ABSTRACT:Birdcage coils are used widely in nuclear magnetic resonance (NMR) applications for their ability to operate in transmit/receive mode with high signal-to-noise ratio (SNR) and wide field homogeneity. When used at low frequencies, a number of drawbacks arise that drastically reduce their overall performances. In this article we show how the choice of the conductors cross-geometry and capacitor quality affects the coil overall performance. Several birdcage coils, tuned at 7.66 MHz, were designed and built, using conductors with different shapes and capacitors with different quality factors. Realized coils were tested and characterized by laboratory workbench in order to extract several quality indices. In addition, experiments were done in a real magnetic resonance (MR) scanner to evaluate the field homogeneity. We show that an optimized choice of conductor shape and capacitors allows us to obtain high-quality birdcage coils tuned at frequencies in the megahertz range. Our results are applicable in several situations as MR imaging (MRI) in low-field scanners, imaging of nuclear species at intermediate fields, Overhauser systems, and prepolarized NMR.

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ABSTRACT:In this article, undersampling technique theory and design, realization, and workbench test of a low-cost digital MRI (magnetic resonance imaging) receiver are reported. The authors first discuss classic analog receiver architecture, reporting its disadvantages, and then present a receiver with a complete description of the electronic circuit. The intended use of the subsystem should be as a radiofrequency (RF) receiver chain in a low-cost dedicated MRI scanner (e.g., suitable for musculoskeletal limbs studies). The digital receiver consists of a passband (antialiasing) filter, an ADC (analog to digital converter) to digitalize the signal with the undersampling technique, and a DDC (digital down converter) for the frequency translation and filtering to process the signal using a PC. The use of the DDC guarantees perfect signals quadrature and high-performance filtering; moreover, it is adaptable to each modification of the system because of its programmability. Detailed specifications and hardware design of the digital receiver that is designed to be used in a dedicated MR scanner is provided, guaranteeing top performance and low cost.