NEW DOUBLE-QUANTUM FILTERING SCHEMES

New double-quantum filtering (DQF) schemes are theoretically developed by reformulating the equations describing the double-quantum (DQ) sig nal. The equations describing the second- and third-rank DQ signals ar e simplified by restricting the RF phases as required for DQF. The equ ations are then factorized into two terms representing the separate co ntribution to the DQ signal from the RF pulses involved in the prepara tion and evolution times. This allows analysis of the DQ signal of a p articular DQF scheme separately for each of these times in a concise m anner. By use of the reformulated equations, the conventional DQF sche me is shown to be only one of four possible DQF schemes. The three new DQF schemes offer some desirable properties over the conventional DQF scheme. In the conventional DQF scheme, the third-rank DQ signal decl ines rapidly to null as the flip angles of the creation and readout RF pulses deviate from 90 degrees to 54.7 degrees or 125.3 degrees. In a ddition, the second- and third-rank DQ signals in the conventional DQF scheme are opposite in their polarities, resulting in attenuation of the total DQ signal due to destructive interference between them. In o ne of three new DQF schemes, the DQ signal does not vanish at 54.7 deg rees and 125.3 degrees, but varies smoothly with the same functional d ependence on the RF flip angles as the second-rank DQ and triple-quant um signals. Furthermore, in two of the three new DQF schemes, the seco nd- and third-rank DQ signals have the same polarity so that the total DQ signal may be enhanced through constructive interference between t hem. These features of new DQF schemes have been confirmed experimenta lly. (C) 1996 Academic Press, Inc.