Fast proton spectroscopic imaging using steady‐state free precession methods

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Abstract
Various pulse sequences for fast proton spectroscopic imaging (SI) using the steady‐state free precession (SSFP) condition are proposed. The sequences use either only the FID‐like signal S1, only the echo‐like signal S2, or both signals in separate but adjacent acquisition windows. As in SSFP imaging, S1 and S2 are separated by spoiler gradients. RF excitation is performed by slice‐selective or chemical shift‐selective pulses. The signals are detected in absence of a B0 gradient. Spatial localization is achieved by phase‐encoding gradients which are applied prior to and rewound after each signal acquisition. Measurements with 2D or 3D spatial resolution were performed at 4.7 T on phantoms and healthy rat brain in vivo allowing the detection of uncoupled and J‐coupled spins. The main advantages of SSFP based SI are the short minimum total measurement time (Tmin) and the high signal‐to‐noise ratio per unit measurement time (SNRt). The methods are of particular interest at higher magnetic field strength B0, as TR can be reduced with increasing B0 leading to a reduced Tmin and an increased SNRt. Drawbacks consist of the limited spectral resolution, particularly at lower B0, and the dependence of the signal intensities on T1 and T2. Further improvements are discussed including optimized data processing and signal detection under oscillating B0 gradients leading to a further reduction in Tmin. Magn Reson Med 50:453–460, 2003.