The radiative detection of n.m.r. in thermally oriented radioactive nuclei by observing the effects of a resonant RF field on the anisotropic angular distribution of the ')I-ray radiations was first suggested by Bloembergen and Temmer (1953). The main advantage of radiative detection over conventional n.m.r. techniques, which detect the EMF induced in an RF coil by the precessing nuclei, would be that far fewer nuclei are required. Furthermore, such experiments should lead to new information on the fundamentals of the influence of resonant fields on the emission of radiations by nuclei (Shirley 1968). Thus, following the suggestion of Bloembergen and Temmer, several unpublished attempts were made to detect n.m.r. in nuclei that were oriented in paramagnetic dielectric materials at low temperatures. These attempts failed, probably because of the combination of nonresonant RF heating and the low thermal conductivity of the host materials. More recently, it has been realized that ferromagnetic metals offer significant advantages as host materials, e.g. large hyperfine magnetic fields generally act on the nuclei, the thermal conductivity is relatively high and, relative to the applied RF fields, there is a large ferromagnetic enhancement (Portis and Gossard 1960) of the RF fields which act on the nuclei.
Subject-loaded quadrifilar helical-antenna RF coil with high B1+field uniformity and large FOV for 3-T MRI