Intrinsic Brain Tumors and Functional Mapping

Image-guided surgery has become an integral component in the surgical management of brain tumors. The clear advantages include: the possibility of performing a smaller craniotomy centered directly over the pathology; localization of tumors deep within the cortical surface with minimal disturbance of the surrounding brain tissue; definition of the relationships of the tumour to eloquent cortex and other important structures; and definition of the tumor-brain plane, especially when this is poorly visible at surgery, such as in low-grade astrocytomas. The completeness of resection may also be enhanced, although this is controversial unless some form of intraoperative image updating is also used. The opportunity to plan the optimal approach to the tumor pre-operatively is also valuable.

The integration of functional data into the anatomical image data set was the next logical step in the development of image-guided surgery systems. If the function of relevant eloquent brain in the vicinity of a lesion can be mapped onto the MR image on the workstation, maximal resection with minimal neurological morbidity is facilitated. The potential to increase the safety of the procedure is evident. It is not always possible to accurately identify eloquent brain regions perioperatively with reference to standard anatomical landmarks, as tumors may cause significant gyral displacement and distortion of surface cortical anatomy.

Intraoperative, invasive, functional mapping using cortical stimulation techniques is time consuming and requires modification of the anesthetic regimen. Patients need to be awake for assessment of language function, but light anesthesia without muscle relaxation is adequate for motor mapping. Unlike pre-operative non-invasive mapping, this technique does not allow pre-operative planning and risk evaluation.

Magnetoencephalography (MEG) and functional MRI (fMRI) have been successfully used to obtain functional information that was subsequently integrated with the pre-operatively acquired image data set. In the former [14], cortical neuromagnetic signals are generated by repetitive sensory and motor stimulation; in practice, this involves mechanical stimulation of the index finger or repetitive finger-tapping movements. These signals are picked up and localized by a biomagnetometer, consisting of two 37-channel sensors placed over the scalp. Electrical sources can be localized with high spatial and temporal accuracy. The locations of the sensory and motor cortex are therefore identified, and, using a contour-fit algorithm, the MEG results can be overlaid onto the MR images. Lesions close to the motor cortex were successfully removed in 50 patients [14], with procedure-related neurological impairment in only one patient. Perioperative somatosensory evoked potentials agreed with the co-registered MEG localizations to a high level of concordance.

Functional MRI is non-invasive and uses widely available equipment [15]. Unlike MEG, it does not detect neural activation directly, but identifies a region within the cortex that is metabolically activated during the repetitive performance of an activity such as finger tapping. Performance of the task causes a substantial increase in cerebral blood flow in the corresponding brain region, sufficient in fact to lead to a decrease in the level of deoxyhemo-globin in the veins draining that region. This is detected as an increase in the T2-weighted MR signal. The signal-to-noise ratio is low, and many repetitions are required to generate each image slice. The fMRI image is then fused with the anatomical data set. In a study in which 12 patients underwent excision of lesions near the motor cortex, the prediction error ranged from 0 mm to 10 mm. This was considered to be satisfactory, as the target was the precentral gyrus rather than a single point on it. Intraoperative confirmation using somatosen-sory evoked potentials and phase reversal to identify the sensory and motor cortices respectively showed that fMRI identified the region correctly in each case. All of the tumors were excised without causing new neurological

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