To develop a 16-channel transceiver body imaging array at 7.0T with improved transmit, receive and SAR performance by combining loop and dipole elements and utilizing their complementary field characteristics.

The 16-channel loop-dipole transceiver array (16LD) consisted of 8 identical loop-dipole building blocks (LD), with each block containing a fractionated dipole antenna [1] and an 8x18cm² rectangular loop coil (Figure 1.a). The dipole and loop elements were etched on separate FR4 boards, tuned/matched using fixed value capacitors and aligned along their long-axes at their center locations. Precise relative placement of the two elements was crucial to achieve decoupling performance <-15dB.

Two housings made of vinyl fabric each contained four LD blocks with a center-to-center distance of ~11.5cm between the neighboring blocks (Figure 1.b-c). Flexibility of the housing was essential in conforming the elements to subjects with differing body sizes and shapes to maintain performance by providing designed loading conditions.

The 16LD was modeled around anatomically relevant locations on the torso and pelvis of an anatomically correct human model (Duke, Virtual Family [2]). EM-field distributions of each coil element were computed using an FDTD solver in SEMCAD X software (SPEAG, Zürich, Switzerland) and were imported to Matlab (Mathworks, Natick, MA) to investigate shim-dependent B1+ and SAR distributions. Time-average power (TAP) limits yielding 20W/kg peak 10g averaged SAR and 4W/kg partial body SAR were calculated for safe operation in the prostate, kidney and heart. Furthermore, B1+ transmit efficiency normalized to unit total coil power, B1+ SAR efficiency normalized to peak 10g-averaged SAR, and SNR in the prostate were calculated for comparison with in vivo measurements.

MRI experiments were conducted on a whole-body Magnetom 7.0T scanner (Siemens Healthcare, Erlangen, Germany) equipped with sixteen 1kW power amplifiers. B1+ transmit efficiency and SNR of the 16LD was measured inside the prostate of five healthy consented subjects, using the methods in Ref. [3]. The experimental transmit/receive performance of the 16LD was compared against two existing transceiver arrays: 16-channel microstrip-line (16ML) [4] and a 10-channel version of the fractionated dipole antenna (10DA) arrays [1]. The subjects for the 16ML and 10DA data included 5 healthy male volunteers with BMIs and AP dimensions covering a similar range. Anatomical MRI of the prostate, kidneys and the heart were acquired on healthy male volunteers with the 16LD using TAP limits determined from simulations.

Simulated transmit and receive performance of the 16LD, 16ML and 10DA are listed in Table 1. TAP limits for the 16LD were imposed by local 10g-averaged SAR when the excitation voltage of the loop elements were >-7dB of the dipole elements, and by partial-body SAR when the excitation voltages of the loop elements were ≤-7dB of the dipole elements. B1+ transmit efficiency of the 16LD was >30% higher than the 16ML and 10DA. Compared to the 16ML and 10DA, the B1+ local SAR efficiency of the 16LD was 45.8% and 38.5% higher, and the SNR of the 16LD was 49.5% and 36.0% higher, respectively. The 16LD outperformed both 16ML and 10DA in terms of B1+ transmit efficiency, B1+ local SAR efficiency, peak B1+, and SNR by at least 30% in simulations on Duke.

Experimentally measured B1+ transmit efficiency and SNR of the 16LD inside the prostate are plotted in Figure 2 in comparison to the 16ML and 10DA with the horizontal axis showing the AP dimension of the subjects. The 16LD had on average, 29.1% and 20.0% higher B1+ transmit efficiency, 29.1% and 51.8% higher peak B1+, and 26.9% and 20.4% higher SNR compared to 16ML and 10DA, respectively.

Anatomical prostate MRI along axial and coronal views are shown in Figure 3. Coronal MRI of the kidneys and four-chamber view of the heart acquired using 16LD are shown in Figure 4.

Combining loop and dipole elements maintained both the higher transmit and receive performance of the loop elements at shallower depths and the improved performance of the dipole elements at greater depths while simultaneously improving SAR performance due to the distinct E-field distributions of the loop and dipole elements. Furthermore, the loop-dipole combination increased channel count and density beyond what can be currently achieved when using dipole elements alone if relying on distance-based decoupling. The 16LD outperformed 16ML and 10DA in both simulations and experiments in terms of transmit and receive performance inside the prostate. MRI of the prostate, kidneys and the heart were acquired using the 16LD on healthy subjects showing the potential of this RF coil design to successfully image targets throughout the body at 7.0T.