Application of Undersampling Technique for the Design of an NMR Signals Digital Receiver GIULIO GIOVANNETTI, 1 VALENTINA HARTWIG, 1 VITTORIO VITI, 2 GIUSEPPE GAETA, 1 RAFFAELLO FRANCESCONI, 1 LUIGI LANDINI, 3 ANTONIO BENASSI 1 1 Institute of Clinical Physiology,National Council of Research,Via Moruzzi 1, 56124 S. Cataldo,Pisa,Italy 2 Esaote Biomedica s.p.a., Genova,Italy 3 Department of Information Engineering,University of Pisa,Pisa,Italy ABSTRACT: In this article, undersampling technique theory and design, realization, and workbench test of a low-cost digital MRI (magnetic resonance imaging) receiver are re- ported. The authors first discuss classic analog receiver architecture, reporting its disad- vantages, 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 programmabil- ity. 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. © 2006 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 29B: 107–114, 2006 KEY WORDS: undersampling; digital demodulator; radiofrequency signal; signal-to-noise ratio INTRODUCTION An MRI receiver is employed to convert the received RF signal from the coil into a suitable form for an ADC converter. Generally, the receiver is a superhet- erodyne circuit that demodulates the RF signal into a low-frequency band in accordance with a reference frequency equal to the emitted RF radiation (1). The scheme of such classic analog MRI receiver is shown in Fig. 1. The free induction decay (FID) signal emitted by the sample is picked up by a RF coil and then processed by a circuit that maximizes the energy to transfer to the amplifier. The pream- plifier amplifies the signal while minimizing the noise. The RF signal is then sent to a phase-quadra- ture detector system. This type of receiver is simple, but it presents many problems. Filters and amplifiers must have the same phase and frequency characteristics, and there must be an extreme accuracy on 90° phase difference between the two reference signals. If not, a first-type distortion can occur (“ghost” artifacts on the image) (2). For these reasons, we implemented a phase- quadrature digital detector system that eliminates phase errors, unbalance errors, and ghost artifacts. Received 28 November 2005; revised 24 February 2006; accepted 27 February 2006 Correspondence to: Ing. G. Giovannetti, I.F.C., C.N.R.; E-mail: giovannetti@ifc.cnr.it Concepts in Magnetic Resonance Part B (Magnetic Resonance Engineering), Vol. 29B(3) 107–114 (2006) Published online in Wiley InterScience (www.interscience.wiley. com). DOI 10.1002/cmr.b.20065 © 2006 Wiley Periodicals, Inc. 107