Optimization of improved motion-sensitized driven-equilibrium (iMSDE) blood suppression for carotid artery wall imaging

Abstract

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 T2 decay, eddy current effects and B1 + 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 (m1) values for iMSDE prepared T1- and T2- weighted (T1- and T2-w) imaging are presented.

Methods: Twelve healthy volunteers and six patients with carotid artery disease underwent iMSDE and double inversion recovery (DIR) prepared T1- and T2-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 m1. 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 m1 (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 m1 of 487 mT*ms2/m (p<0.001). In T1-w iMSDE, there was a SNR decrease and an increased T2 weighting with increasing m1. In T2-w iMSDE, by matching the effective echo time (TE), the SNR was equivalent when m1 was <= 1518 mT*ms2/m, however, higher m1 values (2278 - 3108 mT*ms2/m) reduced the SNR. In the patient study, iMSDE improved blood suppression but reduced vessel wall CNR efficiency in both T1-w and T2-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 T1-w and T2-w images. The optimized m1s are 487 mT*ms2/m for T1-w imaging and 1518 mT*ms2/m for T2-w imaging. Composite 180° refocusing pulses and sinusoidal gradients improve SNR performance. iMSDE further improves the inherent blood suppression of CUBE.

Figure 1

Diagram of iMSDE preparation with trapezoid and sinusoid gradients. Gradient amplitude (G) and duration (d) of the flow sensitive gradient are defined.

Figure 2

PDw phantom images using varying iMSDE RF pulse types with identical windowing. Composite 180° only (CP180), composite -90° only (CP90), composite 180° and composite -90° (CP180_CP90), and no composite RF pulses (NOCP). Sinusoidal gradients with a first order moment of 487 mT*ms²/m were used.

Figure 3

Percentage SNR decrease using iMSDE relative to the standard sequence without blood suppression (in gel phantoms). Comparison of SNR change is plotted against the idealized case (computing theoretical signal intensity change as a function of the increased effective echo time (TEeff)). Experimental observations of signal intensity change using trapezoidal and sinusoidal gradients for motion sensitization are illustrated using two calibrated gel phantoms with T2 values of 50 ms (A) and 89 ms (B) [1.5 T @ 21°C] to represent typical carotid vessel wall and muscle tissue respectively. A composite 180° pulse was used for all iMSDE preparations.

Figure 4

Comparison of muscle and lumen SNR with varying iMSDE RF pulse type in T₁-w images (6 volunteers, 48 locations). Composite 180° only (CP180), composite -90° only (CP90), composite 180° and composite -90° (CP180_CP90), and no composite RF pulses (NOCP). Sinusoidal gradients with a first order moment of 487 mT*ms²/m were used.

Figure 5

Comparison of muscle SNR change using iMSDE preparation with sinusoidal (SG) and trapezoidal (TG) gradients (6 volunteers, 48 locations).

Figure 6

iMSDE first moment optimization for T ₁ -w (A) and T ₂ -w (B) contrast (6 volunteers, 48 locations).

Figure 7

Examples of iMSDE first order moment optimization in volunteers. T₁-w imaging with 485 mT*ms²/m and T₂-w imaging with 1518 mT*ms²/m produced excellent blood suppression and SNR.

Figure 8

A patient study using DIR and iMSDE prepared T₁- and T₂-w FSE. Residual blood flow artefacts (arrows) observed following DIR preparation were suppressed using the optimized iMSDE module.

Figure 9

Example images of 3D T₁-w CUBE and iMSDE prepared CUBE images in a volunteer. Blood flow artefacts (arrows) were further suppressed with the iMSDE preparation, however, a reduction in SNR was noted.