Loop, microstrip ^{1, 2} and dipole ^{3, 4} transmit/receive arrays are the most common RF coil configurations at 7T and higher. Dipole and microstrip coils produce different and somewhat complementary B₁ patterns³ and hybrid E-field distributions. Based this observation, we developed a 16-channel transmit/receive array for 7T head imaging by interleaving dipole and microstrip elements. Mutual coupling among any elements is <-14 dB without including any other decoupling. Compared with 8-channel microstrip-only and dipole-only arrays, the proposed 16-ch dipole+microstrip array has a higher SNR gain and lower g-factor.

Coil Construction: A 16-channel array with 8 dipole and 8 microstrip elements was constructed on a cylindrical acrylic tube (diameter 24.1 cm and length 25.4 cm), as shown in Fig. 1. The dipole elements and microstrip elements were placed alternately. Each microstrip element was a typical half-wavelength resonator with a Teflon substrate (18×4×1.27 cm³) ¹. The width of the strip conductors and grounds were 1 cm and 4 cm, respectively. One end of each microstrip resonator was terminated with a 3.3 pF capacitor and the other end was terminated with trimmer capacitors for tuning and matching. Each dipole was electrically shortened by two lumped inductors and matched by a trimmer capacitor. The width and length of the dipole conductors were 0.75 cm and 21 cm, respectively. All dipole and microstrip elements were used for transmit and receive, and were tuned to 298 MHz and matched to 50 ohm (S₁₁ better than -25 dB). Floated bazooka baluns were used for all elements to avoid “cable resonance”.

Bench test and MR experiments: The S-parameters of the 16-ch array loaded with a cylindrical water phantom (diameter 15 cm and height 20 cm) were measured with an Agilent 5071C network analyzer. GRE images of the water phantom were obtained with a human 7T Philips Achieva scanner (Philips Healthcare, Cleveland, Ohio, USA). MR images were also acquired using each channel individually. The parameters of the GRE sequence were: FA=25⁰, TR/TE=500/10ms, FOV=180×180mm², matrix=192×192, thickness=5mm. The SNR on root-sum-of-squares (RSOS)-combined GRE images was calculated as: signal/std(noise)*0.655. G-factor maps were calculated using Pruessmann’s method ⁵. B₁⁺ profiles of each channel were measured with the DREAM method ⁶.

Fig. 2A shows the S₁₁ and S₂₁ plots of adjacent dipole-microstrip, dipole-dipole and microstrip-microstrip elements. Fig. 2B shows the S-parameter matrix of the 16-ch array loaded with the water phantom. The worst isolation between any two elements was better than -14 dB, indicating excellent decoupling. In our experience, the coupling between most channels will in practice be lower than -20 dB in heavier loading cases, e.g., with a human head. Fig. 3 shows the GRE images and B₁⁺ maps (both magnitude and phase) of the individual channels. As expected from the S-parameter measurements, each channel produces quite distinct images and B₁ profiles. Due to the relatively confined nature of EM fields from the microstrips, their B₁ fields changed little when the dipole elements were placed adjacent to them. The symmetry and deep penetration of the dipole elements’ B₁ fields were also maintained when they were placed next to the microstrip elements, as shown in Fig. 3. However, their imaging coverage at surface areas was slightly limited by the presence of the microstrip elements. Fig. 4 and Fig. 5 show the calculated SNR and g-factor maps in a central slice of the imaged phantom. Both SNR and g-factor are improved using the new hybrid design.Interleaving dipole and microstrip elements provides a higher overall SNR and a better g-factor when compared to dipole-only or microstrip-only arrays. Note that the mixed array does not need preamp decoupling and can also be used as a transmit-only array. The diverse B₁⁺ field patterns generated by this configuration may be advantageous for RF shimming. No decoupling treatment is needed for the mixed dipole and microstrip array, so it can be used as a flexible transceiver array at ultrahigh field.