Magnet resonance imaging (MRI), is frequently used, along with other data, to generate a diagnosis of epilepsy. The MRI is a noninvasive imaging tool that scans the structure of the whole brain to identify abnormalities. The MRI, alone, gives no information on physiological and pathological functions, only structure. Fortunately, adjuncts and variants of the MRI and related techniques used to image epilepsy record changes in biological function. Progress in imaging epilepsy improve diagnostic accuracy as well as expand understanding of epilepsy. This blog will discuss progress in imaging epilepsy.
Progress in Imaging Epilepsy – MRI-related
MRI dependent imaging include 1) functional MRI and associated improvements, 2) diffusion MRI and 3) 1H-magnetic resonance spectroscopic imaging (MRSI). The original MRI, using magnetic fields, detects subtle changes in brain structure, e.g. volume and contour differences in neuronal clusters particularly when newer technical adjustments are utilized (1). Thus MRI imaging successfully identified differences in brains of normal individuals and those with temporal lobe epilepsy (1). It is equally useful in detecting tumors and areas of traumatic brain injury.
1) Functional MRI and advancements.
The functional MRI (fMRI), a variant of the MRI, is also a non invasive technique. It tracks blood oxygen levels in the brain. The method relies on the observation that active neurons (e.g. those involved in brain-related activities such as thinking) consume more oxygen than quiescent neurons. Thus, areas of high blood oxygen use pinpoint sites of activated functional neurons. This technique succeeded in mapping areas involved in cognition, stress, and emotions (1). Therefore, combining an EEG with a fMRI improves location of epileptogenic neurons triggering and supporting a seizure. Such information is especially important in ablation surgery for drug-resistant patients with severe epilepsy (1). fMRI is successfully used to follow the path of propagation of the seizure throughout the brain, providing evidence on how one part of the brain influences another during epilepsy progression (1).
2) Diffusion-weighted MRI/Diffusion Tensor MRI.
These imaging techniques measure tissue water movement in cells and surrounding space in the brain. Each technique does it slightly differently with the oldest of the two as the diffusion-weighted MRI (3). These techniques are advantageous in the diagnosis of pediatric epilepsy (4) and assessment of acute stroke(3) . Research results using this imaging track and define the connectivity between neurons during brain-related activities, expanding knowledge of brain physiology (3).
3) 1H-magnetic resonance spectroscopic imaging (MRSI).
This is an old technique that is generating renewed interest due to its enhanced magnetic resonance sensitivity (5). MRSI detects substances whose molecular components contain high amounts of 1H protons. The main substances are N-acetyl-aspartate (NAA), creatine, and choline. Results of numerous studies show that the ratio of NAA to creatine provides information on abnormal neuronal activity associated with oxidative stress. This imaging opens up research on the relation between oxidative stress and neurotransmitters such as GABA (gamma amino butyric acid), a neurotransmitter implicated in epilepsy (5).
Progress in Imaging Epilepsy – Radioactive Imaging.
In contrast to the MRI, PET and SPECT are invasive imaging methods that inject radioactive substances to study brain activity in disease.
1) PET stands for Positron Emission Tomography.
This imaging technique employs a positron emitting chemical (tracer) such as a radioactive variant of glucose (fluorodeoxyglucose). Active neurons and auxiliary cells take up the sugar thus highlighting an area of brain activity. This is used to identify tumors which are more metabolically active than normal tissue. PET is also valuable in tracing blood flow in metabolic regions during a seizure. While capture of a seizure during a PET scan is rare, measurement of activity between seizures is more likely and provides important information. Here, areas of low metabolism (hypometabolism) indicate sites of epileptic origin and are useful aids in surgical ablations (1).
The PET also uses radioactive analogues of neurotransmitters to uncover changes in brain receptor activity in epilepsy (6). Additionally, PET imaging reveals an increase in a specific marker of brain inflammation, the translocator protein (TSPO). TSPO is elevated in epilepsy (7). Its elevation indicates that auxiliary cells (glial cells) are active and contributing to brain inflammation (8). See earlier blog on the role of inflammation in epilepsy (Neuroinflammation of Epilepsy – new diagnostic tools)
2) SPECT stands for Single photon emission computed tomography.
This imaging technique is similar to PET but differs in that the radioactive tracers emit a different type of radiation. Gamma rays are detected by SPECT. For epilepsy diagnosis, SPECT is used secondarily to PET (9). It is considered important for surgical candidates with drug resistant epilepsy in which no structural lesions are found. This technique is highly successful in localization of the site of seizure origin in pediatric epilepsy (10).
Progress in Imaging Epilepsy – Magnetoencephalography
Magnetoencephalography (MEG) differs radically from the above discussed techniques. Specifically, it does not use magnetism or radioactivity to “shake up or interact with” with molecules in brain to define structure and function (11). Rather, the MEG detects magnetic fields generated or given off from nerves after they transmit their signal (termed postsynaptic activity). Because magnetic waves are received without distortion, the location of epileptogenic foci are more accurately identified. MEG is considered an advanced version of the EEG. Similar to the EEG, it is non invasive. However, the EEG records electrical activity without the ability to assign origin to such activity. In contrast, the MEG accurately locates the origin of abnormal nerve activity. MEG associated with PET or MRI data is of great assistance in surgical ablations for severe epilepsy (11). Additionally, findings from this technique suggest the existence of connectivity between several areas of the brain during seizures, expanding understanding of the complexity of epilepsy (1).
Conclusions
This blog describes a number of sophisticated imaging techniques and methods used to assist with the diagnosis of epilepsy, illustrating progress in imaging epilepsy. These include variations of the classic MRI, radioactive imaging and an upgraded version of the EEG. The majority of these methods provide critical data to pinpoint epileptogenic foci for surgical ablations in cases of drug-resistant severe epilepsy. Although accessed less frequently for research, the insights that have been gained thus far are invaluable. These techniques offer potential for deeper understanding of the pathology of epilepsy and hence better therapy and a possible cure.
References http://pubmed
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