Spatial dependence of a differential shading artifact in images from coil arrays with reactive cross-talk at 1.5 T

Reactive cross-talk causes leakage of the reception signal between neighbor ing coils of a receiver array. We present here experimental and computer-si mulated NMR images (based upon a simple theory) to show, for an array of tw o coils, that the leakage (or secondary) signal is combined phase sensitive ly with the primary signal in each coil, to produce tin certain geometries) a differential shading artifact, manifest as a divot of missing intensity in the image derived from one land only one) of the two coils. The asymmetr y of this effect arises from the sense of the nuclear precession, and the a fflicted coil may be swapped with its mate by reversing the direction of th e static magnetic field. The artifact appears most clearly in transaxial im ages and is shown to be forbidden in certain types of saggital images. In a simplified theory for an array of two meshes (i.e., with only two degrees of freedom) the severity of the artifact depends upon the normalized coeffi cient of coupling (denoted eta and related to the cross-talk in decibels, p si, by psi = -20 log eta.) While the presence of input trap circuits in a t ypical array doubles the degrees of freedom and complicates both the circui t theory and the circuit measurements, the cross-talk is nonetheless shown to be given by an expression of the form psi = --20 log eta', where the new primed parameter eta' embodies the impedance-matching capacitance and the resistance of the scanner's preamplifiers, as well as the mutual reactance responsible for the cross-talk. The values of cross-talk inferred from the computer simulations of the image artifact are somewhat higher (by an estim ated 3 to 6 dB) than those obtained by bench top measurements; but, given t hat the simulations unmistakably reproduce the unique and highly characteri stic visual appearance of the artifact, the proposed model for its formatio n is claimed to be essentially correct. Finally, it is suggested that the a rtifact could be corrected by means of the filtered, edge-completed, recept ion profile described by Wald and co-workers (Wald et at, Magn. Reson. Med. 34,433(1995)). (C) 2001 Academic Press.