Magnetic resonance imaging and temporal lobe epilepsy

Structural Neuroimaging of the Medial Temporal Lobe in Alzheimer's Disease Clinical Trials

Atrophy in the medial temporal lobe (MTA) is being used as a criterion to support a diagnosis of Alzheimer's disease (AD). There are several structural neuroimaging approaches for quantifying MTA, including semiquantitative visual rating scales, volumetry (3D), planimetry (2D), and linear measures (1D). Current applications of structural neuroimaging in Alzheimer's disease clinical trials (ADCTs) incorporate it as a tool for improving the selection of subjects for enrollment or for stratification, for tracking disease progression, or providing evidence of target engagement for new therapeutic agents. It may also be used as a surrogate marker, providing evidence of disease-modifying effects. However, despite the widespread use of volumetric magnetic resonance imaging (MRI) in ADCTs, there are some important challenges and limitations, such as difficulties in the interpretation of results, limitations in translating results into clinical practice, and reproducibility issues, among others. Solutions to these issues may arise from other methodologies that are able to link the results of volumetric MRI from trials with conventional MRIs performed in routine clinical practice (linear or planimetric methods). Also of potential benefit are automated volumetry, using indices for comparing the relative rate of atrophy of different regions instead of absolute rates of atrophy, and combining structural neuroimaging with other biomarkers. In this review, authors present the existing structural neuroimaging approaches for MTA quantification. They then discuss solutions to the limitations of the different techniques as well as the current challenges of the field. Finally, they discuss how the current advances in AD neuroimaging can help AD diagnosis.

A low-field intraoperative MRI system for glioma surgery: is it worthwhile?

As intraoperative MRI expands its presence, its use will undoubtedly increase in glioma surgery. The foregoing discussion makes it clear that its benefits are unsurpassed by any other existing system. Because of their radiographic characteristics and gross appearance, gliomas are particularly suited for intraoperative MRI-guided surgery. It enables us to localize gliomas and define tumor margins precisely when, during surgery, the difference between tumor and brain is not easy to discern. The images generated during surgery serve as a detailed and updated map within which navigation is performed with utmost precision. Its significance is further highlighted when dealing with tumors in eloquent areas of the brain, where uncertainties over the location of tumor in relation to important brain structures can hinder the removal of tumor. By providing accurate positional information and in conjunction with cortical mapping techniques, intraoperative MRI enhances the confidence of the surgeon to go forward with resection or to stop when reaching important cortex. It allows us to perform the resection to the desired limit without causing injury to nearby important structures, thereby preventing postoperative neurologic deficits. The tracking system guides us in targeting each minute part of the tumor with unprecedented accuracy, and the ability to update images makes possible the constant evaluation of the progress of surgery. This near-real-time imaging can eliminate the errors brought about by the brain shifting that occurs throughout surgery. It also serves the important purpose of verifying the presence and position of any remaining tumor in the operative field. By means of sequential imaging, additional resection can be performed on any remaining tumor until imaging shows completion. The unwanted occurrence of finding residual tumor on a postoperative scan is thus practically eliminated. As a result, the surgical goal of complete or optimal resection can be achieved without any guesswork. Ultimately, what this means for the glioma patient is increased likelihood of longer survival brought about by a more thorough tumor resection. Intraoperative MRI addresses many of the surgical challenges posed by gliomas. As it becomes more available, there will come a point when the prevailing persuasion will be that some poorly defined tumors near eloquent cortex should not be operated on without intraoperative MRI. In the final analysis, not only is intraoperative MRI worthwhile but it will, in all likelihood, become a standard of care for many glioma cases.

Magnetic resonance imaging of changes elicited by status epilepticus in the rat brain: diffusion-weighted and T2-weighted images, regional blood volume maps, and direct correlation with tissue and cell damage

The rat brain was investigated with structural and functional magnetic resonance imaging (MRI) 12 h after the arrest of pilocarpine-induced status epilepticus lasting 4 h. Histopathological data, obtained immediately after MRI analysis, were correlated with the images through careful evaluation of tissue shrinkage. Diffusion-weighted and T2-weighted imaging showed changes throughout the cerebral cortex, hippocampus, amygdala, and medial thalamus. However, only T2-weighted imaging, based on rapid acquisition relaxation-enhanced sequences, revealed in the cortex inhomogeneous hyperintensity that was highest in a band corresponding to layer V. Regional cerebral blood volume (rCBV) maps were generated using T2*-weighted gradient-echo images and an ultrasmall superparamagnetic iron oxide contrast agent. In the cortex, rCBV peaked in superficial and deep bands exhibiting a distribution complementary to the highest T2-weighted intensity. Selective rCBV increase was also documented in the hippocampus and subcortical structures. In tissue sections, alterations indicative of marked edema were found with Nissl staining in areas corresponding to the highest T2-weighted intensity. Degenerating neurons, revealed by FluoroJadeB histochemistry, were instead concentrated in tissue exhibiting hyperperfusion in rCBV maps, such as hippocampal subfields and dentate gyrus, cortical layers II/III and VI, and medial thalamus. The data indicate that:(i) T2-weighted imaging provides a sensitive tool to investigate edematous brain alterations that follow sustained seizures; (ii) rCBV maps reveal regional hyperperfusion; (iii) rCBV peaks in tissue exhibiting marked neurodegeneration, which may not be selectively revealed by structural MRI. The findings provide an interpretation of the brain response to sustained seizures revealed in vivo by different strategies of MRI analysis.

Complex Partial Seizures: Surgical Treatment

Any patient who continues to have complex partial seizures following an adequate trial of antiepileptic drugs (AEDs) should be considered for evaluation for surgery in a comprehensive epilepsy center. Investigations should include 24-hour electroencephalographic (EEG) monitoring, a high-resolution magnetic resonance imaging (MRI) study, and neuropsychological assessment. Surgery is most effective in patients with unilateral hippocampal atrophy or a structural lesion and concordant EEG findings. With modern microsurgical techniques, surgery is associated with a low rate of major morbidity. The vagal nerve stimulator is a device recently approved by the United States Food and Drug Administration that may offer control equivalent to that of AEDs in patients who are not candidates for surgical resection.