Fiber Tract Visualization and Intraoperative Functional Neuronavigation(<SPECIAL ISSUE>Operation Suite in 21st Century)

Abstract

Visualization of neural fibers using diffusion tensor imaging (DTI) is becoming an indispensable tool in neurosurgery. Currently, two techniques, three-dimensional anisotropy contrast (3DAC) imaging and tractography, are frequently utilized. 3DAC images provide three-dimensional fiber-directional information in true color contrast and exquisitely high anatomical resolution sensitive to physiological anisotropism. Images are obtained by performing processes in image form (pictorial mathematics) without the necessity of eigenvalue estimation. Hence, image quality is identical to conventional magnetic resonance images such as T1 weighted and T2 weighted images. 3DAC has been shown to provide images with extraordinary contrast to detect not only major neural tracts but also fine neural fibers and nuclei, including in the brainstem and spinal cord. Tractography is widely used in neurosurgical settings, especially for presurgical planning to identify neural tracts. The direction of the largest diffusion determined by DTI parallels the dominant fiber orientation in each voxel, representing the mean longitudinal direction of neural fibers, which can be utilized for fiber tract tracing, i.e., tractography. Tractography is generally performed in two different ways, i.e., by deterministic or probabilistic methods. Advantages of the latter method over the former include the ability to clearly represent uncertainty and to reconstruct crossing fibers in the data, though the former is computationally intensive because of its requirement of thousands of iterations. There is widespread misunderstanding that tractography images actually represent real tracts and their pathology. In order to avoid such a potentially serious mistake, clinicians should clearly understand that, unlike 3DAC, tractography is in fact an artificial image. Recently, functional neuronavigation, i. e., intraoperative navigation integrated with preoperative functional images, was introduced into neurosurgical practice. Functional neuronavigation combined with visualization of the fiber tract is also expected to significantly reduce postoperative neurological deficits. In this venue, accuracy of the navigation is critical. It is, therefore, imperative that neurosurgeons understand the pros and cons of each of the techniques to be incorporated. The most reliable scenario will be the combination of intraoperative images, such as computed tomographic images and magnetic resonance images, appropriately integrated with functional data obtained preoperatively.