Transmit B1-field correction at 7 T using actively tuned coupled inner elements.

When volume coils are used for (1)H imaging of the human head at 7 T, wavelength effects in tissue cause a variation in intensity, that is typically brighter at the center of the head and darker in the periphery. Much of this image nonuniformity can be attributed to variation in the effective transmit B(1) field, which falls by ∼ 50% to the left and right of center at mid-elevation in the brain. Because most of this B(1) loss occurs in the periphery of the brain, we have explored use of actively controlled, off-resonant loop elements to locally enhance the transmit B(1) field in these regions. When tuned to frequencies above the NMR frequency, these elements provide strong local enhancement of the B(1) field of the transmit coil. Because they are tuned off-resonance, some volume coil detuning results, but resistive loading of the coil mode remains dominated by the sample. By digitally controlling their frequency offsets, the field enhancement of each inner element can be placed under active control. Using an array of eight digitally controlled elements placed around a custom-built head phantom, we demonstrate the feasibility of improving the B(1) homogeneity of a transmit/receive volume coil without the need for multiple radio frequency transmit channels.