WASHINGTON, DC—New and emerging imaging techniques will greatly aid neurologists who treat patients with epilepsy, according to an overview presented at the 67th Annual Meeting of the American Epilepsy Society. “We’re going through another paradigm shift,” said Graeme Jackson, MD. Among the most “extraordinary” developments is highly sensitive MRI that detects lesions more easily and helps neurologists understand genetic expression and gene interactions, he added.
Graeme Jackson, MD
“Techniques for the use of data are crucial,” continued Dr. Jackson, Senior Deputy Director of the Brain Research Institute at the Florey Institute of Neuroscience and Mental Health in Victoria, Australia. “Surgery success is much better when you’ve found the lesion and you understand what that lesion is. That really should be one of the primary purposes [of imaging].”
Advanced Imaging Improves Detection of Encephaloceles
Good image acquisition enables neurologists to conduct a range of tests and analyses. The higher the MRI field strength, the more information that a scan can provide, and the more likely the scan will reveal subtle abnormalities. A T1-weighted image from a 1.5-T MRI indicated bottom-of-sulcus dysplasia in patients with DEPDC5 mutations. A 3-T image of the same patients revealed a small subcortical band that was not noticeable on the 1.5-T image, said Dr. Jackson.
Advanced imaging is enabling neurologists to find bottom-of-the-sulcus dysplasia and encephaloceles more often. When these conditions are identified and the lesions are removed, the patients are likely to have good outcomes. One patient with bottom-of-the-sulcus dysplasia whose lesion was removed has been seizure-free and off medication for approximately 20 years, said Dr. Jackson.
For one patient, super-high-resolution 3-T MRI revealed cauliflower-shaped outpouching on the temporal lobe that extended through the floor of the temporal fossa. The outpouching was visible only as a blurred outline on 1.5-T MRI. “We’ve now found five [encephaloceles], based on lesioning in temporal lobe epilepsy,” said Dr. Jackson. Patients with an encephalocele “only need a small operation, and they all become seizure-free.”
New Method for Examining Junction of Gray and White Matter
Curved linear reconstruction can be a more accurate representation of the brain than flat images that have a greater problem with partial volume effects, said Dr. Jackson. This technique indicates well the boundary between gray matter and white matter and enables neurologists to examine tissue layers from 8 mm to 18 mm below the cortical surface. Curved linear reconstruction, particularly when used with FLAIR imaging, also can reveal lesions that are difficult to detect using other methods.
Advances in image acquisition that enable closer examination of the junction of gray matter and white matter also improve the measurement of cortical thickness. For this application, 3-T MRI has proven to be a particularly sensitive technique, said Dr. Jackson. Researchers who detected a slight excess of cortical thickness at the bottom of a patient’s sulcus were able to find cortical dysplasia in the same area on 3-T MRI.
The Limitations of DTI
The technique that a neurologist uses to perform fiber tracking is not as important as the anatomic accuracy and biologic truth of the ultimate image, said Dr. Jackson. Diffusion tensor imaging (DTI) creates a spheroid mathematic model of one main direction of a fiber. DTI is helpful for single fiber directions within a voxel, but it does not define the direction well for instances in which multiple fibers cross within a single voxel, which happens in more than 80% of all voxels. Some emerging models such as constrained spherical deconstruction (CSD) can deconstruct the basic diffusion-weighted images into multiple directions when fibers cross and give more accurate images.
The consensus emerging in the physics literature and among MRI technicians is that “we need to move beyond DTI,” said Dr. Jackson. The technique’s main shortcoming is that DTI tractography of corticospinal tracts only shows fibers that connect to the midline motor cortex and misses lateral fibers because these tracks would pass through regions of crossing fibers. The limitations of DTI thus may mislead neurologists in the presence of a corticospinal lesion.
Super-Resolution Imaging Provides Great Detail
The ability to track the brain accurately is a prerequisite for super-resolution imaging, “which uncouples MRI from the signal-to-noise constraint,” said Dr. Jackson. The technique can provide an isotropic resolution of 200 µm, which, in combination with 3-T MRI, can demonstrate white matter structure in remarkable detail. Super-resolution imaging of white matter tracts can show the structure of thalamic bundles, as well as crossing fibers. The technique also enables neurologists to identify small structures and examine tract bundles and tract organization. Clinicians can apply the technique between areas that may be causing seizures to understand how seizures spread. “This level of imaging is really extraordinary,” said Dr. Jackson.