It has been shown that simple custom-tailored transmit/receive surface coil assemblies which are dedicated and optimized for studies of tissue close to the surface are superior in sensitivity when compared to circumscribing coils in these areas. We have investigated the design of a three loop surface coil assembly which is capable of transmitting and receiving circularly polarized radio frequency in deep central regions of the human brain. Only one receiver channel is needed which underlines the minimal hardware requirement (quadrature hybrid) for this design. Furthermore two of these structures tuned to different frequencies can be combined with negligible RF interaction due to its coil topology. Using a single quadrature set tuned to hydrogen frequency(169 MHz) and a spatial localization pulse sequence (STEAM) we measured the power requirement and S/N obtained in the specified region. The results were compared with results obtained with a birdcage resonator on the same phantom (cylinder of 3.8 l of 100 mM NaCl and 250 mM glucose). We found a decreased power requirement of ~1.5 dB for a 90~ pulse and an increased S/N of ~12%. The homogeneity was sufficient to image a whole human brain (Figure 2), however, as expected a 22 cm long birdcage coil showed better overall homogeneity in a phantom. The structure is easy to construct and shows a single mode on the surface coils only. The dual loops exhibit a series resonance sufficiently far away from the frequency of interest. The results suggest that this coil structure has a definite advantage for heteronuclear studies or for spectroscopy and imaging of the inner structures of the brain since the power requirements for heteronuclear studies( e.g. NOE and decoupling) can approach the SAR limits at high fields when circumscribing probes are used.
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