The surface-coil NMR receiver in the presence of homogeneous B₁ excitation

The signal intensity and inherent spatial localization in a single-pulse steady-state experiment using a circular surface receiver coil in the presence of a homogeneous rf excitation magnetic field (B₁) is examined theoretically by computer simulation assuming zero interaction between the transmission-coil and the surface-coil radiofrequency circuits. The theoretical analysis is verified by NMR experiments employing appropriate "phantom" samples with the surface coil orthogonal to the transmitter coil to minimize interaction between the two. Under slow pulse repetition conditions the signal intensity of the homogeneous B₁ transmission-surface-coil reception experiment is essentially the same as that obtained in the single-coil configuration surface-coil experiment and is independent of the orientation of the homogeneous B₁ with respect to the surface coil. A significant consequence of this result is that the optimum performance of multiple-pulse sequences which require specific flip angles over the whole sample should be attained while retaining the spatially restrictive sensitivity profile of the surface coil. The homogeneous B₁ transmission-surface-coil reception sensitive volume penetrates deeper into the sample than the surface-coil single-coil configuration sensitive volume, and therefore may be a more appropriate shape for in vivo NMR spectroscopy. The homogeneous B₁ transmission actually induces more xy magnetization than surface-coil transmission does, but differences in signal phase from different regions of the sample result in an incoherent addition when the signal from all parts of the sample are summed together. In imaging experiments, each volume element of the sample is uniquely characterized by its Larmor frequency and phase, thus, signal reduction due to phase differences is absent.