Novel radio-frequency NMR pulse sequences are presented and their application to imaging of solids with use of rf field gradients is discussed. The sequences cause a nuclear spin to precess about the static field direction at a rate proportional to the strength of certain of the pulses. This forced precession is independent of the resonance offset and of couplings to other spins. The pulse-sequence design is described by means of coherent averaging theory and is confirmed experimentally and numerically. Imaging of the spatial distribution of nuclear spins by the recording of their magnetic resonance signals in field gradients is a well-known concept with a growing body of applications, particularly in medicine. l ' 2 The central idea is that the linear dependence of the frequency of spin precession on the magnetic field at the nucleus can be converted to a measurement of position along a known field gradient. 1 Nearly all NMR imaging has been done on liquids at resolutions of lO^-lO"" 1 cm, but the large line widths in solid samples preclude the direct extension of liquid-state imaging methods to solids. The excellent chemical, orientational, and motional specificity of solid-state NMR, as well as its noninvasive, penetrating nature, suggests that NMR imaging methods applicable to solids 3 " 10 would be valuable as well, for example, in materials science.This Letter introduces a new approach to NMR imaging suitable for solids. A radio-frequency (rf) pulse sequence is described which suppresses dipolar (homonuclear and heteronuclear) and chemical-shift contributions to the linewidth. In addition, the sequence results in a frequency shift of the narrowed line position proportional to the rf field strength of certain of the pulses. Given the line narrowing and the linear dependence of line position on rf field amplitude, a coil capable of producing a switched rf field gradient could be used to achieve spatial discrimination of nuclear spins by observing their forced precession frequency in analogy to a related liquid-state experiment. 11 Here we demonstrate the solid-state pulse sequences with a homogeneous coil by varying the pulse nutation angle and observing the predicted linear variation in the position of the narrowed line.In order to resolve two points in the spin distribution separated by a distance A A-, a gradient of magnitude G>M 2 1/2 /Ar is required, where M\ 12 is the square root of the second moment of the presumably unresolved solid-state NMR spectrum. A typical value for protons in a solid would be G > 50 mT/cm (M 2 1/2 =0.5 mT, Ar = l0~2 cm). Rapid switching of the amplitude and direction of a gradient of this size within the bore of a high-field magnet is impractical. A time-domain view of this inequality is that the precession in the field gradient must last long enough to create a significant difference in phase angle between the magnetization from adjacent picture elements, but that during this time the decay of the magnetization at each point should be minimal. Imaging has been demonstrated w...
