A review of various imaging techniques for diagnosis of the epilepsy

Background: Epilepsy is one of the most common neurological disorders with an estimated prevalence of 0.4-1.4% in human populations. 61% of epilepsy patients suffer from Localization Related Epilepsy (LRE). Also, Focal Cortical Dysplasia (FCD) or Cerebral cortex Dysplasia is known to be the main cause of disorder in a huge fraction of the epilepsy patients. About 30% of epilepsies are resistant to drug treatments. Therefore, their only treatment is to remove the lesion by surgery. Accordingly, the exact diagnosis of epilepsy cores in the brain is vital. For the detection of various types of epilepsy, in addition to recording electrical and magnetic brain signals such as Electroencephalography (EEG) and Magneto Encephalogram (MEG), different imaging techniques such as magnetic resonance imaging (MRI), positron emission tomography (PET), single positron emission tomography (SPECT), magnetic resonance spectroscopy (MRS) and etc. are applied. Since, the precision, sensitivity and the application of these diagnosis methods are different, the aim of this study was to investigate various detection techniques for diagnosing an epileptic region and to examine the advantages and disadvantages of each of these methods. Methods: the presented study is a review. The literature search was performed by using two different databases, which included PubMed and Google scholar to obtain the relevant publications. The search terms were EEG and epilepsy diagnosis or detection . MEG and epilepsy diagnosis or detection , MRI and epilepsy diagnosis or detection , MRS and epilepsy diagnosis or detection , PET and epilepsy diagnosis or detection and SPECT and epilepsy diagnosis or detection . Any primary and secondary papers pertaining to epilepsy diagnosis included in this review. 29 articles were obtained in the first stage. After removing the repetitive articles and reviewing the abstracts 6 articles were selected and reviewed full text according to the aim of the study. Findings: MEG recording is a reliable way to detect epileptogenic lesions. Implementing this method with MRI, can have a significant role in determining the exact location of the lesions before surgery. MRI is a pre-operative standard to detect epilepsy, which neurologists emphasize on using it. According to studies, MR imaging with higher magnetic fields (e.g. 7 Tesla) can detect epileptogenic regions that are not recognized in other detection techniques. Due to higher signal to noise ratio in the MRI system with higher magnetic field intensity, it can detect abnormal areas in the brain tissue more sensitively. In addition to better diagnose of the epileptic lesions, the MRI method avoids unnecessary surgery. PET may be useful in LRE patients which had before been diagnosed with EEG. In this technique, the FDG_PET (Fludeoxyglucose Positron Emission Tomography) method is used to identify the abnormal regions. In the some of these cases the MRI of the patient may not show any abnormal status. Also, FDG_PET is important in the diagnosis of epilepsy when clinical information does not match the individual EEG, or aggressive focus of epilepsy are more than one. The sensitivity of the FDG_PET method can be increased with the help of statistical analysis tools such as Statistical Parametric Mapping (SPM).SPECT is very valuable in the study of LRE patients especially for patients with abnormal MRI. MRS is also used to investigate functional disorders of neurons. This technique can be helpful in the epilepsy detecting. The MRS method can measure the amount of NAA (N.Acetylaspartate acid) in the brain tissue. This diagnostic method, shows the reduction of NAA levels in the epileptogenic regions of the brain tissue. Also, MRS can display metabolic disturbances between the different parts of the brain. The information obtained from this method can be useful in identifying the network distribution patterns (interface) in locating epilepsy-related points (LREs). The results of this test can indicate the area for surgery. Conclusion:In this paper, a variety of techniques that are used to diagnose epilepsy were studied. The high-field MRI is useful in identifying nonlesional epilepsy cases. However, in some cases, the use of MRI with a high magnetic field in epileptogenic area detection may have errors. Therefore, other methods such as PET, SPECT and Functional MRI may be useful. PET can help to identify the epileptic area by implementing the suitable radiotracer. This method has a high sensitivity in the detection of external lesions of the temporal lobe. But, the moderate signal-to-noise ratio of PET, beside using the expensive, unstable and short half-life radiotracers, are the limitations of this imaging modality in epilepsy diagnosis. In addition to the above, this method does not play a role in identifying nonlesional epilepsy areas. In the SPECT imaging method, more accessible and more stable radiotracer than the PET method is used. But this method also has some diagnostic limitations. The MRS technique also detects epileptogenic areas, similar to the FDG_PET method, through the metabolic abnormality of that area. In some cases, the use of the MRS method after an epilepsy surgery provides valuable information that is not provided in the post-operative MRI image. But the MRS technique also has limitations. In this way, the entire brain tissue is not covered, and the accumulation of adipose tissue in the cerebral cortex prevents its proper implementation.

Keywords:

Epilepsy , MRI , EEG , PET , SPECT , MRS , MEG.