Fully reconstructed top-antitop-events are used to determine
the fractions of right-handed and longitudinally polarized W bosons
produced in top-quark decays. The helicity fractions are sensitive to
the couplings and the Dirac structure of the Wtb vertex. This paper
reports measurements of the W-boson helicity fractions from two different
methods using data corresponding to an integrated luminosity of 1.9
fb-1 of proton-antiproton collisions collected by the CDF II detector
operating at the Fermilab Tevatron.