Improved Blood Suppression of Motion-Sensitized Driven Equilibrium in High-Resolution Whole-Brain Vessel Wall Imaging: Comparison of Contrast-Enhanced 3D T1-Weighted FSE with Motion-Sensitized Driven Equilibrium and Delay Alternating with Nutation for Tailored Excitation

DISCUSSION

In this study, we compared the BB effect of CE 3D-T1-FSE with MSDE and with DANTE. In the qualitative analysis, CE 3D-T1-FSE with MSDE had a significantly greater BB effect than CE 3D-T1-FSE with DANTE in the small intracranial arteries and even in the cerebral veins, cortical veins, internal cerebral veins, and transverse sinus. Both qualitative and quantitative analyses of the BB effect in the M1 of the MCA did not show significant differences between CE 3D-T1-FSE with MSDE and with DANTE. Thus, CE 3D-T1-FSE with MSDE may improve lesion conspicuity for pathologic enhancement of the vessel wall and intraluminal enhancing lesions in small vessels by decreasing venous contamination and suppressing signals from slow-flowing blood.

DANTE suppresses signals from both the blood and CSF using an alternative arrangement of low flip angle nonselective radiofrequency pulses with gradient pulses, causing static spin systems to fall into steady states.^2,12,16 Signal suppression of the blood and CSF results from the failure of flowing spins to establish this steady state due to the spoiling effect.^2,16 Previous studies that compared HR-VWI with or without DANTE reported that DANTE demonstrated an improved BB effect.^3,16 Several previous studies comparing the BB effects of DANTE with those of MSDE reported the superiority of DANTE over MSDE;^{3,14,15,24,25} however, these were based on evaluations of the carotid artery,^14,24 a phantom model of an aneurysm,³ or vessels of the neck or lower extremities.²⁵

Our study evaluated small-sized intracranial vessels using contrast enhancement because the CE sequence gives essential clinical information on vasculopathy. An adequate BB effect is crucial in avoiding incomplete blood flow suppression, which may mimic atherosclerotic plaque enhancement or vessel wall enhancement. In contrast to previous phantom results,³ we found that DANTE demonstrated a lesser BB effect in the small intracranial vessels than MSDE. Cornelissen et al³ indicated that the use of preparation pulses when conducting HR-VWI improved the BB effect in an aneurysmal phantom, especially when using 3D-T1-turbo spin-echo (TSE) with DANTE compared with 3D-T1-TSE with MSDE. However, blood-signal suppression is a delicate task that is affected not only by preparation pulses but also by the complex interactions of certain imaging parameters, such as voxel size, TR, TE, receiver bandwidth, and flow rate.^26,27 Pravdivtseva et al²⁷ demonstrated the complicated relationships among flow rate, spatial resolution, and preparation pulses regarding blood-signal suppression in 3D-T1-TSE. In a scenario with a slow blood flow rate, a larger voxel size was efficient in suppressing the blood signals, more from the dominant intravoxel dephasing effect than the flow effect of the blood from the imaging section. However, when we used MSDE, the BB effect was similar between voxel sizes of 0.5 and 0.9 mm.^3,27 Because the highest clinically feasible resolution is recommended in HR-VWI to minimize the partial volume effect,⁹ using preparation pulses with a high-spatial-resolution setting may help achieve a greater BB effect in a scenario with slow blood flow.

MSDE consists of block-shaped radiofrequency pulses with 90°–180°–90° flip angles and motion-sensitizing gradients, which dephase all moving blood spins before imaging, thus, suppressing signals from the blood.^17,18 Previous studies have discussed the efficacy of MSDE in suppressing signals from the blood and CSF, with results indicating MSDE as an effective tool to delineate vessel walls.^11,18,28,29 Because there is a substantial intraluminal signal increase after gadolinium injection, Lindenholz et al⁵ emphasized careful interpretation of slow-flow artifacts mostly induced by venous flow near the artery, which may cause misinterpretation of arterial wall enhancement. In our study, the visual scores of the BB effect, in the cortical veins as well as the ICV, were noticeably higher in CE 3D-T1-FSE with MSDE than in CE 3D-T1-FSE with DANTE (3.30 versus 2.74, P < .001; 3.72 versus 2.32, P < .001). We speculated that the basic difference in the composition of the 2 preparation pulse diagrams may account for our result, which is the CE situation.

DANTE is a low flip angle gradient-recalled acquisition in steady state sequence, which has T1-weighted contrast. Turbulence flow breaks the steady state, and its signal is suppressed. However, if the flow is slow or laminar, signal suppression becomes less. When the contrast agent is used, the blood-signal suppression effect may become less because of fast T1 recovery. On the other hand, the MSDE preparation consists of a 90° excitation pulse, two 180° refocusing pulses with each pulse sandwiched by bipolar motion-sensitizing gradients, and a −90° flip back pulse;²⁹ in other words, it has the characteristics of a spin-echo sequence with a motion-sensitized gradient like the diffusion-weighted spin-echo sequence. The fast T1 recovery of the contrast agent may have a low impact on the flow suppression of MSDE because MSDE has spin-echo-based T2-weighted contrast. Moreover, the gradient size is much larger than that of DANTE, and the large motion-sensitized gradient can suppress slow laminar flow.

In the qualitative analysis, although the overall image quality of MSDE was notable, CE 3D-T1-FSE with DANTE received a significantly higher score than CE 3D-T1-FSE with MSDE (3.54 versus 3.42, P = .013). Li et al¹⁴ speculated that compared with DANTE, MSDE reduces SNR due to T2 decay; however, our quantitative analysis revealed no significant differences in the SNR_wall and SNR_csf between CE 3D-T1-FSE with DANTE and with MSDE. There is a concern that the vessel wall SNR would be decreased when using preparation pulses for 3D-T1-FSE.²⁰ Cho et al¹³ reported that preparation pulses may not be a requisite for MR imaging sequences with a long echo-train length and an appropriate TR (below 1160 ms; which enhances the differentiation of vessel walls from CSF and brain simultaneously clinically feasible acquisition time) in vessel wall imaging. However, we believe that preparation pulses, especially those of MSDE, need to be applied in the CE 3D-T1-FSE sequence because the intracranial blood flow is often slow, stagnant, or turbulent; thus, artifacts may pose problems in the interpretation of possible vessel wall pathologies,^5,30 particularly when gadolinium shortens the T1 relaxation times of blood.

This study had several limitations. First, this was a retrospective study that included patients who underwent HR-VWI at a single center during a short time period. Therefore, we included only a limited number of patients. However, we used precisely controlled MR imaging settings for all patients with a randomized order of image acquisition to minimize potential bias, which could be magnified in a limited sample size. Nevertheless, the optimal sample size requires further validation. Second, we did not conduct quantitative analysis in the smaller vessels, A3, M3, P2 to P3, and the ICV, unlike in the M1 segment of the MCA. Due to the excessively thin walls of these vessels, the signal intensity acquired by measuring the ROI was easily altered by partial volume artifacts. As a substitute, we performed a qualitative analysis of the small-sized vessels using a 4-grade visual scoring system. Third, although image analysis of the lesion-free portion of the vessels was performed, the patients included in our study were those who satisfied the clinical indications of the HR-VWI examination, rather than healthy volunteers. In actual clinical settings, most individuals examined with HR-VWI are patients with pathologies. Hence, our cohort study might serve as a representative sample. Future studies including both healthy volunteers and patients with pathologies may clarify the optimal preparation pulse that could be widely used for HR-VWI.