Quadrature detection in the laboratory frame

Abstract

A theory of quadrature detection in the laboratory frame is developed. It is shown that the geometry of the two orthogonal coil systems needed for quadrature detection is radically different from that used with saddle-shaped coils, and that the homogeneity of the B₁ field produced upon transmission is marginally better. The opposing quadrature phase shifts needed for transmission and reception are emphasized, and the use of a quadrature hybrid is advocated as a simple and inexpensive means of interfacing the transmitter, probes, and preamplifier. Experimental results are presented which confirm the theoretical predictions, and show that up to a 40% improvement in sensitivity and a twofold reduction in transmitter power are possible, particularly in those instances where the sample is conductive—namely, imaging of humans and in vivo spectroscopy.