Optimization of Improved Motion-sensitized Driven-equilibrium (iMSDE) blood suppression for carotid artery wall imaging

BACKGROUND: Improved motion-sensitized driven-equilibrium (iMSDE) preparations have been successfully used in carotid artery wall imaging to achieve blood suppression, but it causes notable signal loss, mostly due to inherent T(2) decay, eddy current effects and B(1)(+) inhomogeneity. In this study, we investigate the signal to noise ratio (SNR) and blood suppression performance of iMSDE using composite RF pulses and sinusoidal gradients. Optimized first moment (m(1)) values for iMSDE prepared T(1)- and T(2)- weighted (T(1)- and T(2)-w) imaging are presented. METHODS: Twelve healthy volunteers and six patients with carotid artery disease underwent iMSDE and double inversion recovery (DIR) prepared T(1)- and T(2)-w fast spin echo (FSE) MRI of the carotid arteries. Modified iMSDE module using composite RF pulses and sinusoidal gradients were evaluated with a range of m(1). SNR of adjacent muscle, vessel wall and the lumen were reported. The optimized iMSDE module was also tested in a 3D variable flip angle FSE (CUBE) acquisition. RESULTS: The SNR of muscle was highest using sinusoidal gradients, and the relative improvement over the trapezoidal gradient increased with higher m(1) (p<0.001). Optimal SNR was observed using an iMSDE preparation scheme containing two 180° composite pulses and standard 90° and -90° pulses (p=0.151). iMSDE produced better blood suppression relative to DIR preparations even with a small m(1) of 487 mT*ms(2)/m (p<0.001). In T(1)-w iMSDE, there was a SNR decrease and an increased T(2) weighting with increasing m(1.) In T(2)-w iMSDE, by matching the effective echo time (TE), the SNR was equivalent when m(1) was <= 1518 mT*ms(2)/m, however, higher m(1) values (2278 – 3108 mT*ms(2)/m) reduced the SNR. In the patient study, iMSDE improved blood suppression but reduced vessel wall CNR efficiency in both T(1)-w and T(2)-w imaging. iMSDE also effectively suppressed residual flow artifacts in the CUBE acquisition. CONCLUSIONS: iMSDE preparation achieved better blood suppression than DIR preparation with reduced vessel wall CNR efficiency in T(1-)w and T(2)-w images. The optimized m(1)s are 487 mT*ms(2)/m for T(1)-w imaging and 1518 mT*ms(2)/m for T(2)-w imaging. Composite 180° refocusing pulses and sinusoidal gradients improve SNR performance. iMSDE further improves the inherent blood suppression of CUBE.