ULtrasound Imaging in Focal Cortical dYSplasia: a New Approach to Delineating the Dysplastic Cortex During Neurosurgery

Three decades since its comprehensive description (Taylor et al., 1971), focal cortical dysplasia (FCD) remains an enigmatic condition. FCD may cause severe refractory epilepsy that can be directly life threatening. Preoperative neuroimaging usually includes high-resolution MR imaging, which can reveal only 60 to 80% of cortical abnormalities in patients with FCD. When antiepileptic drugs fail to bring complete seizure freedom in FCD patients, surgical resection of the FCD is inevitable. Many patients, especially those with normal MR imaging results, undergo additional diagnostic procedures. Scalp EEG is frequently used and was one of the more important modalities during early surgical series. Approximately one half to two thirds of patients with abnormal EEG findings have a regional ictal abnormality. In some cases, intracranial electrophysiological recordings, most commonly with grid arrays, are used. Chronic recording allows identification of eloquent cortex areas, in addition to defining the epileptogenic region. The "eloquent brain" refers to the parts of the brain that allows the interaction with and the process of surrounding environment, via the senses, motion, language, memory and the purposeful use of tools. Nevertheless, all these techniques are either invasive or have a spatiotemporal resolution too poor to identify precisely the epileptic lesion deep in the brain. Hence, large resection of lesion areas, such as lobectomies and even hemispherectomies, are performed with a high risk of side effects including aphasia, partial face paralysis and hemiplegia depending on the localization of the lesion.

Navigable three-dimensional (3D)-MRI (based on Neuronavigation system) is currently used at the Sainte Anne hospital for planning and guiding during resection but neurosurgeons often complains about poor resolution and non-real-time imaging. While the use of surgical navigation has been an important advance in brain surgery, its utility is limited by the phenomenon known as brain shift. Whenever the brain is exposed, cerebral spinal fluid (CSF) is lost. Additionally, after the start of resectioning, the position of the surgical field can shift by centimeters, compared to the pre-surgery position. Brain shift makes it potentially hazardous to rely on preoperative images to determine the location of residual tumors. The only way to deal with brain shift and maintain accurate neuronavigation is with intraoperative imaging to enhance resection of the pathologic tissue in FCD.

Previously, the investigators demonstrated the feasibility of their approach by monitoring the hemodynamic responses during drug-induced epileptic seizures in preclinical models using functional micro-Doppler Sonography (fmDS).

The investigators are now developing this new tool combining a navigable three-dimensional (3D)-ultrasound interface to correct in real-time the brain shift (B-mode) with the near-real-time identification with unprecedented resolution of the dysplasia foci based on the specific hemodynamic signature of abnormal neurons.