Page 135 - Assessing right ventricular function and the pulmonary circulation in pulmonary hypertension Onno Anthonius Spruijt

irreversible or reversible two‐tissue model was evaluated using Akaike Information Criterion (AIC) [28]. For all patients and controls, AIC showed a significantly improved fit using a reversible 2T4k model and consequently, all results below are obtained using this model. In this model, k1 represents the rate constant for the transport of 18FLT from blood into tissue, k2 represents the rate constant of the transport of 18FLT from tissue back to blood, k3 reflects the rate constant of phosphorylation of 18FLT and k4 describes the rate of 18FLT dephosphorylation [29].
Input Functions
The lung has a dual circulation with perfusion coming from both the pulmonary and systemic (bronchial) circulation. Because systemic perfusion becomes quantitatively more important in PAH [30], we first determined model fits separately with pulmonary, systemic and combined input functions. Because the best model fit was obtained using a pulmonary input function, this was used for further calculations. Five 1cm diameter regions of interest (ROI) were drawn in the pulmonary artery at the level of the pulmonary trunk on early frames showing the first‐pass of the bolus through the pulmonary circulation. Average activity through time CPA(t) was obtained from this region and assumed to represent the total supply of radioactivity to the lungs CA(t) in eq. (1). To obtain the arterial input function CPL(t), CPA(t) was multiplied with a sigmoid function fitted through the ratio of plasma and whole‐blood activity concentrations and a second sigmoid function fitted through the fraction of non‐metabolized tracer. This correction was performed as described previously [26]. A second set of ROIs was drawn in left atrial cavity in at least three slices. Average activity concentrations CLA(t) were obtained and used as correction factor for venous blood concentrations in the lungs CV(t) in eq. (1).
Lung Segmentation
Pulmonary activity concentrations were obtained using the low‐dose CT scan. Using automated region‐growing methods, a region of low tissue density between 0.05 and 0.6 g/mL was obtained for each lung. A layer of 2 voxels (8mm) was eroded from the borders of each lung region to reduce the effects of respiratory motion, spill‐in of activity from the liver and other partial volume effects. To avoid any effects of a gradient signal in the lungs and of differences in the field of view of the scanner and patient positioning, data from a 2cm region at the level of the bifurcation of the pulmonary artery were used for analysis using the ROIs drawn in the pulmonary artery.
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