Abstract
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Objectives: Accurate voxel-wise estimates of the (fractional) blood volume are essential for quantification of pulmonary PET studies due to the large amount of blood in the lungs. CO imaging is considered to be the gold standard method for measuring blood volume. However, due to the short half-life, this method requires a cyclotron in close proximity to the scanner and remains unavailable for most imaging facilities. Dynamic FDG-PET has previously been used to determine blood volume on a voxel basis in the lungs [1], however, no validation of this technique has been performed. This study aims to validate this approach by comparing the two different methods.
Methods: Dynamic 18F-FDG-PET, dynamic 15O-CO-PET, and 20-min 68Ge transmission scans were acquired for six sedated pigs on a Siemens ECAT HR PET scanner in a previous study [2]. The animals were maintained and handled in accordance with guidelines for animal research [3]. The study protocol was approved by the local Committee for Laboratory Animal Welfare, National Cardiovascular Center, Osaka, Japan. All volumes were reconstructed with Filtered Back-Projection with all corrections enabled. For both the FDG and CO studies, the ascending aorta was segmented on early time frames to measure the blood concentration. To derive the blood volume map from the FDG acquisition, this blood time activity curve was first fitted [4] to obtain the input function, and the regional fractional blood volume was derived from a reversible one-tissue compartmental model. The ‘reference’ blood volume map was obtained from the CO acquisition, using a single time frame after equilibrium, dividing the lung voxel concentration by the arterial concentration multiplied by an estimation of the hematocrit ratio (0.9 [5]). Six spherical Regions of Interest (ROIs) (with a volume of 1213 mm3) were drawn in the upper, dorsal base and ventral base of the left and right lungs to compare the results from each processing method.
Results: The blood volume maps obtained from the dynamic FDG acquisition and from the CO acquisition are comparable, with a mean blood volume for the whole lung in the range of 0.21 ± 0.06 for both maps. Figure 1 shows a scatter plot of the mean values within the spheres for the two different blood volume maps demonstrating a strong correlation (Pearson correlation: p=0. 0.9289).
Conclusion: The results indicate that the method used to derive the blood volume map from dynamic FDG-PET acquisitions provides a valid estimate of the fractional blood volume in the lungs. Research Support: We acknowledge funding support from GlaxoSmithKline to UCL (BIDS3000030921, STU100028576 and COL29165). This project is also supported by researchers at the National Institute for Health Research, University College London Hospitals Biomedical Research Centre.