Transmit/Receive (T/R) loop coils are used at multiple field strengths for imaging and spectroscopy applications. At 7T, T/R loop coils have challenges in transmit |B₁⁺|-field efficiency and receive |B₁^-|-field sensitivity in a spatial context, due to the increased tissue dielectric/wavelength effect. This presents an opportunity for improvement in their usage at 7T. Here we propose a revised T/R loop coil schema which improves the B₁-field non-uniformity at 7T. The proposed modification may make the T/R loop coil construct yet more useful at ultra-high fields.Figure 1(a) shows a coil model (XFDTD software package, Remcom, Inc., State College, PA) [1, 2] of a conventional T/R loop coil with a circular shape. The loop coil is modeled as a conducting strip with a diameter of 14cm and strip width of ~ 1.3 cm. The loop coil is shielded by a 20cm-diameter circular conducting plate which is gaped ~2.5 cm from the loop conductor. Eight lumped capacitors are equally distributed along the loop conductor to tune the coil to the uniform current distribution mode at 298MHz (¹H at 7T). Figure 1(b) shows the revised T/R loop coil model which has a base structure as per convention. However, two additional straight conductors are connected across the loop, offset from loop center. The distance from each straight conductor center to loop center is 4cm. The straight conductors are parallel to each other and to the main magnet B₀ direction when in use. There is an RF switch in each straight conductor. The revised loop coil is tuned to the resonant mode at 298MHz where current flows in the same direction along the circular conductor path, as shown in the circuit diagram of Figure 1(c). The circular conductor plane is substantially parallel to a plane containing the B₀ direction. To compare the transmit |B₁⁺|-field and receive |B₁^-|-field distributions of the conventional and revised T/R loop coils, a uniform cylindrical phantom with material properties set to average values of select human tissue is modeled (conductivity of 0.694S/m, relative permittivity of 80 and mass density 1000kg/m3). The phantom has a diameter of 16cm and height (length) of 10cm. The loop coil models are placed 1cm above the flat surface of the phantom model as shown in Figure 2(a). For the revised T/R loop coil, the two RF switches are simulated as resistors placed in the middle of straight conductors, as marked S1 and S2 in Figure 2(a). During the transmit phase, S1 is set to 1000W to simulate a switch-off state, while S2 is set to 0.5W to simulate a switch-on state. During the receive phase, S1 is set to 0.5W to simulate a switch-on state, while S2 is set to 1000W to simulate a switch-off state. Transmit |B₁⁺|-field and receive |B₁^-|-field are then calculated from formulas in reference [3]. A region of interest (RoI) in the center slice of the phantom is defined as shown in Figure 2(b), where average transmit and receive B₁-field over the marked semi-circle-area is calculated for comparison.

Figure 3 shows the normalized plots of transmit |B₁⁺|-field (|B₁⁺|/|B₁⁺|_avg) and receive |B₁^-|-field (|B₁^-|/|B₁^-|_avg) over the RoI in center slice of the phantom for the conventional and revised T/R loop, respectively, where |B₁⁺|_avg and |B₁^-|_avg is the average B₁-field over the area of RoI. As seen, for the conventional loop coil, the spatial non-uniformity of B₁-field within RoI is noteworthy for both transmit |B₁⁺| and receive |B₁^-|. As also seen, the transmit |B₁⁺| and receive |B₁^-| is improved for the revised T/R loop coil. The combination of transmit |B₁⁺| uniformity and receive |B₁^-| sensitivity plays a role in performance. The revised T/R loop coil appears to have a better use-volume at 7T than a conventional loop coil. Table 1 lists the B₁-field quantitative comparison of these two T/R loop coils. For transmit |B₁⁺|, the values are scaled to absorbed average power P_abs of 1W. For receive |B₁^-|, the values are scaled to current amplitude of 1A in loop conductor near the RF drive source. Normalized standard deviation (standard deviation divided by mean value, no unit) and the dynamic ratios of |B₁⁺|_max/|B₁⁺|_min and |B₁^-|_max/|B₁^-|_min are also calculated. As seen, the revised T/R loop coil has higher transmit efficiency (+17%), more uniform transmit |B₁⁺|-field (standard deviation -36%, dynamic ratio -44%) and better receive sensitivity (+15%).

Here we propose a revised T/R loop coil for 7T. As simulation results show, the revised T/R loop coil is improved across-the-board compared with a similar sized conventional T/R loop coil. In practice, the distance between the added straight conductors to the center of the loop coil can be adjusted for optimization toward specific aim. Here we use a circular loop coil as an example. The same concept is also applicable for other loop shapes such as rectangular or oval. It is also possible to make transmit/receive array coils by using multiple revised loops for multi-channel applications at ultra-high fields.