Molecular imaging for pediatric lung diseases

Molecular Imaging of Pulmonary Inflammation and Infection

Infectious and inflammatory pulmonary diseases are a leading cause of morbidity and mortality worldwide. Although infrequently used in this setting, molecular imaging may significantly contribute to their diagnosis using techniques like single photon emission tomography (SPET), positron emission tomography (PET) with computed tomography (CT) or magnetic resonance imaging (MRI) with the support of specific or unspecific radiopharmaceutical agents. ¹⁸F-Fluorodeoxyglucose (¹⁸F-FDG), mostly applied in oncological imaging, can also detect cells actively involved in infectious and inflammatory conditions, even if with a low specificity. SPET with nonspecific (e.g., ⁶⁷Gallium-citrate (⁶⁷Ga citrate)) and specific tracers (e.g., white blood cells radiolabeled with ¹¹¹Indium-oxine (¹¹¹In) or ^99mTechnetium (^99mTc)) showed interesting results for many inflammatory lung diseases. However, ⁶⁷Ga citrate is unfavorable by a radioprotection point of view while radiolabeled white blood cells scan implies complex laboratory settings and labeling procedures. Radiolabeled antibiotics (e.g., ciprofloxacin) have been recently tested, although they seem to be quite unspecific and cause antibiotic resistance. New radiolabeled agents like antimicrobic peptides, binding to bacterial cell membranes, seem very promising. Thus, the aim of this narrative review is to provide a comprehensive overview about techniques, including PET/MRI, and tracers that can guide the clinicians in the appropriate diagnostic pathway of infectious and inflammatory pulmonary diseases.

In vivo fluorescence imaging of bacteriogenic cyanide in the lungs of live mice infected with cystic fibrosis pathogens

BACKGROUND

Pseudomonas aeruginosa (PA) and Burkholderia cepacia complex (Bcc), commonly found in the lungs of cystic fibrosis (CF) patients, often produce cyanide (CN), which inhibits cellular respiration. CN in sputa is a potential biomarker for lung infection by CF pathogens. However, its actual concentration in the infected lungs is unknown.

METHODS AND FINDINGS

This work reports observation of CN in the lungs of mice infected with cyanogenic PA or Bcc strains using a CN fluorescent chemosensor (4',5'-fluorescein dicarboxaldehyde) with a whole animal imaging system. When the CN chemosensor was injected into the lungs of mice intratracheally infected with either PA or B. cepacia strains embedded in agar beads, CN was detected in the millimolar range (1.8 to 4 mM) in the infected lungs. CN concentration in PA-infected lungs rapidly increased within 24 hours but gradually decreased over the following days, while CN concentration in B. cepacia-infected lungs slowly increased, reaching a maximum at 5 days. CN concentrations correlated with the bacterial loads in the lungs. In vivo efficacy of antimicrobial treatments was tested in live mice by monitoring bacteriogenic CN in the lungs.

CONCLUSIONS

The in vivo imaging method was also found suitable for minimally invasive testing the efficacy of antibiotic compounds as well as for aiding the understanding of bacterial cyanogenesis in CF lungs.

18F-fluorodeoxyglucose-PET/CT imaging of lungs in patients with cystic fibrosis

BACKGROUND

Airway inflammation plays a critical role in the progression of cystic fibrosis (CF) lung disease, and in the destruction of airways and lung parenchyma. Current methods to assess CF lung disease (BAL, spirometry, and high-resolution CT scanning), do not always accurately reflect actual disease states. Fluorodeoxyglucose (FDG)-PET scanning has been used previously to image infection and inflammation. In this study, we assessed the use of (18)F-FDG PET/CT scanning to evaluate and monitor lung inflammation and/or infection in patients with CF.

METHODS

PET/CT scans were performed in 20 patients with CF (age range, 14 to 54 years); 7 of 20 patients underwent repeat PET/CT scans during and after acute exacerbations. The results were compared with clinical information and with images from eight control subjects with no known lung disease.

RESULTS

Foci of enhanced activity were observed on FDG-PET scans of patients with CF but not those of control subjects. Higher focal activity (standardized uptake value, > 3.0) was seen during disease exacerbation and infection. Coregistered CT scan images assisted in the localization of PET foci and showed corresponding CT scan findings, with many additional findings on CT scans that were not seen on PET scans. Foci seen on high-intensity PET scans during exacerbations disappeared after antibiotic therapy and the resolution of exacerbation, while corresponding CT scan findings remained unchanged.

CONCLUSIONS

PET/CT imaging demonstrated the presence of foci of enhanced uptake that may reflect active focal infectious or inflammatory processes in the lungs. These foci can be cleared with antibiotic therapy. Further studies are needed to validate these results and to determine whether FDG-PET/CT scanning can predict the nature/severity of disease in patients with CF.

A prospective study of macrophage migration inhibitory factor as a marker of inflammatory detection

This study was to evaluate whether macrophage migration inhibitory factor (MIF) can be used as a better marker of inflammatory detection through the biodistribution and inflammatory imaging study with ¹³¹I-labelled anti-MIF McAb and control antibody in inflammatory model mice. The mRNA and protein expression of MIF in inflammatory lesions were proved by RT-PCR and immunohistochemistry. The model mice were injected with 3.7 MBq of each agent and killed at 24, 48 and 72 hrs after injection. Whole-body images were obtained with storage phosphor screen. The organs, blood, abscesses muscles were removed, weighed and counted with a γ counter. The percentage of uptake by organs and per gram tissues and abscess/normal tissue (%ID/g) concentration ratios were calculated. The abscesses in mice were well visualized from 24 hrs. The target-to-non-target (T/NT) ratios were 6.71 ± 1.09 (24 hrs), 8.57 ± 0.81 (48 hrs) and 11.41 ± 0.37 (72 hrs) for ¹³¹I-labelled anti-MIF McAb group; while in control group of ¹³¹I-IgG, T/NT ratios were 4.65 ± 0.63 (24 hrs), 6.44 ± 0.60 (48 hrs) and 8.23 ± 0.35 (72 hrs) (P < 0.05). MIF mRNA expression was threefold increased in inflammatory tissues at 24 hrs compared with normal tissues, and twofold increased at 48 hrs. MIF protein expression was stronger in the inflammatory tissues at 48 hrs after focal inflammation occurred. Our findings suggest that the ¹³¹I-labelled anti-MIF McAb appears to be more specific and suitable than ¹³¹I-labelled IgG for targeting focal inflammation, which means MIF can be used as a better marker of inflammatory detection.

Imaging pulmonary inflammation with positron emission tomography: a biomarker for drug development

Methods currently used to assess lung and airway inflammation are often poorly quantitative, invasive, nonspecific, or insensitive. Positron emission tomography (PET) with [18F]fluorodeoxyglucose [18F]FDG), on the other hand, is a noninvasive, highly sensitive imaging technique that can be used to quantify pulmonary inflammation. [18F]FDG, an analogue of glucose, is taken up by the same transporters that take up glucose into the cell; therefore, [18F]FDG uptake tracks cellular glucose transport, which is highly correlated to the rate of cellular glucose metabolism. Recent studies in animal models of neutrophilic lung inflammation, as well as in patients with inflammatory lung disease, indicate that increased [18F]FDG uptake by the lungs correlates with the number of activated neutrophils recovered from the lungs. Therefore, the in vivo measurement of pulmonary glucose metabolism is a measure of neutrophil burden within the lungs. We propose that FDG-PET imaging can be used as a measurable biomarker in the development of drug therapies targeting lung inflammation.

Visualizing lung function with positron emission tomography

Positron emission tomography (PET) provides three-dimensional images of the distributions of radionuclides that have been inhaled or injected into the lungs. By using radionuclides with short half-lives, the radiation exposure of the subject can be kept small. By following the evolution of the distributions of radionuclides in gases or compounds that participate in lung function, information about such diverse lung functions as regional ventilation, perfusion, shunt, gas fraction, capillary permeability, inflammation, and gene expression can be inferred. Thus PET has the potential to provide information about the links between cellular function and whole lung function in vivo. In this paper, recent advancements in PET methodology and techniques and information about lung function that have been obtained with these techniques are reviewed.