PurposeThis paper describes a technique that can be used in vivo to measure the dipolar relaxation time, T1D, of macromolecular protons contributing to magnetization transfer (MT) in tissues and to produce quantitative T1D maps.Theory and MethodsThe technique builds upon the inhomogeneous MT (ihMT) technique that is particularly sensitive to tissue components with long T1D. A standard ihMT experiment was altered to introduce a variable time for switching between positive and negative offset frequencies for RF saturation. A model for the dependence of ihMT was developed and used to fit data acquired in vivo.ResultsApplication of the method to images from brains of healthy volunteers produced values of T1D = (5.9 ± 1.2) ms in gray matter and T1D = (6.2 ± 0.4) ms in white matter regions and provided maps of the T1D parameter.ConclusionThe model and experiments described provide access to a new relaxation characteristic of tissue with potentially unique diagnostic information. Magn Reson Med 78:1362–1372, 2017. © 2016 International Society for Magnetic Resonance in Medicine.