Microfocal X-ray CT imaging and pulmonary arterial distensibility in excised rat lungs

MicroCT analysis of vascular morphometry: a comparison of right lung lobes in the SUGEN/hypoxic rat model of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a rare disease characterized by significant vascular remodeling within the lung. Clinical computed tomography (CT) scans are routinely used to aid in PAH diagnosis. Animal models, including the Sugen-hypoxic rat model (SU/hyp), of PAH closely mimic human PAH development. We have previously used micro-computed tomography (microCT) to find extensive right lung vascular remodeling in the SU/hyp. We hypothesized that the individual right lung lobes may not contribute equally to overall lung vascular remodeling. Sprague-Dawley rats were subjected to a subcutaneous injection of vascular endothelial growth factor receptor blocker (Sugen 5416) and subsequently exposed to chronic hypoxic conditions (10% O₂) for three weeks. Following perfusion of the lung vasculature with an opaque resin (Microfil), the right lung lobes were microCT-imaged with a 10-µm voxel resolution and 3D morphometry analysis was performed separately on each lobe. As expected, we found a significantly lower ratio of vascular volume to total lobe volume in the SU/hyp compared with the control, but only in the distal lobes (inferior: 0.23 [0.21–0.30] versus 0.35 [0.27–0.43], P = 0.02; accessory: 0.27 [0.25–0.33] versus 0.37 [0.29–0.43], P = 0.06). Overall, we observed significantly fewer continuous blood vessels and reduced vascular density while having greater vascular lumen diameters in the distal lobes of both groups (P < 0.05). In addition, the vascular separation within the SU/hyp lobes and the vascular surface area to volume ratio were significantly greater in the SU/hyp lobes compared with controls (P < 0.03). Results for the examined parameters support the overall extensive vascular remodeling in the SU/hyp model and suggest this may be lobe-dependent.

Physiological and pathological angiogenesis in the adult pulmonary circulation

Angiogenesis occurs during growth and physiological adaptation in many systemic organs, for example, exercise-induced skeletal and cardiac muscle hypertrophy, ovulation, and tissue repair. Disordered angiogenesis contributes to chronic inflammatory disease processes and to tumor growth and metastasis. Although it was previously thought that the adult pulmonary circulation was incapable of supporting new vessel growth, over that past 10 years new data have shown that angiogenesis within this circulation occurs both during physiological adaptive processes and as part of the pathogenic mechanisms of lung diseases. Here we review the expression of vascular growth factors in the adult lung, their essential role in pulmonary vascular homeostasis and the changes in their expression that occur in response to physiological challenges and in disease. We consider the evidence for adaptive neovascularization in the pulmonary circulation in response to alveolar hypoxia and during lung growth following pneumonectomy in the adult lung. In addition, we review the role of disordered angiogenesis in specific lung diseases including idiopathic pulmonary fibrosis, acute adult distress syndrome and both primary and metastatic tumors of the lung. Finally, we examine recent experimental data showing that therapeutic enhancement of pulmonary angiogenesis has the potential to treat lung diseases characterized by vessel loss.

Structure and composition of pulmonary arteries, capillaries, and veins

The pulmonary vasculature comprises three anatomic compartments connected in series: the arterial tree, an extensive capillary bed, and the venular tree. Although, in general, this vasculature is thin-walled, structure is nonetheless complex. Contributions to structure (and thus potentially to function) from cells other than endothelial and smooth muscle cells as well as those from the extracellular matrix should be considered. This review is multifaceted, bringing together information regarding (i) classification of pulmonary vessels, (ii) branching geometry in the pulmonary vascular tree, (iii) a quantitative view of structure based on morphometry of the vascular wall, (iv) the relationship of nerves, a variety of interstitial cells, matrix proteins, and striated myocytes to smooth muscle and endothelium in the vascular wall, (v) heterogeneity within cell populations and between vascular compartments, (vi) homo- and heterotypic cell-cell junctional complexes, and (vii) the relation of the pulmonary vasculature to that of airways. These issues for pulmonary vascular structure are compared, when data is available, across species from human to mouse and shrew. Data from studies utilizing vascular casting, light and electron microscopy, as well as models developed from those data, are discussed. Finally, the need for rigorous quantitative approaches to study of vascular structure in lung is highlighted.

MDCT-based quantification of porcine pulmonary arterial morphometry and self-similarity of arterial branching geometry

The pig is frequently used as an experimental model for studies of the pulmonary circulation, yet the branching and dimensional geometry of the porcine pulmonary vasculature remains poorly defined. The purposes of this study are to improve the geometric definition of the porcine pulmonary arteries and to determine whether the arterial tree exhibits self-similarity in its branching geometry. Five animals were imaged using thin slice spiral computed tomography in the prone posture during airway inflation pressure at 25 cmH2O. The luminal diameter and distance from the inlet of the left and right pulmonary arteries were measured along the left and right main arterial pathway in each lung of each animal. A further six minor pathways were measured in a single animal. The similarity in the rate of reduction of diameter with distance of all minor pathways and the two main pathways, along with similarity in the number of branches arising along the pathways, supports self-similarity in the arterial tree. The rate of reduction in diameter with distance from the inlet was not significantly different among the five animals (P > 0.48) when normalized for main pulmonary artery diameter and total main artery pathlength, which supports intersubject similarity. Other metrics to quantify the tree geometry are strikingly similar to those from airways of other quadrupeds, with the exception of a significantly larger length to diameter ratio, which is more appropriate for the vascular tree. A simplifying self-similar model for the porcine pulmonary arteries is proposed to capture the important geometric features of the arterial tree.

An in-vivo computed tomography approach for quantifying porcine pulmonary arterial morphometry

The objective of this study was to develop an in vivo CT imaging-based approach for pulmonary arterial morphometry measurement, and to improve the geometrical basis for studies of the porcine vasculature. The luminal diameter and distance from the inlet of left and right pulmonary arteries, and pulmonary arteries within the lungs of two porcine subjects were measured at inflation pressure of 25 cmH(2)O. The results suggest that the porcine pulmonary arteries have geometric self-similarity, and that this approach will have utility for systematically quantifying pulmonary arterial vessel dimensions in vivo in a larger group of animals.

Effects of acute Rho kinase inhibition on chronic hypoxia-induced changes in proximal and distal pulmonary arterial structure and function

Hypoxic pulmonary hypertension (HPH) is initially a disease of the small pulmonary arteries. Its severity is usually quantified by pulmonary vascular resistance (PVR). Acute Rho kinase inhibition has been found to reduce PVR toward control values in animal models, suggesting that persistent pulmonary vasoconstriction is the dominant mechanism for increased PVR. However, HPH may also cause proximal arterial changes, which are relevant to right ventricular (RV) afterload. RV afterload can be quantified by pulmonary vascular impedance, which is obtained via spectral analysis of pulsatile pressure-flow relationships. To determine the effects of HPH independent of persistent pulmonary vasoconstriction in proximal and distal arteries, we quantified pulsatile pressure-flow relationships before and after acute Rho kinase inhibition and measured pulmonary arterial structure with microcomputed tomography. In control lungs, Rho kinase inhibition decreased 0 Hz impedance (Z₀), which is equivalent to PVR, from 2.1 ± 0.4 to 1.5 ± 0.2 mmHg·min·ml⁻¹ (P < 0.05) and tended to increase characteristic impedance (Z(C)) from 0.21 ± 0.01 to 0.22 ± 0.01 mmHg·min·ml⁻¹. In HPH lungs, Rho kinase inhibition decreased Z₀ (P < 0.05) without affecting Z(C). Microcomputed tomography measurements performed on lungs after acute Rho kinase inhibition demonstrated that HPH significantly decreased the unstressed diameter of the main pulmonary artery (760 ± 60 vs. 650 ± 80 μm; P < 0.05), decreased right pulmonary artery compliance, and reduced the frequency of arteries of diameter 50-100 μm (both P < 0.05). These results demonstrate that acute Rho kinase inhibition reverses many but not all HPH-induced changes in distal pulmonary arteries but does not affect HPH-induced changes in the conduit arteries that impact RV afterload.

Automation process for morphometric analysis of volumetric CT data from pulmonary vasculature in rats

With advances in medical imaging scanners, it has become commonplace to generate large multidimensional datasets. These datasets require tools for a rapid, thorough analysis. To address this need, we have developed an automated algorithm for morphometric analysis incorporating A Visualization Workshop computational and image processing libraries for three-dimensional segmentation, vascular tree generation and structural hierarchical ordering with a two-stage numeric optimization procedure for estimating vessel diameters. We combine this new technique with our mathematical models of pulmonary vascular morphology to quantify structural and functional attributes of lung arterial trees. Our physiological studies require repeated measurements of vascular structure to determine differences in vessel biomechanical properties between animal models of pulmonary disease. Automation provides many advantages including significantly improved speed and minimized operator interaction and biasing. The results are validated by comparison with previously published rat pulmonary arterial micro-CT data analysis techniques, in which vessels were manually mapped and measured using intense operator intervention.

How to measure peripheral pulmonary vascular mechanics

Pulmonary hypertension (PH) is initially a disease of the small, peripheral resistance arteries. Changes in these vessels are best assessed by measurement of pulmonary artery pressure at several levels of flow to generate multi-point pressure-flow curves. This approach is superior to the traditional single-point measurement of pulmonary vascular resistance (PVR) because it allows a flow-independent definition of the resistive properties of that portion of the pulmonary vascular bed and also provides information on its distensibility. In animal models, multi-point pressure-flow curves can be obtained using an isolated, ventilated, perfused lung system. Clinically, cardiopulmonary exercise testing (CPET) with non-invasive echocardiography is feasible and provides realistic values of the resistance and peripheral compliance. Together, these values can be used to better understand and screen for PH and exercise-induced PH.

Irradiation of varying volumes of rat lung to same mean lung dose: a little to a lot or a lot to a little?

PURPOSE

To investigate whether irradiating small lung volumes with a large dose or irradiating large lung volumes with a small dose, given the same mean lung dose (MLD), has a different effect on pulmonary function in laboratory animals.

METHODS AND MATERIALS

WAG/Rij/MCW male rats were exposed to single fractions of 300 kVp X-rays. Four treatments, in decreasing order of irradiated lung volume, were administered: (1) whole lung irradiation, (2) right lung irradiation, (3) left lung irradiation, and (4) irradiation of a small lung volume with four narrow beams. The irradiation times were chosen to accumulate the same MLD of 10, 12.5, or 15 Gy with each irradiated lung volume. The development of radiation-induced lung injury for < or =20 weeks was evaluated as increased breathing frequency, mortality, and histopathologic changes in the irradiated and control rats.

RESULTS

A significant elevation of respiratory rate, which correlated with the lung volume exposed to single small doses (> or =5 Gy), but not with the MLD, was observed. The survival of the rats in the whole-lung-irradiated group was MLD dependent, with all events occurring between 4.5 and 9 weeks after irradiation. No mortality was observed in the partial-volume irradiated rats.

CONCLUSIONS

The lung volume irradiated to small doses might be the dominant factor influencing the loss of pulmonary function in the rat model of radiation-induced lung injury. Caution should be used when new radiotherapy techniques that result in irradiation of large volumes of normal tissue are used for the treatment of lung cancer and other tumors in the thorax.

Optimization of a retrospective technique for respiratory-gated high speed micro-CT of free-breathing rodents

The objective of this study was to develop a technique for dynamic respiratory imaging using retrospectively gated high-speed micro-CT imaging of free-breathing mice. Free-breathing C57Bl6 mice were scanned using a dynamic micro-CT scanner, comprising a flat-panel detector mounted on a slip-ring gantry. Projection images were acquired over ten complete gantry rotations in 50 s, while monitoring the respiratory motion in synchrony with projection-image acquisition. Projection images belonging to a selected respiratory phase were retrospectively identified and used for 3D reconstruction. The effect of using fewer gantry rotations--which influences both image quality and the ability to quantify respiratory function--was evaluated. Images reconstructed using unique projections from six or more gantry rotations produced acceptable images for quantitative analysis of lung volume, CT density, functional residual capacity and tidal volume. The functional residual capacity (0.15 +/- 0.03 mL) and tidal volumes (0.08 +/- 0.03 mL) measured in this study agree with previously reported measurements made using prospectively gated micro-CT and at higher resolution (150 microm versus 90 microm voxel spacing). Retrospectively gated micro-CT imaging of free-breathing mice enables quantitative dynamic measurement of morphological and functional parameters in the mouse models of respiratory disease, with scan times as short as 30 s, based on the acquisition of projection images over six gantry rotations.

Three-dimensional imaging and morphometric analysis of alveolar tissue from microfocal X-ray-computed tomography

We evaluated microfocal X-ray-computed tomography (micro-CT) as a method to visualize lung architecture two and three dimensionally and to obtain morphometric data. Inflated porcine lungs were fixed by formaldehyde ventilation. Tissue samples (8-mm diameter, 10-mm height) were stained with osmium tetroxide, and 400 projection images (1,024 × 1,024 pixel) were obtained. Continuous isometric micro-CT scans (voxel size 9 μm) were acquired to reconstruct two- and three-dimensional images. Tissue samples were sectioned (8-μm thickness) for histological analysis. Alveolar surface density and mean linear intercept were assessed by stereology-based morphometry in micro-CT scans and corresponding histological sections. Furthermore, stereology-based morphometry was compared with morphometric semi-automated micro-CT analysis within the same micro-CT scan. Agreement of methods was assessed by regression and Bland-Altman analysis. Comparing histology with micro-CT, alveolar surface densities (35.4 ± 2.4 vs. 33.4 ± 1.9/mm, P < 0.05) showed a correlation ( r = 0.72; P = 0.018) with an agreement of 2 ± 1.6/mm; the mean linear intercept (135.7 ± 14.5 vs. 135.8 ± 15 μm) correlated well ( r = 0.97; P < 0.0001) with an agreement of −0.1 ± 3.4 μm. Semi-automated micro-CT analysis resulted in smaller alveolar surface densities (33.4 ± 1.9 vs. 30.5 ± 1/mm; P < 0.01) with a correlation ( r = 0.70; P = 0.023) and agreement of 2.9 ± 1.4/mm. Non-destructive micro-CT scanning offers the advantage to visualize the spatial tissue architecture of small lung samples two and three dimensionally.

A Reinvestigation of Murine Cranial Suture Biology: Microcomputed Tomography versus Histologic Technique

BACKGROUND

Histology remains the standard form to analyze cranial suture in murine models, but this technique provides only limited "snapshots" of the entire suture and requires animal euthanasia with tissue destruction. Because of the bone complex microarchitecture, better methods are required to study the behavior of the cranial suture and its surrounding environment. The authors compared microcomputed tomography and histology as techniques to evaluate murine cranial sutures.

METHODS

A total of 360 microcomputed tomography images and 160 to 170 histologic sections were processed from a mouse at postnatal days 22 and 45, respectively. After euthanasia, the posterior frontal and sagittal sutures were imaged with a microcomputed tomography system and subsequently processed for histologic analysis. Quantitative analysis of two-dimensional images was performed to determine the percentage of bone in a 1-mm sample.

RESULTS

Quantitative analysis of the percentage of bone within the sutures showed identical patterns by microcomputed tomography and histology techniques. Both methods demonstrated the posterior frontal suture to have heavier fusion patterns in the anterior and endocranial portions, with variable skip areas of complete patency on the endocranial surface, ectocranial surface, or both at day 45.

CONCLUSIONS

Cranial suture fusion in the murine model is not an "all-or-none" phenomenon. The posterior frontal suture, previously thought to be completely fused on day 45 by histological analysis, showed variable fusion along the length of the suture by both methods. Quantitative assessment of the percentage of bone within the posterior frontal and sagittal sutures and morphologic assessment of these sutures demonstrated similar findings by both methods. Whereas thorough histologic evaluation of an entire suture would be extremely labor intensive and impractical, these findings help to validate microcomputed tomography as a rapid and reliable method of examining the entire suture in murine models.

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.

Prospective respiratory-gated micro-CT of free breathing rodents

Microcomputed tomography (Micro-CT) has the potential to noninvasively image the structure of organs in rodent models with high spatial resolution and relatively short image acquisition times. However, motion artifacts associated with the normal respiratory motion of the animal may arise when imaging the abdomen or thorax. To reduce these artifacts and the accompanying loss of spatial resolution, we propose a prospective respiratory gating technique for use with anaesthetized, free-breathing rodents. A custom-made bed with an embedded pressure chamber was connected to a pressure transducer. Anaesthetized animals were placed in the prone position on the bed with their abdomens located over the chamber. During inspiration, the motion of the diaphragm caused an increase in the chamber pressure, which was converted into a voltage signal by the transducer. An output voltage was used to trigger image acquisition at any desired time point in the respiratory cycle. Digital radiographic images were acquired of anaesthetized, free-breathing rats with a digital radiographic system to correlate the respiratory wave form with respiration-induced organ motion. The respiratory wave form was monitored and recorded simultaneously with the x-ray radiation pulses, and an imaging window was defined, beginning at end expiration. Phantom experiments were performed to verify that the respiratory gating apparatus was triggering the micro-CT system. Attached to the distensible phantom were 100 microm diameter copper wires and the measured full width at half maximum was used to assess differences in image quality between respiratory-gated and ungated imaging protocols. This experiment allowed us to quantify the improvement in the spatial resolution, and the reduction of motion artifacts caused by moving structures, in the images resulting from respiratory-gated image acquisitions. The measured wire diameters were 0.135 mm for the stationary phantom image, 0.137 mm for the image gated at end deflation, 0.213 mm for the image gated at peak inflation, and 0.406 mm for the ungated image. Micro-CT images of anaesthetized, free-breathing rats were acquired with a General Electric Healthcare eXplore RS in vivo micro-CT system. Images of the thorax were acquired using the respiratory cycle-based trigger for the respiratory-gated mode. Respiratory gated-images were acquired at inspiration and end expiration, during a period of minimal respiration-induced organ motion. Gated images were acquired with a nominal isotropic voxel spacing of 44 microm in 20-25 min (80 kVp, 113 mAs, 300 ms imaging window per projection). The equivalent ungated acquisitions were 11 min in length. We observed improved definition of the diaphragm boundary and increased conspicuity of small structures within the lungs in the gated images, when compared to the ungated acquisitions. In this work, we have characterized the externally monitored respiratory wave form of free-breathing, anaesthetized rats and correlated the respiration-induced organ motion to the respiratory cycle. We have shown that the respiratory pressure wave form is an excellent surrogate for the radiographic organ motion. This information facilitates the definition of an imaging window at any phase of the breathing cycle. This approach for prospectively gated micro-CT can provide high quality images of anaesthetized free-breathing rodents.

Microfocal X-ray computed tomography post-processing operations for optimizing reconstruction volumes of stented arteries during 3D computational fluid dynamics modeling

Restenosis caused by neointimal hyperplasia (NH) remains an important clinical problem after stent implantation. Restenosis varies with stent geometry, and idealized computational fluid dynamics (CFD) models have indicated that geometric properties of the implanted stent may differentially influence NH. However, 3D studies capturing the in vivo flow domain within stented vessels have not been conducted at a resolution sufficient to detect subtle alterations in vascular geometry caused by the stent and the subsequent temporal development of NH. We present the details and limitations of a series of post-processing operations used in conjunction with microfocal X-ray CT imaging and reconstruction to generate geometrically accurate flow domains within the localized region of a stent several weeks after implantation. Microfocal X-ray CT reconstruction volumes were subjected to an automated program to perform arterial thresholding, spatial orientation, and surface smoothing of stented and unstented rabbit iliac arteries several weeks after antegrade implantation. A transfer function was obtained for the current post-processing methodology containing reconstructed 16 mm stents implanted into rabbit iliac arteries for up to 21 days after implantation and resolved at circumferential and axial resolutions of 32 and 50 microm, respectively. The results indicate that the techniques presented are sufficient to resolve distributions of WSS with 80% accuracy in segments containing 16 surface perturbations over a 16 mm stented region. These methods will be used to test the hypothesis that reductions in normalized wall shear stress (WSS) and increases in the spatial disparity of WSS immediately after stent implantation may spatially correlate with the temporal development of NH within the stented region.

Anatomically based finite element models of the human pulmonary arterial and venous trees including supernumerary vessels

Studies of the origin of pulmonary blood flow heterogeneity have highlighted the significant role of vessel branching structure on flow distribution. To enable more detailed investigation of structure-function relationships in the pulmonary circulation, an anatomically based finite element model of the arterial and venous networks has been developed to more accurately reflect the geometry found in vivo. Geometric models of the arterial and venous tree structures are created using a combination of multidetector row X-ray computed tomography imaging to define around 2,500 vessels from each tree, a volume-filling branching algorithm to generate the remaining accompanying conducting vessels, and an empirically based algorithm to generate the supernumerary vessel geometry. The explicit generation of supernumerary vessels is a unique feature of the computational model. Analysis of branching properties and geometric parameters demonstrates close correlation between the model geometry and anatomical measures of human pulmonary blood vessels. A total of 12 Strahler orders for the arterial system and 10 Strahler orders for the venous system are generated, down to the equivalent level of the terminal bronchioles in the bronchial tree. A simple Poiseuille flow solution, assuming rigid vessels, is obtained within the arterial geometry of the left lung, demonstrating a large amount of heterogeneity in the flow distribution, especially with inclusion of supernumerary vessels. This model has been constructed to accurately represent available morphometric data derived from the complex asymmetric branching structure of the human pulmonary vasculature in a form that will be suitable for application in functional simulations.

Distensibility of the normal human lung circulation during exercise

Increasing pulmonary arterial (Ppa) and wedge (Pw) pressures at high flow (Q) during exercise could distend the thin-walled vessels. A mechanical descriptor of vascular distension, the distensibility (α, fractional diameter change/mmHg pressure), has been reported to be ∼0.02 for isolated large and small arteries, i.e., a 2% change in diameter per millimeter mercury pressure. In this review we used a pulmonary hemodynamic model to estimate α for data from exercising humans to determine whether interpretable results might be obtained. In 59 normal sea level subjects having published measurements of Ppa and Pw over a range of Q, we found values of α (0.02 ± 0.002) giving calculated Ppa, which matched measured Ppa to within 1.3 ± 0.1 (SE) mmHg. When subjects were exposed to chronic hypoxia ( n = 6, in Operation Everest II), α decreased (0.022 ± 0.002 vs. 0.008 ± 0.001, P < 0.05), but when subjects were exposed to acute hypoxia (Duke chamber study, n = 8), α did not decrease (0.014 ± 0.002 vs. 0.012 ± 0.002, P = not significant). Values of α tended to decrease with age in men >60 yr. Thus at rest and during exercise, normal values of α in young persons were similar to those measured in vitro, and the values decreased in chronic hypoxia and with aging where vascular remodeling or vascular wall stiffening was expected. We propose that the estimation of pulmonary vascular distensibility in humans may be a useful descriptor of pulmonary hemodynamics.

Alterations in wall shear stress predict sites of neointimal hyperplasia after stent implantation in rabbit iliac arteries

Restenosis resulting from neointimal hyperplasia (NH) limits the effectiveness of intravascular stents. Rates of restenosis vary with stent geometry, but whether stents affect spatial and temporal distributions of wall shear stress (WSS) in vivo is unknown. We tested the hypothesis that alterations in spatial WSS after stent implantation predict sites of NH in rabbit iliac arteries. Antegrade iliac artery stent implantation was performed under angiography, and blood flow was measured before casting 14 or 21 days after implantation. Iliac artery blood flow domains were obtained from three-dimensional microfocal X-ray computed tomography imaging and reconstruction of the arterial casts. Indexes of WSS were determined using three-dimensional computational fluid dynamics. Vascular histology was unchanged proximal and distal to the stent. Time-dependent NH was localized within the stented region and was greatest in regions exposed to low WSS and acute elevations in spatial WSS gradients. The lowest values of WSS spatially localized to the stented area of a theoretical artery progressively increased after 14 and 21 days as NH occurred within these regions. This NH abolished spatial disparity in distributions of WSS. The results suggest that stents may introduce spatial alterations in WSS that modulate NH in vivo.

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.

Dynamic small animal lung imaging via a postacquisition respiratory gating technique using micro-cone beam computed tomography

RATIONALE AND OBJECTIVES

Micro computed tomography is an important tool for small animal imaging. On many occasions, it is desirable to image lungs in a live instead of postmortem small animal to perform a pulmonary physiology study. Because the lungs are moving, gating with respect to the ventilatory phase has to be performed to reduce motion artifacts. Precapture ventilation gating may be difficult to achieve in some situations, which motivates us to propose and implement a simple postacquisition gating method.

MATERIALS AND METHODS

Rats were used as the subjects in this study. A sequence of low-dose projection images were acquired at 30 frames per second for each view angle. During each capture sequence the rat undergoes multiple ventilation cycles. Using the sequence of projection images, an automated region of interest algorithm, based on integrated grayscale intensity, tracts the ventilatory phase of the lungs. In the processing of an image sequence, multiple projection images are identified at a particular phase and averaged to improve the signal-to-noise ratio. The resulting averaged projection images from different view angles are input to a Feldkamp cone-beam algorithm reconstruction algorithm to obtain isotropic image volumes.

RESULTS

Reconstructions with reduced movement artifacts are obtained. In the gated reconstruction, registration of the bone is much better, the edge of the lung is clearly defined, and structures within the lung parenchyma are better resolved. Also, different phases of a breathing cycle can be reconstructed from one single tomographic scan by the proposed gating method.

CONCLUSION

A postacquisition gating method using the phase information encoded in the 2-dimensional cone beam projections is proposed. This method is simple to implement and does not require additional experimental set-up to monitor the respiration. It may find applications in lung tumor detection, dynamic pulmonary physiology studies, and the respiratory systems modeling. Minimal motion artifact data sets improve qualitative and quantitative analysis techniques that are useful in physiologic studies of pulmonary structure and function.

Quantitative models of the rat pulmonary arterial tree morphometry applied to hypoxia-induced arterial remodeling

Little is known about the constituent hemodynamic consequences of structural changes that occur in the pulmonary arteries during the onset and progression of pulmonary arterial remodeling. Many disease processes are known to be responsible for vascular remodeling that leads to pulmonary arterial hypertension, cor pulmonale, and death. Histology has been the primary tool for evaluating pulmonary remodeling, but it does not provide information on intact vascular structure or the vessel mechanical properties. This study is an extension of our previous work in which we developed an alternative imaging technique to evaluate pulmonary arterial structure. The lungs from Sprague-Dawley rats were removed, perfusion analysis was performed on the isolated lungs, and then an X-ray contrast agent was used to fill the arterial network for imaging. The lungs were scanned over a range of intravascular pressures by volumetric micro-computed tomography, and the arterial morphometry was mapped and measured in the reconstructed isotropic volumes. A quantitative assessment of hemodynamic, structural, and biomechanical differences between rats exposed for 21 days to hypoxia (10% O(2)) or normoxia (21.0% O(2)) was performed. One metric, the normalized distensibility of the arteries, is significantly (P < 0.001) larger [0.025 +/- 0.0011 (SE) mmHg(-1)] (n = 9) in normoxic rats compared with hypoxic [0.015 +/- 0.00077 (SE) mmHg(-1)] (n = 9). The results of the study show that these models can be applied to the Sprague-Dawley rat data and, specifically, can be used to differentiate between the hypoxic and the control groups.

[Cardio-pulmonary vascular system. Three-dimensional quantitative evaluation by microcomputed tomography]

In recent years microcomputed tomography (microCT) has become more and more important in basic research. Now commercial microCT scanners are available. Thus, it is very likely that this new, accurate and promising method for three-dimensional and non-destructive quantitative evaluation of intact tissues including vessels will be applied more frequently. The review provides a survey of the basic technology of microCT and its current use for high resolution three-dimensional morphometric and functional analysis within the cardio-pulmonary vascular system.

Microfocal CT: a method for evaluating murine cranial sutures in situ

INTRODUCTION

The murine model is a well-established surrogate for studying human cranial suture biology. In mice, all sutures with the exception of the posterior frontal (PF) suture remain patent throughout life. Histology is regarded as the gold standard for analyzing sutures. On this basis, PF suture fusion begins on day of life 25 and is complete by day 45. Cranial suture histology, however, requires sacrifice of the animal to obtain tissue for analysis. As a result, knowledge of the kinetics of cranial suture fusion is based on a patchwork analysis of many sutures from many different animals. The behavior of a single suture through time is unknown. Our goal is to develop a noninvasive means to repeatedly image mouse cranial sutures in vivo. As a first step, the present study was performed to evaluate microfocal computer tomography (micro-CT) technology for the use of capturing images of a mouse cranium in situ.

METHODS

The micro-CT system consists of a microfocal X-ray source and a large format CCD camera optically coupled to a high-resolution X-ray image intensifier, digitally linked to a computer. The PF and sagittal sutures lie in continuity along the midline of the skull. Holes were drilled in the calvaria on both sides of the PF and sagittal sutures of a 45-day-old euthanized mouse. A micro-CT scan of this animal was performed and hundreds of cross-sectional images were generated for the cranium. These images were used to reconstruct three-dimensional volumetric images of the entire cranium. Comparisons were made between (1). the gross specimen and the three dimensional reconstructions; (2). two-dimensional coronal images obtained by micro-CT and those obtained by histology.

RESULTS

Analysis of PF and sagittal sutures demonstrated the following: (1). The drilled holes were accurately rendered by micro-CT, when compared to both the gross specimen and the histology. (2). The sagittal suture was found to be patent by both micro-CT and histology. (3). The PF suture is fused by histology, but unexpectedly, the PF suture appears incompletely fused by micro-CT. By micro-CT, however, the anterior and endocranial regions appear more extensively fused than the remainder of the PF suture, a finding consistent with published histologic analysis.

CONCLUSIONS

We successfully imaged 45-day-old mouse cranial sutures in situ using micro-CT technology. Precise correlation between histologic sections and radiologic images is difficult, but convincing similarities exist between the gross specimen and images from micro-CT and histology. PF suture fusion in a 45-day-old animal appears different by micro-CT than by histology. One possible explanation for this apparent discrepancy is that suture fusion in histology is determined based on the appearance of bone morphology and not tissue density, as the specimens are necessarily decalcified to section the bone. Micro-CT, on the other hand, distinguishes tissues on the basis of density. Newly forming bone may require bone matrix formation prior to complete calcification; PF suture in 45-day-old mice may be morphologically complete but incompletely ossified. Studies correlating histologic and micro-CT assessment of suture development are underway. Micro-CT appears to be a promising method for noninvasive imaging of mouse cranial suture.

Flow and pressure distributions in vascular networks consisting of distensible vessels

We examine the influence of vessel distensibility on the fraction of the total network flow passing through each vessel of a model vascular network. An exact computational methodology is developed yielding an analytic proof. For a class of structurally heterogeneous asymmetric vascular networks, if all the individual vessels share a common distensibility relation when the total network flow is changed, this methodology proves that each vessel will continue to receive the same fraction of the total network flow. This constant flow partitioning occurs despite a redistribution of pressures, which may result in a decrease in the diameter of one and an increase in the diameter of the other of two vessels having a common diameter at a common pressure. This theoretical observation, taken along with published experimental observations on pulmonary vessel distensibilities, suggests that vessel diameter-independent distensibility in the pulmonary vasculature may be an evolutionary adaptation for preserving the spatial distribution of pulmonary blood flow in the face of large variations in cardiac output.

Pulmonary arterial morphometry from microfocal X-ray computed tomography

The objective of this study was to develop an X-ray computed tomographic method for pulmonary arterial morphometry. The lungs were removed from a rat, and the pulmonary arterial tree was filled with perfluorooctyl bromide to enhance X-ray absorbance. At each of four pulmonary arterial pressures (30, 21, 12, and 5.4 mmHg), the lungs were rotated within the cone of the X-ray beam that was projected from a microfocal X-ray source onto an image intensifier, and 360 images were obtained at 1 degrees increments. The three-dimensional image volumes were reconstructed with isotropic resolution with the use of a cone beam reconstruction algorithm. The luminal diameter and distance from the inlet artery were measured for the main trunk, its immediate branches, and several minor trunks. These data revealed a self-consistent tree structure wherein the portion of the tree downstream from any vessel of a given diameter has a similar structure. Self-consistency allows the entire tree structure to be characterized by measuring the dimensions of only the vessels comprising the main trunk of the tree and its immediate branches. An approach for taking advantage of this property to parameterize the morphometry and distensibility of the pulmonary arterial tree is developed.