Abstract
Super-resolution ultrasound imaging (SRUS) enables in vivo microvascular imaging of deeper-lying tissues and organs, such as the kidneys or liver. The technique allows new insights into microvascular anatomy and physiology and the development of disease-related microvascular abnormalities. However, the microvascular anatomy is intricate and challenging to depict with the currently available imaging techniques, and validation of the microvascular structures of deeper-lying organs obtained with SRUS remains difficult. Our study aimed to directly compare the vascular anatomy in two in vivo 2D SRUS images of a Sprague–Dawley rat kidney with ex vivo μCT of the same kidney. Co-registering the SRUS images to the μCT volume revealed visually very similar vascular features of vessels ranging from ~ 100 to 1300 μm in diameter and illustrated a high level of vessel branching complexity captured in the 2D SRUS images. Additionally, it was shown that it is difficult to use μCT data of a whole rat kidney specimen to validate the super-resolution capability of our ultrasound scans, i.e., validating the actual microvasculature of the rat kidney. Lastly, by comparing the two imaging modalities, fundamental challenges for 2D SRUS were demonstrated, including the complexity of projecting a 3D vessel network into 2D. These challenges should be considered when interpreting clinical or preclinical SRUS data in future studies.
Highlights
Super-resolution ultrasound imaging (SRUS) enables in vivo microvascular imaging of deeper-lying tissues and organs, such as the kidneys or liver
Within the SRUS image and μCT overlap of scan 1, we found up to nine individual superposed vasa recta bundles in the μCT that could reasonably contribute with a microbubble track to a single SRUS image pixel (Supplementary Fig. S3)
Due to the high complexity of the microvascular s ystems[32], comparing the structures depicted in the microbubble-based SRUS images with other microvessel imaging modalities and showing their similarities would further support the technique
Summary
Super-resolution ultrasound imaging (SRUS) enables in vivo microvascular imaging of deeper-lying tissues and organs, such as the kidneys or liver. The chorioallantoic membrane of ex ovo chicken embryos has been used as an in vivo ‘phantom’ for validation of SRUS; like the mouse ear, the anatomy of the membrane’s thin-layered vascular bed allows co-registration of the SRUS images with optical imaging of the same field-of-view[18,19,20]. The distribution of vessel diameters measured with μCT and SRUS were in good agreement, with a peak diameter between 10 and 20 μm[21] In these two studies, the imaging modalities were not directly co-registered and compared. Our study aimed to directly compare the vascular anatomy shown in two in vivo 2D SRUS images of a Sprague–Dawley rat kidney with ex vivo μCT of the same kidney For this comparison, we aimed at estimating the proportion of vessels in the SRUS images that were captured in the μCT. Reaching these goals required imaging the same areas of the same rat kidney with both in vivo SRUS and ex vivo μCT, co-registering the two modalities, segmenting the vessels in each of the modalities, and quantifying the overlap of the two segmentations
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