 Therefore to observe an NOE on the resonance intensity of I, the contribution of formula_17 and formula_18 must be important.These pathways, known as "cross-relaxation" pathways, only make a significant contribution to the spin-lattice relaxation when the relaxation is dominated by dipole-dipole or scalar coupling interactions, but the scalar interaction is rarely important and is assumed to be negligible.In the homonuclear case where formula_19, if formula_18 is the dominant relaxation pathway, then saturating formula_14 increases the intensity of the formula_4 resonance and the NOE is "positive", whereas if formula_23 is the dominant relaxation pathway, saturating formula_14 decreases the intensity of the formula_4 resonance and the NOE is "negative".