Measurement of three-dimensional anatomy and function of pulmonary arteries with high-speed x-ray computed tomography

A semi-automatic framework of measuring pulmonary arterial metrics at anatomic airway locations using CT imaging

Pulmonary vascular dysfunction has been implicated in smoking-related susceptibility to emphysema. With the growing interest in characterizing arterial morphology for early evaluation of the vascular role in pulmonary diseases, there is an increasing need for the standardization of a framework for arterial morphological assessment at airway segmental levels. In this paper, we present an effective and robust semi-automatic framework to segment pulmonary arteries at different anatomic airway branches and measure their cross-sectional area (CSA). The method starts with user-specified endpoints of a target arterial segment through a custom-built graphical user interface. It then automatically detect the centerline joining the endpoints, determines the local structure orientation and computes the CSA along the centerline after filtering out the adjacent pulmonary structures, such as veins or airway walls. Several new techniques are presented, including collision-impact based cost function for centerline detection, radial sample-line based CSA computation, and outlier analysis of radial distance to subtract adjacent neighboring structures in the CSA measurement. The method was applied to repeat-scan pulmonary multirow detector CT (MDCT) images from ten healthy subjects (age: 21-48 Yrs, mean: 28.5 Yrs; 7 female) at functional residual capacity (FRC). The reproducibility of computed arterial CSA from four airway segmental regions in middle and lower lobes was analyzed. The overall repeat-scan intra-class correlation (ICC) of the computed CSA from all four airway regions in ten subjects was 96% with maximum ICC found at LB10 and RB4 regions.

Volumetric xenon-CT imaging of conventional and high-frequency oscillatory ventilation

RATIONALE AND OBJECTIVES

For mechanical ventilation of patients with pulmonary injuries, it has been proposed that high-frequency oscillatory ventilation (HFOV) offers advantages over conventional ventilation (CV); however, these advantages have been difficult to quantify. We used volumetric, dynamic imaging of Xenon (Xe) washout of the canine lung during both HFOV and CV to compare regional ventilation in the two modalities.

MATERIALS AND METHODS

Three anesthetized, mechanically ventilated animals were studied, each at three different ventilator settings. Imaging was performed on an experimental Toshiba 256-slice scanner at 80 kV, 250 mAs, and 0.5-second scans, yielding 12.8 cm of Z-axis coverage. Repeated images were acquired at increasing intervals between 1 and 10 seconds for 90 seconds during HFOV and using retrospective respiratory gating to end-expiration for 60 seconds during CV. Image series were analyzed to quantify regional specific ventilation (sV ) from the regional density washout time constants.

RESULTS

High-quality, high-resolution regional ventilation maps were obtained during both CV and HFOV. Overall ventilation decreased at smaller tidal volume, as expected. Regional sV was more uniform during HFOV compared to CV, but the underlying distribution of lung aeration was similar.

CONCLUSIONS

High-resolution volumetric ventilation maps of the lung may be obtained with the 256-slice multidetector computed tomographic scanner. There is a marked difference in the distribution of regional ventilation between CV and HFOV, with a significant gravitational ventilation gradient in CV that was not present during HFOV. This technique may be useful in exploring the mechanisms by which HFOV improves gas exchange.

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.

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.

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.

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

The objective of this study was to develop an X-ray computed tomographic method for measuring pulmonary arterial dimensions and locations within the intact rat lung. Lungs were removed from rats and their pulmonary arterial trees were filled with perfluorooctyl bromide to enhance X-ray absorbance. The lungs were rotated within the cone of the X-ray beam 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 using a cone beam reconstruction algorithm. The vessel diameters were obtained by fitting a functional form to the image of the vessel circular cross section. The functional form was chosen to take into account the point spread function of the image acquisition and reconstruction system. The diameter measurements obtained over a range of vascular pressures were used to characterize the distensibility of the rat pulmonary arteries. The distensibility coefficient alpha [defined by D(P) = D(0)(1 + alphaP), where D(P) is the diameter at intravascular pressure (P)] was approximately 2.8% mmHg and independent of vessel diameter in the diameter range (about 100 to 2,000 mm) studied.

Effect of site and rate of contrast material injection on pulmonary vascular distention

RATIONALE AND OBJECTIVES

The authors performed this study to determine if there were differences in vascular caliber measured on angiograms obtained with the injection protocol used for spiral computed tomography (CT) versus that used for pulmonary angiography.

MATERIALS AND METHODS

The authors studied seven juvenile anesthetized pigs by using a prospective repeated measures experimental design. All pigs received injections of nonionic contrast material via catheters in the brachial vein, superior vena cava, main pulmonary artery, and left pulmonary artery. Weight-adjusted injection rates and volumes ranged from 0.05 mL/kg/sec (3.5 mL/sec, spiral CT protocol) to 0.56 mL/kg/sec (40 mL/sec, pulmonary angiography protocol). Heart rate and pulmonary artery and systemic artery pressures were recorded. During each injection, identically positioned pulmonary angiograms were obtained at full inspiration. Vessel diameters were measured at identical locations after each injection by two observers. The relationship between vessel diameter and hemodynamic parameters and injection site and rate was assessed with analysis of variance.

RESULTS

At suspended full inspiration, no statistically significant difference (P > .05) in vessel diameter or hemodynamic parameters was found between the different injection sites or rates. There was no difference in vascular caliber between systole and diastole.

CONCLUSION

The improved detection of subsegmental pulmonary emboli at pulmonary angiography compared with contrast material-enhanced spiral CT is not due to differences in vascular distention.

Structure-function relationships in the pulmonary arterial tree

Knowledge of the relationship between structure and function of the normal pulmonary arterial tree is necessary for understanding normal pulmonary hemodynamics and the functional consequences of the vascular remodeling that accompanies pulmonary vascular diseases. In an effort to provide a means for relating the measurable vascular geometry and vessel mechanics data to the mean pressure-flow relationship and longitudinal pressure profile, we present a mathematical model of the pulmonary arterial tree. The model is based on the observation that the normal pulmonary arterial tree is a bifurcating tree in which the parent-to-daughter diameter ratios at a bifurcation and vessel distensibility are independent of vessel diameter, and although the actual arterial tree is quite heterogeneous, the diameter of each route, through which the blood flows, tapers from the arterial inlet to essentially the same terminal arteriolar diameter. In the model the average route is represented as a tapered tube through which the blood flow decreases with distance from the inlet because of the diversion of flow at the many bifurcations along the route. The taper and flow diversion are expressed in terms of morphometric parameters obtained using various methods for summarizing morphometric data. To help put the model parameter values in perspective, we applied one such method to morphometric data obtained from perfused dog lungs. Model simulations demonstrate the sensitivity of model pressure-flow relationships to variations in the morphometric parameters. Comparisons of simulations with experimental data also raise questions as to the "hemodynamically" appropriate ways to summarize morphometric data.

Utility of CT scan evaluation for predicting pulmonary hypertension in patients with parenchymal lung disease. Medical College of Wisconsin Lung Transplant Group

OBJECTIVE

To determine the utility of CT-determined main pulmonary artery diameter (MPAD) for predicting pulmonary hypertension (PH) in patients with parenchymal lung disease.

DESIGN

Retrospective review of right-heart hemodynamic data and chest CT scans in 45 patients.

SETTING

Tertiary-referral teaching hospital and VA hospital.

PATIENTS

Between October 1990 and December 1995, 36 patients referred for evaluation of parenchymal lung disease or possible pulmonary vascular disease were found to have PH, as defined by mean pulmonary artery pressure (mPAP) > or =20 mm Hg. Nine control patients (mPAP <20 mm Hg) were also identified (4 from hospital records search, 5 after evaluation for possible PH).

RESULTS

CT-determined MPAD was 35+/-6 mm in patients with PH and 27+/-2 mm in control subjects. In our group of patients, MPAD > or =29 mm had a sensitivity of 87%, specificity of 89%, positive predictive value (PPV) of 0.97, and positive likelihood ratio (LR) of 7.91 for predicting PH; in the subgroup of patients with parenchymal lung disease (n=28, PH and control subjects), MPAD > or =29 mm had a sensitivity of 84%, specificity of 75%, PPV of 0.95, and positive LR of 3.36 for predicting PH. The most specific findings for the presence of PH were both MPAD > or =29 mm and segmental artery-to-bronchus ratio > 1:1 in three or four lobes (specificity, 100%). There was no linear correlation between the degree of PH and MPAD (r=0.124).

CONCLUSIONS

CT-determined MPAD has excellent diagnostic value for detection of PH in patients with advanced lung disease. Therefore, standard chest CT scans can be used to screen for PH as a cause of exertional limitation in patients with parenchymal lung disease. Because CT is commonly used to evaluate parenchymal lung disease, this information is readily available.

Matching pulmonary structure and perfusion via combined dynamic multislice CT and thin-slice high-resolution CT

Taking advantage of two scan modes of an electron beam CT scanner (Imatron), we have developed a method utilizing x-ray CT for relating pulmonary perfusion to global and regional anatomy. A high temporal resolution mode, used to follow bolus contrast agent, is combined with a high spatial resolution mode to obtain the structure-function fusion. A software module has been developed for our image analysis package (VIDA) to automatically calculate physiologic parameters of flow and integrate these color coded functional measurements into a corresponding high spatial resolution data set. We present the scanning methodology details and give examples from our physiologic based research to demonstrate strengths of combining dynamic and high resolution CT to uniquely characterize pulmonary normal and pathophysiology.

A method for measurement of cross sectional area, segment length, and branching angle of airway tree structures in situ

Accurate quantitative measurements of airway and vascular dimensions are essential for evaluating function in both the normal and in the diseased lung. This report describes a new integrated method for three-dimensional (3D) extraction and analysis of pulmonary tree structures using data from High Resolution Computed Tomography (HRCT). Serially scanned two-dimensional (2D) slices of the lower left lobe of isolated dog lungs were stacked to create a volume of data. Airway and vascular trees were extracted using a 3D seeded region-growing algorithm based on differences in CT number between wall and lumen. In the region-growing step, voxels in the lumen are tagged with a distance descriptor to identify points along the tree structure equidistant from the seed point. To obtain quantitative data, we reduced each tree to its central axis. From the central axis, branch length was measured as the distance between two successive branch points, branch angle was measured as the angle produced by two daughter branches, and cross-sectional area was measured from a plane perpendicular to the central axis point. Data derived from these methods can be used to localize and quantify structural differences both during different physiologic conditions and in pathologic lungs.

Fast computed tomography for quantitative cardiac analysis--state of the art and future perspectives

Two fast computed tomographic scanners, designed primarily for imaging cardiac structures and function, have been in use since the early 1980s. The technical aspects of both systems have been described previously in detail, and a considerable body of scientific literature now documents the biomedical capabilities of these scanners. This review examines these biomedical capabilities as applied to quantitative analysis of the heart and pulmonary circulations. On the basis of this overview, some speculations about the current strengths and possible further developments of the fast computed tomographic approach in these applications are made.

Three-dimensional canine renovascular structure and circulation visualized in situ with the dynamic spatial reconstructor

The dynamic spatial reconstructor--a unique, high speed, volume-scanning, X-ray computed tomographic imaging system--was utilized to examine canine renovascular anatomy and renal circulation in situ. In each of the four kidneys examined in this study initial scans were done during bolus injections of angiographic contrast material into the renal artery. A subsequent scan was then performed following an injection of methyl-methacrylate-based casting compound that had been contrast enhanced with ethiodol. After the scans, each kidney was removed, and its parenchyma was digested in potassium hydroxide to expose the vascular cast. Comparison of casts with their reconstructed images and with images obtained during injection of contrast material showed that interlobar arteries and occasionally arcuate arteries could be clearly detected. Although discrete vessels less than 1 mm in diameter could not be resolved, dynamic changes in parenchymal distribution of density during passage of contrast material allowed interpretation of flow through the multiple capillary beds of the kidney. Such analysis indicated that maximal density was in the outer-middle zone of the cortex throughout the duration of the scan. Analysis of artery-to-vein transit time showed arrival of contrast material in the renal vein as soon as 3 sec, and continuation for longer than 8 sec, after the renal artery bolus. In conclusion, renal circulation in the dog can be discretely visualized with the dynamic spatial reconstructor up to the level of the arcuate arteries; however, capillary flow as a whole can be followed through the cortex, and the results suggest the presence of both rapid and slow components of peritubular circulation.