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 investigated. The coil geometry is that of a "split and opened" birdcage coil. The optimum leg positions for such a U-shaped coil have been theoretically predicted using a novel technique that considers both the signal-to-noise ratio and the homogeneity of the field-of-view. Two particular coils of this type have been constructed, each optimum for a different size of image space and assuming a different source of image noise (sample-dominated or coil-dominated). The experimentally determined radiofrequency fields produced by the coils correspond well to theory. Either coil can be used to obtain good-quality, high-resolution (130 microns x 130 microns in-plane) images in vivo of the first, second, or fifth metacarpophalangeal joints of the hand, sites of particular interest in the study of arthritis.