Simple model of double adiabatic inversion (DAI) efficiency

Multislice continuous arterial spin labeling (CASL) using alternating single adiabatic inversion (SAI) and double adiabatic inversion (DAI) (ASD) suffers from control inefficiencies that reduce the perfusion signal-to-noise ratio (SNB). A simple analytic model for DAI efficiency (χ) is presented, and compared with simulation and experimental data. The DAI efficiency (defined as the ratio of uninverted to equilibrium spins) depends primarily on the effective adiabaticity (β′) and the inversion time (both determined from the labeling RF field and gradient amplitudes, and flow velocities). In this study, χ behaved like a damped oscillator, starting from 1 at β′ = 0 (no spin inversion), dropping to 0.5 at β′ = 0.5 (spin saturation), rising again to a peak at β′ ≃ 3, and subsequently oscillating at higher β′. The simple model matched the flow phantom data for β′ < 2, with an accuracy of better than 10%, but it diverged from the flow phantom data as the data oscillated at higher β′. The relative inversion efficiencies of ASD and simultaneous proximal and distal irradiation (SPDI) were measured in 28 elderly volunteers. The ratio of CASL difference (control - label) signals measured in gray matter (= 1.16 ± 0.05), and the simple efficiency models (= 1.13 ± 0.02) had a correlation coefficient of 0.974.