MR architecture advances place an ever-increasing emphasis on protocol speed and improved patient acceptance, creating continuous increases in coils’ internal complexity. New methods, such as multi-band parallel imaging, also place significant demands on the available B­­₁. Bore size increases, to allow larger patients, have forced the hardware between magnet and patient into smaller volumes, increasing transmit coil size and decreasing efficiency, and therefore, maximum B₁. Without increasing amplifier power, available B₁ can be increased by bringing the transmit coil nearer the region of interest, and numerous examples of local transmit coils exist for commercial systems. However, given modern coils’ increasing channel count and increasing internal complexity, adding a traditional local transmit coil outside the receive array increases the size, and reduces visibility through the coil.

In response, many examples of phased-array transmit coils exist in the literature [1] that dispense with an extra local transmit structure at the cost of an increase in electronics (e.g. added Tx/Rx switching, independent amplification and/or phase control per channel). Clinical systems typically support only single-channel local transmit. Passive “secondary resonators” have also been used to couple to the primary transmit coil and concentrate B₁ energy, coupling it more efficiently to imaging samples [2, 3]. This study resembles the latter, coupling a secondary resonant structure to the system transmit coil. However, this approach is based on the observation that roughly half of the B₁ field arising from the traditional birdcage resonator is due to the current in the end rings. A structure similar in shape to the end rings of birdcage, supporting a current distribution like that of a birdcage, should produce a significant portion of the equivalent birdcage’s excitation field. This represents a compromise between the B₁ field lost from the birdcage rungs, and the vastly simplified structure, which can be more easily incorporated into phased-array receive coils.

The ideal birdcage has two end ring azimuthal currents, each with uniform amplitude and a linear 360° phase variation around the circumference, and referenced 180° apart (Figure 1). To achieve the required electrical length, a toroidal conductor was chosen, though this need not be the case. This choice coupled the structure not only to the end rings, but also to the rungs of the body coil, such that each ring’s “handedness” set its azimuthal direction of current. This was exploited by creating two separate toroids, wound in opposing directions. Toroid diameter was approximately 185mm and winding diameter approximately 8mm. Each ring’s resonant frequency was coarse-adjusted by removing turns and fine-tuned by the addition of small capacitive reactances in series with the windings at 90° intervals. Unloaded Q of each ring was approximately 140 (in bore). Optimal ring separation was determined by iterative ring spacing, noting the effect on required RF amplifier gain during preparation phases. Measurements were repeated over a range of tunings and on multiple phantoms, progressing to a volunteer subject. Once proper tuning and separation were determined, imaging was performed, both on phantoms and on the volunteer, to assess the rings’ effect on required transmit gain, SNR, and uniformity.The structure was tested on Philips Ingenia 3T. Phantom experiments demonstrated that reduction in required transmit power (>10%) was possible, even with only one ring, and much larger improvements (>30%) were possible with two (Figures 2, 3). Improper spacing of the rings resulted in both decreased transmitter efficiency and loss in SNR. With proper spacing, an SNR increase of around 6% was observed near the center of a phantom chosen to model the head and shoulders of an adult. Significant effects on uniformity were not observed. Typical T1- and T2-weighted imaging protocols were used for volunteer testing. Measurements indicated a 25%-30% reduction in required power, while SNR was maintained or improved without notable changes in imaging uniformity (Figures 4, 5).A novel structure for concentrating B₁ field for the purpose of increasing transmitter efficiency and boosting available B₁ has been presented in which a “rungless” birdcage was employed. The structure was shown to lower the RF power required to achieve clinically-relevant B₁ levels in typical head imaging MR sequences (or boost the amount of B₁ available, potentially shortening protocol length). Uniformity was not adversely affected and an improvement in SNR was observed, though this is not expected to persist when an external phased-array coil is used for receive. Significantly, the structure used was geometrically simple and not prohibitively large, and did not require extensive cabling or support circuitry, improving its viability for inclusion into a multi-element phased-array receive coil design.