A direct demonstration of both structure and function in the visual system: Combining diffusion tensor imaging with functional magnetic resonance imaging

The relationships between functional activation patterns and the structural properties of brain pathways have not been widely studied. The recently de veloped magnetic resonance imaging technique of diffusion tensor imaging (D TI) allows a full characterization of water molecule mobility in three dime nsions, providing new structural information that is not available from oth er in vivo imaging techniques. The directional bias of diffusion (anisotrop y) may be derived from the diffusion tensor, and is related to fiber tract integrity and orientation. Since DTI and functional magnetic resonance imag ing (fMRI) both require rapid (generally echoplanar) imaging it is possible to obtain geometrically matched images from the two modalities. fMRI and D TI were combined in a visual system study using photic stimulation to demon strate the feasibility of combining the two methods and to investigate the structural properties of activated regions compared to the white matter tra cts. Blood oxygenation level-dependent (BOLD) fMRI activation maps were dir ectly overlayed upon fractional anisotropy (FA) maps, avoiding registration and spatial transformation by carefully matching acquisition parameters. A ctivated regions had lower FA than optic radiation white matter, supporting the hypothesis that most BOLD signal change occurs within the relatively i sotropic cortical grey matter. The combination of these modalities in futur e studies may provide further insights into relationships between brain str ucture and function in both health and disease. (C) 1999 Academic Press.