Introduction

The evaluation of perforating arteries (particularly the lenticulostriate artery, LSA) is important for the diagnosis of small vessel disease¹. However, there is no suitable MRI imaging technique for both the lumen and vessel wall of perforating arteries because of their small caliber size (50 – 400 um), which requires a high spatial resolution. Inner volume (IV) imaging² reduced field-of-view (FOV) by using a multidimensional selective pulse³, enabling a high-resolution image with small acquisition matrix to achieve an acceptable acquisition time. In this study, 2D spatially selective excitation (SSE) RF pulses were used to replace the excitation pulses of the conventional 3D turbo spin-echo (TSE, named SPACE by Siemens) sequence (IV-SPACE). The LSA lumen and vessel wall were imaged by the IV-SPACE sequence with black-blood 0.30 mm isotropic resolution.

Methods

The 2D SSE pulse was designed with a spiral transmit k-space trajectory, 24 turns, and a pulse length of 12.57 ms (Figure 1). The calculation method was described in Pauly et al³ and demonstrated the feasibility in subsequent literatures⁴. The disk diameter was 5 cm, and the excitation FOV was 17 cm to avoid excitation aliasing. A phantom scan was used to verify the designed pulses by comparing the excitation profile with a simulated one. IV-SPACE with 0.30 mm isotropic resolution and conventional SPACE with 0.40 mm isotropic resolution were used for healthy volunteers, scanning on a 7T research system (Siemens Healthcare, Erlangen, Germany) with a 1TX/32RX head coil. Key parameters are listed in Table 1. The data of eight volunteers were collected.

Results

The phantom result of the excitation profile is shown in Figure 1(d). It is consistent with the simulation in Figure 1(c). Examples of IV-SPACE and conventional SPACE images are summarized in Figure 2. An axial minimum intensity projection (MinIP, slab thickness = 7.2 mm) and curved multiplanar reconstruction along the MCA are shown. The lumen and the orifice of an LSA are depicted in the IV-SPACE image, whereas it appears blurry in the conventional SPACE image. In Figure 3, the section view of an LSA is shown and analyzed. The LSA vessel wall is obvious in IV-SPACE (red arrow) but almost invisible in the conventional SPACE image. With a cut line though the vessel center, IV-SPACE shows a significant signal drop at the lumen, which is almost absent in conventional SPACE. Figure 4 shows a case where the LSA wall is visible in both sequences; the IV-SPACE image has a higher contrast, and there is sharper delineation of the vessel wall than in the conventional SPACE image.

Discussion

Conventional SPACE uses non-selective pulse with whole-brain coverage. Slab-selective SPACE reduces the matrix size only in the head-foot direction. In contrast, the 2D SSE pulse reduces FOV in both the anterior-posterior and head-foot directions. Therefore, IV-SPACE needs a smaller acquisition matrix to cover MCA and LSA than conventional SPACE to achieve the same resolution. IV-SPACE can be used to reduce acquisition times or have a higher resolution. Moreover, the long and symmetric slab-selective pulse requires an extra 180。pulse to prevent stimulated echoes so that the arrival time of the first echo is prolonged and the signal-to-noise ratio is decreased. However, the 180。pulse is not necessary for a spiral-in 2D SSE pulse, which results in a stronger signal that is suitable for high-resolution imaging. This is the first time intracranial black-blood images with isotropic 0.30 mm resolution within ten minutes have been demonstrated. The 2D SSE pulse is robust, and no obvious signal folding from outer excitation FOV was observed, even though the B0 and B1+field maps were not considered in the pulse design. A higher spatial resolution reduces the partial volume effect, so the LSA delineation is significantly improved in the IV-SPACE images in Figure 2. The reduction of the echo train length from 50 to 30 might also contribute to sharpness of the vessel walls due to less signal decay (Figures 3 and 4). The sharper vessel wall potentially benefits the diagnosis and also the study of cerebral vascular disease, such as the microvasculopathy in small vessel disease, quantitative evaluation of the aneurysmal wall, and microstructure of atherosclerotic plaques. In our future research, B0 and B1+ field variations should be incorporated into the pulse design, and better suppression of aliasing should be optimized. Also, patients with cerebral vascular diseases should be evaluated.

Conclusion

The inner-volume 3D TSE sequence was developed to achieve isotropic 0.30 mm black-blood images within a clinically acceptable time. A higher resolution produces sharper delineation of the vessel wall and lumen of intracranial perforating arteries. The technique is promising for the evaluation of microvasculopathies of cerebral vascular diseases.

Acknowledgements

This work was supported in part by the Beijing Municipal Natural Science Foundation (7184226), Young Elite Scientists Sponsorship Program by CAST (2017QNRC001), Ministry of Science and Technology of China grant (2015CB351701), and the Chinese Academy of Sciences grant (XDBS01000000).