Schematic of Shemesh et al.’s technique. In a conventional spin-echo measurement, spins aligned in a magnetic field are flipped by an rf field pulse (red) and then refocused by a second pulse (orange). A pickup coil measures the signal (white curve) from the precessing spins, obtaining information about their environment by measuring the rate at which they lose phase with one another. (Top) In single pulse-gradient spin-echo (PGSE), a gradient magnetic field pulse (G), which labels the spins, is applied before and after the refocusing pulse. (Bottom) In double PGSE, two pairs of field pulses, G1 and G2, are applied. Shemesh et al. take the ratio of the results of the single and double PGSE spin-echo sequences to gain information about the average size and shape of pores in a porous structure containing water.