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

Low attenuation area is associated with airflow limitation and airway hyperresponsiveness

BACKGROUND

Asthma is a chronic inflammatory disorder of the airways characterized by airflow limitation and airway hyperresponsiveness. Lung density indices on quantitative computed tomography (QCT) are assumed to reflect the degree of air trapping originated from airflow limitation in airway diseases.

PURPOSE

The present study investigated the availability of lung density indices on QCT in clinical evaluation of asthma.

METHODS

Eleven asthmatic patients and 48 healthy control subjects were prospectively evaluated by QCT, pulmonary function testing, and a methacholine challenge test. High-resolution computed tomography scans were performed at full-inspiratory and full-expiratory phases, and percentage of lung field occupied by low attenuation area (LAA%) and mean lung density (MLD) at both inspiratory and expiratory phases were measured.

RESULTS

MLD values at inspiratory phase were significantly increased in asthmatic patients compared with those in healthy control subjects. Inspiratory LAA% values were significantly decreased in asthmatics compared with the values in control subjects. On expiratory scans, MLD values of asthmatics were significantly lower than the values of control subjects. Expiratory LAA% values of asthmatics were significantly higher than the values of control subjects. The LAA% in the expiratory phase showed significant negative correlation with forced expiratory volume in 1 second (FEV(1)), FEV(1)/forced vital capacity, and the provocative dose of methacholine causing a 20% decrease in FEV(1) in asthmatic patients.

CONCLUSION

These results suggest that lung density indices on QCT may be useful for clinical evaluation of asthmatic patients and increased LAA% in the expiratory phase is associated with airflow limitation and airway hyperresponsiveness in asthma.

Rapid and efficient mapping of regional ventilation in the rat lung using hyperpolarized 3He with Flip Angle Variation for Offset of RF and Relaxation (FAVOR)

A novel imaging method is presented, Flip Angle Variation for Offset of RF and Relaxation (FAVOR), for rapid and efficient measurement of rat lung ventilation using hyperpolarized helium-3 (3He) gas. The FAVOR technique utilizes variable flip angles to remove the cumulative effect of RF pulses and T1 relaxation on the hyperpolarized gas signal and thereby eliminates the need for intervening air wash-out breaths and multiple cycles of 3He wash-in breaths before each image. The former allows an improvement in speed (by a factor of approximately 30) while the latter reduces the cost of each measurement (by a factor of approximately 5). The FAVOR and conventional ventilation methods were performed on six healthy male Brown Norway rats (190-270 g). Lobar measurements of ventilation, r, obtained with the FAVOR method were not significantly different from those obtained with the conventional method for the right middle and caudal and left lobes (P>0.05 by a Wilcoxon matched pairs test). A methacholine challenge test was also administered to an animal and reduction and recovery of r was detected by the FAVOR method. The reduced 3He consumption and the improvement in speed provided by FAVOR suggest that it may allow measurement of ventilation in human subjects not previously possible.

Localization and quantification of regional and segmental air trapping in asthma

OBJECTIVE

To assess the feasibility of volumetric and densitometric software to localize and quantify signs of regional air trapping after methacholine bronchoprovocations in asthma.

METHODS

Eight atopic subjects with mild-to-moderate asthma using short-acting beta2-agonists only, with hyperresponsiveness to methacholine, were evaluated. Low-dose baseline expiratory 16-slice multidetector computed tomography scans before and after a methacholine bronchoprovocation were acquired. MeVisPULMO3D software (Bremen, Germany) was applied to the scans, providing quantitative information on volume and density measures of the total lung and each lobe.

RESULTS

After methacholine, the expiratory scan showed a median (interquartile range) increase in volume of 534 mL (357-1279 mL), a decrease in lung density (mean and 15th percentile) of 52 Hounsfield Units (HU) (116-39 HU) and 34 HU (78-25 HU), respectively, and an increase in percentage low attenuation areas of 3% (2%-6%) for the total lung, with similar patterns in individual lung lobes. The right and left lower lung lobes showed the largest increases in air trapping, 211 mL (117-363 mL) and 229 mL (155-315 mL), respectively, versus a volume increase of 70 mL (20-249 mL), 26 mL (-16-92 mL), and 91 mL (-28-241 mL) for the right upper, middle, and left upper lobes, respectively. Volume changes in the lower lobes were associated with baseline forced expiratory flow between 25% and 75% of forced vital capacity, whereas low attenuation areas changes in the lower lobes were not.

CONCLUSIONS

This study suggests that multidetector computed tomography scans are able to localize and quantify regional air trapping in asthma after methacholine bronchoprovocations. Volumetric measurements of the lobes as compared to densitometric measurements are superior in detecting local air trapping in gravity-dependent areas of the lung.

MDCT assessment of airway wall thickness in COPD patients using a new method: correlations with pulmonary function tests

Quantitative assessment of airway-wall dimensions by computed tomography (CT) has proven to be a marker of airway-wall remodelling in chronic obstructive pulmonary disease (COPD) patients. The objective was to correlate the wall thickness of large and small airways with functional parameters of airflow obstruction in COPD patients on multi-detector (MD) CT images using a new quantification procedure from a three-dimensional (3D) approach of the bronchial tree. In 31 patients (smokers/COPD, non-smokers/controls), we quantitatively assessed contiguous MDCT cross-sections reconstructed orthogonally along the airway axis, taking the point-spread function into account to circumvent over-estimation. Wall thickness and wall percentage were measured and the per-patient mean/median correlated with FEV1 and FEV1%. A median of 619 orthogonal airway locations was assessed per patient. Mean wall percentage/mean wall thickness/median wall thickness in non-smokers (29.6%/0.69 mm/0.37 mm) was significantly different from the COPD group (38.9%/0.83 mm/0.54 mm). Correlation coefficients (r) between FEV1 or FEV1% predicted and intra-individual means of the wall percentage were -0.569 and -0.560, respectively, with p < 0.001. Depending on the parameter, they were increased for airways of 4 mm and smaller in total diameter, being -0.621 (FEV1) and -0.537 (FEV1%) with p < 0.002. The wall thickness was significantly higher in smokers than in non-smokers. In COPD patients, the wall thickness measured as a mean for a given patient correlated with the values of FEV1 and FEV1% predicted. Correlation with FEV1 was higher when only small airways were considered.

Electron beam CT scan is a valuable and safe imaging tool for the pediatric surgical patient

PURPOSE

Electron beam computed tomography (EBCT) is a relatively new technology that has been used primarily to detect coronary artery calcification in adult patients. EBCT has several potential advantages over traditional CT: (1) fast acquisition times resulting in less need for sedation, (2) decreased radiation exposure, and (3) robust software enabling real-time interactive 3-dimensional visualization of anatomic relationships. In this series of case reports, the authors describe their initial experience with the use of EBCT in pediatric noncardiac imaging.

METHODS

Children with a variety of thoracic and abdominal disease processes were evaluated by EBCT.

RESULTS

All patients underwent EBCT scanning without the need for sedation, and 3-dimensional images of the data sets were rendered in minutes after their acquisition. The diagnostic images provided equivalent spatial resolution to the multislice CT scanner but without motion artifacts and lower radiation exposure.

CONCLUSIONS

EBCT is a safe, effective, and user- and patient-friendly alternative to traditional CT in the care of pediatric surgical diseases. The diagnostic yield of EBCT will continue to improve with new technologic advances and clinical experience.

Dynamic lung morphology of methacholine-induced heterogeneous bronchoconstriction

Hyperpolarized (HP) 3helium (3He) dynamic MRI was used to investigate airway response in rats following intravenous (i.v.) bolus administration of a contractile agent, methacholine (MCh). The method provides direct visualization of the ventilated regions within the lung. Heterogeneous bronchoconstriction following the i.v. MCh injection was evident using this technique. These 3He dynamic lung images revealed that the inspired fresh air was shunted to the less-constricted regions after the MCh challenge in a similar manner as described by Laplace's relationship for the stability between adjacent alveoli. The airways in the more-constricted regions became nearly closed, resulting in air trapping, while the airways in the less-constricted regions remained effectively open, leading to overinflation. These data suggest a lung model of airway constriction partitioned into ventilated and nonventilated regions. These nonventilated regions are heterogeneously distributed in the lung and this distribution cannot be deduced from spirometric measurement of the whole lung. We demonstrate that a combination of functional 3He images and anatomical 1H images provide an effective method to diagnose regional lung abnormalities in rats.

Functional imaging of airway narrowing

The report will focus on studies that illustrate how high resolution computed tomography can be used to provide new insights into airway and lung function, that cannot be obtained with any other methodology in humans or animal models. In one series of experiments, we have clearly demonstrated that even large cartilaginous airways are capable of complete closure in vivo. These unequivocal in vivo results invalidate the ubiquitous concept that there is a limit to airway narrowing in normal subjects. In another series of experiments, we have investigated potential reasons why asthmatic subjects might show airway constriction following deep inspiration instead of the normal dilation. Experimental results show that a constrictor response to deep inspiration can be generated in normal airways simply by minimizing tidal stresses. The absence of these normal rhythmic stresses alters the smooth muscle throughout the airway tree, such that subsequent large stresses lead to a further constriction. These results also offer a possible mechanism by which the response to deep inspiration is altered in asthmatic subjects. By allowing accurate measurement of the size of individual airways, computed tomography with modern commercially available scanners thus provides a unique opportunity to evaluate specific hypotheses regarding mechanisms underlying lung disease.

Effects of childhood respiratory diseases on the anatomical and functional development of the respiratory system

The anatomical and functional development of the lung appears especially vulnerable to a whole range of insults during gestation and the first few years of life. A significant proportion of adult lung disease originates in utero or early infancy. Most publications on this topic are descriptive retrospective studies. An important limitation of these is that structural changes may precede abnormalities in lung function and development of symptoms. Little is known with certainty with respect to the long-term effects of early insults to the respiratory system. Furthermore, the reversibility of the functional and/or structural defects is hardly ever adequately investigated and it is probably not correct to extrapolate findings from adult studies to paediatric pulmonary diseases. Promoting or facilitating optimal lung growth in fetuses and infants and reducing the incidence of lower respiratory tract infection in infancy may reduce the incidence of adult chronic lung disease in generations to come.