The phenomenon of multiple echoes following a two-pulse stimsulting from the nuclear dipolar demagnetizing field. This ulated-echo experiment in highly polarized systems is reinvestifield is proportional to a sample's magnetization and is negligated and explained using the Bloch equations. A semiquantitative gible when normal magnetic fields (õ12 T) and concentraformalism based on the nuclear contributions to the magnetic sustions (Ӷ55 M H 2 O) are used. It has long been understood ceptibility throughout a multiple-pulse sequence is outlined and that the back reaction of the dipolar demagnetizing field on used to understand the formation of the echoes. This theory is a sample's magnetization can lead to unexpected results. used to direct the design of phase-cycled and pulsed-field-gradient The production of multiple echoes by a stimulated echo experiments to control the echo formation. The theory also sug-(p/2-t-p/2) pulse sequence as first observed in solid gests a means of indirectly detecting a heteronucleus contained in 3 He is just one example. Deville and co-workers (4) demona different molecule. This kind of detection is accomplished by strated that these multiple echoes could be explained by modifying the static magnetic field with a stimulated-echo experiment on one set of the spins. Heteronuclear experiments show that adding terms to account for the dipolar demagnetizing contrithis classical semiquantitative formalism is a useful tool in the butions of the nuclear spins to the static field in the Bloch understanding of spin dynamics in highly polarized systems. equations. More recent work by Bowtell (5) showed that