In this study, contrast-enhanced X-ray tomographic imaging for monitoring and quantifying respiratory disease in preclinical rodent models is proposed. A K-edge imaging method has been developed at the Canadian Light Source to very accurately obtain measurements of the concentration of iodinated contrast agent in the pulmonary vasculature and inhaled xenon in the airspaces of rats. To compare the iodine and xenon concentration maps, a scout projection image was acquired to define the region of interest within the thorax for imaging and to ensure the same locations were imaged in each K-edge subtraction (KES) acquisition. A method for triggering image acquisition based on the real-time measurements of respiration was also developed to obtain images during end expiration when the lungs are stationary, in contrast to other previously published studies that alter the respiration to accommodate the image acquisition. In this study, images were obtained in mechanically ventilated animals using physiological parameters at the iodine K-edge in vivo and at the xenon K-edge post mortem (but still under mechanical ventilation). The imaging techniques were performed in healthy Brown Norway rats and in age-matched littermates that had an induced lung injury to demonstrate feasibility of the imaging procedures and the ability to correlate the lung injury and the quantitative measurements of contrast agent concentrations between the two KES images. The respiratory-gated KES imaging protocol can be easily adapted to image during any respiratory phase and is feasible for imaging disease models with compromised lung function.

SummaryAir trapping is a common finding radiologists encounter on CT imaging of the thorax. This term is used when there are geographic areas of differing attenuation within the lung parenchyma. Most commonly, this is the result of abnormal retention of air due to complete or partial airway obstruction from small airway pathologies. Perfusional differences due to underlying vascular diseases could also result in these appearances, and hence, inspiratory and full expiratory phase CT studies are required to accurately diagnose air trapping. It is important to note that this can occasionally be present in healthy patients. Multiple diseases are associated with air trapping. Determining the aetiology relies on accurate patient history and concomitant findings on CT. There is currently no consensus on accurate assessment of the severity of air trapping. The ratio of mean lung density between expiration and inspiration on CT and the change in lung volume have demonstrated a positive correlation with the presence of small airway disease. Treatment and resultant patient outcome depend on the underlying aetiology, and hence, radiologists need to be familiar with the common causes of air trapping. This paper outlines the most common disease processes leading to air trapping, including Constrictive bronchiolitis, Hypersensitivity pneumonitis, DIPNECH, and Post‐infectious (Swyer‐James/Macleod). Various diseases result in the air trapping pattern seen on the expiratory phase CT scan of the thorax. Combining patient history with other concomitant imaging findings is essential for accurate diagnosis and to further guide management.

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Dual-energy micro-CT imaging techniques have been developed to enable accurate identification and segmentation of different tissues. Using dual-energy techniques for thoracic imaging requires obtaining images at multiple respiratory or cardiac phases, and may require images obtained pre-and post-contrast enhancement for each energy. In this study, we investigated if the multiple images obtained during dual-energy imaging resulted in an x-ray dose sufficiently high to interfere with or mask symptoms of respiratory disease. We performed a dual-energy micro-CT study (5 images in a single session, with a cumulative entrance dose of 0.47 Gy) to image the thorax of healthy male Brown Norway rats at 8 weeks of age. Groups of 5 rats were euthanized at 1 day, 1, 2, 3, and 4-weeks postexposure and the lungs were excised and examined by histology (H&E stained slides). Positive controls were exposed to an entrance dose of 1.5 Gy and euthanized at 2 weeks and negative controls were not exposed to x-rays. There was no evidence of alveolar damage or inflammation for any of the animals exposed to the dual-energy imaging session compared with the negative control group. Inflammation was evident for the positive controls. This study concludes that the dual-energy imaging protocol developed in this study does not contribute to lung tissue damage. For preclinical respiratory research, these results show that any inflammation and alveolar damage observed in the lungs would be attributed to the disease model under investigation, and not be affected by obtaining 3D dual-energy micro-CT images.

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