study comparing 3T and 1.5T imaging in patients with MTS showed no substantial difference in surgical outcome between the two groups.
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A: A 30-year-old male with complex partial seizures arising from the right supplemental motor cortex and superior parietal region, with 7-Tesla (7T) axial spoiled gradient echo study showing a small focal area of cortical thinning in the right parietal lobe. The magnified view of the lesion in the right panel shows cortical dysplasia (arrowhead); B: 7T fast spin echo (FSE) showing increased signal in the area of thinned cortex.
However, the 3D short T1 inversion recovery (STIR) sequence using 3T is useful for evaluating the extent of hippocampal sclerosis and in identifying or clarifying uncertain findings in the 20 % of patients with previously unremarkable scans.22
In a recent prospective study performed by Henry and colleagues, 11 healthy subjects and eight patients with TLE underwent neuroimaging with 7T MRI. The authors found that ultrahigh-field-strength MRI permitted detection of selectively greater Ammon horn atrophy and paucity of digitations in the hippocampal head in patients with TLE.24
As we employ more 7T MRIs in the future,
these high-resolution images will undoubtedly provide more information regarding the subtle anatomical abnormalities seen in TLE and other subtle epileptogenic lesions.
Malformation of Cortical Development
Besides having an important role in identifying MTS, conventional MRI is crucial in identifying neocortical lesions in malformation of cortical development (MCD). MCD encompasses a spectrum of developmental disorders that are thought to result from abnormal neuronal proliferation and/or migration during cortical development. These disorders include mild MCD, focal cortical dysplasia, polymicrogyria, hemimegencephalay, schizencephaly and lissencephaly. MCD is one of the most common causes of medically refractory epilepsy in children and young adults. Some of the characteristic findings on imaging include abnormal gyration, thickened cortex, loss of gray–white junction, heterotopia, and signal changes on T2 (see Figure 3).25–27 a recent retrospective study of 143 patients with MCD, complete
In EUROPEAN NEUROLOGICAL REVIEW
higher) MR systems offer a greatly increased signal-to-noise ratio, allowing for increased spatial resolution and increased tissue contrast (see Figure 2).23
Ultrahigh-field-strength (7T and
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