fMRI of human visual pathways

Abstract

Functional magnetic resonance imaging (fMRI) of the human brain provides images of changes in local blood flow and oxygenation that are evoked by sensory, motor, or cognitive events. Functional MRI has been used since 1991 [1] to identify areas of the brain that respond to visual stimulation and the performance of vision-related tasks. Increasingly, fMRI is accompanied by diffusion tensor imaging (DTI), which provides images of the speed and direction of diffusion of water molecules in the brain. Fortuitously, this allows remarkable differentiation of cerebral white mater and the delineation of a variety of major white matter tracts including vision-related pathways such as the optic radiations. This chapter focuses primarily on fMRI, but DTI data are also discussed where relevant. Together, the two methods provide a wealth of information about the anatomical and functional status of key components of the visual system in individual patients even in the presence of pathology. For example, an imaging-based map of the visual system can be helpful for planning and guiding surgical resection of tumors impacting critical vision-related brain structures. This is especially true when mass effects or previous surgeries have distorted the normal anatomy making it difficult to know where key structures are located and if they are still functional. In difficult cases, identifying the region of “closest approach” of a planned resection to the cortical representation of central vision or to the optic radiations can help to minimize the risk to eloquent neural tissue and thereby avoid significant treatment-induced vision loss while still permitting maximum therapeutic effect.