We report the first demonstration of spatial mapping of proton chemical shifts using a modified projection reconstruction/echo-planar hybrid technique (PREP) ( 1, 2). PREP is a fast chemical-shift imaging technique which allows the generation of a full three-dimensional data set (x-y-6) with Npoints in each of the spatial dimensions and L points in the chemical-shift dimension, from data gathered in Nseparate experiments. The technique uses a single rapidly switched magnetic field gradient and this imposes hardware requirements similar to those of standard echo-planar imaging (EPI) (3). It therefore offers a compromise between the rapid echo-planar shift mapping technique (EPSM) ( I, 4) which can produce a three-dimensional data set in a single experiment and the standard 3DFT technique (5). 3DFT imposes lesser hardware requirements but requires the execution of N X N or N X L experiments for the generation of an N X N X L data array. The timing sequence of the PREP technique is shown in Fig. 1. This was slightly altered by the addition of a nonselective 180” pulse and an r-gradient prepulse of duration 7p in order to ensure that all spins regardless of spatial position or chemicalshift value are in phase when sampling begins at time 7 T/ 2. After slice selection a rapidly switched field gradient G,, with period 47, is applied in the direction r . This produces a signal S(t) comprising a series of echoes with peaks occurring at times 27, 47, etc., after slice selection. These are progressively affected by dephasing due to chemical-shift differences in the sample. As in standard double EPI (3), data processing requires the formation of a set of echoes which evolve in a purely positive (S+(t)) or purely negative gradient (S-(t)). This is accomplished by appropriately splicing together the signals from two experiments each with opposite starting phases of G,, as shown in Fig. 1. Fourier transformation of this spliced signal yields a discrete stick spectrum projection profile of the object which, for a common chemical-shift species, has equally spaced sticks. However, when other chemical species are present, the sticks in appro-