Z-Spectrum Analysis Provides Proton Environment Data (ZAPPED): A New Two-Pool Technique for Human Gray and White Matter

A new technique – Z-spectrum Analysis Provides Proton Environment Data (ZAPPED) – was used to map cross-relaxing free and restricted protons in nine healthy subjects plus two brain tumor patients at 3T. First, MT data were acquired over a wide symmetric range of frequency offsets, and then a trio of quantitative biomarkers, i.e., the apparent spin-spin relaxation times (T(2,f), T(2,r)) in both free and restricted proton pools as well as the restricted pool fraction F(r), were mapped by fitting the measured Z-spectra to a simple two-Lorentzian compartment model on a voxel-by-voxel basis. The mean restricted exchangeable proton fraction, F(r), was found to be 0.17 in gray matter (GM) and 0.28 in white matter (WM) in healthy subjects. Corresponding mean values for apparent spin-spin relaxation times were 785 µs (T(2,f)) and 17.7 µs (T(2,r)) in GM, 672 µs (T(2,f)) and 23.4 µs (T(2,r)) in WM. The percentages of F(f) and F(r) in GM are similar for all ages, whereas F(r) shows a tendency to decrease with age in WM among healthy subjects. The patient ZAPPED images show higher contrast between tumor and normal tissues than traditional T(2)-weighted and T(1)-weighted images. The ZAPPED method provides a simple phenomenological approach to estimating fractions and apparent T(2) values of free and restricted MT-active protons, and it may offer clinical useful information.