Fig. 2.

Pulse sequence and image reconstruction scheme. (A) The system is initially prepolarized along the y axis (perpendicular to the paper surface). A train of uniform dc-field π pulses along a direction x̂′ making an angle θ with the x axis manipulates further evolution in the gradient field. The pulse repetition rate has to be fast compared to the maximum local Larmor frequency in the gradient field. Acquisition (Acq) is performed stroboscopically in between pulses. Different acquisition schemes are possible. The one indicated above the pulses (black) symmetrizes the cycle, canceling out all odd terms in the average Hamiltonian expansion. The one below the pulse train (gray) is appropriate for a longer dead time of the detector (the first π pulse has been included for didactical reasons but is not necessary). (B) As the direction x̂′ of the dc-field pulses changes (Left), so do the direction of the effective field (always pointing in the direction of the pulses) and the direction x̂″ of the effective gradient (Right), here indicated by the angle γ. A series of image projections can then be obtained, leading to the full reconstruction of the spatial spin distribution.