Optimizing the inversion efficiency of pseudo-continuous ASL pulse sequence using B0 field map information

Introduction: The recent introduction of pseudo-continuous inversion pulses (pCASL) has the potential to greatly facilitate the use of continuous Arterial Spin Labeling (ASL) (1). However, field inhomogeneities, can compromise the inversion efficiency of pCASL, which causes loss in SNR and severe quantification error (2). We propose a method to restore the loss in inversion efficiency by correcting the phase of the RF pulses in combination with a z-shimming scheme. This will provide more robust perfusion measurements than the conventional pseudo-continuous technique. The method is demonstrated using numerical simulation and In-vivo data. Theory: We model the local field inhomogeneities as a constant shift plus a linear Z-gradient in the local magnetc field (Berror and Gerror) at the tagging plane for the pCASL pulse sequence. Berror and Gerror can be estimated by mapping the field inhomogeneities using a first order linear approximation. Gerror induces an, unwanted, velocity-dependent phase in the magnetization vector during the interval between RF pulses that degrades the adiabatic inversion (2). Berror (offresonance) produces a position-dependent phase error in the magnetization vector that further degrades the inversion. We propose to compensate for Gerror by updating the refocusing lobe of the Gss (Fig 1) using Eq. 1. Unwanted phase caused by Berror will then be compensated by adding a linear phase (φlinear ) defined by Eq. 2 to the RF pulses. ΔAextra = −Gerrorδ (Eq. 1) φ linear = −γBerrorδ (Eq. 2) Methods: We simulated the behavior of the magnetization vector of an ensemble of moving spins (T1=1660ms, T2=250ms, laminar flow with peak velocity=100cm/s) in the presence of the pCASL pulse shown in Fig.1 (Residual fractional moment (η) = 0.12, flip angle = 35°, Hanning-shaped RF of 500μs duration, δ=1.5 ms) using a numerical implementation of the Bloch equations. The effect of field inhomogeneity was simulated by introducing an artifactual gradient and a shift in resonance frequency (Gerror and Berror) to the pulse sequence. This error was then corrected using the proposed method. A pCASL sequence was implemented on a 3.0 T Signa Excite scanner (General Electric, Waukesha, WI) using the same parameters used in the simulation study. Residual fractional moment (η) was first set to the optimum value obtained from a simulation study, assuming no B0 inhomogeneity (η=0.12 ; results not shown here). B0 field inhemogenities were then mapped from two sets of images at the location of tagging plane, acquired with a TE difference of 1 ms (3). Berror and Gerror were estimated from the obtained field map. Refocusing lobe of slice selective gradients and phase of RF pulses were then modified according to Eqs.1 and 2.