Three-dimensional anisotropy contrast magnetic resonance imaging of the rat nervous system: MR axonography.

Anisotropy of apparent diffusivities (Dapp) of water molecules is known to be an excellent means for elucidating detailed information regarding neuronal fiber direction and density in live subjects (axonography). Nevertheless, determination of eigenvalues and eigenvectors of the apparent diffusion tensor, D xi app, the necessary process for the treatment of D xi app in axonography, is cumbersome and the quality of the reconstructed images based on estimated parameter values of D xi app has often been disappointing. We present here a simple algorithm which allows for the extraction of the anisotropic term vector, DAapp, from D xi app in pictorial form, which in turn provides a novel color-coded contrast for magnetic resonance imaging (MRI). MRI images constructed using this technique, three-dimensional anisotropy contrast (3DAC), provided impressive anatomic resolution of the CNS, comparable to in vitro histology. Directional information of DAapp expressed by hue of color clearly correlated to fiber direction, while magnitude of DAapp expressed by intensity of color correlated to fiber diameter. Magnetic resonance axonography (MRX) utilizing 3DAC technique possesses significant potential for various biological and clinical applications.