Gradient-Echo Imaging Considerations for Hyperpolarized 129Xe MR

Magnetic-resonance imaging using the novel MR signal (26% Xe), which was contained in a 25 cm cylindrical glass cell (2 cm diameter, 7.5 cm length) , at 3 atm, along source provided by hyperpolarized noble gases Xe and He may prove to be an important new diagnostic technique with 3 atm N2 buffer gas and a small quantity of solid Rb. Xe was hyperpolarized in the fringe field of a 1.5 T superfor medical imaging (1) . Imaging with a hyperpolarized noble gas is different in many ways from conventional imconducting 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 90–1007C, the glass cell was rapidly cooled in when designing suitable imaging pulse sequences. Every excitation pulse destroys some of the hyperpolarized longituice water to remove the Rb vapor by condensation onto the cell walls. 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 convenMHz 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 gradientexperiments 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 Xe in the gas phase. In this Communication, we investigate gradient-echo Several investigators have used low-flip-angle gradientimaging strategies for hyperpolarized 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 Xe and 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 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 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 longituLet us consider a system of hyperpolarized spins with dinal relaxation time, T1 , of the hyperpolarized Xe. longitudinal magnetization am00 , where m00 is the thermal Hyperpolarization of 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 n th exciyields up to 100,000-fold enhancement in spin polarization tation pulse is given by am00cos u sin u, ignoring T1 relaxand MR detectability (6) . We used natural abundance xenon ation. This relationship is demonstrated in Fig. 1a, which shows observed hyperpolarized Xe signal amplitudes of the xenon pumping cell for a train of 128 127 pulses (filled Ø To whom correspondence should be addressed.