The importance of accounting for voxel-wise delay in blood delivery for accurate determination of kinetic parameters from dynamic PET studies of the lung.

Objectives Pulmonary fractional blood volume (V­­_b) is known to be a marker of lung function and can be determined from kinetic analysis (KA) techniques following dynamic PET/CT acquisitions. The accuracy of V_b estimation depends on accounting for the delay between input function (IF) (fitted blood curve) and arrival time of the tracer bolus in the voxel. As this delay is expected to be variable in the lung, if not corrected for, whole lung kinetic analysis may result in errors. This work investigates improvements in V_b estimation by accounting for tracer delivery delay throughout the lung in patients with idiopathic pulmonary fibrosis (IPF).

Methods 5 patients with IPF underwent dynamic PET/CT with F¹⁸-FMISO. 2 V_b analysis methods were applied; 1) a one tissue compartment model was used to ascertain the average time delay between the whole lung time activity curve (TAC) and an aorta IF. KA was then applied on a voxel-by-voxel basis using this average time delay to determine V_b. 2) The time delay was estimated on a voxel-wise basis and the V_b values re-determined. Comparison was made between these methods.

Results Time delays between the medial and peripheral regions of the lung were found to be 1.1±0.6s, but in fibrotic regions were found to be up to 5.8s. After correcting for the time delay, V_b was increased in all cases. V_b in medial and healthy appearing peripheral lung was increased by up to 9.2% with the new method. In fibrotic regions, V_b increased by a maximum of 39.8% with inclusion of voxel-varying time delay.

Conclusions There is a time delay in blood delivery between the medial and peripheral lung which is exaggerated in fibrotic regions. This time delay between the IF and the tissue TAC affected estimation of fractional blood volume (V­­_b) and will, in turn, influence the estimates of other parameters. Inclusion of this time delay per voxel is therefore vital for ensuring correct kinetic parameter estimation in the lung.

Research Support This work is supported by funding from GSK, Fibrosis DPU, GlaxoSmithKline R&D and EPSRC. This project is supported by researchers at the National Institute for Health Research, University College London Hospitals Biomedical Research Centre.