Microcomputed tomography of kidneys following chronic bile duct ligation

Pathophysiology of Hepatorenal Syndrome

Hepatorenal syndrome type 1 (HRS-1) is a unique form of acute kidney injury that affects individuals with decompensated cirrhosis with ascites. The primary mechanism leading to reduction of kidney function in HRS-1 is hemodynamic in nature. Cumulative evidence points to a cascade of events that led to a profound reduction in kidney perfusion. A state of increased intrahepatic vascular resistance characteristic of advanced cirrhosis and portal hypertension is accompanied by maladaptive peripheral arterial vasodilation and reduction in systemic vascular resistance and mean arterial pressure. As a result of a fall in effective arterial blood volume, there is a compensatory activation of the sympathetic nervous system and the renin-angiotensin system, local renal vasoconstriction, loss of renal autoregulation, decrease in renal blood flow, and ultimately a fall in glomerular filtration rate. Systemic release of nitric oxide stimulated by the fibrotic liver, bacterial translocation, and inflammation constitute key components of the pathogenesis. While angiotensin II and noradrenaline remain the critical mediators of renal arterial and arteriolar vasoconstriction, other novel molecules have been recently implicated. Although the above-described mechanistic pathway remains the backbone of the pathogenesis of HRS-1, other noxious elements may be present in advanced cirrhosis and likely contribute to the renal impairment. Direct liver-kidney crosstalk via the hepatorenal sympathetic reflex can further reduce renal blood flow independently of the systemic derangements. Tense ascites may lead to intraabdominal hypertension and abdominal compartment syndrome. Cardio-hemodynamic processes have also been increasingly recognized. Porto-pulmonary hypertension, cirrhotic cardiomyopathy, and abdominal compartment syndrome may lead to renal congestion and complicate the course of HRS-1. In addition, a degree of ischemic or toxic (cholemic) tubular injury may overlap with the underlying circulatory dysfunction and further exacerbate the course of acute kidney injury. Improving our understanding of the pathogenesis of HRS-1 may lead to improvements in therapeutic options for this seriously ill population.

Pathophysiology of Hepatorenal Syndrome - Acute Kidney Injury

Hepatorenal syndrome is a complication of liver cirrhosis with ascites that results from the complex interplay of many pathogenetic mechanisms. Advanced cirrhosis is characterized by the development of hemodynamic changes of splanchnic and systemic arterial vasodilatation, with paradoxical renal vasoconstriction and renal hypoperfusion. Cirrhosis is also an inflammatory state. The inflammatory cascade is initiated by a portal hypertension-induced increased translocation of bacteria, bacterial products, and endotoxins from the gut to the splanchnic and then to the systemic circulation. The inflammation, whether sterile or related to infection, is responsible for renal microcirculatory dysfunction, microthrombi formation, renal tubular oxidative stress, and tubular damage. Of course, many of the bacterial products also have vasodilatory properties, potentially exaggerating the state of vasodilatation and worsening the hemodynamic instability in these patients. The presence of cardiac dysfunction, related to cirrhotic cardiomyopathy, with its associated systolic incompetence, can aggravate the mismatch between the circulatory capacitance and the circulation volume, worsening the extent of the effective arterial underfilling, with lower renal perfusion pressure, contributing to renal hypoperfusion and increasing the risk for development of acute kidney injury. The presence of tense ascites can exert an intra-abdominal compartmental syndrome effect on the renal circulation, causing renal congestion and hampering glomerular filtration. Other contributing factors to renal dysfunction include the tubular damaging effects of cholestasis and adrenal dysfunction. Future developments include the use of metabolomics to identify metabolic pathways that can lead to the development of renal dysfunction, with the potential of identifying biomarkers for early diagnosis of renal dysfunction and the development of treatment strategies.

Imaging the Renal Microcirculation in Cell Therapy

Renal microvascular rarefaction plays a pivotal role in progressive kidney disease. Therefore, modalities to visualize the microcirculation of the kidney will increase our understanding of disease mechanisms and consequently may provide new approaches for evaluating cell-based therapy. At the moment, however, clinical practice is lacking non-invasive, safe, and efficient imaging modalities to monitor renal microvascular changes over time in patients suffering from renal disease. To emphasize the importance, we summarize current knowledge of the renal microcirculation and discussed the involvement in progressive kidney disease. Moreover, an overview of available imaging techniques to uncover renal microvascular morphology, function, and behavior is presented with the associated benefits and limitations. Ultimately, the necessity to assess and investigate renal disease based on in vivo readouts with a resolution up to capillary level may provide a paradigm shift for diagnosis and therapy in the field of nephrology.

A Review of Ex Vivo X-ray Microfocus Computed Tomography-Based Characterization of the Cardiovascular System

Cardiovascular malformations and diseases are common but complex and often not yet fully understood. To better understand the effects of structural and microstructural changes of the heart and the vasculature on their proper functioning, a detailed characterization of the microstructure is crucial. In vivo imaging approaches are noninvasive and allow visualizing the heart and the vasculature in 3D. However, their spatial image resolution is often too limited for microstructural analyses, and hence, ex vivo imaging is preferred for this purpose. Ex vivo X-ray microfocus computed tomography (microCT) is a rapidly emerging high-resolution 3D structural imaging technique often used for the assessment of calcified tissues. Contrast-enhanced microCT (CE-CT) or phase-contrast microCT (PC-CT) improve this technique by additionally allowing the distinction of different low X-ray-absorbing soft tissues. In this review, we present the strengths of ex vivo microCT, CE-CT and PC-CT for quantitative 3D imaging of the structure and/or microstructure of the heart, the vasculature and their substructures in healthy and diseased state. We also discuss their current limitations, mainly with regard to the contrasting methods and the tissue preparation.

Simultaneous Three-Dimensional Vascular and Tubular Imaging of Whole Mouse Kidneys With X-ray μCT

Concurrent three-dimensional imaging of the renal vascular and tubular systems on the whole-kidney scale with capillary level resolution is labor-intensive and technically difficult. Approaches based on vascular corrosion casting and X-ray micro computed tomography (μCT), for example, suffer from vascular filling artifacts and necessitate imaging with an additional modality to acquire tubules. In this work, we report on a new sample preparation, image acquisition, and quantification protocol for simultaneous vascular and tubular μCT imaging of whole, uncorroded mouse kidneys. The protocol consists of vascular perfusion with the water-soluble, aldehyde-fixable, polymeric X-ray contrast agent XlinCA, followed by laboratory-source μCT imaging and structural analysis using the freely available Fiji/ImageJ software. We achieved consistent filling of the entire capillary bed and staining of the tubules in the cortex and outer medulla. After imaging at isotropic voxel sizes of 3.3 and 4.4 μm, we segmented vascular and tubular systems and quantified luminal volumes, surface areas, diffusion distances, and vessel path lengths. This protocol permits the analysis of vascular and tubular parameters with higher reliability than vascular corrosion casting, less labor than serial sectioning and leaves tissue intact for subsequent histological examination with light and electron microscopy.

Reappraising the spectrum of AKI and hepatorenal syndrome in patients with cirrhosis

The occurrence of acute kidney injury (AKI) in patients with end-stage liver disease constitutes one of the most challenging clinical scenarios in in-hospital and critical care medicine. Hepatorenal syndrome type 1 (HRS-1), which is a specific type of AKI that occurs in the context of advanced cirrhosis and portal hypertension, is associated with particularly high mortality. The pathogenesis of HRS-1 is largely viewed as a functional derangement that ultimately affects renal vasculature tone. However, new insights suggest that non-haemodynamic tubulo-toxic factors, such as endotoxins and bile acids, might mediate parenchymal renal injury in patients with cirrhosis, suggesting that concurrent mechanisms, including those traditionally associated with HRS-1 and non-traditional factors, might contribute to the development of AKI in patients with cirrhosis. Moreover, histological evidence of morphological abnormalities in the kidneys of patients with cirrhosis and renal dysfunction has prompted the functional nature of HRS-1 to be re-examined. From a clinical perspective, a diagnosis of HRS-1 guides utilization of vasoconstrictive therapy and decisions regarding renal replacement therapy. Patients with cirrhosis are at risk of AKI owing to a wide range of factors. However, the tools currently available to ascertain the diagnosis of HRS-1 and guide therapy are suboptimal. Short of liver transplantation, goal-directed haemodynamically targeted pharmacotherapy remains the cornerstone of treatment for this condition; improved understanding of the underlying pathogenic mechanisms might lead to better clinical outcomes. Here, we examine our current understanding of the pathophysiology of HRS-1 and existing challenges in its diagnosis and treatment.

Rat Mesenchymal Stromal Cell Sheets Suppress Renal Fibrosis via Microvascular Protection

Renal fibrosis is one of the largest global health care problems, and microvascular (MV) injury is important in the development of progressive fibrosis. Although conventional cell therapy suppresses kidney injury via the role of vasoprotective cytokines, the effects are limited due to low retention of administered cells. We recently described that transplantation of hepatocyte growth factor (HGF)‐transgenic mesothelial cell sheets showed a remarkable cell survival and strong therapeutic effects in a rat renal fibrosis model. Due to the translational hurdles of transgenic cells, we here applied this technique for allogeneic transplantation using rat bone marrow mesenchymal stromal cells (MSCs). MSC sheets were transplanted onto the kidney surface of a rat renal ischemia–reperfusion‐injury model and the effects were compared between those in untreated rats and those receiving intravenous (IV) administration of the cells. We found that donor‐cell survival was superior in the cell sheet group relative to the IV group, and that the cell sheets secreted HGF and vascular endothelial growth factor (VEGF) up to day 14. Transplantation of cell sheets increased the expression of activated HGF/VEGF receptors in the kidney. There was no evidence of migration of transplanted cells into the kidney parenchyma. Additionally, the cell sheets significantly suppressed renal dysfunction, MV injury, and fibrosis as compared with that observed in the untreated and IV groups. Furthermore, we demonstrated that the MSC sheet protected MV density in the whole kidney according to three‐dimensional microcomputed tomography. In conclusion, MSC sheets strongly prevented renal fibrosis via MV protection, suggesting that this strategy represents a potential novel therapy for various kidney diseases. stem cells translational medicine2019;8:1330&1341

Rat bone marrow mesenchymal stromal cell sheets were transplanted onto the kidney surface. In ischemia–reperfusion‐injury, microvascular density loss caused by endothelial injury resulted in progressive renal fibrosis. Mesenchymal stromal cell sheets remained long‐term on the kidney surface and protected the microvasculature, which resulted in suppression of progressive fibrosis. The therapeutic effects were partially explained by the role of hepatocyte growth factor/vascular endothelial growth factor secreted from mesenchymal stromal cell sheets.

Micro-CT imaging and structural analysis of glomeruli in a model of Adriamycin-induced nephropathy

Glomeruli number and size are important for determining the pathogenesis of glomerular disease, chronic kidney disease, and hypertension. Moreover, renal injury can occur in specific cortical layers and alter glomerular spatial distribution. In this study, we present a comprehensive structural analysis of glomeruli in a model of Adriamycin (doxorubicin) nephropathy. Glomeruli are imaged (micro-CT at 10 × 10 × 10 μm³) in kidney specimens from C57Bl/6 mouse cohorts: control treated with saline ( n = 9) and Adriamycin treated with 20 mg/kg Adriamycin ( n = 7). Several indices were examined, including glomerular number, glomerular volume, glomerular volume heterogeneity, and spatial density at each glomerulus and in each cortical layer (superficial, midcortical, and juxtamedullary). In the Adriamycin-treated animals, glomerular number decreased significantly in the left kidney [control: 8,298 ± 221, Adriamycin: 6,781 ± 630 (mean ± SE)] and right kidney (control: 7,317 ± 367, Adriamycin: 5,522 ± 508), and glomerular volume heterogeneity increased significantly in the left kidney (control: 0.642 ± 0.015, Adriamycin: 0.786 ± 0.018) and right kidney (control: 0.739 ± 0.016, Adriamycin: 0.937 ± 0.023). Glomerular spatial density was not affected. Glomerular volume heterogeneity increased significantly in the superficial and midcortical layers of the Adriamycin cohort. Adriamycin did not affect glomerular volume or density metrics in the juxtamedullary region, suggesting a compensatory mechanism of juxtamedullary glomeruli to injury in the outer cortical layers. Left/right asymmetry was observed in kidney size and various glomeruli metrics. The methods presented here can be used to evaluate renal disease models with subtle changes in glomerular endowment locally or across the entire kidney, and they provide an imaging tool to investigate diverse interventions and therapeutic drugs.

Review article: the role of the microcirculation in liver cirrhosis

BACKGROUND

Intrahepatic microvascular derangements and microcirculatory dysfunction are key in the development of liver cirrhosis and its associated complications. While much has been documented relating to cirrhosis and the dysfunction of the microcirculation in the liver parenchyma, far less is known about the state of the extrahepatic microcirculation and the role this may have in the pathogenesis of multiple organ failure in end stage liver cirrhosis.

AIM

To provide an update on the role of the microcirculation in the pathophysiology of cirrhosis and its associated complications and briefly discuss some of the imaging techniques which may be used to directly investigate the microcirculation.

METHODS

A Medline literature search was conducted using the following search terms: 'cirrhosis', 'microcirculation', 'circulation', 'systemic', 'inflammation', 'peripheral', 'hepatorenal' and 'hepatopulmonary'.

RESULTS

Significant heterogeneous microvascular alterations exist in patients with cirrhosis. Data suggest that the systemic inflammation, associated with advanced cirrhosis, induces microcirculatory dysregulation and contributes to haemodynamic derangement. The resultant vasoconstriction and hypoperfusion in the systemic extrahepatic microvasculature, is likely to be instrumental in the pathophysiology of organ failure in decompensated cirrhosis, however the mechanistic action of vasoactive agents used to correct the circulatory disturbance of advanced cirrhosis is poorly understood.

CONCLUSIONS

Further research into the role of the microcirculation in patients with liver cirrhosis, will improve physicians understanding of the pathophysiology of cirrhosis, and may provide a platform for real time evaluation of an individual's microcirculatory response to vasoactive mediators, thus guiding their therapy.

Quantitative Micro-Computed Tomography Imaging of Vascular Dysfunction in Progressive Kidney Diseases

Progressive kidney diseases and renal fibrosis are associated with endothelial injury and capillary rarefaction. However, our understanding of these processes has been hampered by the lack of tools enabling the quantitative and noninvasive monitoring of vessel functionality. Here, we used micro-computed tomography (µCT) for anatomical and functional imaging of vascular alterations in three murine models with distinct mechanisms of progressive kidney injury: ischemia-reperfusion (I/R, days 1-56), unilateral ureteral obstruction (UUO, days 1-10), and Alport mice (6-8 weeks old). Contrast-enhanced in vivo µCT enabled robust, noninvasive, and longitudinal monitoring of vessel functionality and revealed a progressive decline of the renal relative blood volume in all models. This reduction ranged from -20% in early disease stages to -61% in late disease stages and preceded fibrosis. Upon Microfil perfusion, high-resolution ex vivo µCT allowed quantitative analyses of three-dimensional vascular networks in all three models. These analyses revealed significant and previously unrecognized alterations of preglomerular arteries: a reduction in vessel diameter, a prominent reduction in vessel branching, and increased vessel tortuosity. In summary, using µCT methodology, we revealed insights into macro-to-microvascular alterations in progressive renal disease and provide a platform that may serve as the basis to evaluate vascular therapeutics in renal disease.

Mesenchymal Stromal Cells Improve Renovascular Function in Polycystic Kidney Disease

Polycystic kidney disease (PKD) is a common cause of end-stage renal failure, for which there is no accepted treatment. Progenitor and stem cells have been shown to restore renal function in a model of renovascular disease, a disease that shares many features with PKD. The objective of this study was to examine the potential of adult stem cells to restore renal structure and function in PKD. Bone marrow-derived mesenchymal stromal cells (MSCs, 2.5 × 10(5)) were intrarenally infused in 6-week-old PCK rats. At 10 weeks of age, PCK rats had an increase in systolic blood pressure (SBP) versus controls (126.22 ± 2.74 vs. 116.45 ± 3.53 mmHg, p < 0.05) and decreased creatinine clearance (3.76 ± 0.31 vs. 6.10 ± 0.48 µl/min/g, p < 0.01), which were improved in PKD animals that received MSCs (SBP: 114.67 ± 1.34 mmHg, and creatinine clearance: 4.82 ± 0.24 µl/min/g, p = 0.001 and p = 0.003 vs. PKD, respectively). MSCs preserved vascular density and glomeruli diameter, measured using microcomputed tomography. PCK animals had increased urine osmolality (843.9 ± 54.95 vs. 605.6 ± 45.34 mOsm, p < 0.01 vs. control), which was improved after MSC infusion and not different from control (723.75 ± 56.6 mOsm, p = 0.13 vs. control). Furthermore, MSCs reduced fibrosis and preserved the expression of proangiogenic molecules, while cyst size and number were unaltered by MSCs. Delivery of exogenous MSCs improved vascular density and renal function in PCK animals, and the benefit was observed up to 4 weeks after a single infusion. Cell-based therapy constitutes a novel approach in PKD.

Quantification and characterization of radiation-induced changes to mandibular vascularity using micro-computed tomography

OBJECTIVE

Perhaps the most vexing and exigent problem confronting head and neck cancer reconstruction is overcoming the impediments of collateral damage imposed by radiation therapy (XRT) on normal surrounding tissue. Radiation therapy is detrimental to bone and soft tissue repair resulting in an unacceptably high incidence of devastating wound healing complications as well as the associated morbidity of late pathologic fractures, reduced bone healing, and osteoradionecrosis. The consequences of XRT on bone vasculature, long known to be affected by radiation, have been poorly understood. The purpose of this study was to analyze the degree by which irradiation degrades existing bone vascularity using a powerful micro-computed tomography technique to attain highly precise quantitative metrics of the vascular tree.

METHODS

Fourteen 400-g male Sprague-Dawley rats underwent 35 Gy of fractionated XRT at 7 Gy/d. The animals were euthanized after 28 days, and the left ventricle was fixed and injected with Microfil (MV-122; Flow Tech, Carver, Mass) contrast. Left hemimandibles were dissected and scanned using high-resolution micro-computed tomography (18-μm voxels). The vessel number, thickness, separation, connectivity, and vessel volume fraction were analyzed for the region of interest, defined to be the volume behind the third molar spanning a total distance of 5.1 mm.

RESULTS

Stereologic analysis and subsequent analysis of variance test demonstrated a significant and quantifiable diminution in the irradiated vasculature when compared with control animals. The vessel volume fraction (0.016 vs 0.032, P ≤ 0.003) and vessel thickness (0.042 vs 0.067 mm, P ≤ 0.001) were markedly reduced. Interestingly, further analysis demonstrated no significant differences between vessel separation and vessel number.

CONCLUSIONS

The results of our study specifically quantify the corrosive affects of XRT on the vasculature of the mandible. The data from this novel technique go even further and imply retention of blood vessels but a degradation of their quality and size. Further experiments can now be directed at therapeutic interventions to reverse this process and better understand the underlying mechanism of XRT-induced bone injury.

Susceptibility tensor imaging of the kidney and its microstructural underpinnings

PURPOSE

The purpose of this study was to determine whether susceptibility tensor imaging (STI) could overcome limitations of current techniques to detect tubules throughout the kidney.

METHODS

Normal mouse kidneys (n = 4) were imaged at 9.4T using a three-dimensional gradient multi-echo sequence (55-micron isotropic resolution). Phase images from 12 orientations were obtained to compute the susceptibility tensor. Diffusion tensor imaging (DTI) with 12 encoding directions was compared with STI. Tractography was performed to visualize and track the course of tubules with DTI and STI. Confocal microscopy was used to identify which tubular segments of the nephron were detected by DTI and STI.

RESULTS

Diffusion anisotropy was limited to the inner medulla of the kidney. DTI did not find a significant number of coherent tubular tracks in the outer medulla or cortex. With STI, we found strong susceptibility anisotropy and many tracks in the inner and outer medulla and in limited areas of the cortex.

CONCLUSION

STI was able to track tubules throughout the kidney, whereas DTI was limited to the inner medulla. STI provides a novel contrast mechanism related to local tubule microstructure and may offer a powerful method to study the nephron.

A novel in vivo vascular imaging approach for hierarchical quantification of vasculature using contrast enhanced micro-computed tomography

The vasculature of body tissues is continuously subject to remodeling processes originating at the micro-vascular level. The formation of new blood vessels (angiogenesis) is essential for a number of physiological and pathophysiological processes such as tissue regeneration, tumor development and the integration of artificial tissues. There are currently no time-lapsed in vivo imaging techniques providing information on the vascular network at the capillary level in a non-destructive, three-dimensional and high-resolution fashion. This paper presents a novel imaging framework based on contrast enhanced micro-computed tomography (micro-CT) for hierarchical in vivo quantification of blood vessels in mice, ranging from largest to smallest structures. The framework combines for the first time a standard morphometric approach with densitometric analysis. Validation tests showed that the method is precise and robust. Furthermore, the framework is sensitive in detecting different perfusion levels after the implementation of a murine ischemia-reperfusion model. Correlation with both histological data and micro-CT analysis of vascular corrosion casts confirmed accuracy of the method. The newly developed time-lapsed imaging approach shows high potential for in vivo monitoring of a number of different physiological and pathological conditions in angiogenesis and vascular development.

Polycystic kidneys have decreased vascular density: a micro-CT study

OBJECTIVE

Polycystic kidney disease (PKD) is a common cause of end-stage renal failure and many of these patients suffer vascular dysfunction and hypertension. It remains unclear whether PKD is associated with abnormal microvascular structure. Thus, this study examined the renovascular structure in PKD.

METHODS

PKD rats (PCK model) and controls were studied at 10 weeks of age, and mean arterial pressure (MAP), renal blood flow, and creatinine clearance were measured. Microvascular architecture and cyst number and volume were assessed using micro-computed tomography, and angiogenic pathways evaluated.

RESULTS

Compared with controls, PKD animals had an increase in MAP (126.4 ± 4.0 vs. 126.2 ± 2.7 mmHg) and decreased clearance of creatinine (0.39 ± 0.09 vs. 0.30 ± 0.05 mL/min), associated with a decrease in microvascular density, both in the cortex (256 ± 22 vs. 136 ± 20 vessels per cm2) and medullar (114 ± 14 vs. 50 ± 9 vessels/cm2) and an increase in the average diameter of glomeruli (104.14 ± 2.94 vs. 125.76 ± 9.06 mm). PKD animals had increased fibrosis (2.2 ± 0.2 fold vs. control) and a decrease in the cortical expression in hypoxia inducible factor 1-α and vascular endothelial growth factor.

CONCLUSIONS

PKD animals have impaired renal vascular architecture, which can have significant functional consequences. The PKD microvasculature could represent a therapeutic target to decrease the impact of this disease.

Whole animal imaging

Translational research plays a vital role in understanding the underlying pathophysiology of human diseases, and hence development of new diagnostic and therapeutic options for their management. After creating an animal disease model, pathophysiologic changes and effects of a therapeutic intervention on them are often evaluated on the animals using immunohistologic or imaging techniques. In contrast to the immunohistologic techniques, the imaging techniques are noninvasive and hence can be used to investigate the whole animal, oftentimes in a single exam which provides opportunities to perform longitudinal studies and dynamic imaging of the same subject, and hence minimizes the experimental variability, requirement for the number of animals, and the time to perform a given experiment. Whole animal imaging can be performed by a number of techniques including x-ray computed tomography, magnetic resonance imaging, ultrasound imaging, positron emission tomography, single photon emission computed tomography, fluorescence imaging, and bioluminescence imaging, among others. Individual imaging techniques provide different kinds of information regarding the structure, metabolism, and physiology of the animal. Each technique has its own strengths and weaknesses, and none serves every purpose of image acquisition from all regions of an animal. In this review, a broad overview of basic principles, available contrast mechanisms, applications, challenges, and future prospects of many imaging techniques employed for whole animal imaging is provided. Our main goal is to briefly describe the current state of art to researchers and advanced students with a strong background in the field of animal research.

Novel use of X-ray micro computed tomography to image rat sciatic nerve and integration into scaffold

This paper describes how specimens of nervous tissue can be prepared for successful imaging in X-ray Micro Computed Tomography (microCT), and how this method can be used to study the integration of nervous tissue into a polymeric scaffold. The sample preparation involves staining the biological tissue with osmium tetroxide to increase its X-ray attenuation, and a technique for maintaining the specimen in a moist environment during the experiment to prevent drying and shrinkage. Using this method it was possible to observe individual nerve fascicles and their relationship to the 3-D tissue structure. A scaffold supporting a regenerated sciatic nerve was similarly stained to distinguish the nervous tissue from the scaffold, and to observe how the nerve grew through a 2.5 mm long, 100 microm x 100 microm cross-section channel polyimide array. Furthermore, blood vessels could be identified in these images, and it was possible to monitor how a large proximal blood vessel split through the channel scaffold and proceeded down individual channels. This paper explains how microCT is a useful tool both for studying the location and extent of growth into a polymeric scaffold, and for determining whether the regenerated tissue has blood supply.

Microcomputed tomography characterization of neovascularization in bone tissue engineering applications

Vasculogenesis and angiogenesis have been studied for decades using numerous in vitro and in vivo systems, fulfilling the need to elucidate the mechanisms involved in these processes and to test potential therapeutic agents that inhibit or promote neovascularization. Bone tissue engineering in particular has benefited from the application of proangiogenic strategies, considering the need for an adequate vascular supply during healing and the challenges associated with the vascularization of scaffolds implanted in vivo. Conventional methods of assessing the in vivo angiogenic response to tissue-engineered constructs tend to rely on a two-dimensional assessment of microvessel density within representative histological sections without elaboration of the true vascular tree. The introduction of microcomputed tomography (micro-CT) has recently allowed investigators to obtain a diverse range of high-resolution, three-dimensional characterization of structures, including renal, coronary, and hepatic vascular networks, as well as bone formation within healing defects. To date, few studies have utilized micro-CT to study the vascular response to an implanted tissue engineering scaffold. In this paper, conventional in vitro and in vivo models for studying angiogenesis will be discussed, followed by recent developments in the use of micro-CT for vessel imaging in bone tissue engineering research. A new study demonstrating the potential of contrast-enhanced micro-CT for the evaluation of in vivo neovascularization in bony defects is described, which offers significant potential in the evaluation of bone tissue engineering constructs.

Evaluation of angiogenesis using micro-computed tomography in a xenograft mouse model of lung cancer

Quantitative evaluation of lung tumor angiogenesis using immunohistochemical techniques has been limited by difficulties in generating reproducible data. To analyze intrapulmonary tumor angiogenesis, we used high-resolution micro-computed tomography (micro-CT) of lung tumors of mice inoculated with mouse Lewis lung carcinoma (LLC1) or human adenocarcinoma (A549) cell lines. The lung vasculature was filled with the radiopaque silicone rubber, Microfil, through the jugular vein (in vivo application) or pulmonary artery (ex vivo application). In addition, human adenocarcinoma lung tumor-bearing mice treated site-specifically with humanized monoclonal antibody (bevacizumab) against vascular endothelial growth factor. Quantitative analysis of lung tumor microvessels imaged with micro-CT showed that more vessels (mainly small, <0.02 mm(2)) were filled using the in vivo (5.4%) compared with the ex vivo (2.1%) method. Furthermore, bevacizumab-treated lung tumor-bearing mice showed significantly reduced lung tumor volume and lung tumor angiogenesis compared with untreated mice as assessed by micro-CT. Interestingly, microvascularization of mainly the smaller vessels (<0.02 mm(2)) was reduced after bevacizumab treatment. This observation with micro-CT was nicely correlated with immunohistochemical measurement of microvessels. Therefore, micro-CT is a novel method for investigating lung tumor angiogenesis, and this might be considered as an additional complementary tool for precise quantification of angiogenesis.

2-Arachidonoylglycerol, an endogenous cannabinoid receptor agonist, in various rat tissues during the evolution of experimental cholestatic liver disease

BACKGROUND/AIM

Changes in tissue levels of 2-arachidonoylglycerol (2-AG), an endocannabinoid, during the evolution of bile duct ligation (BDL) may indicate that endocannabinoids have a role in the hemodynamic changes that occur in this condition.

METHODS

2-AG levels, in various organs and vascular beds of BDL rats, 2 and 4 weeks post surgery, were determined. Untouched and sham-operated (SO) rats were used as controls.

RESULTS

2-AG content of a specific organ was not a static finding and depended on the rat's age, the time from the surgical procedure and the type of procedure. The most pronounced changes were observed in BDL rats 4 weeks post surgery. In these rats, hepatic, pulmonary, cardiac and renal medullary and papillary 2-AG levels were highest observed. No changes in splenic, aortic and renal cortical 2-AG levels were observed. In addition a stepwise increase in 2-AG levels from the cortex to the papilla was detected and was followed by a decrease in creatinine clearance.

CONCLUSIONS

2-AG probably has a role in the pathophysiologic changes in the liver, heart, lung and kidney that follows BDL.

Estimation of regional myocardial mass at risk based on distal arterial lumen volume and length using 3D micro-CT images

The determination of regional myocardial mass at risk distal to a coronary occlusion provides valuable prognostic information for a patient with coronary artery disease. The coronary arterial system follows a design rule which allows for the use of arterial branch length and lumen volume to estimate regional myocardial mass at risk. Image processing techniques, such as segmentation, skeletonization and arterial network tracking, are presented for extracting anatomical details of the coronary arterial system using micro-computed tomography (micro-CT). Moreover, a method of assigning tissue voxels to their corresponding arterial branches is presented to determine the dependent myocardial region. The proposed micro-CT technique was utilized to investigate the relationship between the sum of the distal coronary arterial branch lengths and volumes to the dependent regional myocardial mass using a polymer cast of a porcine heart. The correlations of the logarithm of the total distal arterial lengths (L) to the logarithm of the regional myocardial mass (M) for the left anterior descending (LAD), left circumflex (LCX) and right coronary (RCA) arteries were log(L)=0.73log(M)+0.09 (R=0.78), log(L)=0.82log(M)+0.05 (R=0.77) and log(L)=0.85log(M)+0.05 (R=0.87), respectively. The correlation of the logarithm of the total distal arterial lumen volumes (V) to the logarithm of the regional myocardial mass for the LAD, LCX and RCA were log(V)=0.93log(M)-1.65 (R=0.81), log(V)=1.02log(M)-1.79 (R=0.78) and log(V)=1.17log(M)-2.10 (R=0.82), respectively. These morphological relations did not change appreciably for diameter truncations of 600-1400microm. The results indicate that the image processing procedures successfully extracted information from a large 3D dataset of the coronary arterial tree to provide prognostic indications in the form of arterial tree parameters and anatomical area at risk.

3D visualisation and quantification by microcomputed tomography of late gestational changes in the arterial and venous feto-placental vasculature of the mouse

This study evaluates microcomputed tomography (micro-CT) as a method to obtain quantitative three-dimensional (3D) information on the arterial and venous vasculature of the mouse placenta. Surface renderings at embryonic days (E) 13.5, 15.5, and 18.5 (full term) revealed that the arterial and venous vasculature branched within the chorionic plate whereas only the arterial vasculature deeply penetrated the placenta. Umbilical vessel diameters measured by micro-CT did not significantly differ from those measured non-invasively in vivo by ultrasound biomicroscopy. Variability in umbilical diameters, and surface area and volume measurements of arterial and venous vascular trees due to experimental error was low relative to biological variability, and significant inter-litter differences within gestational ages were detected. Furthermore, umbilical vessel diameter increased significantly and incrementally to an arterial diameter of 0.631+/-0.009 mm and a venous diameter of 0.690+/-0.018 mm at E18.5. Umbilical vein diameter was 3-9% greater than the artery, and both were significantly correlated with embryonic body weight (R> or =0.96). Surface area and volume were determined for vessels greater than the minimum resolvable diameter of 0.03 mm which therefore excluded capillaries. Arterial surface area and volume were unchanged from E13.5-15.5 but then more than doubled at E18.5 (to 170+/-13 mm(2) and 7.2+/-0.8mm(3), respectively). Venous surface areas and volumes changed similarly with development although surface areas were lower than their arterial counterparts. We conclude that micro-CT has sufficient accuracy and precision to quantify late gestational changes in the 3D structure of the arterial and venous vasculature of the mouse placenta.

Review – 3D Micro CT Imaging of Renal Micro-Structural Changes

Appropriate nephron function is dependent on the detailed spatial interrelationship of blood vessels with the tubular components. However, because of methodological limitations,the three-dimensional anatomical complexity of the renal vasculature and its geometrical relationship with specific tubular segments along the nephron has been difficult to study in a quantitative manner. Three-dimensional microcomputed tomography (3D micro CT) offers the unique opportunity to image kidney sample volumes with a high spatial resolution (of up to 5 microm cubic voxel size) without physically sectioning them, thereby allowing accurate calculations of vessel tortuosity and density, as indices of neovascularization, as well as volume and distribution of various kidney structures. In conjunction with molecular biology techniques, valuable associations between renal microstructures and activation of local molecular pathways can be drawn to elucidate mechanisms of renal disease and design therapeutic approaches. For example, recent studies in animal models of renal hypertension, hypercholesterolemia, and diabetes mellitus have shown that medical intervention to decrease oxidative stress and micro-inflammation may preserve renal microstructures as well as renal function in these chronic renal diseases. Future developments will be needed to establish the place of 3D micro CT in developing, directing, and monitoring the treatment of chronic kidney diseases.

Structural morphology of renal vasculature

An automatic segmentation technique has been developed and applied to two renal micro-computer tomography (CT) images. With the use of a 20-microm voxel resolution image, the arterial and venous trees were segmented for the rat renal vasculature, distinguishing resolving vessels down to 30 microm in radius. A higher resolution 4-microm voxel image of a renal vascular subtree, with vessel radial values down to 10 microm, was segmented. Strahler ordering was applied to each subtree using an iterative scheme developed to integrate information from the two segmented models to reconstruct the complete topology of the entire vascular tree. An error analysis of the assigned orders quantified the robustness of the ordering process for the full model. Radial, length, and connectivity data of the complete arterial and venous trees are reported by order. Substantial parallelism is observed between individual arteries and veins, and the ratio of parallel vessel radii is quantified via a power law. A strong correlation with Murray's Law was established, providing convincing evidence of the "minimum work" hypothesis. Results were compared with theoretical branch angle formulations, based on the principles of "minimum shear force," were inconclusive. Three-dimensional reconstructions of renal vascular trees collected are made freely available for further investigation into renal physiology and modeling studies.

Micro-CT of the Human Lung: Imaging of Alveoli and Virtual Endoscopy of an Alveolar Duct in a Normal Lung and in a Lung with Centrilobular Emphysema—Initial Observations

The appearance of human lung parenchyma at the structural level of alveoli was investigated by the use of micro-computed tomography (CT). Approval for use of autopsy lungs was given by the head of the pathology institute of the university, in accordance with the requirements of the State Ministry of Science and Arts and without the need for institutional review board approval. Two human lungs (one normal lung and one lung with centrilobular emphysema of a mild to moderate degree) were inflated and fixed with hot formalin vapor. Lung specimens excised from the superior segment of the left lower lobe (B6) were stained with silver nitrate in a vacuum and investigated at a volume of interest of 4 mm for each side with a voxel size of 14 mum. Normal-size and enlarged alveoli became visible. A three-dimensional reconstruction of the terminal airspaces made virtual endoscopy of the alveolar ducts possible.

Acute Rat Lung Injury: Feasibility of Assessment with Micro-CT

PURPOSE

To evaluate the feasibility of micro-computed tomography (CT) for analysis of the lung fine structure and its alterations during endotoxin-induced lung injury.

MATERIALS AND METHODS

Intravital perfusion-fixed rat lungs with (n = 5) and without (n = 5) endotoxin perfusion were scanned with micro-CT. Three imaging modalities (conventional histology, intravital microscopy, and electron microscopy) were used to document the effect of endotoxin and the in vivo application of contrast agent (a mixture of barium sulfate, gelatin, and thymol). The effect of endotoxin on structural changes of the lung was evaluated with analysis of variance.

RESULTS

Intravital microscopy, conventional histology, and electron microscopy demonstrated capillary perfusion of contrast agent, inflated alveoli, and no extravasation of barium sulfate in the extravascular space. Systemic application of endotoxin led to a significant increase in the soft-tissue volume of the lungs (ie, tissue edema) (58.09 microm(3)+/- 4.6 [standard error of the mean] vs 8.31 microm(3)+/- 1.63, P <.001) and significant thickening of the alveolar walls (34.01 microm +/- 4.5 vs 14.83 microm +/- 2.5, P <.001) at micro-CT. Simultaneously, endotoxin-treated rat lungs showed a significant reduction in total air space (49.74 microm(3)+/- 1.72 vs 100.99 microm(3)+/- 1.16, P <.001).

CONCLUSION

These findings indicate that micro-CT is feasible for structural evaluation of the lung fine structure and its alterations during endotoxin-induced lung injury.

Analysis of 3D bone ingrowth into polymer scaffolds via micro-computed tomography imaging

This paper illustrates the utility of micro-computed tomography (micro-CT) to study the process of tissue engineered bone growth. A micro-CT facility for imaging and visualising biomaterials in three dimensions (3D) is described. The facility is capable of acquiring 3D images made up of 2000(3) voxels on specimens up to 60mm in extent with resolutions down to 2 microm. This allows the 3D structure of tissue engineered materials to be imaged across three orders of magnitude of detail. The capabilities of micro-CT are demonstrated by imaging the Haversian network within human femoral cortical bone (distal diaphysis) and bone ingrowth into a porous scaffold at varying resolutions. Phase identification combined with 3D visualisation enables one to observe the complex topology of the canalicular system of the cortical bone. Imaging of the tissue engineered bone at a scale of 1cm and resolutions of 10 microm allows visualisation of the complex ingrowth of bone into the polymer scaffold. Further imaging at 2 microm resolution allows observation of bone ultra-structure. These observations illustrate the benefits of tomography over traditional techniques for the characterisation of bone morphology and interconnectivity and performs a complimentary role to current histomorphometric techniques.

Quantitative microcomputed tomography analysis of collateral vessel development after ischemic injury

Transgenic mouse models are increasingly being used to investigate the functions of specific growth factors or matrix proteins to design therapeutic strategies for controlling blood vessel growth. However, the available methodologies for evaluating angiogenesis and arteriogenesis in these models are limited by animal size, user subjectivity, the power to visualize the three-dimensional vessel networks, or the capability to employ a vigorous quantitative analysis. In this study, we employed contrast-enhanced microcomputed tomography imaging to assess collateral development after induction of hindlimb ischemia in the mouse. The morphological parameters vessel volume, connectivity, number, thickness, thickness distribution, separation, and degree of anisotropy were evaluated in control and surgery limbs 0, 3, and 14 days postsurgery. Results indicate that the vascular volume of the surgically manipulated limb was reconstituted as early as 3 days after femoral artery excision through development of a series of highly connected, small caliber, closely spaced, and isotropically oriented collateral vessels. Parametric analyses were completed to assess the sensitivity of the calculated morphological parameters to variations in image binarization threshold and voxel size. Images taken at the 36-μm voxel size were found to be optimal for evaluating collateral vessel formation, whereas 8- to 16-μm voxel sizes were needed to resolve smaller vascular structures. This study demonstrates the utility of microcomputed tomography as a robust method for quantitative, three-dimensional analysis of blood vessel networks. Whereas these initial efforts focused on the mouse hindlimb ischemia model, the developed techniques may be applied to a variety of model systems to investigate mechanisms of angiogenesis and arteriogenesis.

Atherosclerotic lesions at micro CT: feasibility for analysis of coronary artery wall in autopsy specimens

PURPOSE

To evaluate the feasibility of micro computed tomography (CT) for analysis of the coronary artery wall.

MATERIALS AND METHODS

With micro CT, two-dimensional transverse images were generated from 10 human autopsy specimens of coronary arteries (2.5-3.5 cm long), with section thickness of 6 microm. Vessel wall perimeter, plaque area, calcified lesion area, media area, and lumen area were determined by three experienced radiologists. Results were compared with those obtained from a detailed conventional histomorphometric analysis of corresponding cross sections. Hotelling T(2) test (a multivariate generalization of the univariate Student t test) and Pearson correlation coefficient were used to assess the correlation between micro CT findings and conventional histologic measurements. The significance of differences in gray-scale measurements was tested with analysis of variance.

RESULTS

Micro CT provided quantitative information about plaque morphology equivalent to that provided with histomorphometric analysis. Hotelling T(2) test revealed significantly smaller values for vessel wall perimeter and lumen area with histologic sections (P <.001). Gray-scale measurements were established with which lesions could be categorized after histologic classification.

CONCLUSION

Micro CT is feasible for analysis of the coronary artery wall.

Analysis of the Fetal Placental Vascular Tree by X-ray Micro-computed Tomography

The current understanding of the placental vascular tree largely derives from time-consuming morphometric analyses performed by conventional histology, electron microscopy of corrosion casts and three-dimensional reconstructions based on physical tissue sections. In the present study, we demonstrate for the first time that micro-computed tomography (micro-CT) emerges as a new, non-destructive and fast tool for imaging and quantifying fetoplacental vasculature. Term placentae (n=5) were perfused with contrast agent consisting of barium-sulfate, gelatine and thymol shortly after Caesarean-section-delivery. Samples (1 cm(3)) from eight different regions of the placenta were subsequently scanned in a micro-CT. Using tomographic reconstruction algorithms, three dimensional images were obtained by micro-CT allowing total stereoscopic visualization and continuous quantitative analysis of the vascular structure of the investigated samples. These samples were compared regarding vascular surface (VS) and vascular density (vascular volume fraction, TCVF). Quantitative assessment showed an average vascular density of 16 per cent (SD+/-0.4) and a vascular surface of 475 mm(2)(SD+/-8) per total tissue volume (including intervillous space) of 125 mm(3). Micro-CT image-analysis showed no significant differences in the fetal vascularization among term placentae. Micro-CT imaging is feasible for imaging and analysis of the villous vascular tree, allows further morphologic studies and immunohistochemistry of the placental specimens and may emerge as an additional tool in the investigation of the physiology and pathophysiology of the placental vasculature.

Effect of losartan on renal microvasculature during chronic inhibition of nitric oxide visualized by micro-CT

Chronic inhibition of nitric oxide (NO) synthase with the competitive l-arginine analog NG-nitro-l-arginine methyl ester (l-NAME) leads to an elevated systemic blood pressure and reduction in renal blood flow without significant changes in urinary sodium and water excretion. Simultaneous administration of ANG II AT1 receptor antagonist losartan and l-NAME prevents the alterations in blood pressure and renal hemodynamics. Microcomputed tomography (micro-CT) was used to investigate the role of ANG II in the changes of renal microvasculature during chronic NO inhibition. Sprague-Dawley rats were given l-NAME with or without AT1 receptor antagonist losartan (40 mg. kg-1. day-1 each) in their drinking water for 19 days. Kidneys from each group (control, l-NAME-, and l-NAME + losartan-treated rats) were perfusion-fixed in situ, infused with a silicon-based polymer containing lead chromate, and scanned by micro-CT. The microvasculature in the reconstructed three-dimensional renal images was studied using computerized analytic techniques. Kidneys of l-NAME-treated rats had significantly fewer normal glomeruli (28,824 +/- 838) than those of control rats (36,266 +/- 3,572). Losartan normalized the number to control values (34,094 +/- 1,536). The amount of vasculature in the cortex, outer medulla, and inner medulla of l-NAME-treated rats was about two-thirds that of control rats; losartan normalized the values to control levels. These data indicate that chronic treatment with the NO synthase inhibitor l-NAME produces a generalized rarefaction of renal capillaries. Because simultaneous AT1 receptor blockade abolished those changes, the data suggest that the reduction in vasculature is mediated by ANG II through AT1 receptors.

Vitamin E prevents renal dysfunction induced by experimental chronic bile duct ligation

BACKGROUND

The mechanisms by which prolonged cholestasis alters renal hemodynamics and excretory function are unknown but may be related to increased oxidative stress, with subsequent formation of lipid peroxidation-derived products (e.g., F2-isoprostanes) and endothelin (ET). We investigated whether antioxidant therapy prevents chronic bile duct ligation (CBDL)-induced alterations in systemic and renal hemodynamics, and reduces F2-isoprostane and ET levels.

METHODS

Sprague-Dawley rats were placed on either a normal or a high vitamin E diet for 7 days and then underwent either CBDL or sham surgery. They were then maintained on their respective diets for 21 more days, at which time the physiologic studies were performed.

RESULTS

Thirty-three percent of the CBDL rats died by day 21. The remaining rats had a lower mean arterial pressure (MAP), renal blood flow (RBF), glomerular filtration rate (GFR), and sodium and water excretion than control rats. CBDL rats had higher portal pressure, renal venous pressure, and renal vascular resistance (RVR). These changes were associated with increased levels of systemic and renal venous F2-isoprostanes and ET. Vitamin E normalized MAP, RBF, GFR, RVR, and sodium and water excretion, and improved the 21-day survival without altering portal or renal venous pressures. Surprisingly, vitamin E did not alter the systemic levels of F2-isoprostanes but markedly reduced their levels in the renal venous circulation.

CONCLUSION

Vitamin E improves MAP and renal function in CBDL rats, and selectively decreases renal levels of oxidative stress and ET, suggesting that local redox balance is implicated in CBDL-induced renal dysfunction.

Decreased renal heme oxygenase-1 expression contributes to decreased renal function during cirrhosis

Heme oxygenase (HO) is the rate-limiting enzyme in the degradation of heme, catalyzing the oxidative cleavage of heme molecules to biliverdin, carbon monoxide, and iron. The present study was designed to investigate the role of HO-1 in the pathogenesis of renal dysfunction during cirrhosis. Biliary cirrhosis was induced in rats by common bile duct ligation (CBDL). Animals were studied 2 and 5 wk after surgery. In kidney from CBDL rats, HO-1 protein expression increased slightly at 2 wk but was abolished at 5 wk. In addition, we confirmed histologically that HO-1 expression was suppressed in renal tubules and interlobular arterioles in 5-wk-old CBDL rats. Conversely, HO-1 expression in liver was strongly increased. Consistent with the development of cirrhosis and renal dysfunction mean arterial pressure (MAP), glomerular filtration rate (GFR), and renal blood flow (RBF) were decreased in CBDL rats compared with sham-operated controls. In sham rats, treatment with the selective HO inhibitor zinc protoporphyrin markedly decreased GFR and RBF to values similar to those measured in CBDL rats without decreasing MAP. In conclusion, decreased renal HO-1 expression contributes to deteriorated renal function and hemodynamics during cirrhosis. This finding provides a novel mechanism for the pathophysiology of renal dysfunction during cirrhosis.

The use of microcomputed tomography to study microvasculature in small rodents

Appropriate nephron function is dependent on the intrarenal arrangement of blood vessels. The preferred and primary means to study the architecture of intrarenal circulation has been by filling it with opaque substances such as india ink, radio-opaque contrast material, or various polymers for study by light or scanning electron microscopy. With such methodologies, superficial vessels may obscure deep vessels and little quantitative information may be obtained. Serial-section microtomy has not been practical because of problems relating to alignment and registration of adjacent sections, lost sections, and preparation time and effort. Microcomputed tomography (micro-CT) overcomes such limitations and provides a means to study the three-dimensional architecture of filled vessels within an intact rodent kidney and to obtain more quantitative information. As an example of micro-CT's capabilities, we review the use of micro-CT to study the alterations in renal microvasculature caused by the development of liver cirrhosis after chronic bile duct ligation. In this example, micro-CT evidence shows a selective decrease in cortical vascular filling in the kidney, with a maintenance of medullary vascular filling. These changes may contribute to the salt and water retention that accompanies cirrhosis. These results indicate that micro-CT is a promising method to evaluate renal vascular architecture in the intact rodent kidney relative to physiological and pathological function.