Individual white matter fibers cannot be resolved by current magnetic resonance (MR) technology. Many fibers of a fiber bundle will pass through an individual volume element (voxel). Individual visualized fiber tracts are thus the result of interpolation on a relatively coarse voxel grid, and an infinite number of them may be generated in a given volume by interpolation. This paper aims at creating a level set representation of a fiber bundle to describe this apparent continuum of fibers. It further introduces a coordinate system warped to the fiber bundle geometry, allowing for the definition of geometrically meaningful fiber bundle measures. 1 Motivation and Background Stroke, Alzheimer's disease, epilepsy, multiple sclerosis, schizophrenia, and various other neuropsychiatric disorders have been implicated with white matter abnormalities [1]. Diffusion weighted magnetic resonance imaging (DW-MRI) measures the directional diffusivity of water molecules and has been used to detect...