A coil combination for magnetic resonance perfusion imaging of mice in vivo at 7 T

Abstract

A radio frequency (rf) coil combination of a birdcage resonator and a receive-only surface coil was developed for in vivo magnetic resonance imaging of mice at 7.0 T. Since this coil was designed for spin labeling perfusion measurements, the length of the birdcage resonator needed to be 110 mm at a diameter of 35 mm. This was challenging since this length extended 1/10 of the wavelength at the spectrometer Larmor frequency of 300.3 MHz. Symmetric drive with homogeneous B1 field was achieved by introduction of a new rf coupling scheme using an additional conductor path ring at zero potential. This design allowed a balanced drive of the coil without the use of an additional balun. The receive-only surface coil was realized as a single loop with a diameter of 24 mm. To avoid coupling between the coils, active decoupling using p-i-n diode switches was integrated. These switches showed good characteristics and the coil combination was not sensitive to effects such as contrast alterations, rf shielding of the transmit pulse by the receive coil, and possible receive coil destruction during transmission. The improved performance of the coil combination with respect to a stand-alone surface coil or a stand-alone birdcage resonator was demonstrated in phantoms and mice. In comparison with a stand-alone suface coil, the coil combination provided more uniform contrast behavior and an extended depth of view. In addition, the combination showed an improved signal-to-noise ratio with respect to a stand-alone birdcage resonator.