3 mLVs were found in the coronal T2-weighted 3-dimensional fluid-attenuated inversion recovery sequence (3D-T2-FLAIR) image of the same coronal plane as shown in figure 1 of Absinta et al., 2 mLVs could be identified in the coronal plane at the bilateral superior lateral corners of the SSS as well as below the SSS (Fig. 1B). The mLVs were visible in multiple coronal sections at different regions of interest at the dorsal side of the brain surface. Figure 1D shows 2 image sequences where the course of the mLVs is visible.
Coronal T1-weighted 3-dimensional magnetization-prepared rapid gradient-echo sequence (3D-T1- MPRAGE) image of the same section shows no visible mLVs (as expected) but the SSS as a triangular area filled with an intermediate intensity (as usually observed) (Fig. 1A).
Transverse 2D-T2-weighted images showed clearly high-intensity regions at the occipital part of the brain surface most likely to also resemble mLVs (Fig. 1C).
The diameter of the mLVs was determined from successive 3D-T2-FLAIR images of the superior region shown in figure 1B to be 700 ± 300 µm and at the occipital region based on the 2D-T2 images to be 800 ± 300 µm. This is in general agreement with the results of Goodman et al. who determined the diameter of mLVs from extracted human meningeal samples to be 354 ± 55 µm (healthy adults) and 381 ± 76 (subjects with Alzheimer's disease) within a range of 19 to 470 µm.
With our study, we could replicate the findings of Absinta et al. and Kuo et al.. In particular, we showed that mLVs are visible in high-resolution 3D-T2-FLAIR and 2D-T2 MR images. To the best of our knowledge, our report is the fourth published report (with Absinta et al. being the first, Kuo et al. being the second and Naganawa et al. being the third) demonstrating the successful in vivo MRI-based detection of mLVs in humans. Since no gadolinium-based contrast agents (GBCA) were intravenously administered for MR imaging in our study – in contrast to the study of Absinta et al. – our findings also highlight the fact that mLVs can be observed in T2-weighted MR images even without GBCA contrast enhancement. This finding is also in agreement with the results of Kuo et al. where also no GBCA was used.
Our findings of the position of the mLVs in the dura as well as the number of mLVs around the SSS are aligned with the findings of Absinta et al.. Furthermore, the diameter of the mLVs estimated by us based on the MRI images is in good agreement with the results of Absinta et al.. Our findings also support those of Naganawa et al. who used a 3D-real IR sequence; however, Naganawa et al. did not report finding mLVs below the SSS as we and Absinta et al. did. A mLV below the SSS was also detected by Kuo et al., strengthening our findings that indeed 3 mLVs surround the SSS. Interestingly, in the ex vivo analysis of specimens of human dura conducted by Goodman et al. more than 5 vessels were found around the SSS.
We found mLVs in the occipital part of the head in 2D-T2 images. The presence of mLVs occipitally in humans is also in agreement with the findings of Absinta et al. that showed the presence of putative mLVs at the same position using 3D rendering (see Suppl. Fig. 1 of Absinta et al.).
We think it is unlikely that the mLVs identified by us are actually arachnoid granulations since the structures found had a tubular form where arachnoid granulations do not have this feature.