Transmit B1 Field Correction at 7T using Actively Tuned Coupled Inner Elements

When volume coils are used for 1H imaging of the human head at 7T, wavelength effects in tissue cause intensity variations that are typically brighter at the center of the head and darker in the periphery. Much of this image non-uniformity can be attributed to variation in the effective transmit B1 field, which falls by about 50% to the left and right of center at mid-elevation in the brain. Because most of this B1 loss occurs in the periphery of the brain, we have explored use of actively controlled, off-resonant loop elements to locally enhance the transmit B1 field in these regions. When tuned to frequencies above the NMR frequency, these elements provide strong local enhancement of the B1 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 B1 homogeneity of a transmit/receive volume coil without the need for multiple RF transmit channels.