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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

View larger version (11K): [in this window] [in a new window]   FIGURE 1.  Iterative algorithm for calculating model-adjusted input function (). Method is based on 2-compartment model. PET signal from blood-pool ROI is separated in blood and activity spillover component by estimating RC and SC. Index [i] refers to number of iterations. CROI(t) = raw PET signal from blood-pool ROI; estimate of plasma activity; Cp(t1) and Cp(t2) = calibration measurements of plasma activity, obtained at times t1 and t2.

View larger version (39K): [in this window] [in a new window]   FIGURE 2.  Delineation (white outlines) of blood-pool ROIs in descending aorta (AO) and in RH cavities (RHC) and LH cavities (LHC) on representative image slice: regional activity after intravenous single-bolus injection of 13N-nitrogen dissolved in saline (A) and 18F-FDG activity, on identical scale, shortly after tracer injection and at end of imaging (B and C, respectively). The 3 ROIs are shown together only for illustration purposes.

View larger version (11K): [in this window] [in a new window]   FIGURE 3.  Evaluation of model fit and effect of ROI volume on estimated parameter SC: representative model fit according to Equation 2 for dynamic PET data measured in blood-pool ROI over LH cavities (A), average SC obtained for blood-pool ROIs of varying size, illustrating how algorithm adjusts SC to different conditions (B); normalized SC/(1 – RC), illustrating differences in 18F-FDG uptake rates (C). Error bars are ±1 x SE. ROIs with lowest volume were used for analyses.

View larger version (7K): [in this window] [in a new window]   FIGURE 4.  Convergence of iterative algorithm (Fig. 1) for model parameters RC and SC. Plot was generated for blood-pool ROIs in LH. Error bars are ±1 x SE.

View larger version (11K): [in this window] [in a new window]   FIGURE 5.  mse between model-adjusted input function and manually sampled input function as function of time point of first calibration measurement, t1. Plot shows results for blood-pool ROIs in RH and LH cavities (RHC and LHC, respectively) and descending aorta (AO). Values are out of scale for t1 = 0.25 min, t1 = 0.75 min, and t1 > 25 min (not shown).

View larger version (12K): [in this window] [in a new window]   FIGURE 6.  Comparison of activity in manual blood samples of pulmonary artery plasma, raw PET signal from blood-pool ROI in LH cavities, and model-adjusted input function: example of calibration measurements at t1 = 4.5 min and t2 = 50 min (A); example of longer tracer injection time (90 s instead of 1 min) and shorter initial PET frames (15 s instead of 30 s), with t1 = 15 min and t2 = 50 min (B). Discontinuity during peak was due to delay between end of tracer injection and flush of saline used to remove remaining tracer from infusion system.

View larger version (16K): [in this window] [in a new window]   FIGURE 7.  Evaluation of 18F-FDG uptake rates (Ki) for raw image–derived input functions and for model-adjusted input functions in comparison to Ki values obtained with manual blood sampling. Ki values include exposed and nonexposed lungs and were obtained with input functions from blood-pool ROIs in RH cavities (RHC), LH cavities (LHC), and aorta (AO). Shown are Ki obtained with raw image–derived input functions (A) and corresponding Bland–Altman plot (B), and Ki values calculated with model-adjusted input functions (C) and corresponding Bland–Altman plot (D). All values are in 10–3 mL blood/mL lung/min. Parameters of linear regression analysis are summarized in Table 2.