  Magnetic resonance imaging (MRI) is the method of choice for noninvasive
studies of micrometer-scale structures in biological tissues via their effects
on the time/frequency-dependent ("restricted") and anisotropic self-diffusion
of water. Traditional MRI relies on pulsed magnetic field gradients to encode
the signal with information about translational motion in the direction of the
gradient, which convolves fundamentally different aspects-such as bulk
diffusivity, restriction, anisotropy, and flow-into a single effective
observable lacking specificity to distinguish between biologically plausible
microstructural scenarios. To overcome this limitation, we introduce a formal
analogy between measuring rotational correlation functions and interaction
tensor anisotropies in nuclear magnetic resonance (NMR) spectroscopy and
investigating translational motion in MRI, which we utilize to convert data
acquisition and analysis strategies from NMR of rotational dynamics in
macromolecules to MRI of diffusion in biological tissues, yielding
model-independent quantitative metrics reporting on relevant microstructural
properties with unprecedented specificity. Our model-free approach advances the
state-of-the-art in microstructural MRI, thereby enabling new applications to
complex multi-component tissues prevalent in both tumors and healthy brain.
