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Image-Derived Input Function for Assessment of ¹⁸F-FDG Uptake by the Inflamed Lung

¹ Department of Anesthesia and Critical Care, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts; and ² Department of Medicine (Pulmonary and Critical Care Unit), Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts

Correspondence: For correspondence or reprints contact: Tobias Schroeder, Department of Anesthesia and Critical Care, Massachusetts General Hospital, 55 Fruit St., Boston, MA 02114. E-mail: tschroder{at}partners.org

Pulmonary uptake of ¹⁸F-FDG assessed with PET has been used to quantify the metabolic activity of inflammatory cells in the lung. This assessment involves modeling of tracer kinetics and knowledge of a time–activity curve in pulmonary artery plasma as an input function, usually acquired by manual blood sampling. This paper presents and validates a method to accurately derive an input function from a blood-pool region of interest (ROI) defined in dynamic PET images. Methods: The method is based on a 2-parameter model describing the activity of blood and that from spillover into the time–activity curve for the ROI. The model parameters are determined using an iterative algorithm, with 2 blood samples used to calibrate the raw PET-derived activity data. We validated both the 2-parameter model and the method to derive a quantitative input function from ROIs defined for the cavities of the right and left heart and for the descending aorta by comparing them against the time–activity curve obtained by manual blood sampling from the pulmonary artery in lungs with acute inflammation. Results: The model accurately described the time–activity curve from sampled blood. The 2-sample calibration method provided an efficient algorithm to derive input functions that were virtually identical to those sampled manually, including the fast kinetics of the early phase. The ¹⁸F-FDG uptake rates in acutely injured lungs obtained using this method correlated well with those obtained exclusively using manual blood sampling (R² > 0.993). Within some bounds, the model was found quite insensitive to the timing of calibration blood samples or the exact definition of the blood-pool ROIs. Conclusion: Using 2 mixed venous blood samples, the method accurately assesses the entire time course of the pulmonary ¹⁸F-FDG input function and does not require the precise geometry of a specific blood-pool ROI or a population-based input function. This method may substantially facilitate studies involving modeling of pulmonary ¹⁸F-FDG in patients with viral or bacterial infections, pulmonary fibrosis, and chronic obstructive pulmonary disease.

Key Words: PET • Massachusetts General Hospital • ¹⁸F-FDG • acute lung injury • inflammation

COPYRIGHT © 2007 by the Society of Nuclear Medicine, Inc.

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