High-resolution maps of regional ventilation utilizing inhaled fluorescent microspheres

Measuring Anatomical Distributions of Ventilation and Aerosol Deposition with PET-CT

In disease, lung function and structure are heterogeneous, and aerosol transport and local deposition vary significantly among parts of the lung. Understanding such heterogeneity is relevant to aerosol medicine and for quantifying mucociliary clearance from different parts of the lung. In this chapter, we describe positron emission tomography (PET) imaging methods to quantitatively assess the deposition of aerosol and ventilation distribution within the lung. The anatomical information from computed tomography (CT) combined with the PET-deposition data allows estimates of airway surface concentration and peripheral tissue dosing in bronchoconstricted asthmatic subjects. A theoretical framework is formulated to quantify the effects of heterogeneous ventilation, uneven aerosol ventilation distribution in bifurcations, and varying escape from individual airways along a path of the airway tree. The framework is applied to imaging data from bronchoconstricted asthmatics to assess the contributions of these factors to the unevenness in lobar deposition. Results from this analysis show that the heterogeneity of ventilation contributes on average to more than one-third of the variability in interlobar deposition. Actual contribution of ventilation in individual lungs was variable and dependent on the breathing rate used by the subject during aerosol inhalation; the highest contribution was in patients breathing slowly. In subjects breathing faster, contribution of ventilation was reduced, with more expanded lobes showing lower deposition per unit ventilation than less expanded ones in these subjects. The lobar change in expansion measured from two static CT scans, which is commonly used as a surrogate for ventilation, did not correlate with aerosol deposition or with PET-measured ventilation. This suggests that dynamic information is needed to provide proper estimates of ventilation for asthmatic subjects. We hope that the enhanced understanding of the causes of heterogeneity in airway and tissue dosing using the tools presented here will help to optimize therapeutic effectiveness of inhalation therapy while minimizing toxicity.

Combined Assessment of Pulmonary Ventilation and Perfusion with Single-Energy Computed Tomography and Image Processing

RATIONALE AND OBJECTIVES

To establish a proof-of-principle for combined assessment of pulmonary ventilation and perfusion using single-energy computed tomography (CT) and image processing/analysis (denoted as single-energy CT ventilation/perfusion imaging).

MATERIALS AND METHODS

Breath-hold CT scans were acquired at end-expiration and end-inspiration before injection of iodinated contrast agents, and repeated at end-inspiration after contrast injection for 17 canines (8 normal and 9 diseased lung subjects). Ventilation images were calculated with deformable image registration to map the end-expiratory and end-inspiratory CT images and quantitative analysis for regional volume changes as surrogates for ventilation. Perfusion images were calculated by subtracting the end-inspiratory precontrast CT from the deformably registered end-inspiratory postcontrast CT, yielding a map of regional Hounsfield unit enhancement as a surrogate for perfusion. Ventilation-perfusion matching, spatial heterogeneity, and gravitationally directed gradients were compared between two groups using a Wilcoxon rank-sum test.

RESULTS

The normal group had significantly higher Dice similarity coefficients for spatial overlap of segmented functional volumes between ventilation and perfusion (median 0.40 vs. 0.33, p = 0.05), suggesting stronger ventilation-perfusion matching. The normal group also had greater Spearman's correlation coefficients based on 16 regions of interest (median 0.58 vs. 0.40, p = 0.09). The coefficients of variation were comparable (median, ventilation 0.71 vs. 0.91, p = 0.60; perfusion 0.63 vs. 0.75, p = 0.27). The linear regression slopes of gravitationally directed gradient were also comparable for ventilation (median, ventilation -0.26 vs. -0.18, p = 0.19; perfusion -0.17 vs. -0.06, p = 0.11).

CONCLUSION

These findings provide proof-of-principle for single-energy CT ventilation/perfusion imaging.

Using the prone position could help to combat the development of fast hypoxia in some patients with COVID-19

The world is facing an explosive COVID‐19 pandemic. Some cases rapidly develop deteriorating lung function, which causes deep hypoxaemia and requires urgent treatment. Many centres have started treating patients in the prone position, and oxygenation has improved considerably in some cases. Questions have been raised regarding the mechanisms behind this. The mini review provides some insights into the role of supine and prone body positions and summarises the latest understanding of the responsible mechanisms. The scope for discussion is outside the neonatal period and entirely based on experimental and clinical experiences related to adults. The human respiratory system is a complex interplay of many different variables. Therefore, this mini review has prioritised previous and ongoing research to find explanations based on three scientific areas: gravity, lung structure and fractal geometry and vascular regulation. It concludes that gravity is one of the variables responsible for ventilation/perfusion matching but in concert with lung structure and fractal geometry, ventilation and regulation of lung vascular tone. Since ventilation distribution does not change between supine and prone positions, the higher expression of nitric oxide in dorsal lung vessels than in ventral vessels is likely to be the most important mechanism behind enhanced oxygenation in the prone position.

Ventilation/Perfusion Relationships and Gas Exchange: Measurement Approaches

Ventilation-perfusion ( V ˙ A / Q ˙ ) matching, the regional matching of the flow of fresh gas to flow of deoxygenated capillary blood, is the most important mechanism affecting the efficiency of pulmonary gas exchange. This article discusses the measurement of V ˙ A / Q ˙ matching with three broad classes of techniques: (i) those based in gas exchange, such as the multiple inert gas elimination technique (MIGET); (ii) those derived from imaging techniques such as single-photon emission computed tomography (SPECT), positron emission tomography (PET), magnetic resonance imaging (MRI), computed tomography (CT), and electrical impedance tomography (EIT); and (iii) fluorescent and radiolabeled microspheres. The focus is on the physiological basis of these techniques that provide quantitative information for research purposes rather than qualitative measurements that are used clinically. The fundamental equations of pulmonary gas exchange are first reviewed to lay the foundation for the gas exchange techniques and some of the imaging applications. The physiological considerations for each of the techniques along with advantages and disadvantages are briefly discussed. © 2020 American Physiological Society. Compr Physiol 10:1155-1205, 2020.

Gravity outweighs the contribution of structure to passive ventilation-perfusion matching in the supine adult human lung

Gravity and matched airway/vascular tree geometries are both hypothesized to be key contributors to ventilation-perfusion (V̇/Q̇) matching in the lung, but their relative contributions are challenging to quantify experimentally. We used a structure-based model to conduct an analysis of the relative contributions of tissue deformation (the "Slinky" effect), other gravitational mechanisms (weight of blood and gravitational gradient in tissue elastic recoil), and matched airway and arterial tree geometry to V̇/Q̇ matching and therefore to total lung oxygen exchange. Our results showed that the heterogeneity in V̇ and Q̇ were lowest and the correlation between V̇ and Q̇ was highest when the only mechanism for V̇/Q̇ matching was either tissue deformation or matched geometry. Heterogeneity in V̇ and Q̇ was highest and their correlation was poorest when all mechanisms were active (that is, at baseline). Eliminating the contribution of matched geometry did not change the correlation between V̇ and Q̇ at baseline. Despite the much larger heterogeneities in V̇ and Q̇ at baseline, the contribution of in-common (to V̇ and Q̇) gravitational mechanisms provided sufficient compensatory V̇/Q̇ matching to minimize the impact on oxygen transfer. In summary, this model predicts that during supine normal breathing under gravitational loading, passive V̇/Q̇ matching is predominantly determined by shared gravitationally induced tissue deformation, compliance distribution, and the effect of the hydrostatic pressure gradient on vessel and capillary size and blood pressures. Contribution from the matching airway and arterial tree geometries in this model is minor under normal gravity in the supine adult human lung. NEW & NOTEWORTHY We use a computational model to systematically analyze contributors to ventilation-perfusion matching in the lung. The model predicts that the multiple effects of gravity are the predominant mechanism in providing passive ventilation-perfusion matching in the supine adult human lung under normal gravitational loads, while geometric matching of airway and arterial trees plays a minor role.

Inhibition of Constitutive Nitric Oxide Synthase Does Not Influence Ventilation-Perfusion Matching in Normal Prone Adult Sheep With Mechanical Ventilation

BACKGROUND

Local formation of nitric oxide in the lung induces vasodilation in proportion to ventilation and is a putative mechanism behind ventilation-perfusion matching. We hypothesized that regional ventilation-perfusion matching occurs in part due to local constitutive nitric oxide formation.

METHODS

Ventilation and perfusion were analyzed in lung regions (≈1.5 cm) before and after inhibition of constitutive nitric oxide synthase with N-nitro-L-arginine methyl ester (L-NAME) (25 mg/kg) in 7 prone sheep ventilated with 10 cm H2O positive end-expiratory pressure. Ventilation and perfusion were measured by the use of aerosolized fluorescent and infused radiolabeled microspheres, respectively. The animals were exsanguinated while deeply anesthetized; then, lungs were excised, dried at total lung capacity, and divided into cube units. The spatial location for each cube was tracked and fluorescence and radioactivity per unit weight determined.

RESULTS

After administration of L-NAME, pulmonary artery pressure increased from a mean of 16.6-23.6 mm Hg, P = .007 but PaO2, PaCO2, and SD log(V/Q) did not change. Distribution of ventilation was not influenced by L-NAME, but a small redistribution of perfusion from ventral to dorsal lung regions was observed. Perfusion to regions with the highest ventilation (fifth quintile of the ventilation distribution) remained unchanged after L-NAME.

CONCLUSIONS

We found minimal or no influence of constitutive nitric oxide synthase inhibition by L-NAME on the distributions of ventilation and perfusion, and ventilation-perfusion in prone, anesthetized, ventilated, and healthy adult sheep with normal gas exchange.

Bridging the Gap Between Science and Clinical Efficacy: Physiology, Imaging, and Modeling of Aerosols in the Lung

Development of a new drug for the treatment of lung disease is a complex and time consuming process involving numerous disciplines of basic and applied sciences. During the 2015 Congress of the International Society for Aerosols in Medicine, a group of experts including aerosol scientists, physiologists, modelers, imagers, and clinicians participated in a workshop aiming at bridging the gap between basic research and clinical efficacy of inhaled drugs. This publication summarizes the current consensus on the topic. It begins with a short description of basic concepts of aerosol transport and a discussion on targeting strategies of inhaled aerosols to the lungs. It is followed by a description of both computational and biological lung models, and the use of imaging techniques to determine aerosol deposition distribution (ADD) in the lung. Finally, the importance of ADD to clinical efficacy is discussed. Several gaps were identified between basic science and clinical efficacy. One gap between scientific research aimed at predicting, controlling, and measuring ADD and the clinical use of inhaled aerosols is the considerable challenge of obtaining, in a single study, accurate information describing the optimal lung regions to be targeted, the effectiveness of targeting determined from ADD, and some measure of the drug's effectiveness. Other identified gaps were the language and methodology barriers that exist among disciplines, along with the significant regulatory hurdles that need to be overcome for novel drugs and/or therapies to reach the marketplace and benefit the patient. Despite these gaps, much progress has been made in recent years to improve clinical efficacy of inhaled drugs. Also, the recent efforts by many funding agencies and industry to support multidisciplinary networks including basic science researchers, R&D scientists, and clinicians will go a long way to further reduce the gap between science and clinical efficacy.

Mechanical ventilation of acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) has been intensively and continuously studied in various settings, but its mortality is still as high as 30-40 %. For the last 20 years, lung protective strategy has become a standard care for ARDS, but we still do not know the best way to ventilate patients with ARDS. Tidal volume itself does not seem to have an important role to develop ventilator-induced lung injury (VILI), but the driving pressure, which is inspiratory plateau pressure-PEEP, is the most important to predict and affect the outcome of ARDS, though there is no safe limit for the driving pressure. There is so much controversy regarding what the best PEEP is, whether collapsed lung should be recruited, and what parameters should be measured and evaluated to improve the outcome of ARDS. Since the mechanical ventilation for patients with respiratory failure, including ARDS, is a standard care, we need more dynamic and regional information of ventilation and pulmonary circulation in the injured lungs to evaluate the efficacy of new type of treatment strategy. In addition to the CT scanning of the lung as the gold standard of evaluation, the electrical impedance tomography (EIT) of the lung has been clinically available to provide such information non-invasively and at the bedside. Various parameters have been tested to evaluate the homogeneity of regional ventilation, and EIT could provide us with the information of ventilator settings to minimize VILI.

What Causes Uneven Aerosol Deposition in the Bronchoconstricted Lung? A Quantitative Imaging Study

BACKGROUND

A previous PET-CT imaging study of 14 bronchoconstricted asthmatic subjects showed that peripheral aerosol deposition was highly variable among subjects and lobes. The aim of this work was to identify and quantify factors responsible for this variability.

METHODS

A theoretical framework was formulated to integrate four factors affecting aerosol deposition: differences in ventilation, in how air vs. aerosol distribute at each bifurcation, in the fraction of aerosol escaping feeding airways, and in the fraction of aerosol reaching the periphery that is exhaled. These factors were quantified in 12 of the subjects using PET-CT measurements of relative specific deposition sD*, relative specific ventilation sV* (measured with dynamic PET or estimated as change in expansion between two static HRCTs), average lobar expansion F_VOL, and breathing frequency measured during aerosol inhalation f_N.

RESULTS

The fraction of the variance of sD* explained by sV* (0.38), by bifurcation effects (0.38), and by differences in deposition along feeding airways (0.31) were similar in magnitude. We could not directly estimate the contribution of aerosol that was exhaled. Differences in expansion did not explain any fraction of the variability in sD* among lobes. The dependence of sD* on sV* was high in subjects breathing with low f_N, but weakened among those breathing faster. Finally, sD*/sV* showed positive dependence on F_VOL among low f_N subjects, while the dependence was negative among high f_N subjects.

CONCLUSION

The theoretical framework allowed us to analyze experimentally measured aerosol deposition imaging data. When considering bronchoconstricted asthmatic subjects, a dynamic measurement of ventilation is required to evaluate its effect on aerosol transport. The mechanisms behind the identified effects of f_N and F_VOL on aerosol deposition need further study and may have important implications for aerosol therapy in subjects with heterogeneous ventilation.

Gas exchange and pulmonary hypertension following acute pulmonary thromboembolism: has the emperor got some new clothes yet?

Patients present with a wide range of hypoxemia after acute pulmonary thromboembolism (APTE). Recent studies using fluorescent microspheres demonstrated that the scattering of regional blood flows after APTE, created by the embolic obstruction unique in each patient, significantly worsened regional ventilation/perfusion (V/Q) heterogeneity and explained the variability in gas exchange. Furthermore, earlier investigators suggested the roles of released vasoactive mediators in affecting pulmonary hypertension after APTE, but their quantification remained challenging. The latest study reported that mechanical obstruction by clots accounted for most of the increase in pulmonary vascular resistance, but that endothelin-mediated vasoconstriction also persisted at significant level during the early phase.

Relationship between quantitative CT of pulmonary small vessels and pulmonary perfusion

OBJECTIVE

The relationship between morphologic alterations of pulmonary small vessels and pulmonary perfusion has not been clarified. The purpose of this study was to evaluate the relationship between the cross-sectional area (CSA) of pulmonary small vessels alterations measured on CT images and pulmonary perfusion on lung perfusion scintigraphy.

MATERIALS AND METHODS

This study comprised 46 subjects who underwent both CT and lung perfusion scintigraphy. We measured CSA of pulmonary small vessels less than 5 mm(2) and 5-10 mm(2) using CT images and obtained the percentage of the right lung to whole lung in each CSA group (CSA<5,R/W and CSA5-10,R/W, respectively). Using (99m)Tc-macroaggregated albumin (MAA) lung perfusion scintigraphy, we obtained right and total lung counts and calculated the percentage of the right to whole-lung counts (MAAR/W). Those CT and scintigraphy measurements were also calculated separately each in right upper, right lower, left upper, and left lower zones. The correlations of CSA<5,R/W and CSA5-10,R/W with MAAR/W, the correlation between the percentage of each lung zone to whole-lung CSA<5 and the percentage of each corresponding lung zone to whole-lung MAA were evaluated.

RESULTS

The mean CSA<5,R/W was 58.1% ± 11.2%, and the mean MAAR/W was 59.3% ± 17.9%. CSA<5,R/W had a significant correlation with MAAR/W (ρ = 0.865, p < 0.0001), whereas significant correlation was found but was relatively weak between CSA5-10,R/W and MAAR/W (ρ = 0.512, p = 0.0003). The percentage of each lung zone to whole-lung CSA<5 had significant correlations with the percentage of each corresponding lung zone to whole-lung MAA.

CONCLUSION

Pulmonary small vessels alteration, as measured by CSA on CT images, significantly correlated with pulmonary perfusion.

Pulmonary vascular diseases

Diseases of the pulmonary vasculature are a cause of increased pulmonary vascular resistance (PVR) in pulmonary embolism, chronic thromboembolic pulmonary hypertension (CTEPH), and pulmonary arterial hypertension or decreased PVR in pulmonary arteriovenous malformations on hereditary hemorrhagic telangiectasia, portal hypertension, or cavopulmonary anastomosis. All these conditions are associated with a decrease in both arterial PO2 and PCO2. Gas exchange in pulmonary vascular diseases with increased PVR is characterized by a shift of ventilation and perfusion to high ventilation-perfusion ratios, a mild to moderate increase in perfusion to low ventilation-perfusion ratios, and an increased physiologic dead space. Hypoxemia in these patients is essentially explained by altered ventilation-perfusion matching amplified by a decreased mixed venous PO2 caused by a low cardiac output. Hypocapnia is accounted for by hyperventilation, which is essentially related to an increased chemosensitivity. A cardiac shunt on a patent foramen ovale may be a cause of severe hypoxemia in a proportion of patients with pulmonary hypertension and an increase in right atrial pressure. Gas exchange in pulmonary arteriovenous malformations is characterized by variable degree of pulmonary shunting and/or diffusion-perfusion imbalance. Hypocapnia is caused by an increased ventilation in relation to an increased pulmonary blood flow with direct peripheral chemoreceptor stimulation by shunted mixed venous blood flow.

Spatial distribution of ventilation and perfusion: mechanisms and regulation

With increasing spatial resolution of regional ventilation and perfusion, it has become more apparent that ventilation and blood flow are quite heterogeneous in the lung. A number of mechanisms contribute to this regional variability, including hydrostatic gradients, pleural pressure gradients, lung compressibility, and the geometry of the airway and vascular trees. Despite this marked heterogeneity in both ventilation and perfusion, efficient gas exchange is possible through the close regional matching of the two. Passive mechanisms, such as the shared effect of gravity and the matched branching of vascular and airway trees, create efficient gas exchange through the strong correlation between ventilation and perfusion. Active mechanisms that match local ventilation and perfusion play little if no role in the normal healthy lung but are important under pathologic conditions.

Heterogeneity and matching of ventilation and perfusion within anatomical lung units in rats

Prior studies exploring the spatial distributions of ventilation and perfusion have partitioned the lung into discrete regions not constrained by anatomical boundaries and may blur regional differences in perfusion and ventilation. To characterize the anatomical heterogeneity of regional ventilation and perfusion, we administered fluorescent microspheres to mark regional ventilation and perfusion in five Sprague-Dawley rats and then using highly automated computer algorithms, partitioned the lungs into regions defined by anatomical structures identified in the images. The anatomical regions ranged in size from the near-acinar to the lobar level. Ventilation and perfusion were well correlated at the smallest anatomical level. Perfusion and ventilation heterogeneity were relatively less in rats compared to data previously published in larger animals. The more uniform distributions may be due to a smaller gravitational gradient and/or the fewer number of generations in the distribution trees before reaching the level of gas exchange, making regional matching of ventilation and perfusion less extensive in small animals.

Reproducibility of registration-based measures of lung tissue expansion

PURPOSE

Lung function depends on lung expansion and contraction during the respiratory cycle. Respiratory-gated CT imaging and 3D image registration can be used to locally estimate lung tissue expansion and contraction (regional lung volume change) by computing the determinant of the Jacobian matrix of the image registration deformation field. In this study, the authors examine the reproducibility of Jacobian-based measures of lung tissue expansion in two repeat 4DCT acquisitions of mechanically ventilated sheep and free-breathing humans.

METHODS

4DCT image data from three white sheep and nine human subjects were used for this analysis. In each case, two 4DCT studies were acquired for each subject within a short time interval. The animal subjects were anesthetized and mechanically ventilated, while the humans were awake and spontaneously breathing based on respiratory pacing audio cues. From each 4DCT data set, an image pair consisting of a volume reconstructed near end inspiration and a volume reconstructed near end exhalation was selected. The end inspiration and end exhalation images were registered using a tissue volume preserving deformable registration algorithm and the Jacobian of the registration deformation field was used to measure regional lung expansion. The Jacobian map from the baseline data set was compared to the Jacobian map from the followup data by measuring the voxel-by-voxel Jacobian ratio.

RESULTS

In the animal subjects, the mean Jacobian ratio (baseline scan Jacobian divided by followup scan Jacobian, voxel-by-voxel) was 0.9984±0.021 (mean ± standard deviation, averaged over the entire lung region). The mean Jacobian ratio was 1.0224±0.058 in the human subjects. The reproducibility of the Jacobian values was found to be strongly dependent on the reproducibility of the subject's respiratory effort and breathing pattern.

CONCLUSIONS

Lung expansion, a surrogate for lung function, can be assessed using two or more respiratory-gated CT image acquisitions. The results show that good reproducibility can be obtained in anesthetized, mechanically ventilated animals, but variations in respiratory effort and breathing patterns reduce reproducibility in spontaneously-breathing humans. The global linear normalization can globally compensate for breathing effort differences, but a homogeneous scaling does not account for differences in regional lung expansion rates. Additional work is needed to develop compensation procedures or normalization schemes that can account for local variations in lung expansion during respiration.

Effect of regional lung inflation on ventilation heterogeneity at different length scales during mechanical ventilation of normal sheep lungs

Heterogeneous, small-airway diameters and alveolar derecruitment in poorly aerated regions of normal lungs could produce ventilation heterogeneity at those anatomic levels. We modeled the washout kinetics of (13)NN with positron emission tomography to examine how specific ventilation (sV) heterogeneity at different length scales is influenced by lung aeration. Three groups of anesthetized, supine sheep were studied: high tidal volume (Vt; 18.4 ± 4.2 ml/kg) and zero end-expiratory pressure (ZEEP) (n = 6); low Vt (9.2 ± 1.0 ml/kg) and ZEEP (n = 6); and low Vt (8.2 ± 0.2 ml/kg) and positive end-expiratory pressure (PEEP; 19 ± 1 cmH(2)O) (n = 4). We quantified fractional gas content with transmission scans, and sV with emission scans of infused (13)NN-saline. Voxel (13)NN-washout curves were fit with one- or two-compartment models to estimate sV. Total heterogeneity, measured as SD[log(10)(sV)], was divided into length-scale ranges by measuring changes in variance of log(10)(sV), resulting from progressive filtering of sV images. High-Vt ZEEP showed higher sV heterogeneity at <12- (P < 0.01), 12- to 36- (P < 0.01), and 36- to 60-mm (P < 0.05) length scales compared with low-Vt PEEP, with low-Vt ZEEP in between. Increased heterogeneity was associated with the emergence of low sV units in poorly aerated regions, with a high correlation (r = 0.95, P < 0.001) between total heterogeneity and the fraction of lung with slow washout. Regional mean fractional gas content was inversely correlated with regional sV heterogeneity at <12- (r = -0.67), 12- to 36- (r = -0.74), and >36-mm (r = -0.72) length scales (P < 0.001). We conclude that sV heterogeneity at length scales <60 mm increases in poorly aerated regions of mechanically ventilated normal lungs, likely due to heterogeneous small-airway narrowing and alveolar derecruitment. PEEP reduces sV heterogeneity by maintaining lung expansion and airway patency at those small length scales.

Spatial and temporal heterogeneity of regional lung ventilation determined by electrical impedance tomography during pulmonary function testing

Electrical impedance tomography (EIT) is a functional imaging modality capable of tracing continuously regional pulmonary gas volume changes. The aim of our study was to determine if EIT was able to assess spatial and temporal heterogeneity of ventilation during pulmonary function testing in 14 young (37 ± 10 yr, mean age ± SD) and 12 elderly (71 ± 9 yr) subjects without lung disease and in 33 patients with chronic obstructive pulmonary disease (71 ± 9 yr). EIT and spirometry examinations were performed during tidal breathing and a forced vital capacity (FVC) maneuver preceded by full inspiration to total lung capacity. Regional inspiratory vital capacity (IVC); FVC; forced expiratory volume in 1 s (FEV(1)); FEV(1)/FVC; times required to expire 25%, 50%, 75%, and 90% of FVC (t(25), t(50), t(75), t(90)); and tidal volume (V(T)) were determined in 912 EIT image pixels in the chest cross section. Coefficients of variation (CV) were calculated from all pixel values of IVC, FVC, FEV(1), and V(T) to characterize the ventilation heterogeneity. The highest values were found in patients, and no differences existed between the healthy young and elderly subjects. Receiver-operating characteristics curves showed that CV of regional IVC, FVC, FEV(1), and V(T) discriminated the young and elderly subjects from the patients. Frequency distributions of pixel FEV(1)/FVC, t(25), t(50), t(75), and t(90) identified the highest ventilation heterogeneity in patients but distinguished also the healthy young from the elderly subjects. These results indicate that EIT may provide additional information during pulmonary function testing and identify pathologic and age-related spatial and temporal heterogeneity of regional lung function.

Imaging regional PAO2 and gas exchange

Several methods allow regional gas exchange to be inferred from imaging of regional ventilation and perfusion (V/Q) ratios. Each method measures slightly different aspects of gas exchange and has inherent advantages and drawbacks that are reviewed. Single photon emission computed tomography can provide regional measure of ventilation and perfusion from which regional V/Q ratios can be derived. PET methods using inhaled or intravenously administered nitrogen-13 provide imaging of both regional blood flow, shunt, and ventilation. Electric impedance tomography has recently been refined to allow simultaneous measurements of both regional ventilation and blood flow. MRI methods utilizing hyperpolarized helium-3 or xenon-129 are currently being refined and have been used to estimate local PaO(2) in both humans and animals. Microsphere methods are included in this review as they provide measurements of regional ventilation and perfusion in animals. One of their advantages is their greater spatial resolution than most imaging methods and the ability to use them as gold standards against which new imaging methods can be tested. In general, the reviewed methods differ in characteristics such as spatial resolution, possibility of repeated measurements, radiation exposure, availability, expensiveness, and their current stage of development.

Lung imaging in asthmatic patients: the picture is clearer

Imaging of the lungs in patients with asthma has evolved dramatically over the last decade with sophisticated techniques, such as computed tomography, magnetic resonance imaging, positron emission tomography, and single photon emission computed tomography. New insights into current and future modalities for imaging in asthmatic patients and their application are discussed to potentially shed a clearer picture of the underlying pathophysiology of asthma, especially severe asthma, and the proposed clinical utility of imaging in patients with this common disease.

Convective flow dominates aerosol delivery to the lung segments

Most previous computational studies on aerosol transport in models of the central airways of the human lung have focused on deposition, rather than transport of particles through these airways to the subtended lung regions. Using a model of the bronchial tree extending from the trachea to the segmental bronchi (J Appl Physiol 98: 970-980, 2005), we predicted aerosol delivery to the lung segments. Transport of 0.5- to 10-μm-diameter particles was computed at various gravity levels (0-1.6 G) during steady inspiration (100-500 ml/s). For each condition, the normalized aerosol distribution among the lung segments was compared with the normalized flow distribution by calculating the ratio (R(i)) of the number of particles exiting each segmental bronchus i to the flow. When R(i) = 1, particle transport was directly proportional to segmental flow. Flow and particle characteristics were represented by the Stokes number (Stk) in the trachea. For Stk < 0.01, R(i) values were close to 1 and were unaffected by gravity. For Stk > 0.01, R(i) varied greatly among the different outlets (R(i) = 0.30-1.93 in normal gravity for 10-μm particles at 500 ml/s) and was affected by gravity and inertia. These data suggest that, for Stk < 0.01, ventilation defines the delivery of aerosol to lung segments and that the use of aerosol tracers is a valid technique to visualize ventilation in different parts of the lung. At higher Stokes numbers, inertia, but not gravitational sedimentation, is the second major factor affecting the transport of large particles in the lung.

Lung perfusion measured using magnetic resonance imaging: New tools for physiological insights into the pulmonary circulation

Since the lung receives the entire cardiac output, sophisticated imaging techniques are not required in order to measure total organ perfusion. However, for many years studying lung function has required physiologists to consider the lung as a single entity: in imaging terms as a single voxel. Since imaging, and in particular functional imaging, allows the acquisition of spatial information important for studying lung function, these techniques provide considerable promise and are of great interest for pulmonary physiologists. In particular, despite the challenges of low proton density and short T2* in the lung, noncontrast MRI techniques to measure pulmonary perfusion have several advantages including high reliability and the ability to make repeated measurements under a number of physiologic conditions. This brief review focuses on the application of a particular arterial spin labeling (ASL) technique, ASL-FAIRER (flow sensitive inversion recovery with an extra radiofrequency pulse), to answer physiologic questions related to pulmonary function in health and disease. The associated measurement of regional proton density to correct for gravitational-based lung deformation (the "Slinky" effect (Slinky is a registered trademark of Pauf-Slinky incorporated)) and issues related to absolute quantification are also discussed.

High-resolution spatial measurements of ventilation-perfusion heterogeneity in rats

This study was designed to validate a high-resolution method to measure regional ventilation (VA) in small laboratory animals, and to compare regional Va and perfusion (Q) before and after methacholine-induced bronchoconstriction. A mixture of two different colors of 0.04-microm fluorescent microspheres (FMS) was aerosolized and administered to five anesthetized, mechanically ventilated rats. Those rats also received an intravenous injection of a mixture of two different colors of 15-microm FMS to measure regional blood flow (Q). Five additional rats were labeled with aerosol and intravenous FMS, injected with intravenous methacholine, and then relabeled with a second pair of aerosol and intravenous FMS colors. After death, the lungs were reinflated, frozen, and sequentially sliced in 16-microm intervals on an imaging cryomicrotome set to acquire signal for each of the FMS colors. The reconstructed lung images were sampled using randomly placed 3-mm radius spheres. Va within each sphere was estimated from the aerosol fluorescence signal, and Q was estimated from the number of 15-microm FMS within each sphere. Method error ranged from 6 to 8% for Q and 0.5 to 4.0% for Va. The mean coefficient of variation for Q was 17%, and for Va was 34%. The administration of methacholine altered the distribution of both VA and Q within lung regions, with a change in Va distribution nearly twice as large as that seen for Q. The methacholine-induced changes in Va were not associated with compensatory shifts in Q. Cryomicrotome images of FMS markers provide a high-resolution, anatomically specific means of measuring regional VA/Q responses in the rat.

Regional CO2 tension quantitatively mediates homeostatic redistribution of ventilation following acute pulmonary thromboembolism in pigs

Previous studies reported that regional CO(2) tension might affect regional ventilation (V) following acute pulmonary thromboembolism (APTE). We investigated the pathophysiology and magnitude of these changes. Eight anesthetized and ventilated piglets received autologous clots at time = 0 min until mean pulmonary artery pressure was 2.5 times baseline. The distribution of V and perfusion (Q) at four different times (-5, 30, 60, 120 min) was mapped by fluorescent microspheres. Regional V and Q were examined postmortem by sectioning the air-dried lung into 900-1,000 samples of approximately 2 cm(3) each. After the redistribution of regional Q by APTE, but in the scenario assuming that no V shift had yet occurred, CO(2) tension in different lung regions at 30 min post-APTE (P(X)CO(2)) was estimated from the V/Q data and divided into four distinct clusters: i.e., P(X)CO(2) < 10 Torr; 10 < P(X)CO(2) < 25 Torr; 25 < P(X)CO(2) < 50 Torr; P(X)CO(2) > 50 Torr. Our data showed that the clusters in higher V/Q regions (with a P(X)CO(2) < 25 Torr) received approximately 35% less V when measured within 30 min of APTE, whereas, in contrast, the lower V/Q regions showed no statistically significant increases in their V. However, after 30 min, there was minimal further redistribution of V. We conclude that there are significant compensatory V shifts out of regions of low CO(2) tension soon following APTE, and that these variations in regional CO(2) tension, which initiate CO(2)-dependent changes in airway resistance and lung parenchymal compliance, can lead to improved gas exchange.

AEROSOL DELIVERY DURING MECHANICAL VENTILATION TO THE RAT

The authors have adjusted a jet nebulizer to a mechanical ventilator (Servo Ventilator, Siemens) to deliver an aerosol to rats. They aimed to clarify whether a modified jet nebulizer generating particles with a mass median aerodynamic diameter of 2 microm would be effective and safe in intubated ventilated rats. Fluorescent microspheres (diameter: 1.0 microm) were aerosolized to verify qualitatively and quantitatively intrapulmonary deposition. Particle deposition fraction was 3.8% (1.3%) of the delivered dose (median [interquartile range]). There was no evidence for any adverse event as assessed from heart rate, mean arterial pressure, PaO2 and PaCO2 before, during, and after nebulization. No pulmonary tissue trauma was detected histologically.

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.

The difference in ventilation heterogeneity between asthmatic and healthy subjects quantified using hyperpolarized 3He MRI

In this pilot study, algorithms for quantitatively evaluating the distribution and heterogeneity of human ventilation imaged with hyperpolarized (HP) (3)He MRI were developed for the goal of examining structure-function relationships within the asthmatic lung. Ten asthmatic and six healthy human subjects were imaged with HP (3)He MRI before bronchial challenge (pre-MCh), after bronchial challenge (post-MCh), and after a series of deep inspirations (post-DI) following challenge. The acquired images were rigidly coregistered. Local voxel fractional ventilation was computed by setting the sum of the pixel intensity within the lung region in each image to 1 liter of inhaled (3)He mixture. Local ventilation heterogeneity was quantified by computing regional signal coefficient of variation. Voxel fractional ventilation histograms and overall heterogeneity scores were then calculated. Asthmatic subjects had a higher ventilation heterogeneity to begin with (P = 0.025). A methacholine challenge elevated ventilation heterogeneity for all subjects (difference: P = 0.08). After a DI postchallenge, this heterogeneity reversed substantially toward the baseline state for healthy subjects but only minimally in asthmatic subjects. This difference was significant in absolute quantity (difference: P = 0.007) as well as relative to the initial increase (difference: P = 0.03). These findings suggest that constriction heterogeneity is not a characteristic unique to asthmatic airway trees but rather a behavior intrinsic to all airway trees when provoked. Once ventilation heterogeneity is established, it is the lack of reversal following DIs that distinguishes asthmatics from non-asthmatics.

The Unpredictable Effect of Changing Cardiac Output on Hypoxemia after Acute Pulmonary Thromboembolism

Previous studies reported that the degree of hypoxemia following acute pulmonary thromboembolism (APTE) was highly variable and that its mechanism was mainly due to the creation of many high and low ventilation/perfusion (V/Q) units, as a result of the heterogeneous regional blood flow (Q) caused by embolic obstruction. We studied the effect of changing cardiac output (Qt) on gas exchange after APTE in 5 embolized piglets (23 ± 3 Kg), using Dobutamine intermittently at approximately 20 μg/kg/min for 120 minutes. The distribution of ventilation (V) and perfusion (Q) at various times was mapped using fluorescent microspheres in 941 ± 60 lung regions. After APTE, increase in Qt by Dobutamine improved venous oxygen tension (PvO2) but arterial PaO2 did not change consistently. On the other hand, cluster analysis showed that the V/Q ratio of most lung regions was lowered due to increases in Q at the same time. We concluded that the effect of changing cardiac output on gas exchange following APTE was affected by the simultaneous and varying balance between the changing V/Q mismatch and the concomitantly changing PvO2, which might explain the unpredictability of PaO2 in the clinical setting.

Registration-based estimates of local lung tissue expansion compared to xenon CT measures of specific ventilation

The main function of the respiratory system is gas exchange. Since many disease or injury conditions can cause biomechanical or material property changes that can alter lung function, there is a great interest in measuring regional lung ventilation and regional specific volume change. We describe a registration-based technique for estimating local lung expansion from multiple respiratory-gated CT images of the thorax. The degree of regional lung expansion is measured using the Jacobian (a function of local partial derivatives) of the registration displacement field, which we show is directly related to specific volume change. We compare the ventral-dorsal patterns of lung expansion estimated across five pressure changes to a xenon CT based measure of specific ventilation in five anesthetized sheep studied in the supine orientation. Using 3D image registration to match images acquired at 10 cm H(2)O and 15 cm H(2)O airway pressures gave the best match between the average Jacobian and the xenon CT specific ventilation (linear regression, average r(2)=0.73).

Functional imaging: CT and MRI

Numerous imaging techniques permit evaluation of regional pulmonary function. Contrast-enhanced CT methods now allow assessment of vasculature and lung perfusion. Techniques using spirometric controlled multi-detector row CT allow for quantification of presence and distribution of parenchymal and airway pathology; xenon gas can be employed to assess regional ventilation of the lungs, and rapid bolus injections of iodinated contrast agent can provide a quantitative measure of regional parenchymal perfusion. Advances in MRI of the lung include gadolinium-enhanced perfusion imaging and hyperpolarized gas imaging, which allow functional assessment, including ventilation/perfusion, microscopic air space measurements, and gas flow and transport dynamics.

Quantification of lung injury using ventilation and perfusion distributions obtained from gamma scintigraphy

This paper explores the potential of isotope V/Q lung scans to quantify lung disease. Areas of restricted perfusion in subjects with a pulmonary embolus (PE) were identified in 3D reconstructions of V/Q images achieved using anatomical data from the Visible Human Project. From these, the extent of lung damage was quantified. Significant differences in the values of both LogSD V and LogSD Q (p > 0.05) obtained from plots of V and Q against Log(V/Q) were found between normal subjects and subjects with a PE, but no correlation was found between either of these parameters and the degree of lung damage in subjects with a PE (p > 0.05). Whilst V/Q values were log normally distributed, the V/Q distributions from the subjects with a PE failed to show the bimodal distribution predicted from theoretical considerations and MIGET measurements previously reported. There was a statistically significant difference in the mean and standard deviation values of the V/Q distributions between normal subject and subjects with a PE (p < 0.05) but not in the median values (p > 0.05). There was no correlation between the mean, median and standard deviation of the distributions from the subjects with a PE and the percentage of damage present (p > 0.05).

Deposition of differently sized airborne microspheres in the respiratory tract of chickens

As a part of the development of an efficient dry powder aerosol vaccine for poultry, the objective of this study was to accurately determine the deposition pattern of nebulized microspheres in the airways of unanaesthetized chickens of different ages (1 day, 2 weeks and 4 weeks old). In the first part of the study, the aerosol administration method was characterized: the influence of different nebulizers and nebulizing protocols on the relative humidity in the exposure chamber, the particle size distributions, the microsphere output and single microsphere percentage were determined. In the second part, birds were exposed to nebulized fluorescently labelled polystyrene microspheres (1 to 20 microm). Respiratory and gastro-intestinal tract tissue samples were collected and the number of fluorescent microspheres per sample was determined. In 2-week-old and 4-week-old chickens, microspheres of 5 and 10 microm, respectively, were too large for deposition in the lungs and air sacs as less than 5% of these microspheres penetrated into the lower airways. The larger size of microspheres reaching the lower airways of 4-week-old birds was explained by increasing airway dimensions with age. For 1-day-old chickens, deposition in the lungs decreased from 17 to 3% with increasing particle size (1 to 20 microm), but increased in the air sacs from 6 to 20%. Consequently, the total deposition percentage in the lower airways was independent of microsphere size and even 20 microm particles were able to penetrate into the lower airways, which was attributed to mouth breathing of the 1-day-old chickens.

Registration-derived estimates of local lung expansion as surrogates for regional ventilation

The main function of the respiratory system is gas exchange. Since many disease or injury conditions can cause biomechanical or material property changes that can alter lung function, there is a great interest in measuring regional lung ventilation. We describe a registration-based technique for estimating local lung expansion from multiple respiratory-gated CT images of the thorax. The degree of regional lung expansion is measured using the Jacobian of the registration displacement field. We compare lung expansion estimated across five pressure changes to a xenon CT based measure of specific ventilation, and have shown good agreement (linear regression, r2 = 0.89 during gas wash-in) in one animal.

Sporadic coordinated shifts of regional ventilation and perfusion in juvenile pigs with normal gas exchange

Repeated high-resolution measurements of both regional pulmonary ventilation and regional blood flow (r ) have revealed that approximately 6 to 10% of the summed spatial and temporal heterogeneity can be attributed to spontaneous temporal variability. To test the hypothesis that the spontaneous temporal shifts of r and r are coordinated, 12 anaesthetized juvenile pigs had pairs of colours of aerosol and intravenous fluorescent microspheres (FMS) administered simultaneously at 20 min intervals to mark r and r . The animals were killed, the lungs inflated, air-dried and cut into approximately 2 cm(3) cubes. The concentrations of FMS colours from each cube, representing r and r at every 20 min interval, were measured with a fluorescence spectrophotometer. The correlation between per-piece temporal shifts in r and r , calculated as the mean within-piece covariance, was positive (P < 0.001) for every temporally adjacent pair of measurements in every animal, although there were large differences in the magnitude of the mean temporal covariance among animals. The individual cubes with the most positive temporal covariance across all measurement periods usually demonstrated a large single-interval coordinated shift of r and r , with average temporal covariance observed at the other intervals. The largest between-interval shifts in r and r included equal proportions of coordinated increases and coordinated decreases. High-resolution measurements of r and r acquired over 20 min intervals reveal that the overall positive correlation between temporal changes in r and r is driven by relatively infrequent large-magnitude changes within small regions of the lung.

Effect of low-xenon and krypton supplementation on signal/noise of regional CT-based ventilation measurements

Xenon computed tomography (Xe-CT) is used to estimate regional ventilation by measuring regional attenuation changes over multiple breaths while rebreathing a constant Xe concentration ([Xe]). Xe-CT has potential human applications, although anesthetic properties limit [Xe] to ≤35%. We investigate effects of lower [Xe], including a low [Xe]-krypton (Kr) combination, on time constant (TC) determination. Six anesthetized sheep were scanned prone and supine using multidetector row CT. Lungs were imaged by respiratory gating during washin of a 30%, 40%, 55% Xe, and a 30% Xe/30% Kr mixture. Using Kr avoids unwanted effects of Xe. Mean TCs, coefficients of variation (CV), and half confidence intervals (CI)/mean served as indexes of sensitivity to noise. Mean supine and prone TCs of three [Xe] values were not significantly different. Average CVs of TCs increased from 57% (55% Xe), 58% (40% Xe), and 73% (30% Xe) ( P < 0.05: paired t-tests; 30% Xe vs. higher [Xe]). Monte Carlo simulation indicated a CV based on inherent image noise was 8% for 55% Xe and 17% for 30% Xe ( P < 0.05). Adding 30% Kr to 30% Xe gave a washin signal equivalent to 40% Xe. Half CI/mean using the 30% Xe/30% Kr mixture was not significantly different from 55 and 40% Xe. Although average TCs were not affected by changes in [Xe], the higher CV and half CI/mean suggested reduced signal-to-noise ratio at the 30% [Xe]. The 30% Xe/30% Kr mixture was comparable to that of 40% Xe, providing an important agent for CT-based assessment of regional ventilation in humans.

Distribution of blood flow and ventilation in the lung: gravity is not the only factor

Current textbooks in anaesthesia describe how gravity affects the regional distribution of ventilation and blood flow in the lung, in terms of vertical gradients of pleural pressure and pulmonary vascular pressures. This concept fails to explain some of the clinical features of disturbed lung function. Evidence now suggests that gravity has a less important role in the variation of regional distribution than structural features of the airways and blood vessels. We review more recent studies that used a variety of methods: external radioactive counters, measurements using inhaled and injected particles, and computer tomography scans. These give a higher spatial resolution of regional blood flow and ventilation. The matching between ventilation and blood flow in these small units of lung is considered; the effects of microgravity, increased gravity, and different postures are reviewed, and the application of these findings to conditions such as acute lung injury is discussed. Down to the scale of the acinus, there is considerable heterogeneity in the distribution of both ventilation and blood flow. However, the matching of blood flow with ventilation is well maintained and may result from a common pattern of asymmetric branching of the airways and blood vessels. Disruption of this pattern may explain impaired gas exchange after acute lung injury and explain how the prone position improves gas exchange.

Endothelin receptor blockade does not improve hypoxemia following acute pulmonary thromboembolism

We studied the roles of endothelins in determining ventilation (V̇a) and perfusion (Q̇) mismatch in a porcine model of acute pulmonary thromboembolism (APTE), using a nonspecific endothelin antagonist, tezosentan. Nine anesthetized piglets (∼23 kg) received autologous clots (∼20 g) via a central venous catheter at time = 0 min. The distribution of V̇a and Q̇ at five different time points (−30, −5, 30, 60, 120 min) was mapped by fluorescent microspheres of 10 different colors. Five piglets ( group 1) received tezosentan (courtesy of Actelion) starting at time = 40 min for 2 h, and four piglets ( group 2) received only saline and served as control. Our results showed that, in all of the animals at 30 min following APTE but before tezosentan, the mean V̇a/Q̇ was increased, as was V̇a/Q̇ heterogeneity (log SD V̇a/Q̇), which represented a widening of its main peak. Afterwards, tezosentan attenuated the pulmonary hypertension in group 1 but also produced moderate systemic hypotension. However, it did not improve arterial Po2 or V̇a/Q̇ mismatch. We concluded that endothelin antagonism had minimal impact on gas exchange following APTE and confirmed our earlier observation that the main mechanism for hypoxemia in APTE was due to the mechanical redistribution of pulmonary regional blood flow away from the embolized vessels, resulting in the creation of many divergent low and high V̇a/Q̇ regions.

Microsphere maps of regional blood flow and regional ventilation

Systematically mapped samples cut from lungs previously labeled with intravascular and aerosol microspheres can be used to create high-resolution maps of regional perfusion and regional ventilation. With multiple radioactive or fluorescent microsphere labels available, this methodology can compare regional flow responses to different interventions without partial volume effects or registration errors that complicate interpretation of in vivo imaging measurements. Microsphere blood flow maps examined at different levels of spatial resolution have revealed that regional flow heterogeneity increases progressively down to an acinar level of scale. This pattern of scale-dependent heterogeneity is characteristic of a fractal distribution network, and it suggests that the anatomic configuration of the pulmonary vascular tree is the primary determinant of high-resolution regional flow heterogeneity. At approximately 2-cm(3) resolution, the large-scale gravitational gradients of blood flow per unit weight of alveolar tissue account for <5% of the overall flow heterogeneity. Furthermore, regional blood flow per gram of alveolar tissue remains relatively constant with different body positions, gravitational stresses, and exercise. Regional alveolar ventilation is accurately represented by the deposition of inhaled 1.0-microm fluorescent microsphere aerosols, at least down to the approximately 2-cm(3) level of scale. Analysis of these ventilation maps has revealed the same scale-dependent property of regional alveolar ventilation heterogeneity, with a strong correlation between ventilation and blood flow maintained at all levels of scale. The ventilation-perfusion (VA/Q) distributions obtained from microsphere flow maps of normal animals agree with simultaneously acquired multiple inert-gas elimination technique VA/Q distributions, but they underestimate gas-exchange impairment in diffuse lung injury.

Integrating systems biology and medical imaging: understanding disease distribution in the lung model

OBJECTIVE

Many chronic diseases exhibit characteristic pulmonary distribution patterns, but the underlying biologic explanations remain elusive. On the basis of emerging evidence from systems biology, we propose that gradients of T helper immune function exist as an epiphenomenon of the hypoxic pulmonary vasoconstriction response. Regional variation of immune function may contribute to preferential distribution patterning of lung diseases.

CONCLUSION

The lungs represent but one example in which the distribution of immune function throughout the body may explain disease location. This hypothetic framework can apply to diseases outside the realm of pulmonary biology and illustrates the potential benefit of integrating advances in systems biology and medical imaging.

Computed tomography studies of lung ventilation and perfusion

With the emergence of multidetector-row computed tomography (CT) it is now possible to image both structure and function via use of a single imaging modality. Breath-hold spiral CT provides detail of the airway and vascular trees along with texture reflective of the state of the lung parenchyma. Use of stable xenon gas wash-in and/or wash-out methods using an axial mode of the CT scanner whereby images are acquired through gating to the respiratory cycle provide detailed images of regional ventilation with isotropic voxel dimensions now on the order of 0.4 mm. Axial scanning during a breath hold and gating to the electrocardiogram during the passage of a sharp bolus injection of iodinated contrast agent provide detailed images of regional pulmonary perfusion. These dynamic CT methods for the study of regional lung function are discussed in the context of other methods that have been used to study heterogeneity of lung function.

Quantitative lung perfusion mapping at 0.2 T using FAIR True-FISP MRI

Perfusion measurements in lung tissue using arterial spin labeling (ASL) techniques are hampered by strong microscopic field gradients induced by susceptibility differences between the alveolar air and the lung parenchyma. A true fast imaging with steady precession (True-FISP) sequence was adapted for applications in flow-sensitive alternating inversion recovery (FAIR) lung perfusion imaging at 0.2 Tesla and 1.5 Tesla. Conditions of microscopic static field distribution were assessed in four healthy volunteers at both field strengths using multiecho gradient-echo sequences. The full width at half maximum (FWHM) values of the frequency distribution for 180-277 Hz at 1.5 Tesla were more than threefold higher compared to 39-109 Hz at 0.2 Tesla. The influence of microscopic field inhomogeneities on the True-FISP signal yield was simulated numerically. Conditions allowed for the development of a FAIR True-FISP sequence for lung perfusion measurement at 0.2 Tesla, whereas at 1.5 Tesla microscopic field inhomogeneities appeared too distinct. Perfusion measurements of lung tissue were performed on eight healthy volunteers and two patients at 0.2 Tesla using the optimized FAIR True-FISP sequence. The average perfusion rates in peripheral lung regions in transverse, sagittal, and coronal slices of the left/right lung were 418/400, 398/416, and 370/368 ml/100 g/min, respectively. This work suggests that FAIR True-FISP sequences can be considered appropriate for noninvasive lung perfusion examinations at low field strength.

The spatial and temporal heterogeneity of regional ventilation: Comparison of measurements by two high-resolution methods

High-resolution estimates of ventilation distribution in normal animals utilizing deposition of fluorescent microsphere aerosol (FMS technique) demonstrate substantial ventilation heterogeneity, but this finding has not been confirmed by an independent method. Five supine anesthetized sheep were used to compare the spatial and temporal heterogeneity of regional ventilation measured by both the FMS technique and by a ventilation model utilizing the data from computed tomography images of xenon gas washin (CT/Xe technique). An aerosol containing 1 microm fluorescent microspheres (FMS) was administered via a mechanical ventilator delivering a 2-s end-inspiration hold during each breath. Following the aerosol administration, sequential CT images of a transverse lung slice were acquired during each end-inspiration hold during washin of a 65% Xenon/35% oxygen gas mixture (CT/Xe technique). Four paired FMS and CT/Xe measurements were done at 30 min intervals, after which the animals were sacrificed. The lungs were extracted, air-dried and sliced in 1cm transverse sections. The lung section corresponding to the CT image was cut into 1 cm3 cubes, with notation of spatial coordinates. The individual cubes were soaked in solvent and the four fluorescent signals were measured with a fluorescence spectrophotometer. The color signals were normalized by the mean signal for all pieces and taken as the FMS estimate of ventilation heterogeneity. The CT images were clustered into 1 cm3 voxels and the rate of increase in voxel density was used to calculate voxel ventilation utilizing the model of . The regional ventilation voxel measurements were normalized by the mean value to give a CT/Xe estimate of ventilation heterogeneity comparable to the normalized FMS measurements. The overall of heterogeneity of ventilation at the 1 cm3 level of resolution was comparable by both techniques, with substantial differences among animals (coefficient of variation ranging from 37% to 74%). The repeated within-animal measurements by both techniques gave consistent values. Both techniques showed comparable large-scale distribution of regional ventilation in the caudal lobes of the supine animals. There were appreciable differences in the temporal variability of ventilation among animals. This study provides an independent confirmation of the scale-dependent heterogeneity of ventilation described by previous FMS aerosol studies of ventilation heterogeneity.

13C imaging—a new diagnostic platform

The evolution of magnetic resonance imaging (MRI) has been astounding since the early 1980s, and a broad range of applications has emerged. To date, clinical imaging of nuclei other than protons has been precluded for reasons of sensitivity. However, with the recent development of hyperpolarization techniques, the signal from a given number of nuclei can be increased as much as 100,000 times, sufficient to enable imaging of nonproton nuclei. Technically, imaging of hyperpolarized nuclei offers several unique properties, such as complete lack of background signal and possibility for local and permanent destruction of the signal by means of radio frequency (RF) pulses. These properties allow for improved as well as new techniques within several application areas. Diagnostically, the injected compounds can visualize information about flow, perfusion, excretory function, and metabolic status. In this review article, we explain the concept of hyperpolarization and the techniques to hyperpolarize 13C. An overview of results obtained within angiography, perfusion, and catheter tracking is given, together with a discussion of the particular advantages and limitations. Finally, possible future directions of hyperpolarized 13C MRI are pointed out.

Regional hypoxic pulmonary vasoconstriction in prone pigs

Hypoxic pulmonary vasoconstriction (HPV) is known to affect regional pulmonary blood flow distribution. It is unknown whether lungs with well-matched ventilation (V)/perfusion (Q) have regional differences in the HPV response. Five prone pigs were anesthetized and mechanically ventilated (positive end-expiratory pressure = 2 cmH2O). Two hypoxic preconditions [inspired oxygen fraction (FI(O2)) = 0.13] were completed to stabilize the animal's hypoxic response. Regional pulmonary blood Q and V distribution was determined at various FI(O2) (0.21, 0.15, 0.13, 0.11, 0.09) using the fluorescent microsphere technique. Q and V in the lungs were quantified within 2-cm3 lung pieces. Pieces were grouped, or clustered, based on the changes in blood flow when subjected to increasing hypoxia. Unique patterns of Q response to hypoxia were seen within and across animals. The three main patterns (clusters) showed little initial difference in V/Q matching at room air where the mean V/Q range was 0.92-1.06. The clusters were spatially located in cranial, central, and caudal portions of the lung. With decreasing FI(O2), blood flow shifted from the cranial to caudal regions. We determined that pulmonary blood flow changes, caused by HPV, produced distinct response patterns that were seen in similar regions across our prone porcine model.

Hypoxic pulmonary vasoconstriction is heterogeneously distributed in the prone dog

Hypoxic pulmonary vasoconstriction (HPV) is thought to protect gas exchange by decreasing perfusion to hypoxic regions. However, with global hypoxia, non-uniformity in HPV may cause over-perfusion to some regions, leading to high-altitude pulmonary edema. To quantify the spatial distribution of HPV and regional PO2 (PRO2) among small lung regions (approximately 2.0 cm3), five prone beagles (approximately 8.3 kg) were anesthetized and ventilated (PEEP approximately 2 cm H2O) with an F1O2 of 0.21, then 0.50, 0.18, 0.15, and 0.12 in random order. Regional blood perfusion (Q), ventilation (VA) and calculated PRO2 were obtained using iv infusion of 15 microm and inhalation of 1 microm fluorescent microspheres. Lung pieces were clustered by their relative blood flow response to each F1O2. Clusters were shown to be spatially grouped within animals and across animals. Lung piece resistance increased as PRO2 decreased to 60-70 mmHg but dropped at PRO2's < 60mmHg. Regional ventilation changed little with hypoxia. HPV varied more in strength of response, rather than PRO2 response threshold. In initially homogeneous VA/Q lungs, we conclude that HPV response is heterogeneous and spatially clustered.

3He MRI-based assessment of posture-dependent regional ventilation gradients in rats

A recently developed method for quantitative assessment of regional lung ventilation was employed for the study of posture-dependent ventilation differences in rats. The measurement employed hyperpolarized (3)He MRI to detect the build-up of the signal intensity after increasing numbers of (3)He breaths, which allowed for computation of a regional ventilation parameter. A group of six anesthetized rats was studied in both supine and prone postures. Three-dimensional maps of the ventilation parameter were obtained with high spatial resolution (voxel volume approximately 2 mm(3)). Vertical (dorsal-ventral) gradients of the ventilation index, defined as the regional ventilation normalized by the average ventilation within the whole lung, were investigated. Variations in the regional distribution of the ventilation parameter, as well as of the ventilation index, could be detected, depending on the posture of the rats. In supine posture, ventilation was elevated in the dependent parts of the lungs, with a linear gradient of the ventilation index of -0.11 +/- 0.03 cm(-1). In prone posture, the distribution of ventilation was more uniform, with a significantly (P < 0.001) smaller gradient of the ventilation index of -0.01 +/- 0.02 cm(-1). It is concluded that the (3)He MRI-based method can detect and quantify regional ventilation gradients in animals as small as the rat and that these gradients depend on prone or supine posture of the animal.