Measurement of tissue oxygenation in isolated rat hearts using near infrared spectroscopy

Radiofrequency (RF) receivers are key components in magnetic resonance imaging (MRI) systems. Birdcage coils are widely used for their ability to achieve high signal-to-noise ratio (SNR), however, when they are used in low field scanners, both the detected signal SNR and the coil quality factor Q decrease with frequency, thus reducing the image quality. In this paper we initially describe the design of a high quality birdcage coil, demonstrating the possibility of obtaining high quality images even using low field scanners (0.18T). To convert the RF signal from the coil into a suitable form for an analog-to-digital (ADC) converter, a classic analog superheterodyne circuit is generally employed. This kind of receiver is simple but also presents many problems for filters and amplifiers design. In this study we implemented a phase-quadrature digital detector system using an alternative technique called undersampling. Our receiver is constituted by a passband filter, an ADC and a DDC (Digital Down Converter) for the frequency translation and filtering to process the signal using a PC. In this work we give all the detailed specifications and hardware design issues for the receiver that is designed to be used in a dedicated scanner guaranteeing top performances with low cost