| In recent years,the ultra-high field magnetic resonance(MR)has been widely applied in clinical diagnosis and scientific researches.Compared with low-field and high-field MR,ultra-high field MR has a higher image signal-to-noise ratio,better temporal and spatial resolution,and can quickly image small anatomical details,which is conducive to instantly detection of subtle lesions and early diagnosis of diseases.RF power amplifier(RFPA)is one of the core components of ultra-high field magnetic resonance imager,which function is to drive the RF coil to excite magnetic resonance signal.With the increase of field strength,the working frequency of RF power increases,and the power capacity required also increases.Consequently,the transmission loss of coaxial cable between RF power amplifier and RF transmitting coil is rising,which not only causes the performance waste of RF power amplifier,but also increases the cost and dimension of the power amplifier.At the same time,the loss of RF coaxial cable is not conducive to the channel decoupling of ultra-high field magnetic resonance multi-channel parallel transmission system(p Tx),affecting the effect of RF field uniformity.Because the traditional magnetic resonance RFPA is magnetic,it cannot be used directly in the scanning room with strong magnetic field,so it cannot shorten the power transmission distance and reduce cable loss.Aiming at the magnetic problem of traditional power amplifier with magnetic parts which can not reduce power consumption,based on the application requirements of ultra-high field 5T magnetic resonance instrument,this paper designed a 2kW non-magnetic RF power amplifier that can be used directly in scanning room,which effectively shorten the power transmission distance and reduce cable loss.Firstly,utilized theoretical analysis and electromagnetic simulation,a non-magnetic barron which can be used for push-pull configuration of amplifier and a non-magnetic lump-type Wilkinson power divider which can be used for power synthesis are designed.Then,based on these non-magnetized sub-components,through software simulation,fabrication and test,the design of the RF amplification part of the non-magnetic power amplifier is completed.Aiming to the problems of power reflection and load mismatch caused by the failure of magnetic circulator in scanning room,a highly sensitive protection circuit of reflected power is designed,and the analog negative feedback technology is adopted to automatically correct the non-linearity of RF power amplifier.Through the manufacture,integration and debugging of each component,a complete 5T magnetic resonance non-magnetic RFPA was constructed..Test results in laboratory show that the non-magnetic RFPA output power is greater than 2kW,and the basic performance of the input standing wave,flatness,harmonics and noise has not been decreased due to the absence of magnetization.The negative feedback correction scheme improves the tolerance of linearity and load mismatch of power amplifier.In the range of 40 dB dynamic input power,the power amplifier can work normally when the VSWR is ≤3,and with amplitude linearity ≤1dB and phase linearity ≤ 10 DEg.Simulation load mismatch test verifies the effectiveness of the reflection monitoring circuit of RFPA.The proposed non-magnetic RFPA meets the requirements of 5T magnetic resonance RFPA,which has important reference value for the research and development of ultra-high field magnetic resonance non-magnetic radio power amplifier.It lays a certain theoretical and experimental foundation for the localization of non-magnetic high field magnetic resonance radio frequency power amplifier.In the future,it can be applied to the construction of near-magnetic multi-channel p Tx system,improve system energy efficiency,improve the non-uniformity of RF field,and promote the clinical application of 5T ultra-high field magnetic resonance. |
