B₀ and B₁-insensitive uniform T₁-weighting for quantitative, first-pass myocardial perfusion magnetic resonance imaging

Myocardial perfusion can be estimated, in principle, from first-pass MR images by converting the T₁-weighted signal-time curves to contrast agent concentration-time curves. Typically, T₁ weighting is achieved by saturating the magnetization with a nonselective radiofrequency (RF) pulse prior to the imaging sequence. The accuracy of the perfusion estimate derived from the single-point T₁-weighted signal depends on the initial residual longitudinal magnetization (RLM) produced by the saturation pulse. In this study we demonstrate that single-shot, echo-planar imaging can be used to show initial RLM resulting from incomplete saturation due to static magnetic field and RF field inhomogeneities in the heart at 1.5 T. Three saturation pulses, single, composite simple, and composite B₁-insensitive rotation (BIR-4) were evaluated in phantom and cardiac experiments. The RLM image was calculated by normalizing the saturated image by a proton-density-weighted image. Mean RLM produced by the three saturation pulses was significantly different in noncontrast cardiac imaging (RLM_single = 0.108 ± 0.078; RLM_composite = 0.051 ± 0.052; RLM_BIR-4 = 0.011 ± 0.009; P < 0.001; n = 20). Using a BIR-4 pulse to perform saturation of magnetization seems promising for improving the effectiveness and uniformity of T₁ weighting for first-pass perfusion imaging.