  We describe a method for quantitative characterization of radio-frequency
(RF) inhomogeneity using NOON states. NOON states are special multiple-quantum
coherences which can be easily prepared in star-topology spin-systems. In this
method we exploit the high sensitivity of the NOON states for z-rotations. As a
result, Torrey oscillations with NOON states decay much faster than that of
single quantum coherences. Therefore the present method requires shorter pulse
durations, and enables the study of inhomogeneity at higher RF powers. To model
such a inhomogeneity profile, we propose a two-parameter asymmetric Lorentzian
function. The experiments are carried out in 1H channels of two different
high-resolution NMR probes and the results are compared. Finally, we also
extend the NOON state method to characterize the inhomogeneity correlations
between two RF channels. We obtain the correlation profile between 1H and 31P
channels of an NMR probe by constructing a five-parameter asymmetric-3D
Lorentzian model.
