The loss of signal to noise due to imperfect isolation between the channels of a quadrature nuclear magnetic resonance probe

Abstract

We consider a quadrature nuclear magnetic resonance (NMR) probe with imperfect isolation between the two channels arising from a parasitic mutual impedance, and show that the theoretically achievable signal-to-noise ratio is reduced by a factor of (1+η2)−1/2, relative to that of a probe with perfect isolation, where −20 log η is the isolation in dB; provided that all relevant receiver components are matched to a common impedance. The theoretical result is verified by experimental determination of the response of actual NMR imaging probes when driven with coherent signal, and the response of model circuits simulating actual probes when driven with sources of excess noise; the simulated circuits are needed to reduce extraneous interference in the noise measurements. The theory is also confirmed by computer simulations of noise response in coupled resonator circuits.

Spatial Dependence of a Differential Shading Artifact in Images from Coil Arrays with Reactive Cross-talk at 1.5 T

“…To account for the differential shading, we adapt a theory of reactively coupled NMR coils, first given to explain the effect of cross-talk in laboratory-frame quadrature reception (3). Briefly, for a pair of identical resonant coils, coupled by parasitic mutual reactance, the strength of interaction is measured by the normalized coefficient of coupling η, defined as the product of the usual coefficient of coupling (in this case M/L, cf.…”

Abstracts (Continue in Part I)

“…Then, the capacitors on the side rungs were changed to bring the minor axis resonance up to 63. 9 Mhz, thus converging the two linear modes.…”

“…This configuration created two linear modes, one at 63. 9 MHz oriented along the major axis, and the other at 58. 3 MHz oriented along the minor axis.…”

“…In preliminary bench tuning, the cross talk between quadrature channels was corrected with fixed capacitors for the 31P section, and small flux paddles for the l H section. We judged that 15dB of quadrature (9) cross-talk was adequate with the resonator loaded by the equivalent of a human head. A final tuning was performad in the magnet bore with a human subject; the resonator was not retuned from exam to exam.…”

“…9 MHz, with minor axis of 7 cm, major axis of 13 cm, and length of 10 cm. The coil was driven across low reactance capacitors that had been placed around both end rings.…”