Magnetic-resonance imaging using the novel MR signal (26% 129 Xe), which was contained in a 25 cm 3 cylindrical glass cell (2 cm diameter, 7.5 cm length), at 3 atm, along source provided by hyperpolarized noble gases 129 Xe and 3 He may prove to be an important new diagnostic technique with 3 atm N 2 buffer gas and a small quantity of solid Rb. 129 Xe was hyperpolarized in the fringe field of a 1.5 T super-for medical imaging (1). Imaging with a hyperpolarized noble gas is different in many ways from conventional im-conducting magnet. Circularly polarized 795 nm light from diode laser arrays (Optopower, Tucson, Arizona) was used aging. The large nonequilibrium polarization of the nuclei is nonrenewable; hence, special considerations are required for optical pumping of the Rb vapor. After 30 min of optical pumping at 90-100ЊC, the glass cell was rapidly cooled in when designing suitable imaging pulse sequences. Every excitation pulse destroys some of the hyperpolarized longitu-ice water to remove the Rb vapor by condensation onto the cell walls. 129 Xe spectra and images were obtained at 17.7 dinal magnetization, which then cannot be restored by waiting for relaxation back to thermal equilibrium as in conven-MHz on a 1.5 T magnet interfaced with a SMIS spectroscopy/imaging system. Spoiler gradients were used in all tional MRI. This disadvantage is offset by the elimination of long recycle delays. In particular, high-speed gradient-experiments to dephase residual transverse magnetization remaining after each acquisition. This was required because echo techniques can be developed that are especially suitable for imaging hyperpolarized noble gases.
of the long T * 2 values of 129 Xe in the gas phase. In this Communication, we investigate gradient-echo Several investigators have used low-flip-angle gradientimaging strategies for hyperpolarized 129 Xe MRI. We find echo techniques such as fast low-angle shot (FLASH) (7) that the choice of flip angle, sampling order, and resolution for imaging hyperpolarized 129 Xe and 3 He (1, 8-12). The has critical consequences for image quality. Sobering et use of repeated small-flip-angle excitation pulses is espeal. recently suggested a variable-flip-angle approach for cially suitable for imaging hyperpolarized species in experihyperpolarized 129 Xe ( 2 ) , and several authors have prements involving the administration of an initial bolus of viously applied this technique to conventional MRI using hyperpolarized gas. Since repetitive sampling of the residual thermally polarized protons ( 3 -5 ) . Here we report experilongitudinal magnetization causes nonrenewable depletion, ments on the use of different gradient-echo pulse selow-flip-angle excitations are effective in preserving the poquences and find that a variable-flip-angle approach can larization while all rows of k space are sampled. The longituimprove the hyperpolarized 129 Xe signal-to-noise ratio dinal magnetization remaining after n excitation pulses is ( SNR ) and eliminate some typical image artifacts. We proportional to (cos u) n , where u is the pulse flip angle. also demonstrate that, although a constant signal intensity Using too small a flip angle, however, results in unacceptably can be obtained with such an approach, the maximum low SNR. spatial resolution achievable is constrained by the longitu-Let us consider a system of hyperpolarized spins with dinal relaxation time, T 1 , of the hyperpolarized 129 Xe. longitudinal magnetization am 00 , where m 00 is the thermal Hyperpolarization of 129 Xe is achieved by collisional spin equilibrium longitudinal magnetization enhanced by the hyexchange with optically pumped rubidium vapor, which perpolarization factor a. The FID amplitude for the nth exciyields up to 100,000-fold enhancement in spin polarization tation pulse is given by am 00 cos n0 1 u sin u, ignoring T 1 relaxand MR detectability (6). We used natural abundance xenon ation. This relationship is demonstrated in Fig. shows observed hyperpolarized 129 Xe signal amplitudes of the xenon pumping cell for a train of 128 12Њ pulses (filled Ø To whom correspondence should be addressed.