Image-derived input function for assessment of 18F-FDG uptake by the inflamed lung

High-Temporal-Resolution Kinetic Modeling of Lung Tumors with Dual-Blood Input Function Using Total-Body Dynamic PET

The lungs are supplied by both the pulmonary arteries carrying deoxygenated blood originating from the right ventricle and the bronchial arteries carrying oxygenated blood downstream from the left ventricle. However, this effect of dual blood supply has never been investigated using PET, partially because the temporal resolution of conventional dynamic PET scans is limited. The advent of PET scanners with a long axial field of view, such as the uEXPLORER total-body PET/CT system, permits dynamic imaging with high temporal resolution (HTR). In this work, we modeled the dual-blood input function (DBIF) and studied its impact on the kinetic quantification of normal lung tissue and lung tumors using HTR dynamic PET imaging. Methods: Thirteen healthy subjects and 6 cancer subjects with lung tumors underwent a dynamic 18F-FDG scan with the uEXPLORER for 1 h. Data were reconstructed into dynamic frames of 1 s in the early phase. Regional time-activity curves of lung tissue and tumors were analyzed using a 2-tissue compartmental model with 3 different input functions: the right ventricle input function, left ventricle input function, and proposed DBIF, all with time delay and dispersion corrections. These models were compared for time-activity curve fitting quality using the corrected Akaike information criterion and for differentiating lung tumors from lung tissue using the Mann-Whitney U test. Voxelwise multiparametric images by the DBIF model were further generated to verify the regional kinetic analysis. Results: The effect of dual blood supply was pronounced in the high-temporal-resolution time-activity curves of lung tumors. The DBIF model achieved better time-activity curve fitting than the other 2 single-input models according to the corrected Akaike information criterion. The estimated fraction of left ventricle input was low in normal lung tissue of healthy subjects but much higher in lung tumors (∼0.04 vs. ∼0.3, P < 0.0003). The DBIF model also showed better robustness in the difference in 18F-FDG net influx rate [Formula: see text] and delivery rate [Formula: see text] between lung tumors and normal lung tissue. Multiparametric imaging with the DBIF model further confirmed the differences in tracer kinetics between normal lung tissue and lung tumors. Conclusion: The effect of dual blood supply in the lungs was demonstrated using HTR dynamic imaging and compartmental modeling with the proposed DBIF model. The effect was small in lung tissue but nonnegligible in lung tumors. HTR dynamic imaging with total-body PET can offer a sensitive tool for investigating lung diseases.

An update on the use of image-derived input functions for human PET studies: new hopes or old illusions?

BACKGROUND

The need for arterial blood data in quantitative PET research limits the wider usability of this imaging method in clinical research settings. Image-derived input function (IDIF) approaches have been proposed as a cost-effective and non-invasive alternative to gold-standard arterial sampling. However, this approach comes with its own limitations-partial volume effects and radiometabolite correction among the most important-and varying rates of success, and the use of IDIF for brain PET has been particularly troublesome.

MAIN BODY

This paper summarizes the limitations of IDIF methods for quantitative PET imaging and discusses some of the advances that may make IDIF extraction more reliable. The introduction of automated pipelines (both commercial and open-source) for clinical PET scanners is discussed as a way to improve the reliability of IDIF approaches and their utility for quantitative purposes. Survey data gathered from the PET community are then presented to understand whether the field's opinion of the usefulness and validity of IDIF is improving. Finally, as the introduction of next-generation PET scanners with long axial fields of view, ultra-high sensitivity, and improved spatial and temporal resolution, has also brought IDIF methods back into the spotlight, a discussion of the possibilities offered by these state-of-the-art scanners-inclusion of large vessels, less partial volume in small vessels, better description of the full IDIF kinetics, whole-body modeling of radiometabolite production-is included, providing a pathway for future use of IDIF.

CONCLUSION

Improvements in PET scanner technology and software for automated IDIF extraction may allow to solve some of the major limitations associated with IDIF, such as partial volume effects and poor temporal sampling, with the exciting potential for accurate estimation of single kinetic rates. Nevertheless, until individualized radiometabolite correction can be performed effectively, IDIF approaches remain confined at best to a few tracers.

Total-Body Multiparametric PET Quantification of 18F-FDG Delivery and Metabolism in the Study of Coronavirus Disease 2019 Recovery

Conventional whole-body static 18F-FDG PET imaging provides a semiquantitative evaluation of overall glucose metabolism without insight into the specific transport and metabolic steps. Here we demonstrate the ability of total-body multiparametric 18F-FDG PET to quantitatively evaluate glucose metabolism using macroparametric quantification and assess specific glucose delivery and phosphorylation processes using microparametric quantification for studying recovery from coronavirus disease 2019 (COVID-19). Methods: The study included 13 healthy subjects and 12 recovering COVID-19 subjects within 8 wk of confirmed diagnosis. Each subject had a 1-h dynamic 18F-FDG scan on the uEXPLORER total-body PET/CT system. Semiquantitative SUV and the SUV ratio relative to blood (SUVR) were calculated for different organs to measure glucose utilization. Tracer kinetic modeling was performed to quantify the microparametric blood-to-tissue 18F-FDG delivery rate [Formula: see text] and the phosphorylation rate k 3, as well as the macroparametric 18F-FDG net influx rate ([Formula: see text]). Statistical tests were performed to examine differences between healthy subjects and recovering COVID-19 subjects. The effect of COVID-19 vaccination was also investigated. Results: We detected no significant difference in lung SUV but significantly higher lung SUVR and [Formula: see text] in COVID-19 recovery, indicating improved sensitivity of kinetic quantification for detecting the difference in glucose metabolism. A significant difference was also observed in the lungs with the phosphorylation rate k 3 but not with [Formula: see text], which suggests that glucose phosphorylation, rather than glucose delivery, drives the observed difference of glucose metabolism. Meanwhile, there was no or little difference in bone marrow 18F-FDG metabolism measured with SUV, SUVR, and [Formula: see text] but a significantly higher bone marrow [Formula: see text] in the COVID-19 group, suggesting a difference in glucose delivery. Vaccinated COVID-19 subjects had a lower lung [Formula: see text] and a higher spleen [Formula: see text] than unvaccinated COVID-19 subjects. Conclusion: Higher lung glucose metabolism and bone marrow glucose delivery were observed with total-body multiparametric 18F-FDG PET in recovering COVID-19 subjects than in healthy subjects, implying continued inflammation during recovery. Vaccination demonstrated potential protection effects. Total-body multiparametric PET of 18F-FDG can provide a more sensitive tool and more insights than conventional whole-body static 18F-FDG imaging to evaluate metabolic changes in systemic diseases such as COVID-19.

High-Temporal-Resolution Lung Kinetic Modeling Using Total-Body Dynamic PET with Time-Delay and Dispersion Corrections

Tracer kinetic modeling in dynamic PET has the potential to improve the diagnosis, prognosis, and research of lung diseases. The advent of total-body PET systems with much greater detection sensitivity enables high-temporal-resolution (HTR) dynamic PET imaging of the lungs. However, existing models may become insufficient for modeling the HTR data. In this paper, we investigate the necessity of additional corrections to the input function for HTR lung kinetic modeling. Methods: Dynamic scans with HTR frames of as short as 1 s were performed on 13 healthy subjects with a bolus injection of about [Formula: see text] of 18F-FDG using the uEXPLORER total-body PET/CT system. Three kinetic models with and without time-delay and dispersion corrections were compared for the quality of lung time-activity curve fitting using the Akaike information criterion. The impact on quantification of 18F-FDG delivery rate [Formula: see text], net influx rate [Formula: see text] and fractional blood volume [Formula: see text] was assessed. Parameter identifiability analysis was also performed to evaluate the reliability of kinetic quantification with respect to noise. Correlation of kinetic parameters with age was investigated. Results: HTR dynamic imaging clearly revealed the rapid change in tracer concentration in the lungs and blood supply (i.e., the right ventricle). The uncorrected input function led to poor time-activity curve fitting and biased quantification in HTR kinetic modeling. The fitting was improved by time-delay and dispersion corrections. The proposed model resulted in an approximately 85% decrease in [Formula: see text], an approximately 75% increase in [Formula: see text], and a more reasonable [Formula: see text] (∼0.14) than the uncorrected model (∼0.04). The identifiability analysis showed that the proposed models had good quantification stability for [Formula: see text], [Formula: see text], and [Formula: see text] The [Formula: see text] estimated by the proposed model with simultaneous time-delay and dispersion corrections correlated inversely with age, as would be expected. Conclusion: Corrections to the input function are important for accurate lung kinetic analysis of HTR dynamic PET data. The modeling of both delay and dispersion can improve model fitting and significantly impact quantification of [Formula: see text], [Formula: see text], and [Formula: see text].

Prone position decreases acute lung inflammation measured by [11C](R)-PK11195 positron emission tomography in experimental acute respiratory distress syndrome

Whether prone positioning (PP) modulates acute lung inflammation by the modulation of biomechanical forces of ventilator-induced lung injuries (VILIs) remains unclear. We aimed to demonstrate that PP decreases acute lung inflammation in animals with experimental acute respiratory distress syndrome (ARDS). Animals were under general anesthesia and protective ventilation (tidal volume 6 mL·kg^-1, PEEP 5 cmH₂O). ARDS was induced by intratracheal instillation of chlorohydric acid. Animals were then randomized to PP, or to supine position (SP). After 4 h, a positron emission tomography (PET) acquisition with [¹¹C](R)-PK11195 was performed coupled with computerized tomography (CT) acquisitions, allowing the CT quantification of VILI-associated parameters. [¹¹C](R)-PK11195 lung uptake was quantified using pharmacokinetic multicompartment models. Analyses were performed on eight lung sections distributed along the antero-posterior dimension. Six animals were randomized to PP, five to SP (median [Formula: see text]/[Formula: see text] [interquartile range]: 164 [102-269] mmHg). The normally aerated compartment was significantly redistributed to the posterior lung regions of animals in PP, compared with SP. Dynamic strain was significantly increased in posterior regions of SP animals, compared with PP. After 4 h, animals in PP had a significantly lower uptake of [¹¹C](R)-PK11195, compared with SP. [¹¹C](R)-PK11195 regional uptake was independently associated with the study group, dynamic strain, tidal hyperinflation, and regional respiratory system compliance in multivariate analysis. In an experimental model of ARDS, 4 h of PP significantly decreased acute lung inflammation assessed with PET. The beneficial impact of PP on acute lung inflammation was consecutive to the combination of decreased biomechanical forces and changes in the respiratory system mechanics.NEW & NOTEWORTHY Prone position decreases acute lung macrophage inflammation quantified in vivo with [¹¹C](R)-PK11195 positron emission tomography in an experimental acute respiratory distress syndrome. Regional macrophage inflammation is maximal in the most anterior and posterior lung section of supine animals, in relation with increased regional tidal strain and hyperinflation, and reduced regional lung compliance.

Modern Imaging Techniques in the Study and Disease Diagnosis of the Mammary Glands of Animals

The study of the structure and function of the animals' mammary glands is of key importance, as it reveals pathological processes at their onset, thus contributing to their immediate treatment. The most frequently studied mammary diseases are mastitis in cows and ewes and mammary tumours in dogs and cats. Various imaging techniques such as computed tomography, positron emission tomography, magnetic resonance imaging, and ultrasonographic techniques (Doppler, contrast-enchanced, three-dimensional and elastography) are available and can be applied in research or clinical practice in order to evaluate possible abnormalities in mammary glands, as well as to assist in the differential diagnosis. In this review, the above imaging technologies are described, and the perspectives of each method are highlighted. It is inferred that ultrasonographic modalities are the most frequently used imaging techniques for the diagnosis of clinical or subclinical mastitis and treatment guidance on a farm. In companion animals, a combination of imaging techniques should be applied for a more accurate diagnosis of mammary tumours. In any case, the confirmation of the diagnosis is provided by laboratory techniques.

Non-invasive quantification of acute macrophagic lung inflammation with [11C](R)-PK11195 using a three-tissue compartment kinetic model in experimental acute respiratory distress syndrome

PURPOSE

Imaging of acute lung inflammation is pivotal to evaluate innovative ventilation strategies. We aimed to develop and validate a three-tissue compartment kinetic model (3TCM) of [¹¹C](R)-PK11195 lung uptake in experimental acute respiratory distress syndrome (ARDS) to help quantify macrophagic inflammation, while accounting for the impact of its non-specific and irreversible uptake in lung tissues.

MATERIAL AND METHODS

We analyzed the data of 38 positron emission tomography (PET) studies performed in 21 swine with or without experimental ARDS, receiving general anesthesia and mechanical ventilation. Model input function was a plasma, metabolite-corrected, image-derived input function measured in the main pulmonary artery. Regional lung analysis consisted in applying both the 3TCM and the two-tissue compartment model (2TCM); in each region, the best model was selected using a selection algorithm with a goodness-of-fit criterion. Regional best model binding potentials (BP_ND) were compared to lung macrophage presence, semi-quantified in pathology.

RESULTS

The 3TCM was preferred in 142 lung regions (62%, 95% confidence interval: 56 to 69%). BP_ND determined by the 2TCM was significantly higher than the value computed with the 3TCM (overall median with interquartile range: 0.81 [0.44-1.33] vs. 0.60 [0.34-0.94], p < 0.02). Regional macrophage score was significantly associated with the best model BP_ND (p = 0.03). Regional BP_ND was significantly increased in the hyperinflated lung compartment, compared to the normally aerated one (median with interquartile range: 0.8 [0.6-1.7] vs. 0.6 [0.3-0.8], p = 0.03).

CONCLUSION

To assess the intensity and spatial distribution of acute macrophagic lung inflammation in the context of experimental ARDS with mechanical ventilation, PET quantification of [¹¹C](R)-PK11195 lung uptake was significantly improved in most lung regions using the 3TCM. This new methodology offers the opportunity to non-invasively evaluate innovative ventilatory strategies aiming at controlling acute lung inflammation.

Pulmonary functional imaging (PFI): A historical review and perspective

PFI Pulmonary Functional Imaging (PFI) refers to visualization and measurement of ventilation, perfusion, gas flow and exchange as well as biomechanics. In this review, we will highlight the historical development of PFI, describing recent advances and listing the various techniques for PFI offered per modality. Challenges PFI is facing and requirements for PFI from a clinical point of view will be pointed out. Hereby the review is meant as an introduction to PFI.

Measurement of PET Quantitative Bias In Vivo

Quantitative imaging biomarkers are widely used in PET for both research and clinical applications, yet bias in the underlying image data has not been well characterized. In the absence of a readily available reference standard for in vivo quantification, bias in PET images has been inferred using physical phantoms, even though arrangements of this sort provide only a poor approximation of the imaging environment in real patient examinations. In this study, we used data acquired from patient volunteers to assess PET quantitative bias in vivo. Image-derived radioactivity concentrations in the descending aorta were compared with blood samples counted on a calibrated γ-counter. Methods: Ten patients with prostate cancer were studied using 2-(3-(1-carboxy-5-[(6-¹⁸F-fluoro-pyridine-3-carbonyl)-amino]-pentyl)-ureido)-pentanedioic acid PET/CT. For each patient, 3 whole-body PET/CT image series were acquired after a single administration of the radiotracer: shortly after injection as well as approximately 1 and 4 h later. Venous blood samples were obtained at 8 time points over an 8-h period, and whole blood was counted on a NaI γ-counter. A 10-mm-diameter, 20-mm-long cylindric volume of interest was positioned in the descending thoracic aorta to estimate the PET-derived radioactivity concentration in blood. A triexponential function was fit to the γ-counter blood data and used to estimate the radioactivity concentration at the time of each PET acquisition. Results: The PET-derived and γ-counter-derived radioactivity concentrations were linearly related, with an R ² of 0.985, over a range of relevant radioactivity concentrations. The mean difference between the PET and γ-counter data was 4.8% ± 8.6%, with the PET measurements tending to be greater. Conclusion: Human image data acquired on a conventional whole-body PET/CT system with a typical clinical protocol differed by an average of around 5% from blood samples counted on a calibrated γ-counter. This bias may be partly attributable to residual uncorrected scatter or attenuation correction error. These data offer an opportunity for the assessment of PET bias in vivo and provide additional support for the use of quantitative imaging biomarkers.

Inflammatory Activity in Atelectatic and Normally Aerated Regions During Early Acute Lung Injury

RATIONALE AND OBJECTIVES

Pulmonary atelectasis presumably promotes and facilitates lung injury. However, data are limited on its direct and remote relation to inflammation. We aimed to assess regional 2-deoxy-2-[¹⁸F]-fluoro-D-glucose (¹⁸F-FDG) kinetics representative of inflammation in atelectatic and normally aerated regions in models of early lung injury.

MATERIALS AND METHODS

We studied supine sheep in four groups: Permissive Atelectasis (n = 6)-16 hours protective tidal volume (V_T) and zero positive end-expiratory pressure; Mild (n = 5) and Moderate Endotoxemia (n = 6)- 20-24 hours protective ventilation and intravenous lipopolysaccharide (Mild = 2.5 and Moderate = 10.0 ng/kg/min), and Surfactant Depletion (n = 6)-saline lung lavage and 4 hours high V_T. Measurements performed immediately after anesthesia induction served as controls (n = 8). Atelectasis was defined as regions of gas fraction <0.1 in transmission or computed tomography scans. ¹⁸F-FDG kinetics measured with positron emission tomography were analyzed with a three-compartment model.

RESULTS

¹⁸F-FDG net uptake rate in atelectatic tissue was larger during Moderate Endotoxemia (0.0092 ± 0.0019/min) than controls (0.0051 ± 0.0014/min, p = 0.01). ¹⁸F-FDG phosphorylation rate in atelectatic tissue was larger in both endotoxemia groups (0.0287 ± 0.0075/min) than controls (0.0198 ± 0.0039/min, p = 0.05) while the ¹⁸F-FDG volume of distribution was not significantly different among groups. Additionally, normally aerated regions showed larger ¹⁸F-FDG uptake during Permissive Atelectasis (0.0031 ± 0.0005/min, p < 0.01), Mild (0.0028 ± 0.0006/min, p = 0.04), and Moderate Endotoxemia (0.0039 ± 0.0005/min, p < 0.01) than controls (0.0020 ± 0.0003/min).

CONCLUSION

Atelectatic regions present increased metabolic activation during moderate endotoxemia mostly due to increased ¹⁸F-FDG phosphorylation, indicative of increased cellular metabolic activation. Increased ¹⁸F-FDG uptake in normally aerated regions during permissive atelectasis suggests an injurious remote effect of atelectasis even with protective tidal volumes.

Vaping disrupts ventilation-perfusion matching in asymptomatic users

Inhalation of e-cigarette's aerosols (vaping) has the potential to disrupt pulmonary gas exchange, but the effects in asymptomatic users are unknown. We assessed ventilation-perfusion (V̇_A/Q̇) mismatch in asymptomatic e-cigarette users, using magnetic resonance imaging (MRI). We hypothesized that vaping induces V̇_A/Q̇ mismatch through alterations in both ventilation and perfusion distributions. Nine young, asymptomatic "Vapers" with >1-yr vaping history, and no history of cardiopulmonary disease, were imaged supine using proton MRI, to assess the right lung at baseline and immediately after vaping. Seven young "Controls" were imaged at baseline only. Relative dispersion (SD/means) was used to quantify the heterogeneity of the individual ventilation and perfusion distributions. V̇_A/Q̇ mismatch was quantified using the second moments of the ventilation and perfusion versus V̇_A/Q̇ ratio distributions, log scale, LogSDV̇, and LogSDQ̇, respectively, analogous to the multiple inert gas elimination technique. Spirometry was normal in both groups. Ventilation heterogeneity was similar between groups at baseline (Vapers, 0.43 ± 0.13; Controls, 0.51 ± 0.11; P = 0.13) but increased after vaping (to 0.57 ± 0.17; P = 0.03). Perfusion heterogeneity was greater (P = 0.04) in Vapers at baseline (0.53 ± 0.06) compared with Controls (0.44 ± 0.10) but decreased after vaping (to 0.42 ± 0.07; P = 0.005). Vapers had greater (P = 0.01) V̇_A/Q̇ mismatch at baseline compared with Controls (LogSDQ̇ = 0.61 ± 0.12 vs. 0.43 ± 0.12), which was increased after vaping (LogSDQ̇ = 0.73 ± 0.16; P = 0.03). V̇_A/Q̇ mismatch is greater in Vapers and worsens after vaping. This suggests subclinical alterations in lung function not detected by spirometry.NEW & NOTEWORTHY This research provides evidence of vaping-induced disruptions in ventilation-perfusion matching in young, healthy, asymptomatic adults with normal spirometry who habitually vape. The changes in ventilation and perfusion distributions, both at baseline and acutely after vaping, and the potential implications on hypoxic vasoconstriction are particularly relevant in understanding the pathogenesis of vaping-induced dysfunction. Our imaging-based approach provides evidence of potential subclinical alterations in lung function below thresholds of detection using spirometry.

Reproducibility of compartmental modelling of 18F-FDG PET/CT to evaluate lung inflammation

Introduction

Compartmental modelling is an established method of quantifying ¹⁸F-FDG uptake; however, only recently has it been applied to evaluate pulmonary inflammation. Implementation of compartmental models remains challenging in the lung, partly due to the low signal-to-noise ratio compared to other organs and the lack of standardisation. Good reproducibility is a key requirement of an imaging biomarker which has yet to be demonstrated in pulmonary compartmental models of ¹⁸F-FDG; in this paper, we address this unmet need.

Methods

Retrospective subject data were obtained from the EVOLVE observational study: Ten COPD patients (age =66±9; 8M/2F), 10 α₁ATD patients (age =63±8; 7M/3F) and 10 healthy volunteers (age =68±8; 9M/1F) never smokers. PET and CT images were co-registered, and whole lung regions were extracted from CT using an automated algorithm; the descending aorta was defined using a manually drawn region. Subsequent stages of the compartmental analysis were performed by two independent operators using (i) a MIAKAT^TM based pipeline and (ii) an in-house developed pipeline. We evaluated the metabolic rate constant of ¹⁸F-FDG (K_im) and the fractional blood volume (V_b); Bland-Altman plots were used to compare the results. Further, we adjusted the in-house pipeline to identify the salient features in the analysis which may help improve the standardisation of this technique in the lung.

Results

The initial agreement on a subject level was poor: Bland-Altman coefficients of reproducibility for K_im and V_b were 0.0031 and 0.047 respectively. However, the effect size between the groups (i.e. COPD, α₁ATD and healthy subjects) was similar using either pipeline. We identified the key drivers of this difference using an incremental approach: ROI methodology, modelling of the IDIF and time delay estimation. Adjustment of these factors led to improved Bland-Altman coefficients of reproducibility of 0.0015 and 0.027 for K_im and V_b respectively.

Conclusions

Despite similar methodology, differences in implementation can lead to disparate results in the outcome parameters. When reporting the outcomes of lung compartmental modelling, we recommend the inclusion of the details of ROI methodology, input function fitting and time delay estimation to improve reproducibility.

Pharmacokinetics of 99mTc-HMPAO in isolated perfused rat lungs

Lung uptake of technetium-labeled hexamethylpropyleneamine oxime (HMPAO) increases in rat models of human acute lung injury, consistent with increases in lung tissue glutathione (GSH). Since 99mTc-HMPAO uptake is the net result of multiple cellular and vascular processes, the objective was to develop an approach to investigate the pharmacokinetics of 99mTc-HMPAO uptake in isolated perfused rat lungs. Lungs of anesthetized rats were excised and connected to a ventilation-perfusion system. 99mTc-HMPAO (56 MBq) was injected into the pulmonary arterial cannula, a time sequence of images was acquired, and lung time-activity curves were constructed. Imaging was repeated with a range of pump flows and perfusate albumin concentrations and before and after depletion of GSH with diethyl maleate (DEM). A pharmacokinetic model of 99mTc-HMPAO pulmonary disposition was developed and used for quantitative interpretation of the time-activity curves. Experimental results reveal that 99mTc-HMPAO lung uptake, defined as the steady-state value of the 99mTc-HMPAO lung time-activity curve, was inversely related to pump flow. Also, 99mTc-HMPAO lung uptake decreased by ~65% after addition of DEM to the perfusate. Increased perfusate albumin concentration also resulted in decreased 99mTc-HMPAO lung uptake. Model simulations under in vivo flow conditions indicate that lung tissue GSH is the dominant factor in 99mTc-HMPAO retention in lung tissue. The approach allows for evaluation of the dominant factors that determine imaging biomarker uptake, separation of the contributions of pulmonary versus systemic processes, and application of this knowledge to in vivo studies.

NEW & NOTEWORTHY We developed an approach for studying the pharmacokinetics of technetium-labeled hexamethylpropyleneamine oxime (99mTc-HMPAO) in isolated perfused lungs. A distributed-in-space-and-time computational model was fit to data and used to investigate questions that cannot readily be addressed in vivo. Experimental and modeling results indicate that tissue GSH is the dominant factor in 99mTc-HMPAO retention in lung tissue. This modeling approach can be readily extended to investigate the lung pharmacokinetics of other biomarkers and models of lung injury and treatment thereof.

Does Regional Lung Strain Correlate With Regional Inflammation in Acute Respiratory Distress Syndrome During Nonprotective Ventilation? An Experimental Porcine Study

OBJECTIVE

It is known that ventilator-induced lung injury causes increased pulmonary inflammation. It has been suggested that one of the underlying mechanisms may be strain. The aim of this study was to investigate whether lung regional strain correlates with regional inflammation in a porcine model of acute respiratory distress syndrome.

DESIGN

Retrospective analysis of CT images and positron emission tomography images using [F]fluoro-2-deoxy-D-glucose.

SETTING

University animal research laboratory.

SUBJECTS

Seven piglets subjected to experimental acute respiratory distress syndrome and five ventilated controls.

INTERVENTIONS

Acute respiratory distress syndrome was induced by repeated lung lavages, followed by 210 minutes of injurious mechanical ventilation using low positive end-expiratory pressures (mean, 4 cm H2O) and high inspiratory pressures (mean plateau pressure, 45 cm H2O). All animals were subsequently studied with CT scans acquired at end-expiration and end-inspiration, to obtain maps of volumetric strain (inspiratory volume - expiratory volume)/expiratory volume, and dynamic positron emission tomography imaging. Strain maps and positron emission tomography images were divided into 10 isogravitational horizontal regions-of-interest, from which spatial correlation was calculated for each animal.

MEASUREMENTS AND MAIN RESULTS

The acute respiratory distress syndrome model resulted in a decrease in respiratory system compliance (20.3 ± 3.4 to 14.0 ± 4.9 mL/cm H2O; p < 0.05) and oxygenation (PaO2/FIO2, 489 ± 80 to 92 ± 59; p < 0.05), whereas the control animals did not exhibit changes. In the acute respiratory distress syndrome group, strain maps showed a heterogeneous distribution with a greater concentration in the intermediate gravitational regions, which was similar to the distribution of [F]fluoro-2-deoxy-D-glucose uptake observed in the positron emission tomography images, resulting in a positive spatial correlation between both variables (median R = 0.71 [0.02-0.84]; p < 0.05 in five of seven animals), which was not observed in the control animals.

CONCLUSION

In this porcine acute respiratory distress syndrome model, regional lung strain was spatially correlated with regional inflammation, supporting that strain is a relevant and prominent determinant of ventilator-induced lung injury.

Noninvasive quantification of macrophagic lung recruitment during experimental ventilation-induced lung injury

Macrophagic lung infiltration is pivotal in the development of lung biotrauma because of ventilation-induced lung injury (VILI). We assessed the performance of [¹¹C](R)-PK11195, a positron emission tomography (PET) radiotracer binding the translocator protein, to quantify macrophage lung recruitment during experimental VILI. Pigs (n = 6) were mechanically ventilated under general anesthesia, using protective ventilation settings (baseline). Experimental VILI was performed by titrating tidal volume to reach a transpulmonary end-inspiratory pressure (∆P_L) of 35-40 cmH₂O. We acquired PET/computed tomography (CT) lung images at baseline and after 4 h of VILI. Lung macrophages were quantified in vivo by the standardized uptake value (SUV) of [¹¹C](R)-PK11195 measured in PET on the whole lung and in six lung regions and ex vivo on lung pathology at the end of experiment. Lung mechanics were extracted from CT images to assess their association with the PET signal. ∆P_L increased from 9 ± 1 cmH₂O under protective ventilation, to 36 ± 6 cmH₂O during experimental VILI. Compared with baseline, whole-lung [¹¹C](R)-PK11195 SUV significantly increased from 1.8 ± 0.5 to 2.9 ± 0.5 after experimental VILI. Regional [¹¹C](R)-PK11195 SUV was positively associated with the magnitude of macrophage recruitment in pathology (P = 0.03). Compared with baseline, whole-lung CT-derived dynamic strain and tidal hyperinflation increased significantly after experimental VILI, from 0.6 ± 0 to 2.0 ± 0.4, and 1 ± 1 to 43 ± 19%, respectively. On multivariate analysis, both were significantly associated with regional [¹¹C](R)-PK11195 SUV. [¹¹C](R)-PK11195 lung uptake (a proxy of lung inflammation) was increased by experimental VILI and was associated with the magnitude of dynamic strain and tidal hyperinflation.NEW & NOTEWORTHY We assessed the performance of [¹¹C](R)-PK11195, a translocator protein-specific positron emission tomography (PET) radiotracer, to quantify macrophage lung recruitment during experimental ventilation-induced lung injury (VILI). In this proof-of-concept study, we showed that the in vivo quantification of [¹¹C](R)-PK11195 lung uptake in PET reflected the magnitude of macrophage lung recruitment after VILI. Furthermore, increased [¹¹C](R)-PK11195 lung uptake was associated with harmful levels of dynamic strain and tidal hyperinflation applied to the lungs.

Spatial Heterogeneity of Lung Strain and Aeration and Regional Inflammation During Early Lung Injury Assessed with PET/CT

INTRODUCTION

Spatial heterogeneity of lung aeration and strain (change volume/resting volume) occurs at microscopic levels and contributes to lung injury. Yet, it is mostly assessed with histograms or large regions-of-interest. Spatial heterogeneity could also influence regional gene expression. We used positron emission tomography (PET)/computed tomography (CT) to assess the contribution of different length-scales to mechanical heterogeneity and to direct lung injury biological pathway identification.

MATERIALS AND METHODS

Sheep exposed to mild (n = 5, supine and n = 3, prone) and moderate (n = 6, supine) systemic endotoxemia were protectively ventilated. At baseline, 6 hours and 20 hours length-scale analysis was applied to aeration in CT (mild groups) and PET transmission (moderate group) scans; and voxel-level strain derived from image registration of end-inspiratory and end-expiratory CTs (mild). 2-deoxy-2-[(18)F]fluoro-d-glucose (18F-FDG)-PET kinetics parameters in ventral and dorsal regions were correlated with tissue microarray gene expression (moderate).

RESULTS

While aeration and strain heterogeneity were highest at 5-10 mm length-scales, larger length-scales contained a higher fraction of strain than aeration heterogeneity. Contributions of length-scales >5-10 mm to aeration and strain heterogeneity increased as lung injury progressed (p < 0.001) and were higher in supine than prone animals. Genes expressed with regional correlation to 18F-FDG-PET kinetics (|r| = 0.81 [0.78-0.85]) yielded pathways associated with immune system activation and fluid clearance.

CONCLUSION

Normal spatial heterogeneity of aeration and strain suggest larger anatomical and functional determinants of lung strain than aeration heterogeneity. Lung injury and supine position increase the contribution of larger length-scales. 18F-FDG-PET-based categorization of gene expression results in known and novel biological pathways relevant to lung injury.

Improved quantitation and reproducibility in multi-PET/CT lung studies by combining CT information

Background

Matched attenuation maps are vital for obtaining accurate and reproducible kinetic and static parameter estimates from PET data. With increased interest in PET/CT imaging of diffuse lung diseases for assessing disease progression and treatment effectiveness, understanding the extent of the effect of respiratory motion and establishing methods for correction are becoming more important. In a previous study, we have shown that using the wrong attenuation map leads to large errors due to density mismatches in the lung, especially in dynamic PET scans. Here, we extend this work to the case where the study is sub-divided into several scans, e.g. for patient comfort, each with its own CT (cine-CT and ‘snap shot’ CT). A method to combine multi-CT information into a combined-CT has then been developed, which averages the CT information from each study section to produce composite CT images with the lung density more representative of that in the PET data. This combined-CT was applied to nine patients with idiopathic pulmonary fibrosis, imaged with dynamic ¹⁸F-FDG PET/CT to determine the improvement in the precision of the parameter estimates.

Results

Using XCAT simulations, errors in the influx rate constant were found to be as high as 60% in multi-PET/CT studies. Analysis of patient data identified displacements between study sections in the time activity curves, which led to an average standard error in the estimates of the influx rate constant of 53% with conventional methods. This reduced to within 5% after use of combined-CTs for attenuation correction of the study sections.

Conclusions

Use of combined-CTs to reconstruct the sections of a multi-PET/CT study, as opposed to using the individually acquired CTs at each study stage, produces more precise parameter estimates and may improve discrimination between diseased and normal lung.

Quantification of Lung PET Images: Challenges and Opportunities

Millions of people are affected by respiratory diseases, leading to a significant health burden globally. Because of the current insufficient knowledge of the underlying mechanisms that lead to the development and progression of respiratory diseases, treatment options remain limited. To overcome this limitation and understand the associated molecular changes, noninvasive imaging techniques such as PET and SPECT have been explored for biomarker development, with ¹⁸F-FDG PET imaging being the most studied. The quantification of pulmonary molecular imaging data remains challenging because of variations in tissue, air, blood, and water fractions within the lungs. The proportions of these components further differ depending on the lung disease. Therefore, different quantification approaches have been proposed to address these variabilities. However, no standardized approach has been developed to date. This article reviews the data evaluating ¹⁸F-FDG PET quantification approaches in lung diseases, focusing on methods to account for variations in lung components and the interpretation of the derived parameters. The diseases reviewed include acute respiratory distress syndrome, chronic obstructive pulmonary disease, and interstitial lung diseases such as idiopathic pulmonary fibrosis. Based on review of prior literature, ongoing research, and discussions among the authors, suggested considerations are presented to assist with the interpretation of the derived parameters from these approaches and the design of future studies.

Lung Metabolic Activation as an Early Biomarker of Acute Respiratory Distress Syndrome and Local Gene Expression Heterogeneity

BACKGROUND

Acute respiratory distress syndrome (ARDS) is an inflammatory condition comprising diffuse lung edema and alveolar damage. ARDS frequently results from regional injury mechanisms. However, it is unknown whether detectable inflammation precedes lung edema and opacification and whether topographically differential gene expression consistent with heterogeneous injury occurs in early ARDS. The authors aimed to determine the temporal relationship between pulmonary metabolic activation and density in a large animal model of early ARDS and to assess gene expression in differentially activated regions.

METHODS

The authors produced ARDS in sheep with intravenous lipopolysaccharide (10 ng ⋅ kg ⋅ h) and mechanical ventilation for 20 h. Using positron emission tomography, the authors assessed regional cellular metabolic activation with 2-deoxy-2-[(18)F]fluoro-D-glucose, perfusion and ventilation with NN-saline, and aeration using transmission scans. Species-specific microarray technology was used to assess regional gene expression.

RESULTS

Metabolic activation preceded detectable increases in lung density (as required for clinical diagnosis) and correlated with subsequent histologic injury, suggesting its predictive value for severity of disease progression. Local time courses of metabolic activation varied, with highly perfused and less aerated dependent lung regions activated earlier than nondependent regions. These regions of distinct metabolic trajectories demonstrated differential gene expression for known and potential novel candidates for ARDS pathogenesis.

CONCLUSIONS

Heterogeneous lung metabolic activation precedes increases in lung density in the development of ARDS due to endotoxemia and mechanical ventilation. Local differential gene expression occurs in these early stages and reveals molecular pathways relevant to ARDS biology and of potential use as treatment targets.

Allergic Non-Asthmatic Adults Have Regional Pulmonary Responses to Segmental Allergen Challenge

Background

Allergic non-asthmatic (ANA) adults experience upper airway symptoms of allergic disease such as rhinorrhea, congestion and sneezing without symptoms of asthma. The aim of this study was to utilize PET-CT functional imaging to determine whether allergen challenge elicits a pulmonary response in ANA subjects or whether their allergic disease is truly isolated to the upper airways.

Methods

In 6 ANA subjects, bronchoalveolar lavages (BAL) were performed at baseline and 24h after instillation of an allergen and a diluent in separate lung lobes. After instillation (10h), functional imaging was performed to quantify and compare regional perfusion, ventilation, fractional gas content (Fgas), and glucose uptake rate (Ki) between the baseline, diluent and allergen lobes. BAL cell counts were also compared.

Results

In ANA subjects, compared to the baseline and diluent lobes, perfusion and ventilation were significantly lower in the allergen lobe (median [inter-quartile range], baseline vs. diluent vs. allergen: Mean-normalized perfusion; 0.87 [0.85–0.97] vs. 0.90 [0.86–0.98] vs. 0.59 [0.55–0.67]; p<0.05. Mean-normalized ventilation 0.89 [0.88–0.98] vs. 0.95 [0.89–1.02] vs. 0.63 [0.52–0.67], p<0.05). In contrast, no significant differences were found in Fgas between baseline, diluent and allergen lobes or in Ki. Total cell counts, eosinophil and neutrophil cell counts (cells/ml BAL) were significantly greater in the allergen lobe compared to the baseline lobe (all P<0.05).

Conclusions

Despite having no clinical symptoms of a lower airway allergic response (cough and wheeze) allergic non-asthmatic subjects have a pulmonary response to allergen exposure which manifests as reduced ventilation and perfusion.

Lung inflammation persists after 27 hours of protective Acute Respiratory Distress Syndrome Network Strategy and is concentrated in the nondependent lung

OBJECTIVE

PET with [18F]fluoro-2-deoxy-D-glucose can be used to image cellular metabolism, which during lung inflammation mainly reflects neutrophil activity, allowing the study of regional lung inflammation in vivo. We aimed at studying the location and evolution of inflammation by PET imaging, relating it to morphology (CT), during the first 27 hours of application of protective-ventilation strategy as suggested by the Acute Respiratory Distress Syndrome Network, in a porcine experimental model of acute respiratory distress syndrome.

DESIGN

Prospective laboratory investigation.

SETTING

University animal research laboratory.

SUBJECTS

Ten piglets submitted to an experimental model of acute respiratory distress syndrome.

INTERVENTIONS

Lung injury was induced by lung lavages and 210 minutes of injurious mechanical ventilation using low positive end-expiratory pressure and high inspiratory pressures. During 27 hours of controlled mechanical ventilation according to Acute Respiratory Distress Syndrome Network strategy, the animals were studied with dynamic PET imaging of [18F]fluoro-2-deoxy-D-glucose at two occasions with 24-hour interval between them.

MEASUREMENTS AND MAIN RESULTS

[18F]fluoro-2-deoxy-D-glucose uptake rate was computed for the total lung, four horizontal regions from top to bottom (nondependent to dependent regions) and for voxels grouped by similar density using standard Hounsfield units classification. The global lung uptake was elevated at 3 and 27 hours, suggesting persisting inflammation. In both PET acquisitions, nondependent regions presented the highest uptake (p = 0.002 and p = 0.006). Furthermore, from 3 to 27 hours, there was a change in the distribution of regional uptake (p = 0.003), with more pronounced concentration of inflammation in nondependent regions. Additionally, the poorly aerated tissue presented the largest uptake concentration after 27 hours.

CONCLUSIONS

Protective Acute Respiratory Distress Syndrome Network strategy did not attenuate global pulmonary inflammation during the first 27 hours after severe lung insult. The strategy led to a concentration of inflammatory activity in the upper lung regions and in the poorly aerated lung regions. The present findings suggest that the poorly aerated lung tissue is an important target of the perpetuation of the inflammatory process occurring during ventilation according to the Acute Respiratory Distress Syndrome Network strategy.

Modeling of Tracer Transport Delays for Improved Quantification of Regional Pulmonary ¹⁸F-FDG Kinetics, Vascular Transit Times, and Perfusion

¹⁸F-FDG-PET is increasingly used to assess pulmonary inflammatory cell activity. However, current models of pulmonary ¹⁸F-FDG kinetics do not account for delays in ¹⁸F-FDG transport between the plasma sampling site and the lungs. We developed a three-compartment model of ¹⁸F-FDG kinetics that includes a delay between the right heart and the local capillary blood pool, and used this model to estimate regional pulmonary perfusion. We acquired dynamic ¹⁸F-FDG scans in 12 mechanically ventilated sheep divided into control and lung injury groups (n = 6 each). The model was fit to tracer kinetics in three isogravitational regions-of-interest to estimate regional lung transport delays and regional perfusion. ¹³NN bolus infusion scans were acquired during a period of apnea to measure regional perfusion using an established reference method. The delayed input function model improved description of ¹⁸F-FDG kinetics (lower Akaike Information Criterion) in 98% of studied regions. Local transport delays ranged from 2.0 to 13.6 s, averaging 6.4 ± 2.9 s, and were highest in non-dependent regions. Estimates of regional perfusion derived from model parameters were highly correlated with perfusion measurements based on ¹³NN-PET (R² = 0.92, p < 0.001). By incorporating local vascular transports delays, this model of pulmonary ¹⁸F-FDG kinetics allows for simultaneous assessment of regional lung perfusion, transit times, and inflammation.

Human Herpesvirus-6 Pneumonitis around the Engraftment of Cord Blood Transplantation following Foscarnet Prophylaxis in a Patient with Acute Leukemia

Human herpesvirus-6 (HHV-6) reactivation is sometimes observed in immunocompromised patients, especially after allogeneic stem cell transplantation. The complications of HHV-6 reactivation in this setting are mainly recognized as HHV-6 encephalitis. We herein report the case of a patient who developed HHV-6 pneumonitis after cord blood transplantation (CBT). A 35-year-old male underwent CBT for T-cell/myeloid mixed phenotype acute leukemia and achieved neutrophil engraftment on day 31. He had received foscarnet as prophylaxis for HHV-6 reactivation. A computed tomography (CT) scan to evaluate the leukemic tumor showed bilateral interstitial pneumonitis on day 33, although he had no respiratory symptoms. The findings of the CT scan were consistent with those of HHV-6 pneumonitis that were reported previously. HHV-6 DNA, but no other pathogens, was detected in his bronchoalveolar lavage (BAL) fluid. The patient was successfully treated with a therapeutic dose of foscarnet. This case indicates that performing a CT scan around the time of neutrophil engraftment can play an important role in detecting the early phase of HHV-6 pneumonia, and BAL should be considered if features consistent with HHV-6 pneumonitis are observed in patients with a risk of HHV-6 reactivation.

Effect of local tidal lung strain on inflammation in normal and lipopolysaccharide-exposed sheep*

OBJECTIVES

Regional tidal lung strain may trigger local inflammation during mechanical ventilation, particularly when additional inflammatory stimuli are present. However, it is unclear whether inflammation develops proportionally to tidal strain or only above a threshold. We aimed to 1) assess the relationship between regional tidal strain and local inflammation in vivo during the early stages of lung injury in lungs with regional aeration heterogeneity comparable to that of humans and 2) determine how this strain-inflammation relationship is affected by endotoxemia.

DESIGN

Interventional animal study.

SETTING

Experimental laboratory and PET facility.

SUBJECTS

Eighteen 2- to 4-month-old sheep.

INTERVENTIONS

Three groups of sheep (n = 6) were mechanically ventilated to the same plateau pressure (30-32 cm H2O) with high-strain (VT = 18.2 ± 6.5 mL/kg, positive end-expiratory pressure = 0), high-strain plus IV lipopolysaccharide (VT = 18.4 ± 4.2 mL/kg, positive end-expiratory pressure = 0), or low-strain plus lipopolysaccharide (VT = 8.1 ± 0.2 mL/kg, positive end-expiratory pressure = 17 ± 3 cm H2O). At baseline, we acquired respiratory-gated PET scans of inhaled NN to measure tidal strain from end-expiratory and end-inspiratory images in six regions of interest. After 3 hours of mechanical ventilation, dynamic [F]fluoro-2-deoxy-D-glucose scans were acquired to quantify metabolic activation, indicating local neutrophilic inflammation, in the same regions of interest.

MEASUREMENTS AND MAIN RESULTS

Baseline regional tidal strain had a significant effect on [F]fluoro-2-deoxy-D-glucose net uptake rate Ki in high-strain lipopolysaccharide (p = 0.036) and on phosphorylation rate k3 in high-strain (p = 0.027) and high-strain lipopolysaccharide (p = 0.004). Lipopolysaccharide exposure increased the k3-tidal strain slope three-fold (p = 0.009), without significant lung edema. The low-strain lipopolysaccharide group showed lower baseline regional tidal strain (0.33 ± 0.17) than high-strain (1.21 ± 0.62; p < 0.001) or high-strain lipopolysaccharide (1.26 ± 0.44; p < 0.001) and lower k3 (p < 0.001) and Ki (p < 0.05) than high-strain lipopolysaccharide.

CONCLUSIONS

Local inflammation develops proportionally to regional tidal strain during early lung injury. The regional inflammatory effect of strain is greatly amplified by IV lipopolysaccharide. Tidal strain enhances local [F]fluoro-2-deoxy-D-glucose uptake primarily by increasing the rate of intracellular [F]fluoro-2-deoxy-D-glucose phosphorylation.

Early inflammation mainly affects normally and poorly aerated lung in experimental ventilator-induced lung injury*

OBJECTIVE

The common denominator in most forms of ventilator-induced lung injury is an intense inflammatory response mediated by neutrophils. PET with [(18)F]fluoro-2-deoxy-D-glucose can be used to image cellular metabolism, which, during lung inflammatory processes, mainly reflects neutrophil activity, allowing the study of regional lung inflammation in vivo. The aim of this study was to assess the location and magnitude of lung inflammation using PET imaging of [(18)F]fluoro-2-deoxy-D-glucose in a porcine experimental model of early acute respiratory distress syndrome.

DESIGN

Prospective laboratory investigation.

SETTING

A university animal research laboratory.

SUBJECTS

Seven piglets submitted to experimental ventilator-induced lung injury and five healthy controls.

INTERVENTIONS

Lung injury was induced by lung lavages and 210 minutes of injurious mechanical ventilation using low positive end-expiratory pressure and high inspiratory pressures. All animals were subsequently studied with dynamic PET imaging of [(18)F]fluoro-2-deoxy-D-glucose. CT scans were acquired at end expiration and end inspiration.

MEASUREMENTS AND MAIN RESULTS

[(18)F]fluoro-2-deoxy-D-glucose uptake rate was computed for the whole lung, four isogravitational regions, and regions grouping voxels with similar density. Global and intermediate gravitational zones [(18)F]fluoro-2-deoxy-D-glucose uptakes were higher in ventilator-induced lung injury piglets compared with controls animals. Uptake of normally and poorly aerated regions was also higher in ventilator-induced lung injury piglets compared with control piglets, whereas regions suffering tidal recruitment or tidal hyperinflation had [(18)F]fluoro-2-deoxy-D-glucose uptakes similar to controls.

CONCLUSIONS

The present findings suggest that normally and poorly aerated regions--corresponding to intermediate gravitational zones--are the primary targets of the inflammatory process accompanying early experimental ventilator-induced lung injury. This may be attributed to the small volume of the aerated lung, which receives most of ventilation.

Lung [(18)F]fluorodeoxyglucose uptake and ventilation-perfusion mismatch in the early stage of experimental acute smoke inhalation

BACKGROUND

Acute lung injury occurs in a third of patients with smoke inhalation injury. Its clinical manifestations usually do not appear until 48-72 h after inhalation. Identifying inflammatory changes that occur in pulmonary parenchyma earlier than that could provide insight into the pathogenesis of smoke-induced acute lung injury. Furthermore, noninvasive measurement of such changes might lead to earlier diagnosis and treatment. Because glucose is the main source of energy for pulmonary inflammatory cells, the authors hypothesized that its pulmonary metabolism is increased shortly after smoke inhalation, when classic manifestations of acute lung injury are not yet expected.

METHODS

In five sheep, the authors induced unilateral injury with 48 breaths of cotton smoke while the contralateral lung served as control. The authors used positron emission tomography with: (1) [F]fluorodeoxyglucose to measure metabolic activity of pulmonary inflammatory cells; and (2) [N]nitrogen in saline to measure shunt and ventilation-perfusion distributions separately in the smoke-exposed and control lungs.

RESULTS

The pulmonary [F]fluorodeoxyglucose uptake rate was increased at 4 h after smoke inhalation (mean ± SD: 0.0031 ± 0.0013 vs. 0.0026 ± 0.0010 min; P < 0.05) mainly as a result of increased glucose phosphorylation. At this stage, there was no worsening in lung aeration or shunt. However, there was a shift of perfusion toward units with lower ventilation-to-perfusion ratio (mean ratio ± SD: 0.82 ± 0.10 vs. 1.12 ± 0.02; P < 0.05) and increased heterogeneity of the ventilation-perfusion distribution (mean ± SD: 0.21 ± 0.07 vs. 0.13 ± 0.01; P < 0 .05).

CONCLUSION

Using noninvasive imaging, the authors demonstrated that increased pulmonary [F]fluorodeoxyglucose uptake and ventilation-perfusion mismatch occur early after smoke inhalation.

Acute lung injury and pulmonary vascular permeability: use of transgenic models

Acute lung injury is a general term that describes injurious conditions that can range from mild interstitial edema to massive inflammatory tissue destruction. This review will cover theoretical considerations and quantitative and semi-quantitative methods for assessing edema formation and increased vascular permeability during lung injury. Pulmonary edema can be quantitated directly using gravimetric methods, or indirectly by descriptive microscopy, quantitative morphometric microscopy, altered lung mechanics, high-resolution computed tomography, magnetic resonance imaging, positron emission tomography, or x-ray films. Lung vascular permeability to fluid can be evaluated by measuring the filtration coefficient (Kf) and permeability to solutes evaluated from their blood to lung clearances. Albumin clearances can then be used to calculate specific permeability-surface area products (PS) and reflection coefficients (σ). These methods as applied to a wide variety of transgenic mice subjected to acute lung injury by hyperoxic exposure, sepsis, ischemia-reperfusion, acid aspiration, oleic acid infusion, repeated lung lavage, and bleomycin are reviewed. These commonly used animal models simulate features of the acute respiratory distress syndrome, and the preparation of genetically modified mice and their use for defining specific pathways in these disease models are outlined. Although the initiating events differ widely, many of the subsequent inflammatory processes causing lung injury and increased vascular permeability are surprisingly similar for many etiologies.

Use of ¹⁸F-fluorodeoxyglucose positron emission tomography-computed tomography in a miniature pig (Sus scrofa domestica) with pneumonia

A 1-y-old male miniature pig housed in our laboratory facility was evaluated for weight loss and rough coat condition. CBC results revealed neutrophilia. Radiography of the thoracic area showed increased opacity throughout the thoracic cavity except for the right caudal lobe. ¹⁸F-labeled fluorodeoxyglucose positron emission tomography-computed tomography (FDG-PET-CT) revealed elevated standard uptake values in the area corresponding to the radiologic findings. Follow-up thoracic radiography taken 2 wk after FDG-PET-CT showed several interval changes, including markedly decreased opacity throughout the entire thoracic cavity. Necropsy revealed adhesions between the upper portion of the caudal lobe of the left lung and thoracic wall. ELISA for several closely related infectious species confirmed the presence of antibody to Actinobacillus pleuropneumoniae serovar V.

Combining image-derived and venous input functions enables quantification of serotonin-1A receptors with [carbonyl-11C]WAY-100635 independent of arterial sampling

image- derived input functions (IDIFs) represent a promising technique for a simpler and less invasive quantification of PET studies as compared to arterial cannulation. However, a number of limitations complicate the routine use of IDIFs in clinical research protocols and the full substitution of manual arterial samples by venous ones has hardly been evaluated. This study aims for a direct validation of IDIFs and venous data for the quantification of serotonin-1A receptor binding (5-HT(1A)) with [carbonyl-(11)C]WAY-100635 before and after hormone treatment.

METHODS

Fifteen PET measurements with arterial and venous blood sampling were obtained from 10 healthy women, 8 scans before and 7 after eight weeks of hormone replacement therapy. Image-derived input functions were derived automatically from cerebral blood vessels, corrected for partial volume effects and combined with venous manual samples from 10 min onward (IDIF+VIF). Corrections for plasma/whole-blood ratio and metabolites were done separately with arterial and venous samples. 5-HT(1A) receptor quantification was achieved with arterial input functions (AIF) and IDIF+VIF using a two-tissue compartment model.

RESULTS

Comparison between arterial and venous manual blood samples yielded excellent reproducibility. Variability (VAR) was less than 10% for whole-blood activity (p>0.4) and below 2% for plasma to whole-blood ratios (p>0.4). Variability was slightly higher for parent fractions (VARmax=24% at 5 min, p<0.05 and VAR<13% after 20 min, p>0.1) but still within previously reported values. IDIFs after partial volume correction had peak values comparable to AIFs (mean difference Δ=-7.6 ± 16.9 kBq/ml, p>0.1), whereas AIFs exhibited a delay (Δ=4 ± 6.4s, p<0.05) and higher peak width (Δ=15.9 ± 5.2s, p<0.001). Linear regression analysis showed strong agreement for 5-HT(1A) binding as obtained with AIF and IDIF+VIF at baseline (R(2)=0.95), after treatment (R(2)=0.93) and when pooling all scans (R(2)=0.93), with slopes and intercepts in the range of 0.97 to 1.07 and -0.05 to 0.16, respectively. In addition to the region of interest analysis, the approach yielded virtually identical results for voxel-wise quantification as compared to the AIF.

CONCLUSIONS

Despite the fast metabolism of the radioligand, manual arterial blood samples can be substituted by venous ones for parent fractions and plasma to whole-blood ratios. Moreover, the combination of image-derived and venous input functions provides a reliable quantification of 5-HT(1A) receptors. This holds true for 5-HT(1A) binding estimates before and after treatment for both regions of interest-based and voxel-wise modeling. Taken together, the approach provides less invasive receptor quantification by full independence of arterial cannulation. This offers great potential for the routine use in clinical research protocols and encourages further investigation for other radioligands with different kinetic characteristics.

18F-FDG uptake rate is a biomarker of eosinophilic inflammation and airway response in asthma

In asthma, the relationship among airway inflammation, airway hyperresponsiveness, and lung function is poorly understood. Methods to noninvasively assess these relationships in human subjects are needed. We sought to determine whether (18)F-FDG uptake rate (K(i), min(-1)) could serve as a biomarker of eosinophilic inflammation and local lung function.

METHODS

We used PET/CT to assess regional pulmonary perfusion (Q), specific ventilation per unit volume (sV(A)), fractional gas content (Fgas), airway wall thickness, and regional K(i) 10 h after segmental allergen challenge to the right middle lobe in 6 asthmatic subjects with demonstrated atopy. Q, sV(A), and Fgas in the allergen-challenged lobe were compared with the right upper lobe, where diluent was applied as a control. The airway wall thickness aspect ratio (ω) of the allergen-challenged airway was compared with those of similarly sized airways from unaffected areas of the lung. Differences in K(i) between allergen and diluent segments were compared with those in cell counts obtained 24 h after the allergen challenge by a bronchoalveolar lavage of the respective segments.

RESULTS

We found systematic reductions in regional Q, sV(A), and Fgas and increased ω in all subjects. The ratio of eosinophil count (allergen to diluent) was linearly related (R(2) = 0.9917, P < 0.001) to the ratio of K(i).

CONCLUSION

Regional K(i) measured with PET is a noninvasive and highly predictive biomarker of eosinophilic airway inflammation and its functional effects. This method may serve to help in the understanding of allergic inflammation and test the therapeutic effectiveness of novel drugs or treatments.

Effects of surfactant depletion on regional pulmonary metabolic activity during mechanical ventilation

Inflammation during mechanical ventilation is thought to depend on regional mechanical stress. This can be produced by concentration of stresses and cyclic recruitment in low-aeration dependent lung. Positron emission tomography (PET) with (18)F-fluorodeoxyglucose ((18)F-FDG) allows for noninvasive assessment of regional metabolic activity, an index of neutrophilic inflammation. We tested the hypothesis that, during mechanical ventilation, surfactant-depleted low-aeration lung regions present increased regional (18)F-FDG uptake suggestive of in vivo increased regional metabolic activity and inflammation. Sheep underwent unilateral saline lung lavage and were ventilated supine for 4 h (positive end-expiratory pressure = 10 cmH(2)O, tidal volume adjusted to plateau pressure = 30 cmH(2)O). We used PET scans of injected (13)N-nitrogen to compute regional perfusion and ventilation and injected (18)F-FDG to calculate (18)F-FDG uptake rate. Regional aeration was quantified with transmission scans. Whole lung (18)F-FDG uptake was approximately two times higher in lavaged than in nonlavaged lungs (2.9 ± 0.6 vs. 1.5 ± 0.3 10(-3)/min; P < 0.05). The increased (18)F-FDG uptake was topographically heterogeneous and highest in dependent low-aeration regions (gas fraction 10-50%, P < 0.001), even after correction for lung density and wet-to-dry lung ratios. (18)F-FDG uptake in low-aeration regions of lavaged lungs was higher than that in low-aeration regions of nonlavaged lungs (P < 0.05). This occurred despite lower perfusion and ventilation to dependent regions in lavaged than nonlavaged lungs (P < 0.001). In contrast, (18)F-FDG uptake in normally aerated regions was low and similar between lungs. Surfactant depletion produces increased and heterogeneously distributed pulmonary (18)F-FDG uptake after 4 h of supine mechanical ventilation. Metabolic activity is highest in poorly aerated dependent regions, suggesting local increased inflammation.

Analysis of 2-[Fluorine-18]-Fluoro-2-deoxy-D-glucose uptake kinetics in PET studies of pulmonary inflammation

Dynamic positron emission tomography (PET) imaging of the lung using the radiotracer 2-[fluorine-18]-fluoro-2-deoxy-D-glucose ((18)F-FDG) is an emerging method to assess noninvasively the metabolic activity of pulmonary inflammatory cells. Nevertheless, because of the distinct functional and structural characteristics of inflamed lung tissue standard methods of (18)F-FDG analysis can be substantially limited and there is no consensus about the best method for quantification of the (18)F-FDG signal for acute or chronic inflammatory lung diseases. This article gives an overview on recent advances in quantitative analysis of (18)F-FDG uptake kinetics in non-neoplastic inflamed lung tissue.

Cumulative input function method for linear compartmental models and spectral analysis in PET

Compartmental modeling and spectral analysis are often used for tracer kinetic modeling in positron emission tomography (PET). The concentrations in kinetic equations are usually considered to be instantaneous, whereas PET data are inherently integrated over time, which leads to uncertainties in the results. A new formalism for kinetic analysis that uses cumulative tracer concentrations and avoids approximating the image-derived input function and PET measurements with midframe instantanous values was developed. We assessed the improvements of the new formalism over the midframe approximation methods for three commonly used radiopharmaceuticals: [(11)C]raclopride, 2'-deoxy-2'-[(18)F]fluoro-D-glucose (FDG), and 3'-deoxy-3'-[(18)F]fluoro-thymidine (FLT). We found that improvements are case dependent and often not negligible. Improvements for determination of binding potential for [(11)C]raclopride ranged from 5% to 25%. Improvements in estimation accuracy of FDG and FLT microparameters ranged up to 25%. On the other hand, estimation of macroparameter K(i)=K(1)k(3)/(k(2)+k(3)) for FDG or FLT did not show significant benefit with the new method; only modest improvement up to 2% was observed. Assessment of the benefits of using new method is far from being exhaustive, but possibly significant improvement was demonstrated. Therefore, we consider the proposed algorithm a necessary component of any kinetic analysis software.

Assessment of lung inflammation with 18F-FDG PET during acute lung injury

OBJECTIVE

The purpose of this review is to describe the current experimental and clinical data regarding the fundamentals and applications of (18)F-FDG PET during acute lung injury (ALI) and acute respiratory distress syndrome (ARDS).

CONCLUSION

Lung inflammation is a key feature of ALI. During ALI, FDG PET can be used to monitor lung neutrophils, which are essential cells in the pathophysiologic mechanisms of ALI. Pulmonary FDG kinetics are altered during experimental and human ALI and are associated with regional lung dysfunction, histologic abnormalities, and prognosis. FDG PET may be a valuable noninvasive method for gaining comprehensive understanding of the mechanisms of ALI/ARDS and for evaluating therapeutic interventions.

Effect of cigarette smoking on the washout time of sevoflurane anesthesia

Background

Cigarette smoking affects the pharmacodynamic and pharmacokinetic behavior of many drugs and causes deterioration of pulmonary mechanics. We have evaluated the effect of cigarette smoking on washout time after one minimum alveolar concentration-h (1 MAC-h) of sevoflurane anesthesia.

Methods

We investigated the washout time of sevoflurane in 30 non-smoking and 30 healthy cigarette smoking (≥20 cigarettes/day for>1 year) ASA I-II physical status patients, aged 18-63 years, who were candidates for otorhinolaryngologic elective surgery under 1MAC-h standardized sevoflurane anesthesia. At the end of the surgery, the sevoflurane vaporizer was turned off and the time taken for the sevoflurane concentration to decrease to MAC-awake (0.3) and 0.1 MAC levels were recorded. In addition, the ratio of the fractions of inspired concentration (Fi) and expired concentration of sevoflurane (Fexp) at 1 MAC and Fexp of sevoflurane at 0.1MAC were recorded. The patients were mechanically ventilated during the washout time.

Results

We found no difference between the 2 study groups with regard to washout time of sevoflurane. The times of 1MAC down to MAC-awake (106 ± 48 sec in non-smokers vs 97 ± 37 sec in smokers, p > 0.05) and down to 0.1MAC (491 ± 187 sec in non-smokers vs 409 ± 130 sec in smokers, p > 0.05) were similar. Similarly, there were no significant differences in the ratios of Fi/Fexp at 1MAC (1.18 in non-smokers vs. 1.19 in smokers, p > 0.05) and Fexp of sevoflurane at 0.1MAC (0.26 in non-smokers vs. 0.25 in smokers, p > 0.05).

Conclusions

Washout time of 1MAC-h sevoflurane anesthesia is not appear to be effected by cigarette smoking in patients without significant pulmonary disease.

Mild endotoxemia during mechanical ventilation produces spatially heterogeneous pulmonary neutrophilic inflammation in sheep

BACKGROUND

There is limited information on the regional inflammatory effects of mechanical ventilation and endotoxemia on the production of acute lung injury. Measurement of F-fluorodeoxyglucose (F-FDG) uptake with positron emission tomography allows for the regional, in vivo and noninvasive, assessment of neutrophilic inflammation. The authors tested whether mild endotoxemia combined with large tidal volume mechanical ventilation bounded by pressures within clinically acceptable limits could yield measurable and anatomically localized neutrophilic inflammation.

METHODS

Sheep were mechanically ventilated with plateau pressures = 30-32 cm H2O and positive end-expiratory pressure = 0 for 2 h. Six sheep received intravenous endotoxin (10 ng x kg x min), whereas six did not (controls), in sequentially performed studies. The authors imaged with positron emission tomography the intrapulmonary kinetics of infused N-nitrogen and F-FDG to compute regional perfusion and F-FDG uptake. Transmission scans were used to assess aeration.

RESULTS

Mean gas fraction and perfusion distribution were similar between groups. In contrast, a significant increase in F-FDG uptake was observed in all lung regions of the endotoxin group. In this group, F-FDG uptake in the middle and dorsal regions was significantly larger than that in the ventral regions. Multivariate analysis showed that the F-FDG uptake was associated with regional aeration (P < 0.01) and perfusion (P < 0.01).

CONCLUSIONS

Mild short-term endotoxemia in the presence of heterogeneous lung aeration and mechanical ventilation with pressures within clinically acceptable limits produces marked spatially heterogeneous increases in pulmonary neutrophilic inflammation. The dependence of inflammation on aeration and perfusion suggests a multifactorial basis for that finding. F-FDG uptake may be a sensitive marker of pulmonary neutrophilic inflammation in the studied conditions.

Lungs of patients with acute respiratory distress syndrome show diffuse inflammation in normally aerated regions: a [18F]-fluoro-2-deoxy-D-glucose PET/CT study

OBJECTIVE

Neutrophilic inflammation plays a key role in the pathogenesis of acute respiratory distress syndrome (ARDS) and acute lung injury (ALI). Positron emission tomography (PET) with [F]-fluoro-2-deoxy-D-glucose (FDG) can be used to image cellular metabolism that, during lung inflammatory processes, likely reflects neutrophils activity. The aim of this study was to assess the magnitude and regional distribution of inflammatory metabolic activity in the lungs of patients with ALI/ARDS by PET with FDG.

DESIGN

Prospective clinical investigation.

PATIENTS

Ten patients with ALI/ARDS; four spontaneously breathing and two mechanically ventilated subjects, without known lung disease, served as controls.

INTERVENTIONS

In each individual we performed an FDG PET/computed tomography of the thorax.

MEASUREMENTS AND MAIN RESULTS

FDG cellular influx rate constant (Ki) was computed for the imaged lung field and for regions of interest, grouping voxels with similar density. In all patients with ALI/ARDS, Ki was higher than in controls, also after accounting for the increased lung density. Ki values differed greatly among patients, but in all patients Ki of the normally aerated regions was much higher (2- to 24-fold) than in controls. Whereas in some patients the highest Ki values corresponded to regions with the lowest aeration, in others these regions had lower Ki than normally and mildly hypoaerated regions.

CONCLUSION

In patients with ALI/ARDS, undergoing mechanical ventilation since days, the metabolic activity of the lungs is markedly increased across the entire lung density spectrum. The intensity of this activation and its regional distribution, however, vary widely within and between patients.

Modeling pulmonary kinetics of 2-deoxy-2-[18F]fluoro-D-glucose during acute lung injury

RATIONALE AND OBJECTIVES

Dynamic positron emission tomographic imaging of the radiotracer 2-deoxy-2-[(18)F]fluoro-D-glucose ((18)F-FDG) is increasingly used to assess metabolic activity of lung inflammatory cells. To analyze the kinetics of (18)F-FDG in brain and tumor tissues, the Sokoloff model has been typically used. In the lungs, however, a high blood-to-parenchymal volume ratio and (18)F-FDG distribution in edematous injured tissue could require a modified model to properly describe (18)F-FDG kinetics.

MATERIALS AND METHODS

We developed and validated a new model of lung (18)F-FDG kinetics that includes an extravascular/noncellular compartment in addition to blood and (18)F-FDG precursor pools for phosphorylation. Parameters obtained from this model were compared with those obtained using the Sokoloff model. We analyzed dynamic PET data from 15 sheep with smoke or ventilator-induced lung injury.

RESULTS

In the majority of injured lungs, the new model provided better fit to the data than the Sokoloff model. Rate of pulmonary (18)F-FDG net uptake and distribution volume in the precursor pool for phosphorylation correlated between the two models (R(2)=0.98, 0.78), but were overestimated with the Sokoloff model by 17% (P< .05) and 16% (P< .0005) compared to the new one. The range of the extravascular/noncellular (18)F-FDG distribution volumes was up to 13% and 49% of lung tissue volume in smoke- and ventilator-induced lung injury, respectively.

CONCLUSION

The lung-specific model predicted (18)F-FDG kinetics during acute lung injury more accurately than the Sokoloff model and may provide new insights in the pathophysiology of lung injury.