Radio Frequency Front-End Circuitry for an Implantable Multiple Frequency Coil

Introduction: Signal-to-noise ratio (SNR) of nuclear magnetic resonance (NMR) imaging and spectroscopy of implanted bio-artificial tissue constructs can be improved by placing a NMR receiver coil around the implanted construct and receive the NMR signal response through inductive coupling with the external surface coil [1]. The limitation of this approach is that the implanted coil cannot be directly tuned to multiple frequencies. To overcome this limitation, integration of the implantable receiver coil with a digitally-controlled capacitor [2] can be used to form a selective-resonant circuit capable of providing increased signal sensitivity at multiple NMR frequencies associated with important metabolic nuclei. Practical implementation of a fully implantable system requires system miniaturization and the capability of wirelessly controlling the digitally-controlled capacitance value. We propose an integrated chip, fabricated in CMOS technology, which is designed for use as the RF front-end for a finite state machine to provide wireless tuning of the implanted receiver coil resonant frequency through the setting of digitally-controlled capacitor values. The RF front-end is capable of extracting clock and data waveforms from RF pulses transmitted through the external surface coil using a modulation scheme that is compatible with current NMR console technology. Fig. 1a shows the system block diagram, which consists of the integrated RF front-end circuitry, a voltage regulator, an integrated digitally-controlled capacitor array, and a serial-to-parallel finite state machine (FSM). This work describes the design and implementation of the integrated RF broadband receiver circuit for recovering data that will be used to wirelessly control the digitally-controlled capacitor array, thus providing the capability to wirelessly tune the implanted coil’s resonant frequency.