Assessment of methacholine-induced airway constriction by ultrafast high-resolution computed tomography

Imaging of pulmonary embolism and t-PA therapy effects using MDCT and liposomal iohexol blood pool agent: preliminary results in a rabbit model

RATIONALE AND OBJECTIVES

Polyethylene glycol-coated liposomal blood pool contrast agents maintain contrast enhancement over several hours. This study aimed to evaluate (long-term) imaging of pulmonary arteries, comparing conventional iodinated contrast with a liposomal blood pool contrast agent. Also, visualization of the (real-time) therapeutic effects of tissue plasminogen activator (t-PA) on pulmonary embolism (PE) was attempted.

MATERIALS AND METHODS

Six rabbits (weight approximately 4 kg) had autologous blood clots injected through the superior vena cava. Imaging was performed using conventional contrast (iohexol, 350 mg I/ml; GE HealthCare, Princeton, NJ) at a dose of 1400 mg I per animal, and after wash-out, animals were imaged using an iodinated liposomal blood pool agent (88 mg I/mL, dose 900 mg I/animal). Subsequently, five animals were injected with 2 mg of t-PA and imaging continued for up to 4(1/2) hours.

RESULTS

Both contrast agents identified PE in the pulmonary trunk and main pulmonary arteries in all rabbits. Liposomal blood pool agent yielded uniform enhancement, which remained relatively constant throughout the experiments. Conventional agents exhibited nonuniform opacification and rapid clearance postinjection. Three of six rabbits had mistimed bolus injections, requiring repeat injections. Following t-PA, pulmonary embolus volume (central to segmental) decreased in four of five treated rabbits (range 10-57%, mean 42%). One animal showed no response to t-PA.

CONCLUSIONS

Liposomal blood pool agents effectively identified acute PE without need for reinjection. PE resolution following t-PA was quantifiable over several hours. Blood pool agents offer the potential for repeated imaging procedures without need for repeated (nephrotoxic) contrast injections.

Assessment of Airways with Three-dimensional Quantitative Thin-Section CT: In Vitro and in Vivo Validation

PURPOSE

To prospectively validate the ability of customized three-dimensional (3D) software to enable bronchial tree skeletonization, orthogonal reconstruction of the main bronchial axis, and measurement of cross-sectional wall area (WA) and lumen area (LA) of any visible bronchus on thin-section computed tomographic (CT) images.

MATERIALS AND METHODS

Institutional review board approval and patient agreement and informed consent were obtained. Software was validated in a phantom that consisted of seven tubes and an excised human lung obtained and used according to institutional guidelines. In vivo validation was performed with multi-detector row CT in six healthy subjects (mean age, 47 years; range, 20-55 years). Intra- and interobserver agreement and reproducibility over time for bronchial tree skeletonization were evaluated with Bland-Altman analysis. Concordance in identifying bronchial generation was assessed with the kappa statistic. WA and LA obtained with the manual method were compared with WA and LA obtained with validated software by means of the Wilcoxon test and Bland-Altman analysis.

RESULTS

WA and LA measurements in the phantom were reproducible over multiple sessions (P > .90) and were not significantly different from WA and LA assessed with the manual method (P > .62). WA and LA measurements in the excised lung and the subjects were not different from measurements obtained with the manual method (intraclass correlation coefficient > 0.99). All lobar bronchi and 80.8% of third generation bronchi, 72.5% of fourth generation bronchi, and 37.7% of fifth generation bronchi were identified in vivo. Intra- and interobserver agreement and reproducibility over time for airway skeletonization and concordance in identifying bronchial generation were good to excellent (intraclass correlation coefficient > 0.98, kappa > 0.54, respectively).

CONCLUSION

This method enables accurate and reproducible measurement of WA and LA on reformatted CT sections perpendicular to the main axis of bronchi visible on thin-section CT scans.

Assessment of airway caliber in quantitative videobronchoscopy

BACKGROUND

Quantitative assessment of airway caliber is generally confined to indirect methods. Fiberoptic bronchoscopy provides a direct view of the airways, but measurement of the internal size of bronchi in a standard examination is not possible. Using a special image analysis program, we developed a method allowing quantitative assessment of airway caliber by means of videobronchoscopic (VB) examination.

OBJECTIVES

The purpose of the study was toshow that quantitative videobronchoscopy (VB coupled with a computer image analysis) allows direct and accurate measurement of the bronchi diameter.

METHODS

To test our hypothesis, we measured the same areas of a bronchial tree in CT and in VB in 40 patients with diagnostic indications for both the procedures.

RESULTS

We measured the diameters of 149 bronchi. The mean value of the difference between VB and CT measurements was equal to -0.071 mm and was not significantly different from 0 (p = 0.086). There was no obvious relation between the difference and the mean (r = 0.026, p = 0.745). The Bland Altman limits of agreement were L = -1.071 mm and U = 0.929 mm. We also assessed the bronchial diameter after endobronchial challenge and in patients with tracheobronchomalacia to show the application of this method for dynamic measurements.

CONCLUSIONS

Quantitative videobronchoscopy allows the accurate and direct measurement of an airway caliber. It may be useful in clinical setting to quantify changes in a bronchial caliber (endobronchial masses, tracheobronchomalacia). Dynamic visualization of changes in airways may be useful in research, especially to explore the mechanics of airway narrowing.

Measurement of the internal diameter of plastic tubes from projection MR images using a model-based least-squares fit approach

Hyperpolarized (HP) 3He MRI is an emerging tool in the diagnosis and evaluation of pulmonary diseases involving bronchoconstriction, such as asthma. Previously, airway diameters from dynamic HP 3He MR images of the lung were assessed manually and subjectively, and were thus prone to uncertainties associated with human error and partial volume effects. A model-based algorithm capable of fully utilizing pixel intensity profile information and attaining subpixel resolution has been developed to measure surrogate airway diameters from HP 3He MR static projection images of plastic tubes. This goal was achieved by fitting ideal pixel intensity profiles for various diameter (6.4 to 19.1 mm) circular tubes to actual pixel intensity data. A phantom was constructed from plastic tubes of various diameters connected in series and filled with water mixed with contrast agent. Projection MR images were then taken of the phantom. The favorable performance of the model-based algorithm compared to manual assessment demonstrates the viability of our approach. The manual and algorithm approaches yielded diameter measurements that generally stayed within 1 x the pixel dimension. However, inconsistency of the manual approach can be observed from the larger standard deviations of its measured values. The method was then extended to HP 3He MRI, producing encouraging results at tube diameters characteristic of airways beyond the second generation, thereby justifying their application to lung airway imaging and measurement. Potential obstacles when measuring airway diameters using this method are discussed.

Comparison of airway dimensions in different anatomic locations on chest CT in patients with COPD

OBJECTIVE

It is not well known whether there is heterogeneity in the airway dimensions at different anatomic locations in individual patients with COPD. The first objective was to compare airway dimensions of the basal segment bronchus between COPD patients and healthy controls. The second and third objectives were to compare the airway dimensions in two anatomic locations, and to investigate the relationship between CT measurements and pulmonary function among COPD patients.

METHODS

Thirty males with COPD (aged 68.7 +/- 8.1 years) and 18 healthy males (aged 64.9 +/- 14.0 years) were enrolled in the study. COPD was diagnosed according to the criteria of the Global Initiative for Obstructive Lung Disease Workshop Report. Pulmonary function tests and CT scans were performed on all subjects. Airway dimensions and lung attenuation were automatically determined using methods that were validated with a phantom.

RESULTS

Age, smoking index and height did not significantly differ between the COPD patients and healthy controls. The COPD patients had a significantly thicker airway wall than healthy controls. Among the COPD patients, there were no significant differences in the airway dimensions of bronchi in different segments; however, the airway and lung attenuation measurements of the lower lung field were more strongly correlated with FEV(1) than those of the upper lung field.

CONCLUSION

Patients with COPD had no significant heterogeneity in airway dimensions at different anatomic locations. The airway and lung attenuation measurements of the lower lung field were more strongly correlated with airflow limitation than those of the upper lung field.

The structural basis of airways hyperresponsiveness in asthma

We hypothesized that structural airway remodeling contributes to airways hyperresponsiveness (AHR) in asthma. Small, medium, and large airways were analyzed by computed tomography in 21 asthmatic volunteers under baseline conditions (FEV1= 64% predicted) and after maximum response to albuterol (FEV1= 76% predicted). The difference in pulmonary function between baseline and albuterol was an estimate of AHR to the baseline smooth muscle tone (BSMT). BSMT caused an increase in residual volume (RV) that was threefold greater than the decrease in forced vital capacity (FVC) because of a simultaneous increase in total lung capacity (TLC). The decrease in FVC with BSMT was the major determinant of the baseline FEV1( P < 0.0001). The increase in RV correlated inversely with the relaxed luminal diameter of the medium airways ( P = 0.009) and directly with the wall thickness of the large airways ( P = 0.001). The effect of BSMT on functional residual capacity (FRC) controlled the change in TLC relative to the change in RV. When the FRC increased with RV, TLC increased and FVC was preserved. When the relaxed large airways were critically narrowed, FRC and TLC did not increase and FVC fell. With critical large airways narrowing, the FRC was already elevated from dynamic hyperinflation before BSMT and did not increase further with BSMT. FEV1/FVC in the absence of BSMT correlated directly with large airway luminal diameter and inversely with the fall in FVC with BSMT. These findings suggest that dynamic hyperinflation caused by narrowing of large airways is a major determinant of AHR in asthma.

Mechanisms of limited airway dimension with lung inflation

Airways distend with each inspiration, while a sigh or deep inspiration (DI) leads to a significant or a maximum distension of the airways. Distension of the airways is thought to play an important role in maintaining airway patency. Limited distension of the airways with lung inflation may be a major factor in certain lung diseases such as asthma and chronic obstructive pulmonary disease (COPD). High resolution computed tomography (HRCT) has gained wide acceptance as a diagnostic and investigational radiological tool for the evaluation of airway function. HRCT has been used to measure dynamic changes in airway caliber in vivo that are not detectable by conventional global lung measurements such as airway and lung resistance. HRCT is uniquely capable of imaging and quantifying airway size at different lung volumes. The current paper reviews the use of HRCT to examine the role of lung inflation on airway distension in animal models, and discusses potential mechanisms for limited distension of the airways with lung inflation in individuals with asthma and COPD.

Differences in the time course of proximal and distal airway response to inhaled histamine studied by synchrotron radiation CT

We studied the kinetics of proximal and distal bronchial response to histamine aerosol in healthy anesthetized and mechanically ventilated rabbits up to 60 min after histamine administration using a novel xenon-enhanced synchrotron radiation computed tomography imaging technique. Individual proximal airway constriction was assessed by measuring the luminal cross-sectional area. Distal airway obstruction was estimated by measuring the ventilated alveolar area after inhaled xenon administration. Respiratory system conductance was assessed continuously. Proximal airway cross-sectional area decreased by 57% of the baseline value by 20 min and recovered gradually but incompletely within 60 min. The ventilated alveolar area decreased immediately after histamine inhalation by 55% of baseline value and recovered rapidly thereafter. The results indicate that the airway reaction to inhaled histamine and the subsequent recovery are significantly slower in proximal than in distal bronchi in healthy rabbit. The findings suggest that physiological reaction mechanisms to inhaled histamine in the airway walls of large and small bronchi are not similar.

Bronchial reactivity in hyperresponsive patients and healthy individuals: demonstration with high resolution computed tomography

OBJECTIVE

High resolution computed tomography (HRCT) was used to assess the extent of bronchial reactivity after inhalative bronchoprovocation and dilation in hyperresponsive patients and healthy subjects.

PATIENTS AND METHODS

Patients with mild intermittent asthma, 15 with a >20% decrease in FEV1 and a >10 mmHg (PC20+) in PaO2, 12 with a <20% decrease in FEV1 and a >10 mmHg (PC20-) in PaO2 after provocation, and eight healthy humans were included in the study. Changes in cross-sectional area in a total of 1256 bronchi and in bronchial wall area (792 bronchi) were evaluated after histamine-triggered bronchoprovocation and salbutamol-induced bronchodilation at high lung volumes (FVC 80%). Data were compared with the results of pulmonary function tests (FEV1, PaO2, PaCO2).

RESULTS

In all groups, a significant decrease in bronchial cross-sectional area (P<0.001) and a significant increase in bronchial wall area (P<0.001) were observed subsequent to bronchoprovocation. After bronchodilation, the increase in cross-sectional area (P<0.001) and the further increase in airway wall area (P<0.01) were significant in all groups. In PC20+ and PC20- asthmatics, significant differences (P<0.05) in PaO2, >10 mmHg between baseline and provocation were observed. In healthy persons, the PaO2 decrease was <10 mmHg (P>0.05). After histamine provocation, the decrease in FEV1 was measured in the PC20+ group, whereas a <20% FEV1 decrease was found in the PC20- and the control groups, respectively. No significant correlations were observed between radiological data and the results of pulmonary function tests.

CONCLUSIONS

HRCT demonstrated bronchial reactivity in hyperresponsive patients and, unexpectedly, in healthy subjects. The applied pulmonary function tests failed to characterize bronchial reactions in the healthy subjects. Based on these results, HRCT is a useful tool by which to achieve a comprehensive understanding of the pathophysiological processes in asthmatic patients.

Airway Wall Thickness in Cigarette Smokers: Quantitative Thin-Section CT Assessment

PURPOSE

To design and validate a dedicated software tool to measure airway dimensions on thin-section computed tomographic (CT) images and to use the tool to prospectively compare airway wall thickness in nonsmokers with normal lung function with that in smokers with and without chronic obstructive pulmonary disease (COPD).

MATERIALS AND METHODS

All subjects gave written informed consent. The study was approved by local ethics committee. With Laplacian of Gaussian algorithm, software was tested in phantom and excised sheep lung fixed in inflation and validated with Bland-Altman analysis. Study prospectively included nine nonsmokers (six women, three men; mean age, 53 years +/- 5.6 [standard error of the mean]) with normal lung function (group 1), seven smokers (three women, four men; mean age, 56 years +/- 5.6) with normal lung function (group 2), and eight smokers (zero women, eight men; mean age, 65 years +/- 4.0) with COPD. Calculations were determined with spirometrically gated CT: For each selected bronchus, the wall area (WA), internal area (IA), airway caliber (sum of IA and WA), and WA/IA ratio were calculated. For each patient, summation of WA to summation of IA (SigmaWA/SigmaIA) ratio, which reflected normalized airway wall thickness, was calculated. Groups were compared by using analysis of variance with generalized linear model and unpaired t test. Pearson correlation coefficient was used to assess correlation between software measurements and pulmonary function test results.

RESULTS

Comparison of measurements in phantom and excised sheep lung with algorithm measurements revealed that the latter were reliable and repeatable. In clinical study, SigmaWA/SigmaIA ratio was significantly different among three groups (P < .001). Normalized airway wall thickness and IA were significantly related to lung function test data, including forced expiratory volume in 1 second (r = -0.54, P = .006), specific airway conductance (r = -0.45, P = .03), and forced expiratory flow between 25% and 75% of vital capacity (r = -0.65, P < .001).

CONCLUSION

This software provides accurate and reproducible measurements of IA and WA of bronchi on thin-section CT images and demonstrates that in vivo normalized airway wall thickness was larger in smokers with COPD than it was in smokers or nonsmokers without COPD.

Chronic obstructive pulmonary disease: thin-section CT measurement of airway wall thickness and lung attenuation

PURPOSE

To prospectively evaluate airway wall thickness and lung attenuation at spirometrically gated thin-section computed tomography (CT) in patients with chronic obstructive pulmonary disease (COPD) and to correlate gated CT findings with pulmonary function test (PFT) results.

MATERIALS AND METHODS

The ethical committee approved the study, and all patients gave informed consent. Forty-two consecutive patients with COPD (20 with and 22 without chronic bronchitis [CB]) underwent gated thin-section CT and PFTs on the same day. The percentage wall area (PWA) and the thickness-to-diameter ratio (TDR) for all depicted bronchi that were round and larger than 2 mm in diameter, the mean lung attenuation (MLA), and the pixel index (PI) at -950 HU were determined. The reproducibility of the airway measurements was preliminarily tested by performing a five-trial examination in a patient with COPD and in a control patient. Differences in airway and lung attenuation measurements between the patients with and those without CB were evaluated at Mann-Whitney U testing. Simple and multiple regression analyses were used to assess the correlation between thin-section CT and PFT measurements.

RESULTS

The mean intraoperator coefficient of variation for airway measurements was 7.8% (range, 3.8%-13.4%). An average of nine bronchi per patient were assessed. Patients with CB had significantly higher PWAs, TDRs, and MLAs and significantly lower PIs than patients without CB (P < .05 for all values). The combination of PWA, TDR, and PWA normalized to body weight correlated significantly (P < .05) with the forced expiratory volume in 1 second-to-slow vital capacity ratio and the diffusing capacity of the lung for carbon monoxide in patients with but not in patients without CB. PFT results correlated better with MLA and PI in patients without CB.

CONCLUSION

Bronchial wall measurements differ between patients who have COPD with CB and those who have COPD without CB. The correlation between airway dimensions and indexes of airway obstruction in patients with COPD and CB indicates that the bronchial tree is the site of anatomic-functional alterations in this patient group.

Clinical assessment of airway remodeling in asthma: utility of computed tomography

Airway remodeling is an established feature of asthma. Histologic examination is essential in the assessment of remodeling that is a pathologic concept. Examinations of autopsied or resected lung have enabled detailed morphologic and morphometric studies and have provided fundamental knowledge of airway remodeling in asthma. However, such materials are only accidentally available, and clinical information may often be insufficient in autopsied cases. Bronchoscopic mucosal biopsy has been widely used since the 1980s, and has contributed substantially to basic investigations of inflammation and remodeling. However such specimens are limited in size and depth, limited to central airways, and the procedure might be too invasive to be repeated. Remodeling can also be assessed indirectly. Pulmonary function tests to evaluate chronic airflow obstruction are available in clinical settings and suitable for screening or mass studies, but they may be affected by concomitant diseases or short-term asthma control. Computed tomography (CT) has recently been utilized to assess remodeling. It cannot discern pathologic details but provides a broader range of airway/lung morphology and may be less invasive compared to biopsy. In addition to classic subjective evaluations,quantitative assessment has been reported for central airway dimensions, such as airway wall area, luminal area and wall thickness, and for peripheral airway abnormality or air trapping as measured by decreased lung attenuation or increased mosaic perfusion. This article summarizes the merits and limitations of various methods to assess airway remodeling, and describes the details of methodologies, interpretations, pathophysiologic relevance, and future directions of asthmatic airway remodeling assessed by CT.

In vivo evaluation of the effectiveness of bronchial thermoplasty with computed tomography

A recent study has reported that the application of thermal energy delivered through a bronchoscope (bronchial thermoplasty) impairs the ability of airway smooth muscle to shorten in response to methacholine (MCh)(Danek CJ, Lombard CM, Dungworth DL, Cox PG, Miller JD, Biggs MJ, Keast TM, Loomas BE, Wizeman WJ, Hogg JC, and Leff AR. J Appl Physiol 97: 1946-1953, 2004). If such a technique is successful, it has the potential to serve as a therapy to attenuate airway narrowing in asthmatic subjects regardless of the initiating cause that stimulates the smooth muscle. In the present study, we have applied high-resolution computed tomography to accurately quantify the changes in airway area before and after a standard MCh aerosol challenge in airways treated with bronchial thermoplasty. We studied a total of 193 airways ranging from 2 to 15 mm in six dogs. These were divided into treated and control populations. The MCh dose-response curves in untreated airways and soon-to-be-treated airways were superimposable. In contrast, the dose-response curves in treated airways were shifted upward at all points, showing a significantly decreased sensitivity to MCh at both 2 and 4 wk posttreatment. These results thus show that treated airways have significantly increased luminal area at any dose of inhaled MCh compared with untreated airways. The work in this study thus supports the underlying concept that impairing the smooth muscle may be an effective treatment for asthma.

Serial Change in Airway Lumen and Wall Thickness at Thin-Section CT in Asymptomatic Subjects

PURPOSE

To retrospectively analyze serial changes in airway lumen and wall thickness (WT) at multi-detector row computed tomography (CT) in asymptomatic subjects.

MATERIALS AND METHODS

Institutional review board did not require its approval or informed patient consent. Airway dimensions were analyzed in 52 patients (30 men and 22 women) without known cardiopulmonary disease. Contiguous 2-mm CT sections were obtained after reconstruction, extending from origin of right posterior basal segmental bronchi to posterior subsegmental bronchi. Following parameters were determined with semiautomatic image-processing program: luminal area (LA), total airway area (TA), short axis of lumen (LSD), and short axis of total airway (TSD). In airways in which adjacent vessel or branching of small bronchus abutted boundary of airway, extrapolated line was traced by one radiologist. Airway wall area (WA) was calculated as TA - LA, and WT was calculated as (TSD - LSD)/2. Relative WA (WA% = [WA/TA] . 100) and ratio of airway WT to total diameter (D) (WT/D = WT/TSD) were calculated. Linear regression analysis and Spearman rank correlation were used to evaluate relationship between airway parameters (LA, WA%, and WT/D ratio) and distance from origin of segmental bronchi.

RESULTS

LA decreased as CT proceeded from hilum to periphery (r = -0.765, P < .001). In 308 (32.7%) of 943 bronchi, however, LA increased as CT proceeded from hilum to periphery. LA increased by 10% or more in 101 (10.7%) of 943 bronchi. Mean changes in WA% and WT/D ratio between two contiguous sections were 0.66 +/- 5.05 (standard deviation) and 0.003 +/- 0.024, respectively. WA% changed by more than 5% between two contiguous sections in 274 (29.0%) of 943 bronchi. WT/D ratio changed by more than 0.02 between two contiguous sections in 338 (35.8%) of 943 bronchi.

CONCLUSION

Variation of airway lumen and WT is found in asymptomatic subjects without known cardiopulmonary disease.

Morphometric changes after thermal and methacholine bronchoprovocations

To determine whether there are distinctions in the location and pattern of response between different bronchoprovocations, we performed high-resolution computer-assisted tomography in 10 asthmatic subjects before and after isocapnic hyperventilation of frigid air (HV) and methacholine (Meth). The luminal areas of the trachea, main stem, lobar, and segmental bronchi were computed before and after each provocation and blindly compared. Both stimuli reduced the 1-s forced expiratory volume similarly (percent change in 1-s forced expiratory volume HV = 28.1 +/- 5.5%, Meth = 25.8 +/- 5.2%; P = 0.69) but did so in different fashions. Each provocation was associated with the development of both bronchial narrowing and dilation; however, more airways constricted with HV (67.7%) than with Meth (47.0%; P < 0.001). Furthermore, there was little concordance between either the magnitude or direction of change between stimuli in any region of the lung (r = 0.25). In general, the frequency of narrowing increased with branching. Constriction became more prominent in the lobar regions and increased further in the segmental branches, but a wide range of intensity existed. These data demonstrate that provocational stimuli evoke complex morphometric changes within the tracheobronchial tree and that different agonists produce different patterns. Thermal stimuli chiefly influence the segmental level, whereas the response to Meth develops more distally. Even within this distribution, the same airway does not respond in an identical fashion to different stimuli, so there does not appear to be a uniform trigger zone.

Pulmonary disease assessment in cystic fibrosis: comparison of CT scoring systems and value of bronchial and arterial dimension measurements

PURPOSE

To retrospectively compare thin-section computed tomographic (CT) scores obtained with five scoring systems for assessment of pulmonary disease in children with cystic fibrosis and to determine additional value of bronchial and arterial dimension measurements.

MATERIALS AND METHODS

Scores obtained with five thin-section CT scoring systems were compared. A score of 0 indicated the absence of abnormalities; a higher score meant that more structural abnormalities were seen. Three observers assigned scores and then reassigned scores after intervals varying from 1-2 weeks to 1-2 months at review of thin-section CT scans obtained in 25 children with cystic fibrosis. Interobserver and intraobserver reliability was calculated with intraclass correlation coefficients. Quantitative measurements of bronchial and arterial dimensions were obtained. Thin-section CT scores were correlated (Spearman correlation) with bronchial and arterial dimensions and with results of pulmonary function tests (PFTs), such as forced expiratory volume in 1 second (FEV(1)).

RESULTS

Scores with all five scoring systems were reproducible, with intraclass correlation coefficients of 0.74 and higher (P <.05), and showed significant correlations with FEV(1) (R = -0.73 to -0.69, P <.01). Ratio of bronchial diameter to accompanying pulmonary arterial diameter was correlated with thin-section CT scores but not with FEV(1). Ratio of bronchial wall thickness to accompanying pulmonary arterial diameter was not correlated with thin-section CT scores or PFT results.

CONCLUSION

Thin-section CT scores were reproducible and were correlated with PFT results. Measurements of bronchial dimensions were not significantly related to scores or PFT results.

Maximizing quantitative accuracy of lung airway lumen and wall measures obtained from X-ray CT imaging

To objectively quantify airway geometry from three-dimensional computed tomographic (CT) images, an idealized (circular cross section) airway model is parameterized by airway luminal caliber, wall thickness, and tilt angle. Using a two-dimensional CT slice, an initial guess for the airway center, and the full-width-half-maximum principle, we form an estimate of inner and outer airway wall locations. We then fit ellipses to the inner and outer airway walls via a direct least squares fit and use the major and minor axes of the ellipses to estimate the tilt and in-plane rotation angles. Convolving the airway model, initialized with these estimates, with the three-dimensional scanner point-spread function forms the predicted image. The difference between predicted and actual images is minimized by refining the model parameter estimates via a multidimensional, unconstrained, nonlinear minimization routine. When optimization converges, airway model parameters estimate the airway inner and outer radii and tilt angle. Results using a Plexiglas phantom show that tilt angle is estimated to within +/-4 degrees and both inner and outer radii to within one-half pixel when a "standard" CT reconstruction kernel is used. By opening up the ability to measure airways that are not oriented perpendicular to the scanning plane, this method allows evaluation of a greater sampling of airways in a two-dimensional CT slice than previously possible. In addition, by combining the tilt-angle compensation with the deconvolution method, we provide significant improvement over the previous full-width-half-maximum method for assessing location of the luminal edge but not the outer edge of the airway wall.

Duration of deep inspiration and subsequent airway constriction in vivo

The effects of a deep inspiration (DI) in asthmatics differ from those observed in healthy subjects. When considering the effects of a DI, an implicit assumption is that all the airways are distending at the same rate as the lung parenchyma. However, with such rapid lung inflation, the ability of contracted airways to dynamically follow the lung parenchyma was recently shown to significantly lag the lung inflation. Another potentially important variable in the response of the individual airways to a DI that has not been well studied is the duration of the DI maneuver. The current study examines the effects of increasing duration at TLC during a DI on subsequent airway caliber. In five anesthetized and ventilated mongrel dogs, after DIs of increasing duration, changes in airway size were measured over the subsequent 5-minute period using high-resolution computed tomography. Results show that the duration of the maneuver is extremely important, leading to a qualitative change in the airway response. A long DI (> or = 30 seconds) caused subsequent airway dilation, while a shorter DI (< 30 seconds) caused bronchoconstriction. The precise mechanism underlying these observations is uncertain but seems to be related to intrinsic properties within the contracted airway smooth muscle.

HRCT imaging of airway responsiveness: effects of anesthetics

The noninvasive imaging method, high resolution computed tomography (HRCT), has been developed in animal models and applied to humans with obstructive lung disease for assessing regional and individual airway responsiveness. The ability to directly view airway responses during provocations such as tracheal intubation in an asthmatic could greatly enhance our understanding and treatment of airway hyperresponsiveness. HRCT uses increased kilovoltage peak (kVp) and milliamperage (mAs) settings, thin slices, high spacial frequency reconstruction algorithms, and small fields of view to resolve structures as small as 200 microm. Therefore, airways as small as 1-2 mm in diameter can be viewed and measured. HRCT is a more sensitive technique for resolving airway caliber changes than clinical or research methods of pulmonary function tests. HRCT allows direct in vivo measurement of airway responsiveness to pharmacological and physiological stress that induces bronchoconstriction or bronchodilation. Using HRCT, we are able to measure airway dilation at baseline airway tone with inhalation anesthetics, differentiate the bronchodilating properties of inhalational agents in airways with tone, assess bronchodilating agents commonly used as premedications prior to anesthesia, and measure airway heterogeneity at baseline tone and their response to a variety of stimuli. This ability of HRCT to measure airway caliber and response heterogeneity in vivo noninvasively will dramatically improve our understanding of pulmonary physiology in general and the effects of anesthetics on the airways specifically.

Quantitative assessment of airway remodeling using high-resolution CT

Asthma and COPD are the most prevalent of lung diseases and contribute an enormous burden of morbidity in North America and globally. In both conditions, inflammation leads to airway remodeling, which contributes to airway narrowing. To date, airway remodeling has only been assessed using histological examination of airways. However, it may now be possible to assess and quantify the extent of airway remodeling in vivo using high-resolution CT (HRCT). The aim of this article is to review the use of HRCT in the investigation of airway remodeling. A number of investigators have reported techniques to make measurements of airway dimensions using CT and an increasing number of quantitative methods are being developed. Using these techniques, airway dimensions have been examined in patients with asthma and COPD. In patients with asthma, the airway wall area was increased without a decrease in luminal area, whereas in patients with COPD, the airway luminal area was decreased and airway wall area was increased. The different pattern of remodeling may reflect fundamental differences in the inflammatory processes in asthma and COPD and could influence the reversibility of the narrowing. It has also been shown that, by quantifying both the extent of emphysema and of airway remodeling, CT is useful in differentiating COPD patients who have primarily parenchymal disease from those who have primarily airway pathology. With additional advances in technology, it is likely that quantitative assessment of airway wall dimensions will ultimately provide a valuable tool for the study of airway disease.

Three-dimensional human airway segmentation methods for clinical virtual bronchoscopy

RATIONALE AND OBJECTIVES

The segmentation of airways from CT images is a critical first step for numerous virtual bronchoscopic (VB) applications. Automatic or semiautomatic methods are necessary, since manual segmentation is prohibitively time consuming. The methods must be robust and operate within a reasonable time frame to be useful for clinical VB use. The authors developed an integrated airway segmentation system and demonstrated its effectiveness on a series of human images.

MATERIALS AND METHODS

The authors' airway segmentation system draws on two segmentation algorithms: (a) an adaptive region-growing algorithm and (b) a new hybrid algorithm that uses both region growing and mathematical morphology. Images from an ongoing VB study were segmented by means of both the adaptive region-growing and the new hybrid methods. The segmentation volume, branch number estimate, and segmentation quality were determined for each case.

RESULTS

The results demonstrate the need for an integrated segmentation system, since no single method is superior for all clinically relevant cases. The region-growing algorithm is the fastest and provides acceptable segmentations for most VB applications, but the hybrid method provides superior airway edge localization, making it better suited for quantitative applications. In addition, the authors show that prefiltering the image data before airway segmentation increases the robustness of both region-growing and hybrid methods.

CONCLUSION

The combination of these two algorithms with the prefiltering options allowed the successful segmentation of all test images. The times required for all segmentations were acceptable, and the results were suitable for the authors' VB application needs.

Assessment of bronchial wall thickening on posteroanterior chest radiographs in acute asthma

A central bronchus that is readily visible end-on in approximately 50% of normal frontal chest radiographs is the bronchus to the anterior segment of either upper lobe. Bronchial wall thickening, or "cuffing," is considered to be a radiographic sign of an asthmatic exacerbation and is cited as a useful sign in a number of leading textbooks; however, to the authors' knowledge, no prior chest radiographic study has quantitatively assessed this specific sign in a population of asthmatics suffering an acute exacerbation. Posterior chest radiographs were reviewed retrospectively for 51 nonasthmatic, nonsmoking control subjects and for 45 adult asthmatic subjects during an acute exacerbation of moderate to severe asthma. Readers were blinded as to whether the radiograph was from an asthmatic or control subject. If visible end-on, the bronchus to the anterior segment of either upper lobe was assessed by measuring the diameter of the lumen and the thickness of the bronchial wall. At least one clearly defined bronchus to the anterior segment of an upper lobe was visible end-on in 22 patients (43%) in the control group and in 21 patients (47%) in the asthma group (p = NS). Mean wall thickness was 0.7 +/- 0.1 mm in the control group and 0.8 +/- 0.1 mm in the asthma group (p = 0.04). Lumen/wall thickness was 3.1 +/- 0.2 (SEM) in the control group and 2.5 +/- 0.2 in the asthma group (p = 0.055). The presence of bronchial wall thickness does not reliably distinguish radiographs of acutely asthmatic from normal individuals.

High resolution computed tomographic assessment of airway wall thickness in chronic asthma: reproducibility and relationship with lung function and severity

BACKGROUND

In some patients chronic asthma results in irreversible airflow obstruction. High resolution computed tomography (HRCT) has been advocated for assessing the structural changes in the asthmatic lung and permits investigation of the relationships between airway wall thickening and clinical parameters in this condition.

METHODS

High resolution CT scanning was performed in 49 optimally controlled asthmatic patients and measurements of total airway and lumen diameter were made by two independent radiologists using electronic callipers. Wall area as % total airway cross sectional area (WA%) and wall thickness to airway diameter ratio (T/D) were calculated for all airways clearly visualised with a transverse diameter of more than 1.5 mm, with a mean value derived for each patient. Intra- and inter-observer variability was assessed for scope of agreement in a subgroup of patients. Measurements were related to optimum forced expiratory volume in 1 second (FEV1), forced mid expiratory flow, carbon monoxide gas transfer, two scores of asthma severity, airway inflammation as assessed with induced sputum, and exhaled nitric oxide.

RESULTS

Neither observer produced a statistically significant difference between measurements performed on two occasions but there was a significant difference between observers (limits of agreement -2.6 to 6.8 for WA%, p<0.0001). However, mean WA% measured on two occasions differed by no more than 5.4% (limits of agreement -4.0 to 5.4; mean (SD) 0.7 (2.4)). Statistically significant positive associations were observed between both WA% and T/D ratio and asthma severity (r(S)=0.29 and 0.30, respectively, for ATS score), and an inverse association with gas transfer coefficient was observed (r(S)=-0.43 for WA% and r(S)=-0.41 for T/D). No association was identified with FEV1 or airway inflammation.

CONCLUSIONS

The airway wall is thickened in more severe asthma and is associated with gas transfer coefficient. This thickening does not relate directly to irreversible airflow obstruction as measured with FEV1.

Subsecond multisection CT of regional pulmonary ventilation

RATIONALE AND OBJECTIVES

To evaluate the adequacy of multibreath and single-breath stable xenon gas techniques to measure regional ventilation during cardiac-gated, high-speed, multisection imaging, the authors carried out a series of studies using electron-beam computed tomography (CT) and a recently introduced subsecond multisection spiral CT scanner.

MATERIALS AND METHODS

In four anesthetized pigs, the authors implemented single-breath and/or dynamic multibreath wash-in and washout protocols with respiratory-- and cardiac-gated image acquisition. The effects of varying tidal volume and inspiratory flow rate were evaluated independently. Scanning was done at end expiration to avoid artifacts from partial volumed conducting airways, which are filled with inspired gas concentration during inspiration.

RESULTS

A single breath of 100% xenon provides adequate enhancement in the lung parenchyma (mean, 32 HU +/- 1.85 [standard error]) and should not cause unwanted side effects (mean xenon concentration in lung periphery, 21%). The single-breath method is suitable for studies requiring only short periods of apnea. Using the multibreath method, in dependent portions of the lung, there was close agreement between measured changes and predictions based on the xenon calibration data. More than 10 breaths were needed to clear tracer from poorly ventilated areas, and some nondependent regions demonstrated apparently "linear" rather than exponential clearance curves, possibly reflecting longer washout times. Analysis of wash-in and washout curves revealed vertical ventilation gradients and, at higher inspiratory flow rates, redistribution of ventilation to areas of the lung with greater pathway conductance.

CONCLUSION

With careful attention to lung volume and use of cardiac gating, it is now possible to correlate lung structure with function to a degree heretofore not possible.

Decreased pulmonary vascular permeability in aquaporin-1-null humans

The molecular determinants of water permeability in the human lung are incompletely defined. Aquaporins (AQP) are water-specific membrane channel proteins. AQP1 is present in endothelial cells in the lung, including those in the vascular plexus around the airways. Rare individuals have been identified who are deficient in AQP1. High-resolution computed tomography scans of the lung were used to evaluate the response to i.v. fluid challenge in two unrelated AQP1-null individuals and five normal controls. The airways and pulmonary vessels were measured at baseline and after i.v. administration of 3 liters of saline. Increases in airway wall thickness after fluid administration reflect peribronchiolar edema formation. Both control and AQP1 null subjects had approximately a 20% increase in pulmonary vessel area in response to saline infusion, suggesting similar degrees of volume loading. Control subjects had a 44% increase in the thickness of the airway wall, consistent with peribronchiolar edema formation. In marked contrast, airway wall thickness did not change in AQP1-null subjects in response to saline infusion. These studies indicate that AQP1 is a determinant of vascular permeability in the lung, and demonstrate a role for aquaporins in human pulmonary physiology.

Airway response to deep inspiration: role of inflation pressure

Deep inspirations (DIs) have been shown to have both bronchoprotective and bronchodilator effects in healthy subjects; however, the bronchodilator effects of a DI appear to be impaired in asthmatic compared with healthy subjects. Because the ability to generate high transpulmonary pressures at total lung capacity depends on both the lung properties and voluntary effort, we wondered how the response of airways to DI might be altered if the maneuver were done with less than maximal inflation. The present work was undertaken to examine the effects of varying the magnitude of lung inflation during the DI maneuver on subsequent airway caliber. In five anesthetized and ventilated dogs during methacholine infusion, changes in airway size after DIs of increasing magnitude were measured over the subsequent 5-min period using high-resolution computed tomography. Results show that the magnitude of lung inflation is extremely important, leading to a qualitative change in the airway response. A large DI (45 cmH(2)O airway pressure) caused subsequent airway dilation, whereas smaller DIs (< or =35 cmH(2)O) caused bronchoconstriction. The precise mechanism underlying these observations is uncertain, but it seems to be related to an interaction between intrinsic properties of the contracted airway smooth muscle and the response to mild stretch.

Effects of tidal volume stretch on airway constriction in vivo

Tidal stresses are thought to be involved in maintaining airway patency in vivo. The present study examined the effects of normal stresses exerted by the lung parenchyma during tidal ventilation on recovery from agonist-induced airway constriction. In seven anesthetized dogs, one lung was selectively ventilated with a Univent endotracheal tube (Vitaid, Lewiston, NY). Airway tone was increased either transiently (intravenous bolus) or continuously (intravenous infusion) with methacholine (MCh). During one-lung ventilation, changes in the airway size of both lungs were measured for up to 40 min during recovery from constriction by using high-resolution computed tomography. After recovery to baseline, the alternate lung was ventilated, and the protocol was repeated. The absence of tidal stresses led to an attenuated recovery from either transient or steady-state airway constriction. The effectiveness or lack thereof of normal tidal stress in stabilizing airway size may be one factor that contributes to the lack of reversal with tidal breathing and deep inspiration seen in asthmatic subjects.

Airway stability and heterogeneity in the constricted lung

The effect of bronchoconstriction on airway resistance is known to be spatially heterogeneous and dependent on tidal volume. We present a model of a single terminal airway that explains these features. The model describes a feedback between flow and airway resistance mediated by parenchymal interdependence and the mechanics of activated smooth muscle. The pressure-tidal volume relationship for a constricted terminal airway is computed and shown to be sigmoidal. Constricted terminal airways are predicted to have two stable states: one effectively open and one nearly closed. We argue that the heterogeneity of whole lung constriction is a consequence of this behavior. Airways are partitioned between the two states to accommodate total flow, and changes in tidal volume and end-expiratory pressure affect the number of airways in each state. Quantitative predictions for whole lung resistance and elastance agree with data from previously published studies on lung impedance.

Methacholine-induced temporal changes in airway geometry and lung density by CT

PURPOSE

Electron-beam CT (EBCT) was utilized to assess the time course of changes in airways cross-sectional area (CSA) and lung density during methacholine-induced bronchoconstriction.

MATERIALS AND METHODS

EBCT scans (200 ms, 3-mm thickness, 2 mm increments) were obtained before (baseline) and 30 s, 2 min, and 4 min after bolus IV injection of methacholine to pigs receiving mechanical ventilation. A total of seven experiments were analyzed using custom-made image analysis software. With each challenge, five different airways and 50 lung regions of interest were studied.

RESULTS

The time course of lung density changes paralleled the time course for CSA changes. The maximal response to methacholine, measured in terms of both CSA and lung density changes, occurred 30 s after injection. Lung density changes were unaffected by reconstruction algorithm, normal (standard) or sharp (high resolution). Overall, there was increased air content in the lung during bronchoconstriction. This effect was significantly greater at the dependent lung regions.

CONCLUSIONS

EBCT is an effective tool to assess temporal and regional changes in the lung during bronchoconstriction. Measurements of lung density during bronchoconstriction allow for assessment of peripheral changes that are beyond the CT spatial resolution of airways anatomy.

Automatic lung segmentation for accurate quantitation of volumetric X-ray CT images

Segmentation of pulmonary X-ray computed tomography (CT) images is a precursor to most pulmonary image analysis applications. This paper presents a fully automatic method for identifying the lungs in three-dimensional (3-D) pulmonary X-ray CT images. The method has three main steps. First, the lung region is extracted from the CT images by gray-level thresholding. Then, the left and right lungs are separated by identifying the anterior and posterior junctions by dynamic programming. Finally, a sequence of morphological operations is used to smooth the irregular boundary along the mediastinum in order to obtain results consistent with those obtained by manual analysis, in which only the most central pulmonary arteries are excluded from the lung region. The method has been tested by processing 3-D CT data sets from eight normal subjects, each imaged three times at biweekly intervals with lungs at 90% vital capacity. We present results by comparing our automatic method to manually traced borders from two image analysts. Averaged over all volumes, the root mean square difference between the computer and human analysis is 0.8 pixels (0.54 mm). The mean intrasubject change in tissue content over the three scans was 2.75% +/- 2.29% (mean +/- standard deviation).

High-resolution computed tomographic evaluation of airway distensibility and the effects of lung inflation on airway caliber in healthy subjects and individuals with asthma

The effects of a deep inspiration (DI) in individuals with asthma differ from those observed in healthy subjects. It has been postulated that the beneficial effect of lung inflation is mediated by airway stretch. One hypothesis to explain the defects in the function of lung inflation in asthma is that a DI may be unable to stretch the airways. This may result from attenuation of the tethering forces between the airways and the surrounding parenchyma. In the current study, we used high-resolution computed tomography (HRCT) to examine the ability of a DI to distend the airways of subjects with asthma (n = 10) compared with healthy subjects (n = 9) at baseline and after increasing airway tone with methacholine (MCh). We found that both at baseline and after the induction of smooth muscle tone with MCh, a DI distended the airways of healthy and asthmatic subjects to a similar extent, indicating that abnormal interdependence between the lung parenchyma and the airways is unlikely to play a major role in the loss or attenuation of the beneficial effect of lung inflation that characterizes asthma. Furthermore, we observed that after constriction had already been induced by MCh, following a DI, bronchodilation occurred in the healthy subjects but further bronchoconstriction occurred in the subjects with asthma. Our findings suggest that an abnormal excitation contraction mechanism in the airway smooth muscle of subjects with mild asthma counteracts the bronchodilatory effect of a DI. Therefore, the mechanism for reduced bronchodilation after DIs in subjects with mild asthma could be intrinsic to the airway smooth muscle.

Delayed distension of contracted airways with lung inflation in vivo

A deep inspiratory sigh is one of the most severe dynamic stresses that lungs normally experience. It typically is a very transient phenomenon, normally lasting only about 2 to 3 s. The airway response to a deep inspiration has been shown to be different in asthmatic and normal individuals. When airway smooth muscle (ASM) is contracted in normal subjects, a deep inspiration results in a subsequent dilation of the airways. However, in asthmatic subjects, a deep inspiration often results in little change in airway function, and sometimes results in an even further contraction of ASM. The mechanism underlying this difference depends on the dynamic behavior of both ASM and the lung parenchyma. If the contracted muscle had slower dynamic responses than the lung parenchyma, the timing of the deep inspiratory maneuver could affect the airway response. In the present study, we designed an experiment to determine how well matched the dynamic response is of airways to that of the lung parenchyma. The results clearly demonstrate that airways contracted with methacholine dilate at about a rate four times slower than that of the lung parenchyma during rapid lung inflation and deflation. This effect may play a role in the unique response of asthmatic subjects to deep inspiration. The mechanism of this dynamic slowness of contracted airways probably involves intrinsic properties of the smooth-muscle contractile processes.

Computed tomographic measurements of airway dimensions and emphysema in smokers. Correlation with lung function

Chronic obstructive pulmonary disease (COPD) is characterized by the presence of airflow obstruction caused by emphysema or airway narrowing, or both. Low attenuation areas (LAA) on computed tomography (CT) have been shown to represent macroscopic or microscopic emphysema, or both. However CT has not been used to quantify the airway abnormalities in smokers with or without airflow obstruction. In this study, we used CT to evaluate both emphysema and airway wall thickening in 114 smokers. The CT measurements revealed that a decreased FEV(1) (%predicted) is associated with an increase of airway wall area and an increase of emphysema. Although both airway wall thickening and emphysema (LAA) correlated with measurements of lung function, stepwise multiple regression analysis showed that the combination of airway and emphysema measurements improved the estimate of pulmonary function test abnormalities. We conclude that both CT measurements of airway dimensions and emphysema are useful and complementary in the evaluation of the lung of smokers.

Airway closure with high PEEP in vivo

When airway smooth muscle is contracted in vitro, the airway lumen continues to narrow with increasing concentrations of agonist until complete airway closure occurs. Although there remains some controversy regarding whether airways can close in vivo, recent work has clearly demonstrated that, if the airway is sufficiently stimulated with contractile agonists, complete closure of even large cartilaginous conducting airways can readily occur with the lung at functional residual capacity (Brown RH and Mitzner W. J Appl Physiol 85: 2012-2017, 1998). This result suggests that the tethering of airways in situ by parenchymal attachments is small at functional residual capacity. However, at lung volumes above functional residual capacity, the outward tethering of airways should increase, because both the parenchymal shear modulus and tethering forces increase in proportion to the transpulmonary pressure. In the present study, we tested whether we could prevent airway closure in vivo by increasing lung volume with positive end-expiratory pressure (PEEP). Airway smooth muscle was stimulated with increasing methacholine doses delivered directly to airway smooth muscle at three levels of PEEP (0, 6, and 10 cmH(2)O). Our results show that increased lung volume shifted the airway methacholine dose-response curve to the right, but, in many airways in most animals, airway closure still occurred even at the highest levels of PEEP.

Potential mechanism of hyperresponsive airways

It has been known for many years that the response of asthmatic subjects to a deep inspiration differs from that observed in normal healthy subjects. A deep inspiration causes a decrease in airway resistance in normal subjects, whereas asthmatics demonstrate either no change or a slight increase in airway resistance. It has been suggested by several investigators that the inability to dilate airways during lung inflation may be a primary defect in asthma. One study (Skloot and colleagues, J. Clin. Invest. 1995;96:2393- 2403) showed that in the absence of a deep inspiration during methacholine (MCh) challenge, normal subjects had a greatly exaggerated and sustained response to this agonist. It was suggested that asthmatic airways could be modeled by this condition in normal, subjects. Other investigators, however, suggest that there are more intrinsic differences between the responses to lung inflation in airways from asthmatic and normal subjects (Brusasco and colleagues, J. Appl. Physiol. 1999;87:567-573). Resolution of this controversy requires the ability to assess the responses of airways directly, but unfortunately conventional pulmonary function tests in human subjects are not specific enough to allow this evaluation. In the present study, we have performed experiments using a direct imaging approach that allows us to obtain measurements of airway and parenchymal dimensions that can be used to test the responses of individual airways to deep inspiration in vivo. Our results show that the presence of normal tidal stresses allows airway smooth muscle to respond normally to deep inspirations. Removing tidal stresses at FRC after MCh challenge is sufficient to change the normal dilatory response to deep inspiration into an abnormal one of contraction. Altered sensitivity of airway smooth muscle to normal tidal stresses thus may be operative in the development of the asthmatic pathology.

Airway narrowing in healthy humans inhaling methacholine without deep inspirations demonstrated by HRCT

Normal subjects prevented from taking a deep breath show changes in airflow similar to those of asthmatics when challenged with methacholine (MCh). To confirm airway narrowing by MCh in this setting and to determine its location, we concurrently measured changes in airway lumenal area using high resolution computed tomography (HRCT) and airflow using partial spirometry in five normal subjects challenged with increasing doses of MCh under prohibition of deep breaths. In an attempt to improve imaging accuracy, we corrected for the changes in lung volume during bronchoprovocation. At every step of the provocation, scanning was performed at approximately the same lung volume. On the HRCT images, airway area decreased in response to the increasing doses of MCh to 91 +/- 2%, 88 +/- 2%, and 80 +/- 2% of baseline at the doses of MCh 0.25, 0.75, and 2.5 mg/ml, respectively (p < 0.001). Airway narrowing showed no predilection for particular airway sizes and occurred in a heterogeneous pattern. The changes in the mean airway lumenal area as measured by HRCT and the mean partial spirometric outcomes were highly correlated: FEV(1)p (r(2) = 0.46, p = 0.001), FVCp (r(2) = 0.20, p = 0.05), FEV(1)/FVCp (r(2) = 0.55, p = 0.002), MMEFp (r(2) = 0.31, p = 0.01), and taup (r(2) = 0.51, p = 0.0004). We conclude that in normal subjects who are prevented from taking a deep breath, the spirometric changes occurring with aerosol MCh challenge are associated with conducting airway narrowing.

An analysis algorithm for measuring airway lumen and wall areas from high-resolution computed tomographic data

High-resolution computed tomography (HRCT) has been used to examine airway narrowing. We developed an automated computed tomographic image analysis algorithm (computed tomographic airway morphometry; CTAM) to measure airway lumen area (Ai ), airway wall area (Awa), and airway angle of orientation. Tubes of varying size were embedded in Styrofoam and then scanned at angles between 0 degrees and 50 degrees to assess the accuracy of measurements made with CTAM. Two excised pig lungs were fixed in inflation, sectioned, and scanned. Ai and Awa were measured planimetrically from the cut surfaces to optimize CTAM measurement parameters. In CTAM, Ai was defined according to an airway-size-dependent threshold value, and total Awa was determined through a score-guided erosion method. Results were compared with measurements made through a previously validated method (manual method). CTAM provided accurate measurements of the tubes' Ai values at all angles; Awa was overestimated in direct relation to airway size. The manual method underestimated Ai and overestimated Awa in a manner directly related to airway size as well as to airway angle of orientation. In the excised lung, the mean errors of Ai and Awa measurements made with CTAM were 0.52 +/- 0.24 mm(2) and 0.17 +/- 0.32 mm(2) (mean +/- SEM), respectively. Ai errors with the manual method were similar, but Awa was overestimated to a greater degree (6.3 +/- 0.38 mm(2); p < 0.01) and the error was proportional to Awa (r = 0.64; p < 0.01). CTAM allows accurate measurements of airway dimensions and angle of orientation.

Airway reactivity to bronchoconstrictor and bronchodilator: assessment using thin-section and volumetric three-dimensional CT

OBJECTIVE

To determine the extent to which thin-section and volumetric threedimensional CT can depict airway reactivity to bronchostimulator, and to assess the effect of different airway sizes on the degree of reactivity.

MATERIALS AND METHODS

In eight dogs, thin-section CT scans were obtained before and after the administration of methacholine and ventolin. Cross-sectional areas of bronchi at multiple levels, as shown by axial CT, proximal airway volume as revealed by three-dimensional imaging, and peak airway pressure were measured. The significance of airway change induced by methacholine and ventolin, expressed by percentage changes in cross-sectional area, proximal airway volume, and peak airway pressure was statistically evaluated, as was correlation between the degree of airway reactivity and the area of airways.

RESULTS

Cross-sectional areas of the bronchi decreased significantly after the administration of methacholine, and scans obtained after a delay of 5 minutes showed that normalization was insufficient. Ventolin induced a significant increase in cross-sectional areas and an increase in proximal airway volume, while the effect of methacholine on the latter was the opposite. Peak airway pressure increased after the administration of methacholine, and after a 5-minute delay its level was near that of the control state. Ventolin, however, induced no significant decrease. The degree of airway reactivity did not correlate with airway size.

CONCLUSION

Thin-section and volumetric spiral CT with three-dimensional reconstruction can demonstrate airway reactivity to bronchostimulator. The degree of reactivity did not correlate with airway size.

Imaging the airways. Hemoptysis, bronchiectasis, and small airways disease

Advances in technology have increased the contribution of radiology in understanding and evaluating diseases of the airways. In patients with hemoptysis, CT is now established as a complementary technique to bronchoscopy, or as an alternative to bronchoscopy in selected cases. The introduction of high-resolution CT has improved the detection and assessment of bronchiectasis and small airways disease, allowed better correlation between pathologic changes and radiologic appearances, and provided new insights into possible links between small airways disease and bronchial disease.

Site of functional bronchopulmonary anastomoses in sheep

The location of bronchopulmonary anastomoses has long been a topic of discussion, and pre-, post-, and capillary sites have all been demonstrated in postmortem examinations. However, there have been few studies that have provided insight into the patency and function of these anastomoses in the intact lung. To identify these functional sites where the bronchial circulation anastomoses with the pulmonary circulation, we studied sheep lungs in situ serial sectioned with high-resolution computed tomography (CT). Differences in radiodensities of blood, air, and nonionic contrast medium were used to differentiate and localize airways and vessels and to identify the effluent from the bronchial circulation. After an initial series of scans to identify the pulmonary arteries and veins adjacent to airways 2-12 mm in diameter, contrast material was infused into the bronchial artery. In three sheep, the major accumulation of contrast medium was found in pulmonary veins. In one of the sheep, a comparable number of pulmonary arteries and veins contained contrast medium. Serial histologic sections were able to identify small bronchial venules lying within subepithelial bronchial folds that drain directly into pulmonary veins. These results using serial CT and histologic images suggest that drainage from the intraparenchymal bronchial vasculature is predominantly into postcapillary pulmonary vessels.

The myth of maximal airway responsiveness in vivo

A sine qua non of hyperresponsive airway disease in asthmatic subjects is the lack of a maximal response with increasing doses of aerosol agonist challenge. Normal subjects, however, often appear to exhibit an airway response plateau effect even when challenged with high concentrations of agonist. To investigate this question of maximal narrowing in individual airways in vivo, we used high-resolution computed tomography to visualize canine airways narrowed by two routes of agonist challenge. We compared airway narrowing induced by methacholine (MCh) via the conventional aerosol route to that caused by local atomization of MCh directly to individual airways. Our results showed that, with aerosol challenge, airway responses never reached a truly flat plateau even at the highest possible nebulizer concentrations. Airway closure was never observed. However, when MCh was delivered directly to the airway luminal surface, airways could be easily narrowed to complete closure at modest (10 mg/ml) agonist concentrations. Thus neither the elastic recoil of the lung nor limitations of smooth muscle shortening can be responsible for the apparent plateauing of dose-response curves. We suggest that the plateau results from limitations associated with the delivery of high concentration of agonists via the aerosol route.

Segmentation of intrathoracic airway trees: a fuzzy logic approach

Three-dimensional (3-D) analysis of airway trees extracted from computed tomography (CT) image data can provide objective information about lung structure and function. However, manual analysis of 3-D lung CT images is tedious, time consuming and, thus, impractical for routine clinical care. We have previously reported an automated rule-based method for extraction of airway trees from 3-D CT images using a priori knowledge about airway-tree anatomy. Although the method's sensitivity was quite good, its specificity suffered from a large number of falsely detected airways. We present a new approach to airway-tree detection based on fuzzy logic that increases the method's specificity without compromising its sensitivity. The method was validated in 32 CT image slices randomly selected from five volumetric canine electron-beam CT data sets. The fuzzy-logic method significantly outperformed the previously reported rule-based method (p < 0.002).

Individual canine airways responsiveness to aerosol histamine and methacholine in vivo

Inhalational challenges to histamine and methacholine (Mch) are commonly used to measure airway reactivity. In this study, we investigated the serial distribution of individual airway responsiveness in vivo following stimulation with histamine or methacholine by directly measuring airway area with high resolution computed tomography (HRCT). Anesthetized dogs were challenged on separate days to increasing aerosol concentrations of histamine or Mch. Airway areas of 13 or 14 individual airways in each dog ranging in size from 1.1 to 16.9 mm in diameter at baseline were measured. Dose response curves for each agonist showed a similar responsiveness to each agonist over all airway sizes measured. There was a slightly greater individual airway responsiveness to Mch throughout the airway tree, and this greater responsiveness was also present in those airways that are responsible for the decreased Cdyn observed with Mch challenge. We conclude that in vivo aerosol challenges with histamine or methacholine do not lead to preferential construction of large or small airways.

New techniques and developments in physiologic imaging of airways

Recently, noninvasive imaging methods for assessing regional and individual airway responsiveness have been developed in animal models and applied to humans with obstructive lung disease. This article examines the technique of high-resolution CT scan and MR imaging and their applications to the imaging of airways.