1. Introduction
In ultra-high-field magnetic resonance imaging (MRI) systems, radio frequency (RF) power combiners serve as critical components between RF coils and RF power amplifiers, with their core elements including transmission lines, inductors, and capacitors [
1]. These combiners must exhibit exceptional power-handling capabilities: 1.5T MRI RF power amplifier (RFPA) systems require combiners to withstand pulse powers exceeding 20 kW, while 3.0T systems demand handling capacities of up to 40 kW [
2]. Consequently, the key components in high-power MRI combiners must endure large currents and high voltages, resulting in increased physical dimensions and elevated losses. Current mainstream solutions widely adopt microstrip lines or lumped-element components combined with Wilkinson structures [
3], yet they face persistent challenges, such as excessive size, high losses, poor parameter consistency, and difficulties in integrating with printed circuit boards (PCBs). For instance, in 3.0T MRI systems, a conventional one-quarter-wavelength microstrip line segment measures 0.6 m, and a four-way combiner requires six such segments totaling 2.4 m [
4]. This not only leads to volumetric expansion but also exacerbates conductor losses due to increased line resistance. On the other hand, lumped-element solutions, while reducing size, necessitate high-current-tolerant inductors—typically implemented as air-core or magnetic-core structures—which suffer from complex fabrication processes and persistent size constraints [
5].
The existing research has predominantly focused on bridge configurations and Wilkinson structures, with the former commonly used in RF coils and the latter in RFPA [
6]. However, the operating frequency of the MRI system ranges from tens to hundreds of MHz and meets the requirements of high-power output [
7]. The microstrip-based implementation method has a large size and high loss due to the long line length, making it difficult to meet power capacity requirements. The lumped component method faces challenges such as discrete circuit parameters and difficulty in the heat dissipation of various devices. In the traditional design of combiners based on λ/4 microstrip transmission structures, conductor loss and dielectric loss can be effectively reduced by, respectively, increasing the line width of the microstrip and selecting low-loss dielectric materials. However, in high-power, MHz-range (from tens to hundreds of MHz) MRI applications, the quarter-wavelength (λ/4) microstrip lines exhibit excessive physical length, leading to oversized dimensions and significant conductor losses [
8]. For lumped-parameter combiners, enlarging the wire diameter of inductors introduces similar challenges [
9]—increased size, elevated thermal dissipation, and inconsistent inductance values—all of which degrade combiner performance.
Reference [
10] achieved low-loss coaxial combiners under high-bandwidth conditions, but this method remained confined to narrowband, low-power scenarios and is ineffective for MRI systems. Reference [
11] proposed a lumped-element model and tuning technique for narrowband RF combiners, reducing impedance distortion and achieving ultra-high efficiency (13.56 MHz, 1 kW); however, its reliance on ferrite cores introduced consistency challenges and severe heating-induced nonlinearities at 3.0T MRI frequencies. Reference [
12] underscored the critical need for compact, low-loss multi-channel RF combiners in 7.0T systems employing parallel transmission (pTX) technology, emphasizing the growing demand for miniaturization and efficiency in ultra-high-field MRI environments, and the proposed design demonstrated superior performance in this scenario, achieving both loss reduction and compact multi-channel integration that enhances clinical system practicality.
In summary, to address the challenges of bulky dimensions, high loss, and thermal management in high-power, MHz-range MRI applications, this study proposes a capacitor-loaded microstrip hybrid structure by integrating transmission line theory with lumped-element capacitor principles. The compact 3.0T RF combiner was designed by optimizing the microstrip width for a fixed substrate thickness, achieving high power density (8 kW) and low insertion loss (0.4 dB) at the operation bandwidth and minimizing its footprint (320 × 200 × 10 mm). Its high-power capability was analyzed through thermal management. The implemented combiner was successfully integrated into a 3.0T MRI RFPA system and reduced the RFPA’s total dimensions, further improving imaging validation to achieve good quality. Although this approach narrows the bandwidth, the optimized combiner remains fully compatible with MRI systems due to their inherent narrowband operation.
3. Performance Analysis
This work presents a high-power combiner for 3.0T MRI systems, and a comparison of its performance with respect to conventional designs is presented in
Table 10. The proposed combiner achieves superior compactness, efficiency, and integration, enabling system-level high-power output. Integrated into RFPA systems, it enhances power-combining efficiency while reducing system volume.
Table 10 highlights the proposed high-power combiner’s advantages in compactness, performance, and system integration. The combiner enables final-stage power combining to meet high-power requirements and, when integrated into RFPA systems, enhances the combining efficiency while reducing equipment size. To quantify these benefits, a figure of merit (FoM) was established as a weighted product of critical metrics:
where
insertion loss (IL): lower is better → inversely proportional term (1/IL);
combining efficiency (η): direct percentage value (e.g., 91% → 0.91);
integration factor (IF): IF = 1.2 for fully integrated designs (e.g., our work, [
18]);
IF = 1.0 for non-integrated designs (e.g., [
19,
20]);
Normalized volume (V):
The volume was calculated as L × W × H and then normalized to the largest volume in the table (e.g., [
19,
20]: 200 × 200 × 40 = 1.6 × 10
6 mm
3):
This FoM emphasizes low insertion loss, high efficiency, full integration, and compactness.
Table 11 shows that the proposed design achieved an FoM of 6.83, surpassing the next best design ([
19], FoM = 4.0) by 1.5× and non-integrated counterparts ([
20,
21,
22], FoM = 1.45) by 4×. These results highlight its superiority in balancing performance, integration, and miniaturization for high-power MRI systems.
4. Conclusions
This study addresses the challenges of high loss and bulkiness in MRI RFPA combiners. Leveraging the Wilkinson combiner theory, a novel four-way microstrip capacitor-loaded combiner was designed and fabricated. Through optimization of the microstrip’s length and capacitor loading, followed by testing, the combiner achieved an insertion loss reduction of 0.4 dB (30% lower than that of conventional 0.6 dB designs), a power-handling 8 kW peak power, a compact size of 320 × 200 × 10 mm (30% smaller than its traditional counterparts), a port performance VSWR < 1.25 (equivalent to a −25 dB return loss), and isolation >30 dB. Finally, the RF combiner was integrated into the RFPA system to achieve the bulk volume, and then imaging experiments were conducted with the 3.0T MRI system; this RFPA combiner demonstrated high efficiency due to lower loss.
The capacitor-loading technique optimizes the microstrip’s electrical parameters, balancing loss and size for high-field MRI systems. Future research directions include scaling to multi-stage combiners for higher-channel MRI systems, adjusting the capacitor-loaded microstrip length Z0 and structures to make them adaptable to 1.5T~7.0T MRI systems, and microchannel cooling or Low Temperature Co-fire Ceramic (LTCC) processes for 8 kW plus thermal stability. This design methodology can extend to RF ablation and communication systems, advancing high-power RF devices toward miniaturization and low-loss operation.