Three-dimensional morphometric analysis of the renal vasculature

MRI of kidney size matters

OBJECTIVE

To highlight progress and opportunities of measuring kidney size with MRI, and to inspire research into resolving the remaining methodological gaps and unanswered questions relating to kidney size assessment.

MATERIALS AND METHODS

This work is not a comprehensive review of the literature but highlights valuable recent developments of MRI of kidney size.

RESULTS

The links between renal (patho)physiology and kidney size are outlined. Common methodological approaches for MRI of kidney size are reviewed. Techniques tailored for renal segmentation and quantification of kidney size are discussed. Frontier applications of kidney size monitoring in preclinical models and human studies are reviewed. Future directions of MRI of kidney size are explored.

CONCLUSION

MRI of kidney size matters. It will facilitate a growing range of (pre)clinical applications, and provide a springboard for new insights into renal (patho)physiology. As kidney size can be easily obtained from already established renal MRI protocols without the need for additional scans, this measurement should always accompany diagnostic MRI exams. Reconciling global kidney size changes with alterations in the size of specific renal layers is an important topic for further research. Acute kidney size measurements alone cannot distinguish between changes induced by alterations in the blood or the tubular volume fractions-this distinction requires further research into cartography of the renal blood and the tubular volumes.

Impact of renal chimney intra-aortic stent length on branch and end-stent angle in chimney endovascular aneurysm repair and endovascular aneurysm sealing configurations

OBJECTIVE

Practice patterns and durability of parallel stent graft techniques in complex endovascular aneurysm repair (EVAR) remain poorly defined. We aimed to quantify and compare the impact of renal chimney intra-aortic stent length (IASL) on geometric deformations of renal arteries in complex EVAR.

METHODS

Thirty-eight nonconsecutive patients underwent EVAR utilizing parallel stent graft techniques (chimney EVAR [chEVAR], n = 28; chimney endovascular aneurysm sealing [chEVAS], n = 10) between 2010 and 2016. A total of 59 renal chimney stent grafts were used. Geometric quantification was derived from three-dimensional model-based centerline extraction. Renal chimney intra-aortic stent length (IASL) was defined as the length of chimney stent that extended from the proximal edge of the chimney stent to the ostium of the corresponding renal artery.

RESULTS

Mean IASL for both left and right renal arteries in the cohort was 35.7 mm. Renal arteries containing chimney IASL <30 mm trended toward a greater branch angle (135.4 vs. 127.8°, p = .06). Left renal arteries showed significantly greater branch angle among those with IASL <40 mm (135.5 vs. 121.7°, p = .045). Mean IASL for renal arteries in chEVAR was significantly longer compared to chEVAS (39.2 vs. 26.3 mm, p = .003). No difference was noted in overall branch angle or end-stent angle based on procedure type. ChEVAR with IASL <30 mm had significantly greater end-stent angle (48.2 vs. 33.5°, p = .03). In contrast, chEVAS patients showed no difference in end-stent angle based on IASL thresholds, but did have significantly greater branch angle among those with IASL <30 mm when grouped by both all renal arteries (133.5 vs. 113.5°, p = .004) and right renal arteries (134.3 vs. 111.6°, p = .02).

CONCLUSIONS

Renal chimney stents with longer IASL appear to exhibit less renal artery deformation, suggesting a more gradual and perpendicular transition of the chimney stent across the renal ostium.

Parallel generation of extensive vascular networks with application to an archetypal human kidney model

Given the relevance of the inextricable coupling between microcirculation and physiology, and the relation to organ function and disease progression, the construction of synthetic vascular networks for mathematical modelling and computer simulation is becoming an increasingly broad field of research. Building vascular networks that mimic in vivo morphometry is feasible through algorithms such as constrained constructive optimization (CCO) and variations. Nevertheless, these methods are limited by the maximum number of vessels to be generated due to the whole network update required at each vessel addition. In this work, we propose a CCO-based approach endowed with a domain decomposition strategy to concurrently create vascular networks. The performance of this approach is evaluated by analysing the agreement with the sequentially generated networks and studying the scalability when building vascular networks up to 200 000 vascular segments. Finally, we apply our method to vascularize a highly complex geometry corresponding to the cortex of a prototypical human kidney. The technique presented in this work enables the automatic generation of extensive vascular networks, removing the limitation from previous works. Thus, we can extend vascular networks (e.g. obtained from medical images) to pre-arteriolar level, yielding patient-specific whole-organ vascular models with an unprecedented level of detail.

Vitrification and Nanowarming of Kidneys

Vitrification can dramatically increase the storage of viable biomaterials in the cryogenic state for years. Unfortunately, vitrified systems ≥3 mL like large tissues and organs, cannot currently be rewarmed sufficiently rapidly or uniformly by convective approaches to avoid ice crystallization or cracking failures. A new volumetric rewarming technology entitled "nanowarming" addresses this problem by using radiofrequency excited iron oxide nanoparticles to rewarm vitrified systems rapidly and uniformly. Here, for the first time, successful recovery of a rat kidney from the vitrified state using nanowarming, is shown. First, kidneys are perfused via the renal artery with a cryoprotective cocktail (CPA) and silica-coated iron oxide nanoparticles (sIONPs). After cooling at -40 °C min-1 in a controlled rate freezer, microcomputed tomography (µCT) imaging is used to verify the distribution of the sIONPs and the vitrified state of the kidneys. By applying a radiofrequency field to excite the distributed sIONPs, the vitrified kidneys are nanowarmed at a mean rate of 63.7 °C min-1 . Experiments and modeling show the avoidance of both ice crystallization and cracking during these processes. Histology and confocal imaging show that nanowarmed kidneys are dramatically better than convective rewarming controls. This work suggests that kidney nanowarming holds tremendous promise for transplantation.

Decellularization of kidney tissue: comparison of sodium lauryl ether sulfate and sodium dodecyl sulfate for allotransplantation in rat

An automatic decellularization device was developed to perfuse and decellularize male rats' kidneys using both sodium lauryl ether sulfate (SLES) and sodium dodecyl sulfate (SDS) and to compare their efficacy in kidney decellularization and post-transplantation angiogenesis. Kidneys were perfused with either 1% SDS solution for 4 h or 1% SLES solution for 6 h. The decellularized scaffolds were stained with hematoxylin and eosin, periodic acid Schiff, Masson's trichrome, and Alcian blue to determine cell removal and glycogen, collagen, and glycosaminoglycan contents, respectively. Moreover, scanning electron microscopy was performed to evaluate the cell removal and preservation of microarchitecture of both SDS and SLES scaffolds. Additionally, DNA quantification assay was applied for all groups in order to measure residual DNA in the scaffolds and normal kidney. In order to demonstrate biocompatibility of the decellularized scaffolds, human umbilical cord mesenchymal stromal/stem cells (hUC-MSCs) were seeded on the scaffolds. In addition, the allotransplantation was performed in back muscle and angiogenesis was evaluated. Complete cell removal in both SLES and SDS groups was observed in scanning electron microscopy and DNA quantification assays. Moreover, the extracellular matrix (ECM) architecture of rat kidney in the SLES group was significantly preserved better than the SDS group. The hUC-MSCs were successfully migrated from the cell culture plate surface into the SDS and SLES decellularized scaffolds. The formation of blood vessels was observed in the kidney in both SLES and SDS decellularized kidneys. The better preservation of ECM than SDS introduces SLES as the solvent of choice for kidney decellularization.

Age- and gender-dependent variability in the geometry of middle cerebral artery bifurcations

The goal of this study was to analyze trends in the geometric parameters of blood vessels with age in a randomly selected group of 200 computed tomography angiography studies of radiologically healthy individuals using dedicated statistical techniques. A quantitative description of cerebral vascular geometry with numerical parameters (bifurcation angle, branching angle, co-planarity index, average curvature, and average torsion) was proposed. The changes and variability in geometry were analyzed according to age. The bifurcation angle, branching angle, and average curvature increased with age, whereas the co-planarity index and average torsion decreased with age. For equal-diameter branches, women comparing to men presented lower bifurcation angles in the 4th decade of life, and lower values for the co-planarity index in the 3rd and 4th decades of life. In non-equal-diameter branches, the opposite relationship was observed for bifurcation angle in the 4th decade of life comparing both groups. All analyzed parameters showed clearly visible trends with patient age. Deviations in specific decades of life were detected; these deviations could be linked to the development of aneurysms in specific age groups.

Renal oxygenation: From data to insight

Computational models have made a major contribution to the field of physiology. As the complexity of our understanding of biological systems expands, the need for computational methods only increases. But collaboration between experimental physiologists and computational modellers (ie theoretical physiologists) is not easy. One of the major challenges is to break down the barriers created by differences in vocabulary and approach between the two disciplines. In this review, we have two major aims. Firstly, we wish to contribute to the effort to break down these barriers and so encourage more interdisciplinary collaboration. So, we begin with a "primer" on the ways in which computational models can help us understand physiology and pathophysiology. Second, we aim to provide an update of recent efforts in one specific area of physiology, renal oxygenation. This work is shedding new light on the causes and consequences of renal hypoxia. But as importantly, computational modelling is providing direction for experimental physiologists working in the field of renal oxygenation by: (a) generating new hypotheses that can be tested in experimental studies, (b) allowing experiments that are technically unfeasible to be simulated in silico, or variables that cannot be measured experimentally to be estimated, and (c) providing a means by which the quality of experimental data can be assessed. Critically, based on our experience, we strongly believe that experimental and theoretical physiology should not be seen as separate exercises. Rather, they should be integrated to permit an iterative process between modelling and experimentation.

Myoendothelial communication in the renal vasculature and the impact of drugs used clinically to treat hypertension

The renal vasculature has many peculiarities including highly irregular branching. Renal blood flow must sustain adequate perfusion and maintain a high glomerular filtration. Renal autoregulation helps control renal blood flow. The local autoregulatory mechanism, tubuloglomerular feedback, elicits a vasoconstriction that can be found not only in neighboring nephrons but over large areas of the kidney indicating that the renal vasculature supports strong conduction of vascular responses. The basis for conduction is intercellular communication through gap junctions. The endothelium is strongly coupled and serves as a vascular conduction highway leading the signal to the vascular smooth muscle cells through myoendothelial coupling. Extensive intercellular coupling is also found in renin secreting cells where gap junctions seem to tie the cells together to improve control of renin secretion. Lack of coupling leads to dysregulation of renin secretion and hypertension. However, the activity of the renin-angiotensin system also controls gap junction expression in the kidney. Treatment reducing angiotensin II activity, as used in hypertension treatment, can affect expression of renal and vascular gap junction.