Restoration of Signal Polarity in a Set of Inversion Recovery NMR Images

A method is described which enables unambiguous retrieval of sign information in a set of magnetic resonance magnitude images of the inversion recovery type. The proposed method starts from the observation that the inversion recovery curve S is a monotonically increasing function of the inversion time TI, and comes down to finding the zero-crossing time TI0 of this curve for each pixel within the image. ∣S∣ and S are then related by S(TI) = -∣S(TI)∣ for TI ⩽ TI0 and S(TI) = + ∣S(TI)∣ for TI ⩾ TI0. The method, which does not require additional knowledge with respect to any of the NMR parameters involved, is shown to be effective when at least four inversion recovery images with different inversion times of a particular object slice are available. The efficacy of sign retrieval is demonstrated by imaging experiments on phantoms and human subjects. The validity of the polarity restoration method is established by viewing its results against the results of conventional methods, i.e., NMR spectroscopy.