MODIFIED BIRDCAGE COILS FOR TARGETED IMAGING

The design of a radiofrequency coil of U-shaped geometry, specifically intended to image individual knuckle joints of the hand, has been inv estigated, The coil geometry is that of a ''split and opened'' birdcag e coil, The optimum leg positions for such a U-shaped coil have been t heoretically predicted using a novel technique that considers both the signal-to-noise ratio and the homogeneity of the field-of-view. Two p articular coils of this type have been constructed, each optimum for a different size of image space and assuming a different source of imag e noise (sample-dominated or coil-dominated). The experimentally deter mined radiofrequency fields produced by the coils correspond well to t heory, Either coil can be used to obtain good-quality, high-resolution (130 mu m x 130 mu m in-plane) images in vivo of the first, second, o r fifth metacarpophalangeal joints of the hand, sites of particular in terest in the study of arthritis.