The Dual-Angle Method for Fast, Sensitive T1 Measurement in Vivo with Low-Angle Adiabatic Pulses

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A new method for measuring T 1 based on a measurement of the ratio, R, of the steady-state partially saturated NMR signals acquired at two fixed low flip angles (<90°) and a single sequence-repetition period, T R, is presented, The flip angles are chosen to optimize both the signal-to-noise ratio per unit time relative to the best possible Ernst-angle performance and the sensitivity with which a measurement of R can resolve differences in T 1. A flip-angle pair at of around (60°, 15°) yields 70-79% of the maximum achievable Ernst-angle signal-to-noise ratio and a near-linear dependence of R on T R/ T 1 with gradient of about 2:1 over the range 0.1 ≤ T R/ T 1 ≤ 1. Errors in hip-angle and excitation-field ( B 1) inhomogeneity result in roughly proportionate errors in the apparent T 1. The method is best implemented with adiabatic low-angle pulses such as B 1-independent rotation (BIR-4) or BIR-4 phase-cycled (BIRP) pulses, which permit measurements with surface coils. Experimental validation was obtained at 2 T by comparison of unlocalized inversion-recovery and dual-angle proton ( 1H) and phosphorus ( 31P) measurements from vials containing doped water with 0.04 ≤ T 1 ≤ 2.8 s and from the metabolites in the calf muscles of eight human volunteers. Calf muscle values of 6 ± 0.5 s for phosphocreatine and around 3.7 ± 0.8 s for the adenosine triphosphates (ATP) were in good agreement with inversion-recovery T 1 values and values from the literature. Use of the dual-angle method accelerated T 1 measurement time by about fivefold over inversion recovery. The dual-angle method was implemented in a one-dimensional localized surface-coil 31P spectroscopy sequence, producing consistent T 1 measurements from phantoms, the calf muscle, and the human liver. 31P T 1 values of ATP in the livers of six volunteers were about 0.5 ± 0.1 to 0.6 ± 0.2 s: the total exam times were about 35 minutes per subject. The method is ideally suited to low-sensitivity and/or low-concentration moieties, such as in 31P NMR in vivo, where study-time limitations are critical, and for rapid 1H T 1 imaging.