  Extracting reliable and quantitative microstructure information of living
tissue by non-invasive imaging is an outstanding challenge for understanding
disease mechanisms and allowing early stage diagnosis of pathologies. Magnetic
Resonance Imaging is the favorite technique to pursue this goal, but still
provides resolution of sizes much larger than the relevant microstructure
details on in-vivo studies. Monitoring molecular diffusion within tissues, is a
promising mechanism to overcome the resolution limits. However, obtaining
detailed microstructure information requires the acquisition of tens of images
imposing long measurement times and results to be impractical for in-vivo
studies. As a step towards solving this outstanding problem, we here report on
a method that only requires two measurements and its proof-of-principle
experiments to produce images of selective microstructure sizes by suitable
dynamical control of nuclear spins with magnetic field gradients. We design
microstructure-size filters with spin-echo sequences that exploit magnetization
"decay-shifts" rather than the commonly used decay-rates. The outcomes of this
approach are quantitative images that can be performed with current
technologies, and advance towards unravelling a wealth of diagnostic
information based on microstructure parameters that define the composition of
biological tissues.
