Implementation of a modified birdcage resonator for 19F/1H MRI at low fields (0.14 T)

Fluorine-19 nuclear magnetic resonance of perfluorinated blood substitute materials provides a method for determination of oxygen tension (pO2) in vivo. Use of a double resonant 19F/1H radio frequency coil allows convenient correlation between the high resolution anatomic presentation of proton images and the fluorine distribution. However, quantitative 19F measurements require an RF coil with good H1 field homogeneity over the image volume and a high quality factor (Q) to minimize errors caused by the low signal-to-noise levels available in in vivo imaging and image nonuniformities introduced by the large chemical shift of fluorocarbons. The birdcage coil design provides a high Q structure with optimum H1 field uniformity and fill factor. However, at low resonance frequencies, the inherently low inductance of the birdcage geometry requires the use of a large number of chip capacitors giving rise to unwieldy coil fabrication and increased cost. This communication describes a modification to the birdcage design that reduces the chip capacitor requirement by at least a factor of 4 for a given dimension, yet retains the essential characteristics of the birdcage design. The modified structure was tuned for double resonance at 5.7/6.0 MHz for 19F/1H magnetic resonance imaging at 0.14 T. For a coil with a length to diameter ratio of 1.67, an H1 uniformity of +/- 2% for the 19F resonance was obtained over a cylindrical region with radius approximately 0.6r (r = radius of coil) and length approximately 1.8r within the coil.