Hyperpolarized 3He microspheres as a novel vascular signal source for MRI

Hyperpolarized (HP) He-3 can be encapsulated within biologically compatible microspheres while retaining sufficient polarization to be used as a signal source for MRI, Two microsphere sizes were used, with mean diameters of 5.3 +/- 1.3 mu m and 10.9 +/- 3.0 mu m. These suspensions ranged in concentration from 0.9-7.0% gas by volume. Spectroscopic measurements in phantoms at 2 T yielded He-3 relaxation times that varied with gas concentration. At the highest He-3 concentration, the spin-lattice relaxation time, T-1, was 63.8 +/- 9.4 sec, while the transverse magnetization decayed with a time constant of T-2* = 11.0 +/- 0.4 msec, In vivo MR images of the pelvic veins in a rat were acquired during intravenous injection of He-3 microspheres (SNR approximate to 15), Advantages such as intravascular confinement, lack of background signal, and limited recirculation indicate quantitative perfusion measurements may be improved using this novel signal source. (C) 2000 Wiley-Liss, Inc.