Radiofrequency surface coils used as receivers in magnetic resonance imaging (MRI) rely on cables for communication and power from the MRI system. Complex surface coil arrays are being designed for improving acquisition speed and signal-to-noise ratio. This, in-turn makes the cables bulky, expensive, and the currents induced on cables by time-varying magnetic fields of the MRI system may cause patient harm. Though wireless power transfer (WPT) can eliminate cables and make surface coils safer, MRI poses a challenging electromagnetic environment for WPT antennas because the antennas made using long conductors interact with the static and dynamic fields of the MRI system. This paper analyses the electromagnetic compatibility of WPT antennas and reveals that commercially available antennas are not compatible with MRI systems, presenting a safety risk for patients. Even when the risk is minimized, the antennas couple with surface coils leading to misdiagnosis. This paper presents an approach to eliminate safety risks and minimize coupling using a filter named “floating filter.” A WPT antenna without a filter has a distortion of 27%, and floating filters reduce the distortion to 2.3%. Secondly, the floating filter does not affect the power transfer efficiency, and the transfer efficiency of 60% is measured with and without filters.
Decoupler circuits are the primary circuits used to maintain safety and image quality in switching magnetic resonance imaging (MRI) surface coils. Decoupler circuits predominantly employ PIN diodes as a switch and their performance is most commonly calculated on the bench at DC and low power RF conditions. The effects of high-power RF on PIN diode decoupler circuits are not usually measured. Experiments at high RF power levels reveal a decrease in the impedance of a typical decoupler as the PIN diode operates in the nonlinear region, effectively increasing the ON-resistance of the PIN diode. The constraints that dictate the start of nonlinearities are studied, and ways to control these nonlinearities are presented. Furthermore, this work is used as a basis to extend and improve upon previous work that established figure of merit (FOM) for PIN diode decouplers. This study is a comprehensive guide for MRI coil designers who face the task of designing decoupler circuits for surface coils and are looking for tools to accurately estimate the dynamic impedance of the circuit over the course of an MRI sequence.
K E Y W O R D Sdecoupler circuits, high impedance blocking circuits, magnetic resonance imaging, PIN diodes, RF transmit-receiver systems
En este artículo se describe el diseño e implementación de un Sistema Eléctrico y Electrónico para automatizar el proceso de bobinado de fleje plástico de polipropileno en la empresa CODIEMPAQUES del ECUADOR Cía. Ltda. , mediante el uso de motores eléctricos de inducción y motores a pasos que permitirán enrollar y posicionar el fleje plástico en un carrete para su posterior almacenamiento y distribución. El proceso de bobinado comprende dos etapas que trabajan simultamenamente para obtener un rollo de fleje plástico uniforme y compacto. La etapa de enrrollado realizada por el motor de inducción trifásico bajo el control del algoritmo de fuerza; la etapa de posicionamiento realizada por el motor a pasos a través de algoritmos de control de movimiento de posición, velocidad y cambio de giro. Los algoritmos son procesados por autómatas de gama alta y reciben señales provenientes de sensores inductivos y encoders incrementales; posteriormente las señales son transferidas mediante red de autómatas MODBUS para su procesamiento. La etapa de implementación se realizó bajo la norma ISO/IEC 24702 para la distribución, maniobra y conexión de los dispositivos optimizando el tiempo y recursos.
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