Bore size increases can contribute to decreased efficiency
of transmit body coils, but modern protocols often increase the requirements
for B1 power. This paper describes a novel solution for
concentrating B1 power without the use of a traditional local
transmit coil, based on the observation that roughly half of the transmit field
from a traditional birdcage coil comes from the end rings. A dual-ring
structure, modeled on a birdcage without rungs, was built and shown to enable
head imaging with up to a 30% reduction in required RF input power. Uniformity
was maintained.
