This study showed the effectiveness of the FSD magnetization preparation in improving SPACE's blood signal suppression. 3D isotropic high-spatial-resolution carotid arterial wall imaging using FSD-SPACE was achieved with comparable wall-lumen CNR and good agreement in wall and lumen area measurements with multislice T2-weighted 2D SB-TSE.
The carotid bifurcation is a site often afflicted with atherosclerosis. However, blood signal suppression is usually challenging in this location owning to the irregular geometry and complex flow patterns. Consequently, measurements of the vessel wall and plaque size are often inaccurate. In this work, the plaque-mimicking flow artifacts were observed in over half of the volunteers when using the conventional SPACE sequence. Substantial improvement in blood signal suppression was observed in the artifactual regions when incorporating SPACE with FSD preparation. Compared to conventional SPACE imaging, significantly elevated signal contrast (by approximately 6-fold) between those regions and the surrounding vessel wall sufficed to distinguish the lumen from inner wall boundary, effectively minimizing the likelihood of mistaking the flow artifacts for thickening wall.
We observed the overall signal loss when using the FSD preparation, which was also reported by previous studies (21
). The CNR between the entire lumen and arterial wall was reduced by 8% in those cross-sections having flow artifacts. This reduction was contributed to the signal drop of 25% in the arterial wall, which presumably arose from T2 decay and diffusion attenuation associated with the FSD preparation. Several measures were made in this work to alleviate this side-effect. First, the three RF pulses (90°x
) involved in the conventional FSD preparation were not used. Thus, the effective duration (i.e. T2-decay duration) of the FSD preparation was drastically shortened (7 ms), and, concomitantly, signal loss caused by otherwise added local B1
inhomogeneity was avoided. Second, the first-order gradient moment and b
-value applied in our protocol were substantially lower than those employed in previous studies (21
). This is because the SPACE sequence has an inherent BB effect and the FSD preparation simply acts as a supplement to SPACE acquisition for suppressing blood signal.
In this work, 3D FSD-SPACE was compared to multislice 2D SB-TSE with respect to wall-lumen CNR. Previous studies have shown that multislice 2D BB TSE imaging offers a higher tissue SNR efficiency (SNR normalized by imaging time pre section) and comparable wall-lumen CNR relative to single-slice 2D DIR-prepared TSE imaging (15
). Although the DIR preparative sequence is more effective in suppressing blood signal compared to the SB method in the case of single-slice 2D imaging, these two techniques yield no significant difference in lumen SNR and wall-lumen CNR when multislice 2D acquisition is performed (25
). Thus, multislice 2D SB-TSE was used in this comparison study.
The advantage of intrinsically high SNR/CNR with a 3D acquisition was demonstrated in this work. FSD-SPACE is capable of generating a comparable aCNRw-l
while achieving a more than four-fold improvement in slice resolution over 2D SB-TSE. This high-spatial-resolution combined with adequate wall-lumen contrast renders this 3D imaging technique appropriate for plaque imaging in that the dimensions of individual plaque components are typically on the order of submillimeter (4
). In an effort to avoid the partial volume effect, we employed the 3D FSD-SPACE sequence with an isotropic high resolution of 0.63×0.63×0.63 mm3
. This protocol may facilitate plaque assessment with two major benefits. First, the fine plaque structures can be better delineated, and extent and degree of plaques as well as the size of small plaque components can be reliably quantified. Second, misregistration of images obtained in serial examinations can be reduced because of flexible matching between 3D data sets, thereby allowing for accurate monitoring of disease progression and regression (19
Good agreement of arterial wall and lumen area measurements between the 2D and 3D techniques was observed in this study. However, the LA values from FSD-SPACE were, on average, larger than those from SB-TSE; the WA values had an opposite finding. These biases are more likely to be explained by the fact that near-boundary blood signal was present in some transverse sections on 2D TSE images due to inadequate flow suppression by SB, as opposed to in-flow independent flow suppression in FSD-SPACE. It is anticipated that this discrepancy in morphological measurements would decline when comparing FSD-SPACE to single-slice 2D DIR-prepared TSE in which case the lumen can be sufficiently clean (19
). While morphological evaluation was limited to cross-sections in this work, 3D imaging enables flexible viewing of the vessel wall that is desired for accurate quantitative measurement of disease burden in irregularly geometric plaques. (19
) In contrast, it is impossible to acquire true cross-sectional views for all contiguous slices of both carotids within a single scan using a 2D imaging technique.
Large vessel coverage within a clinically practical scan time is another noteworthy advantage with FSD-SPACE imaging over its 2D counterpart. Oblique coronal acquisition and the associated improved BB effect make it possible to image carotids in a longitudinal manner, which is considerably time efficient. With the present SPACE protocol, a 12-cm-long carotid segment can be captured in around 6 min, whereas multislice 2D SB-TSE would take 8 min with slice resolution heavily sacrificed. Thus, this 3D sequence has the promise to serve as a plaque screening imaging approach.
The SPACE sequence used herein is of T2-weighting, but may involve certain T1 contrast due to the longitudinal magnetization recovery over the long echo train. For the purpose of assessing plaque composition, the SPACE sequence may need to be configured into different versions capable of T1-, T2-, and proton-density weighting contrasts (10
). T1-weighted SPACE has been commercially available and investigated for carotid arterial disease imaging (36
). To our knowledge, however, no study has been performed to systematically investigate multi-contrast SPACE imaging of atherosclerotic plaque.
Our study has a notable limitation related to motion that may be present in vessel wall caused by patient body movement, breathing, cardiac systolic pulsation, and swallowing. Compared to 2D imaging, 3D imaging is more sensitive to motion because imaging time is long and motion during a scan can affect the entire dataset. On the other hand, the application of the FSD preparation may exacerbate SPACE's susceptibility to motion owing to the motion-induced dephasing effect, resulting in reduced SNR in the regions with motion. Unfortunately, no compensation for vessel wall motion was performed during 3D scans in this study. We did notice more blurring artifacts or image quality degradation in 3D image sets in comparison to 2D image sets, a finding also reported previously (19
). Although ECG triggering may not be necessary in 2D imaging (25
), a recent work suggested a beneficial effect of ECG in submillimeter high-spatial-resolution 3D MRI of carotid vessel wall (38
). Images acquired during diastole have shown less vessel blurring. In the case of SPACE imaging, ECG-triggering at every two cardiac cycles would not increase total imaging time compared to the present imaging protocol. However, blood flow is relatively slow during diastole, and stronger first-order gradient moment would be necessary to avoid otherwise more pronounced residual flow artifacts. Other motion-compensation methods, such as navigator-echo (39
) or self-gating (40
), have appeared to be able to reduce swallowing- and bulk motion-induced image artifacts and should readily be compatible with SPACE.
In addition, all subjects imaged in this study were healthy volunteers. Blood flow dynamics and subject tolerance to long scans may be quite different for patient group. Considering the complex flow patterns at or distal to a carotid atherosclerotic lesion, it is anticipated that an individually-tailored FSD configuration would be useful. As our ongoing work, for example, a quick scouting scan for appropriate first-order gradient moment may help yield good BB image quality. Nevertheless, the clinical value of FSD-SPACE needs to be investigated through a carotid atherosclerosis patient study.
In conclusion, this work demonstrated the effectiveness of the FSD magnetization preparation in improving blood signal suppression of SPACE for isotropic high-spatial-resolution 3D carotid arterial wall imaging at 3T. The superior spatial resolution and blood flow suppression along with high time-efficiency render this technique promising for vessel wall morphometry, plaque localization, and potentially characterization.