Multiple-quantum vector field imaging by magnetic resonance

We introduce a method for non-invasively mapping fiber orientation in materials and biological tissues using intermolecular multiple-quantum coherences. The nuclear magnetic dipole field of water molecules is configured by a CRAZED sequence to encode spatial distributions of material heterogeneities. At any given point r in space, we obtain the spherical coordinates of fiber orientation (θ,ϕ) with respect to the external field by comparing three signals ∥GX∥, ∥GY∥, and ∥GZ∥ (modulus), acquired with linear gradients applied along the X, Y, and Z axes, respectively. For homogeneous isotropic materials, a subtraction ∥GZ∥ − ∥GX∥ − ∥GY∥ gives zero. With anisotropic materials, we find an empirical relationship relating ∥GZ∥ − ∥GX∥ − ∥GY∥/(∥GX∥ + ∥GY∥ + ∥GZ∥) to the polar angle θ, while ∥GX∥ − ∥GY∥/(∥GX∥ + ∥GY∥ + ∥GZ∥) is related to the azimuthal angle ϕ. Experiments in structured media confirm the structural sensitivity. This technique can probe length scales not accessible by conventional MRI and diffusion tensor imaging.