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A novel longitudinal relaxation time (T1) measurement method using complex amplitude modulation is presented.The method applies a series of inversion pulses to the imaged region in accordance with a binary modulation sequence. The longitudinal magnetization acquired in a given pulse repetition time (TR) interval is the sum of the individual longitudinal magnetization recovered during each previous TR interval, weighted by T1 decay factors and the combined effect of all the radiofrequency pulses they have experienced. The demodulated signal for each voxel is an exponential curve with a decaying rate determined by T1 and the acquisition flip angle Θ.Sequences using a 15-cycle pseudorandom binary code were implemented on Siemens 3T Trio with standard gradient echo readout and multislice gradient echo-planar imaging. The sequences were tested on T1 phantoms and human and compared against inversion recovery method.Our studies on phantoms and a human volunteer show that T1 estimated from this method is very accurate and well reproducible. The average scan time is ˜1.6 s per slice (full k-space gradient echo-planar imaging with matrix size 128 × 128).The current protocol is almost twice as fast as two fastest existing methods. Optimizing protocols and incorporating common acceleration techniques will make it even faster. Magn Reson Med 71:2155–2165, 2014. © 2013 Wiley Periodicals, Inc.