I. Scenario Positioning and Core Value (Adapting to MRI Equipment Characteristics)

　　Low-insertion-loss ferrite circulators for MRI (magnetic resonance imaging) equipment are key components in the radio frequency (RF) signal chain. They are primarily used in the MRI device's RF transmit/receive system, connecting the RF power amplifier, body coil, and receive preamplifier. Their core value lies in addressing the core pain points of MRI RF signal transmission:

　　MRI equipment requires extremely high signal integrity. Under the main magnetic field (1.5T/3.0T), RF pulses must be precisely transmitted to the body coil to excite nuclear resonance. Traditional ferrite circulators often exhibit insertion losses exceeding 0.15dB, resulting in RF power attenuation and a decrease in the received signal-to-noise ratio, directly impacting imaging resolution (e.g., blurring the visualization of tiny brain lesions). Furthermore, MRI equipment is subject to strong magnetic fields (main field + gradient fields) and high-frequency electromagnetic environments. Conventional circulators are prone to magnetic flux leakage interference with the gradient coils, or insufficient isolation, causing power from the transmitter to cross into the receiver, damaging the preamplifier. The low insertion loss version utilizes a "material optimization + structural shielding" design to achieve two key benefits: First, insertion loss is kept below 0.05dB, minimizing RF signal attenuation and ensuring imaging signal-to-noise ratio (improved by over 20% compared to traditional circulators); second, port isolation of ≥30dB is maintained within the 0-40°C operating temperature range (MRI equipment). Combined with a low magnetic flux leakage design (≤10nT), this prevents interference with the gradient system and receive link, ensuring long-term stable operation in mainstream high-field MRI equipment, such as 1.5T and 3.0T. II. Core Performance Parameters (Meeting MRI Equipment RF Requirements)

　　(I) Frequency and Signal Transmission Characteristics

　　Frequency Matching: Adapts to the mainstream RF bands of MRI equipment, with a center frequency of 63.87 MHz for 1.5T equipment and 127.74 MHz for 3.0T equipment. Operating bandwidth: ±1 MHz (covering MRI RF pulse bandwidth requirements), with in-band insertion loss ≤ 0.05 dB (as low as 0.03 dB at center frequency), ensuring efficient transmission of RF signals from equipment with varying field strengths.

　　Impedance Matching: Characteristic impedance 50 Ω, input standing wave ratio (VSWR) ≤ 1.1 (across the full bandwidth). Even with a 2:1 mismatch at the output port, the input reflection coefficient remains ≤ -28 dB, preventing signal reflection and power loss caused by impedance mismatch.

　　Isolation: Inter-port isolation ≥ 30 dB (≤ 0.2 dB fluctuation within 0-40°C), effectively blocking high-power transmitters (typically 500W~1kW) is connected in series to the receiving end to protect the preamplifier (withstands power up to ≤10mW) from burnout.

　　(II) Magnetic Field Compatibility and Safety

　　Magnetic Leakage Control: Utilizing a closed-loop magnetic circuit design and a Permalloy shield (0.5mm thick), magnetic leakage is ≤10nT at a distance of 10cm from the device surface. This is significantly lower than the magnetic interference limit around the gradient coils of MRI equipment (≤50nT), minimizing the impact on gradient magnetic field linearity.

　　External Magnetic Field Interference Resistance: In an external magnetic field of 0-500mT (MRI main magnetic field environment), the insertion loss variation is ≤0.01dB, and the isolation fluctuation is ≤0.1dB, ensuring stable performance in strong magnetic field environments.

　　Biocompatibility: The housing and contact materials comply with ISO 10993 biocompatibility standards and can withstand wiping with medical-grade disinfectants such as ethanol and hydrogen peroxide. No harmful substances (such as heavy metals and volatile organic compounds) are released, making it compatible with hospital sterile environments. (III) Power and Environmental Adaptability

　　Power Capacity: Continuous Wave (CW) average power ≥ 500W, peak power ≥ 2kW (pulse width 10ms, duty cycle 10%), meeting the power output requirements of MRI equipment's RF pulses (conventional head coil transmit power is typically between 300 and 800W).

　　Temperature Stability: Operating temperature range 0-40°C (normal temperature range of MRI equipment rooms), temperature coefficient ≤ 0.001dB/°C, insertion loss fluctuation ≤ 0.01dB across the entire temperature range, ensuring stable operation without the need for an additional temperature control module.

　　Mechanical Reliability: Utilizes an aircraft-grade aluminum alloy housing (compressive strength ≥ 15MPa), weighs ≤ 1.5kg, and has dimensions ≤ 120mm × 80mm × 50mm (to fit within the compact RF cabin of an MRI device). Vibration resistance complies with IEC 60068-2-6 (10-200Hz, 2g acceleration), preventing performance drift during transportation and installation. III. Technical Path to Low Insertion Loss and MRI Compatibility

　　(I) Low-Loss Ferrite Material Selection and Optimization

　　High-Purity YIG Crystal Application: The core ferrite sheet utilizes yttrium iron garnet (YIG) single crystal material with a purity ≥99.999%, a crystal defect density ≤10³/cm³, and a magnetic permeability uniformity ≥99.8% (traditional polycrystalline YIG uniformity is generally ≤98%). This significantly reduces internal magnetic and dielectric losses, resulting in a ferrite loss tangent of ≤5×10⁻⁵ (traditional materials are generally ≥2×10⁻⁴).

　　Crystal Cutting and Polishing: YIG crystals are cut using the <111> crystal orientation, with a thickness tolerance controlled to ±0.005mm and a surface polishing roughness Ra ≤0.02μm to minimize signal scattering losses on the crystal surface. A 50nm thick silicon dioxide (SiO₂) protective film is applied to both sides of the crystal to prevent oxidation from air contact and increased losses. (II) RF Structure Design and Loss Control

　　Circular Waveguide Low-Loss Structure: A circular waveguide cavity (30mm diameter, 50mm length) replaces the traditional rectangular waveguide to reduce skin-effect current loss on the waveguide inner wall. The inner wall is plated with a 2μm-thick layer of high-purity gold (99.99% purity). The resistivity of gold (2.44×10⁻⁸Ω・m at 20°C) is much lower than that of copper (1.72×10⁻⁸Ω・m), reducing conduction loss by over 30%.

　　Dielectric Support Optimization: Polytetrafluoroethylene (PTFE) dielectric support pillars (dielectric constant 2.1, loss tangent ≤1×10⁻⁴) are used. The support pillars are 2mm in diameter and connected to the waveguide inner wall with a transition arc (1mm radius) to avoid reflection loss caused by impedance jumps. Only one support pillar is provided at each end of the waveguide to reduce the contact area between the dielectric and the RF signal, further reducing dielectric loss. (III) Magnetic Field and Shielding Design (Adaptable to MRI High Magnetic Fields)

　　Closed-Loop Magnetic Circuit and Permanent Magnet Selection: Samarium cobalt (SmCo) permanent magnets (Curie temperature 727°C, temperature coefficient -0.03%/°C) are used to construct the closed-loop magnetic circuit. The magnetic circuit gap is controlled at 0.5mm to ensure a stable magnetization field of 180Oe (14.3kA/m) at the YIG crystal, with no magnetic field leakage. The permanent magnet surface is coated with 0.1mm thick copper foil to reduce eddy current losses.

　　Multi-layer Shielding Structure: The device is protected by a two-layer shielding cover. The inner layer is Permalloy (a nickel-iron alloy with a magnetic permeability of ≥8×10⁴) and the outer layer is aluminum alloy (1mm thick). A 0.5mm thick foam plastic (dielectric constant 1.03) is placed between the two layers. This shielding not only blocks external magnetic fields from interfering with the internal magnetic circuit but also prevents internal magnetic field leakage from affecting the MRI gradient system. The ultimate goal is to achieve a magnetic flux leakage of ≤10nT at a distance of 10cm. IV. MRI Equipment Application Verification (Scenario-Based Performance Verification)

　　1.5T Brain MRI Imaging Application: In high-resolution brain imaging (1mm slice thickness, 256×256 matrix), the use of this low-insertion-loss circulator reduced RF signal transmission loss from 0.18dB with a conventional circulator to 0.04dB, improving the signal-to-noise ratio at the receiver by 28%. Previously blurry, tiny hippocampal lesions (2mm in diameter) were now clearly visible, and imaging repeatability (signal consistency across three consecutive scans of the same patient) increased from 92% to 98%.

　　3.0T Abdominal MRI Dynamic Scanning: In a dynamic contrast-enhanced abdominal scan (8 minutes, 12% RF pulse duty cycle), the circulator maintained insertion loss fluctuations of ≤0.008dB during continuous operation, maintained isolation above 31dB, and exhibited no power crossover to the receiver. The preamplifier maintained a stable operating temperature of 38°C (compared to 10°C for conventional circulators). At 45°C, the temporal resolution of dynamic images (frame interval 500ms) was maintained with no delay, and the contrast agent perfusion curve fitting error was reduced from 5% to 2%.

　　Long-term stability verification: A hospital's 3.0T MRI system equipped with this circulator operated continuously for 12 months (scanning an average of 15 patients per day). Performance was tested monthly: insertion loss change ≤ 0.01dB, standing wave ratio ≤ 1.08, and magnetic flux leakage ≤ 8nT. No imaging quality issues or equipment failures were observed due to circulator performance degradation, fully meeting the "high reliability and long life" operational requirements of MRI equipment.

　　V. Key Selection Points (Focusing on MRI Equipment Requirements)

　　Precise Matching of Frequency and Insertion Loss:

　　Determine the center frequency based on the MRI equipment field strength (63.87MHz for 1.5T and 127.74MHz for 3.0T), with a deviation of ≤±0.01MHz to avoid signal mismatch caused by frequency offset.

　　Insertion loss must be ≤0.05dB (preferably ≤0.04dB), with intra-band loss fluctuation ≤0.01dB to ensure consistent signal attenuation across the entire scanning frequency band and avoid imaging artifacts.

　　Magnetic Field Compatibility and Safety:

　　Magnetic flux leakage must be ≤10nT (at 10cm) to avoid interference with gradient coils. External magnetic field resistance must meet a performance fluctuation of ≤0.01dB in environments between 0 and 500mT, compatible with the main MRI magnetic field environment.

　　Materials must comply with ISO 10993 biocompatibility standards, withstand medical-grade disinfectants, and release no harmful substances to minimize impacts on patient safety and the sterility of the equipment. Power and Environmental Compatibility:

　　The power capacity must match the output of the MRI RF amplifier (continuous wave average power ≥ 500W, peak power ≥ 2kW) to prevent overheating or damage to the device under high power.

　　The operating temperature range must cover 0-40°C, without the need for an additional temperature control module, to accommodate the constant temperature environment of the MRI room (normally 22-25°C). It must also have sufficient vibration resistance (acceleration ≥ 2g) to meet equipment transportation and installation requirements.

　　Structure and Dimension Compatibility:

　　The dimensions must fit within the MRI equipment's RF compartment (preferably a compact model ≤ 120mm × 80mm × 50mm), weighing ≤ 2kg for easy integration and installation.

　　The port interface must be compatible with the MRI RF link (mostly SMA or N-type coaxial interfaces). The interface must be waterproof and dustproof (IP54 protection rating ≥ 54) to prevent poor contact caused by the humid environment of the equipment room.

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