To develop a fast and accurate method to measure the dipolar relaxation time, T_1D, in-vivo.The contrast mechanism described as inhomogeneous magnetization transfer (ihMT) is based on the signal difference following saturation at a single offset frequency and that following dual frequency saturation [1-2]. This ihMT signal has been recently modeled based on the interaction between the bound and free pool, with and without inclusion of a dipolar component to the bound pool [3]. In the case of a pulsed saturation preparation, the ihMT signal is found to be dependent on the number of pulses between switching frequencies for the dual offset experiment [4]. In this work, the model that describes the ihMT signal was adapted to describe the effect of a variation in the period between switches in frequencies. In doing so, we provide a robust method by which a dipolar relaxation time, T_1D can be extracted from ihMT data from different substances.A modified ihMT pulse sequence was developed with MT saturation performed using 0.5ms Hann shaped RF pulses every 1.2ms. Dual frequency irradiation is typically achieved by alternating between positive and negative frequency pulses. By increasing the number of pulses performed at the same frequency before switching to the negative frequency, a frequency switching time, w was introduced (Fig.1). An approximate analytic solution to the dipolar ihMT model, described in [3], was derived to enable rapid fitting of the signal decay curves as a function of switching time, w. Data were acquired in 5 healthy volunteers and hair conditioner (hc) on a GE 3T scanner with 500ms of saturation at B_1,RMS=3.5μT, |Δ|=7kHz, and 8 values for w ranging from 1.2-43.2ms, followed by a single-shot spin-echo EPI acquisition. ihMT ratios (ihMTRs) calculated from ROI analysis, e.g. of white matter (WM) and gray matter (GM) (Fig.2a), were fit to the model for ihMT(w). Similarly, ihMT(w) data acquired at 11.75T from WM, GM and muscle (mu) ROIs in 2 mice, 2/4% agarose (aga), and hc #2 with a short TE single-shot FSE sequence and 1ms RF pulses every 1.3ms at |Δ|=8kHz and w=1.3-42.9ms, were also fit to the model. Mouse in-vivo data were collected following saturation at B_1,RMS=4.7;5.8μT for 0.75s; whilst for aga and hc 11.75T data, B_1,RMS=6;8.5;12μT for 1.2s. Initial values for exchange/relaxation parameters were based on previous literature [3,5-6].Initial estimates for parameters other than dipolar relaxation time, T_1D, i.e. R, RM₀^B/R_A, 1/R_AT_2A, and T_2B, showed large variation. Fixing these parameters to the initial values, the ihMT(w) model was modified to have two variable parameters: T_1D and a constant, A by which the resultant ihMTR was multiplied, such that ihMTR=A*ihMT(w,T_1D) (Fig.2b). T_1D remained relatively stable despite factor of 2 changes in the fixed parameters: R, RM0B/R_A, 1/R_AT_2A, and T_2B. The T_1D values found in GM were similar to that in WM for both humans and mice (Fig.3); A in human GM was 0.54 times that of WM in the posterior region (Fig.2c). T_1Ds extracted from fitting were longer in samples associated with (previously reported) higher ihMTR values. Indeed, T_1D was 100 times longer in hc #2 than in aga; hc #1 sample chilled to 3°C showed a 35% increase in T_1D, compared to that at room temperature.Although there was variability in the amplitude and/or other parameters related to the model, the shape of the ihMT signal as a function of w (as defined for this type of experiment, Fig.1) was mostly dictated by the T_1D (Fig.2b). As in previous works, measurements of T_1D in WM/GM were longer than those in mu [3]. Further work is warranted, in particular detailed comparison with alternative measurements of T_1D, e.g. using Jeener-Broekaert echoes [7-8].The experiments and model outlined provided a means by which the dipolar relaxation time, T_1D may easily be measured in-vivo (independent of other MT parameters). This approach may be used to improve modeling of ihMT and to quantitatively characterize pathologic tissues.