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Neuroimaging in Epilepsy

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Author(s): Mahmoud Motamedi

Journal: Iranian Journal of Radiology
ISSN 1735-1065

Volume: 6;
Issue: S1;
Start page: 122;
Date: 2009;
Original page

ABSTRACT
Introduction: The assessment of the problem of seizures requires knowledge of the clinical details and features of the seizures, the functional abnormality in the brain as shown on the EEG, and the structural assessment of the brain with an MRI study optimized for epilepsy. Usually MRI or computed tomographic (CT) scan should be performed in evaluating the cause of a newly diagnosed seizure disorder. MRI is preferred over CT because of its greater sensitivity and specificity for identifying small lesions."nBecause there is an option of surgical excision of the "seizure focus," which may cure the patient, the detection of a focal abnormality of the brain is important for the formulation of the reason for the seizures and the options available for treatment. Knowledge of the brain abnormalities early in the course of treating the patient greatly helps the management of each individual. The challenge to epileptologists is that the problem of epilepsy is a special one, which requires optimized protocols dedicated to it."nMRI interpretation is different when used in a screening way and when viewed in the context of other investigations. This is particularly important when the patient has partial seizures and may be considered for surgical treatment."nMost centers that deal with epilepsy spend a great deal of time in ensuring the quality of their EEG and EEG interpretation. However, unless there is a radiologist with an interest in epilepsy or an epileptologist who spends time with radiologist colleagues, it can be difficult to establish good epilepsy-focused MRI with appropriate sequences, radiography, and interpretation. MRI acquisition and interpretation need to be focused on the problem of epilepsy."nIndication"nThe American academy of neurology has published practice parameters for neuroimaging (NI) studies (MRI, CT) of patients having a first seizure. Emergent NI (scan immediately) should be performed when a health care provider suspects a serious structural lesion. Clinical studies have shown a higher frequency of life-threatening lesions in patients with new focal deficits, persistent altered mental status (with or without intoxication), fever, recent trauma, persistent headache, history of cancer, history of anticoagulation, or suspicion of acquired immunodeficiency syndrome."nUrgent NI (scan is included in the disposition or is performed before disposition when follow-up of the patient's neurological problem cannot be ensured) should be considered for patients who have completely recovered from their seizure and for whom no clear-cut cause (e.g., hypoglycemia, hyponatremia, tricyclic overdose) has been identified to help identify a possible structural source. Because adequate follow-up is needed to ensure a patient's neurological health, urgent NI may be obtained before disposition when timely follow-up cannot be ensured. Additionally, for patients with first-time seizure, emergent NI should be considered if the patient is over 40-years of age or has had partial-onset seizures. "nRadiography"nFor epilepsy protocol images, the orientation of the imaging plane must be according to "in-brain" landmarks. This means that the radiographer must understand the hippocampal axis and ensure that all imaging is in this plane. Because one will need to compare structures on the left and the right, the angulations of the imaging in the coronal plane must also be symmetric, based on in-brain landmark. Images can also be reconstructed in this appropriate axis from three-dimensional (3D) data sets. It is important that the epileptologist reviews the imaging and is satisfied with the quality of the imaging."nInterpretation"nIt is important to identify suspicious as well as definitely abnormal areas. In the interpretation of the MRI in epilepsy, it is not enough to evaluate the images as "abnormal" or, by default, "normal." The diagnosis of "normal" should be an active and positive one. If there is any change that is suspicious but not definitely abnormal, the diagnosis of normal is not appropriate. This "in-between" assessment can be very important in deciding how further investigations progress."nEven when imaging is excellent, the correct interpretation of subtle changes needs to be based on a clear understanding of the features of the abnormality that has been looked for. In the case of epilepsy, the major difficulty that confronts the radiologist, and the one with which the epileptologist must also be confident, is the reliable diagnosis of HS and subtle malformations of cortical development (MCDs)."nThe features of HS are altered signal in a small hippocampus and loss of the normal internal architecture. Not any asymmetry is HS, and an abnormal signal in a large hippocampus is usually hippocampal dysplasia rather than sclerosis. If one only looks for atrophy or asymmetry, the wrong hippocampus may be diagnosed as abnormal."nRequests"nMRI is like the EEG in terms of requirements for basic study, sleep-deprived study, extra electrodes, or video-monitored study. One can acquire many different bits of information using MRI that gives different degrees of assessment of the brain. This also involves having the patient potentially in the scanner for a long time. Although basic MRI is needed in nearly all patients, further studies need to be clinically driven and aimed at solving a specific problem. The epileptologist needs to clearly understand what information is needed and what can be obtained. The more focused the question that is asked of the radiologist, the better focused the assessment of the images will be. This is often the weak link in the otherwise excellent MR imaging. Effort to focus the request is well rewarded and interpretation of an MR depends on the electroclinical findings and is assessed along with other examinations, such as video-EEG telemetry, positron emission tomography (PET), and SPECT."nTypical Imaging Sequences for an Optimized Epilepsy Protocol"nA typical clinical scanning protocol for a patient with refractory epilepsy may include T1-weighted imaging, T2-weighted imaging, FLAIR imaging, and 3D volume acquisition sequences. Imaging must be in hippocampal axis and good signal -to -noise ratio is as important as thin slices. The application of contrast agents is indicated if there is suspicion of a primary or metastatic tumor, infection, or inflammatory lesion. In epilepsy centers, it must be possible to diagnose hippocampal sclerosis with high sensitivity and specificity with visual analysis."nOne should think good "epilepsy MRI" not "MRI." To achieve this, knowledge of MRI is needed not only by the radiologist, but also by the neurologist and epileptologist. It is this continuing interaction that enables optimized imaging to be obtained."nQuantification of Volumes and T2 Relaxometry in Epilepsy"nHigh-resolution T1-weighted 3D volume sequences can be used quantitatively to measure the volume of any particular regions of interest. In the case of epilepsy this is usually the hippocampus. Volumetric measurements require a significant investment in learning the hippocampal boundaries and depend on a large number of variables that need to be understood and controlled for in image analysis and acquisition. Volumetric assessment makes it possible to assess the progression of volumetric changes and may begin to help unravel the effects of the primary disease from the secondary effects of seizures. It can be performed manually or with half- or fully-automated software."nT2 relaxometry is the quantitative determination of the T2 relaxation time. To achieve this, several T2-weighted images are acquired at different echo times, and in each voxel the resultant values are fit with an exponential decay curve to estimate the T2 decay rate of the imaged tissue. T2 relaxometry has been established as a reliable tool that is stable over time. In contrast to elaborate volumetric assessment, the T2 relaxometry is a quick technique with small variance and can be implemented in large-scale studies."nIn epileptic patients with HS, signal increase on T2-weighted images is typically observed in the hippocampus. The measured values of the hippocampal volume and the T2 time are correlated with each other, indicating that a marked volume loss is associated with a significant increase in T2 relaxation, reflecting the complex pathology which is HS."nDiffusion-Weighted Imaging"nA diffusion-weighted signal reflects the molecular motion of water in the extra- and intracellular environments. Diffusion-weighted MR techniques are frequently used to assess early signs of cerebral ischemia. Diffusion changes similar to those observed in ischemia may also be present in tumors or infection."nSummary and Conclusions"nMRI studies are an essential component of the overall assessment of why a patient has epilepsy. This requires us to define and understand the epileptic events, the structural abnormalities in the brain, and the clinical context in which seizures occur. Unless high-quality information is obtained in all three of these domains, the basis of epilepsy in any individual has not been fully assessed."nIn this article, we have dealt with the need to define and understand the structural brain abnormalities by acquiring appropriate epilepsy-focused MRI of high quality and diagnosing the important lesions with high sensitivity and specificity because this is fundamental to good epileptology, albeit often difficult to implement in practice. The clinical context, seizure features, and interpretation of the imaging, with full knowledge of the hypothesized basis of each individual's epilepsy and the other investigations, are the key to the proper use of imaging.

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