Electric dipole based antenna provides a simpler and more efficient design as RF transmitters for 7T spine imaging¹. The transmit efficiency for the dipole antenna is always highest at the center of the antenna and decay as a function of the distance to the center. In clinical practices (e.g. spinal multiple sclerosis studies), the region of interests usually do not coincide with the center of the dipole, led to lower local transmit efficiency and thus higher RF heating penalties. Since it is practically prohibitive to relocate the patients inside the scanner, it is desirable if the transmit field-of-view (FOV) can be re-adjusted remotely in the console room. In addition, patient-specific adjustment of the tuning and the matching of the transmitter can further increase the transmit efficiency. In this context, we propose a new concept of “segmented dipole” in this work, which can be easily reconfigured remotely for each subject. An example transmitter was built for demonstration purpose with the reconfiguration capability demonstrated at a 7T scanner.

The basic idea of segmented dipole is shown in Figure 1. The continuous dipole is divided into several segments. A “length control” unit is inserted between each segment, and an “impedance control” unit is inserted between the balun and the adjacent segment. A simple implementation of the length control unit contains a PIN diode in series of the dipole, a parallel trap to improve isolation, and other supporting components like RF chokes. In this way, each segment can be easily switched on or off. The impedance control unit contains several parallel sub-units, each of which contains a capacitor in series with a PIN diode with corresponding control lines. Depending on the balun design, the impedance control unit can be integrated with the balun itself. All the control DC lines can be connected to a control unit in the console room for remote adjustment.

To demonstrate the feasibility of the proposed segmented dipole design, a segmented dipole with four length control units as well as four impedance control units was built, with its diagram shown in Figure 2. Except for the two segments that connect to the balun, all other segments have same length. Such setup could provide nine different dipoles with various combinations with different tunings. Among these combinations, some of them, such as λ₀₊₁₊₂, λ₀₊₁₊₃, and λ₀₊₂₊₄, have the same resonant frequency but different center locations. Thus, by choosing different combinations, the transmit FOV can be shifted without changing the tuning. The impedance change due to reconfigurations can be further compensated by the impedance control units (with 16 possible combinations) to further improve transmit efficiency.

MR experiments were performed on a Siemens Magnetom 7T-830-AS scanner. The segmented dipole was placed about 2cm above a gel phantom mimicking human muscle dielectric properties at 7T². A control box with eight switches was used to provide the control signal. Gradient-echo images were acquired at the center sagittal plane that going through the dipole, with following parameters: voxel size 1.6x1.6x10 mm³, image matrix size 256x112, TE 3.53ms, TR 500ms, Flip angle 50 degrees.

The capability of shifting transmit FOV is clearly demonstrated in Figure 3, using λ₀₊₁₊₃ configuration (left) and the λ₀₊₂₊₄ configuration (right). Figure 4 shows images acquired with three different impedance control settings with the same length control settings, with significant changes in penetration and sensitivity. In fact, the image intensity dynamic range achievable by the impedance control was more than six folds.

With combination of better control signal generator, e.g. FPGA, system with much finer reconfiguration capability can be implemented. In addition, three-dimensional (rather than the traditional 2D) field steering can be easily realized by combining two segmented dipoles and reconfiguring each individual transmit FOV simultaneously adjusting the phase differences between them.

Similar functionality may be implemented by separate loops along the longitudinal direction. However, in comparison to the segmented dipole setup, it would have lower efficiency¹, as well as considerably increased the amount and the complexity of supporting electronics (e.g. a flexible power divider simultaneously supporting 2-way and 3-way power distribution), or requiring a parallel transmit system.

Segmented dipole framework was proposed for easy remotely reconfiguration of transmit FOV, tuning, and matching of the dipole transmitter. The feasibility was demonstrated with a small-scale implementation. The proposed work can personalize the transmitter and thus improve the transmit efficiency for each subject.