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

A new method for measuring T₁ 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 T₁. 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/T₁ with gradient of about 2:1 over the range 0.1 ≤ T_R/T₁ ≤ 1. Errors in hip-angle and excitation-field (B₁) inhomogeneity result in roughly proportionate errors in the apparent T₁. The method is best implemented with adiabatic low-angle pulses such as B₁-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 ≤ T₁ ≤ 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 T₁ values and values from the literature. Use of the dual-angle method accelerated T₁ 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 T₁ measurements from phantoms, the calf muscle, and the human liver. ³¹P T₁ 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 T₁ imaging.