We have designed and constructed a preliminary 4T whole head volume array RF coil that works within our head gradient set. Previously, it has been demonstrated that substantial SNR gains are possible with a volume coil array consisting of shorter volume coils covering an extended FOV [38, 39] over a single coil covering the same FOV. We utilized this approach using a combination of two short quadrature birdcage coils to cover the whole human head. Applications such as single shot EPI which is routinely used in fMRI studies currently requires the use of the head gradient set in order to achieve sufficiently fast switching times at 4 T. Therefore, one of the requirements was to build such a coil within the confines of a head gradient set. The isolation between the quadrature ports of either coil was below -25 dB. This isolation was sufficient for imaging without further active preamplifier protection. The electromagnetic coupling between the coils could be adjusted to be below -18 dB at the feeding points. Load changes from subject to subject were found not to be critical in regard to coil decoupling. The necessary adjustments for tune and match from subject to subject was accomplished within minutes. Tune and match was easily adjusted for each birdcage coil separately thus also indicating minimal coil interactions. We obtained images to compare the SNR and homogeneity of the birdcage array coil with a high-pass birdcage coil. These images showed that when the subject was appropriately placed, Coil #1 covered the cerebellum, the brain stem and the lower part of the cerebral cortex and Coil #2 imaged the rest of brain. High sensitivity and homogeneity was observed in the combined images over the entire head and especially in the cerebral cortex, resulting in a maximal signal-to noise gain of up to 40% as compared with a 22 cm long high-pass birdcage even though the axially the FOV was substantially larger than then provided by the single high-pass birdcage. The axial homogeneity in the overlap area was excellent and comparable to that of a single birdcage coil.
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