An Integrated B₁ ⁺ Efficient Triple-Tuned (²H/²³Na/³¹P) Body Coil at 7T

The electrical loss mechanisms of multi-tuned birdcages were studied to design an efficient triple-tuned (²H/²³Na/³¹P) body coil at 7T. The sources of RF power loss were investigated in three aspects, namely, resistive losses in copper conductors, dielectric losses of capacitors, and resonance-induced losses in tank circuits of multi-tuned circuits all normalized to the intrinsic losses in tissue of a single-tuned birdcage. A 24-leg triple-tuned (²H/²³Na/³¹P) high-pass birdcage coil with two out of three short circuits in the end rings (i.e., eight effective legs) connected to tuning capacitors (16caps-birdcage) was compared with a classical 24-leg double-tuned (²H/³¹P) birdcage coil (48caps-birdcage). The electric loss in the tissue remains dominant over the coil conductor loss. When loaded with the head in the center and copper and capacitor losses were included, decreasing the number of legs in the birdcage design can increase the B₁ ⁺ field intensity by 6%-41% at the frequency range from ³¹P to ²H. The loss factor of capacitors is amplified when incorporating multi-tuning circuitries. That is, the dominant non-tissue losses are caused by the multi-tuned circuits as they contribute to a 67%/60%/49% decrease in the B₁ ⁺ field intensity when considering a 24-leg design at ²H/²³Na/³¹P frequencies, respectively. Therefore, the triple-tuned wide-leg 16caps-birdcage experimental design employing 16 triple-tuned circuits maintained or even improved the coil efficiency by 0 dB/3.45/3.75 dB at ²H/²³Na/³¹P frequencies compared with the classical 24-leg design utilizing 48 double-tuned circuits. The B₁ ⁺ maps of the triple-tuned wide-leg 16caps-birdcage design showed 5.1, 4.1, and 13.5 μT of B₁ ⁺ field intensity at the center of the body phantom at ²H, ²³Na, and ³¹P frequencies, respectively. Reducing the number of effective legs in a triple-tuned birdcage is an impactful method to maintain relatively high B₁ ⁺ efficiency.