Hyperpolarized (HP) ¹²⁹Xe MRI can reveal information concerning lung ventilation¹, gas exchange^2-4, and microstructural dimensions^5-6. For HP gas imaging to accurately analyze lung function and identify ventilation deficiencies, it is necessary to achieve a homogenous excitation profile. However, scanner bore diameter sets a hard boundary upon coil size and fundamentally limits coil development, necessitating tradeoffs between quality and functionality. For example, birdcages have superior excitation profiles and yield superior SNR to surface coils, but an insertable birdcage would reduce patient comfort and be limited to smaller patients. To simultaneously achieve patient comfort, ease of use and image homogeneity, a dual loop T/R ¹²⁹Xe coil was developed to provide homogenous excitation for a large range of patient sizes. An electromagnetic model was created to analyze the transmit efficiency of this new design compared to a saddle coil design⁷.

Physical Design: To achieve a more homogenous excitation and receive profile two loop coils were constructed with a length of 424mm, a width of 380.5mm for the anterior coil and 409.6mm for the posterior coil. To provide robust mechanical support for subjects and coil electronics, each loop was housed in a lightweight 3D printed polycarbonate frame. The anterior housing was designed as an open loop (Figure 1) and the posterior frame was sandwiched in foam. Each coil loop was constructed with ¼” copper tubing and 8 cuts: 1 cut to accommodate the feedboard attached to a coaxial cable and 7 cuts to accommodate tuning capacitors. Passive proton decoupling was added to each coil. To suppress cable currents one xenon balun and three proton baluns were added to the cable. Both coils were connected to a custom-built Wilkinson divider, which was connected to a custom T/R switch displayed in Figure 2.

Coil simulation: Transmit efficiency was evaluated in an EM Simulation using HFSS (ANSYS) and compared to the efficiency of a previously-built saddle coil⁷. Both coils were modeled after the physical design and loaded with an elliptical phantom measuring 400x210x340mm³. A dielectric constant of 77.53 and a conductivity of 0.7S/m was used for the phantom. Additionally, two air-filled cylinders were placed in the center of the phantom to replicate the lungs. Each coil was resonated at 35.329MHz with a resulting reflection coefficient of -25.6dB for the anterior and -32.7dB for the posterior coil of the dual loop design while being excited with a continuous 1W, 50Ω source. Coupling was calculated with -3.84dB between both loop coils. The setup was surrounded by a replicated RF shield. For safety purposes local SAR values were also analyzed in the simulation.

In-Vivo MRI: Isotopically enriched xenon (86% ¹²⁹Xe) was polarized to 32% using a homebuilt polarizer (50/50 mixture of ¹²⁹Xe/N₂). Axial, multi-slice GRE images (α=11º,TE/TR=4.49/9.36ms, BW=4.2kHz, matrix=92x54, voxel size= 3x3x15mm³, 1 average) were acquired from healthy volunteers in a single breath hold (<16s) using a Philips 3T Achieva™ scanner (Philips Healthcare, Best, Netherlands).

Based on simulations, the B₁⁺ distribution in the center planes of the phantom (Figure 3) includes a larger homogeneous region than the previously designed saddle coil⁷. The simulated B₁⁺ variation through the phantom using the dual loop design showed a minimal fluctuation of 0.2µT over the three axis, whereas the previous design had a much larger variation in its B₁⁺ distribution of more than 1µT. Simulations also showed that local SAR limits of 10W/kg were reached with a 49.24W continuous wave excitation, with a local “hot spot” located on the left and right side of the phantom towards the anterior part of the coil (Figure 4). A maximum B₁⁺ of 2.76µT was found in the center of the phantom with these settings in the simulation. Using this coil, single-breath, HP ¹²⁹Xe ventilation images were successfully acquired in healthy, pediatric volunteers (e.g., Figure 5). These images demonstrate that the coil generated very homogenous signal intensity for diagnostic imaging. Moreover, the SNR of these images was ~25, which is more than sufficient for quantitative analysis of ¹²⁹Xe ventilation.In-vivo HP ¹²⁹Xe imaging demonstrated excellent coil homogeneity for large subject lungs. The dual loop design allowed for comfortable subject breathing and made the coil robust for a much larger range of patient sizes. The dual coil does have lower SNR and efficiency than the earlier saddle coil⁷ design. However, the lower SNR can be explained by the larger size of the loop elements, which will contribute to more noise in the acquired data. High coupling was observed between both loop coils due to the size of the loops and small separation distance compared to the loop diameter.