An Adjustable-Length Dipole Using Forced-Current Excitation for 7T MR

Ultrahigh field imaging of the body and the spine is challenging due to the large field-of-view (FOV) required. It is especially difficult for RF transmission due to its requirement on both the length and the depth of the B₁⁺ field. One solution is to use a long dipole to provide continuous current distribution. The drawback is the natural falloff of the B₁⁺ field toward the ends of the dipole, therefore the B₁⁺ per unit square root of maximum specific absorption rate (B₁⁺/√SAR_max) performance is particularly poor toward the end of the dipole. In this study, a segmented element design using forced-current excitation and a switching circuit is presented. The design provides long FOV when desired and allows flexible FOV switching and power distribution without additional power amplifiers. Different element types and arrangements were explored and a segmented dipole design was chosen as the best design. The segmented dipole was implemented and tested on the bench and with a phantom on a 7T whole body scanner. The switchable mode dipole enabled a large FOV in the long mode and improved B₁⁺/√SAR_max efficiency in a smaller FOV in the short mode.