Hong-Dun Lin

In Vivo Quantitation of Glucose Metabolism in Mice Using Small-Animal PET and a Microfluidic Device

The challenge of sampling blood from small animals has hampered the realization of quantitative small-animal PET. Difficulties associated with the conventional blood-sampling procedure need to be overcome to facilitate the full use of this technique in mice. Methods: We developed an automated blood-sampling device on an integrated microfluidic platform to withdraw small blood samples from mice. We demonstrate the feasibility of performing quantitative small-animal PET studies using 18F-FDG and input functions derived from the blood samples taken by the new device. 18F-FDG kinetics in the mouse brain and myocardial tissues were analyzed. Results: The studies showed that small (∼220 nL) blood samples can be taken accurately in volume and precisely in time from the mouse without direct user intervention. The total blood loss in the animal was <0.5% of the body weight, and radiation exposure to the investigators was minimized. Good model fittings to the brain and the myocardial tissue time–activity curves were obtained when the input functions were derived from the 18 serial blood samples. The R2 values of the curve fittings are >0.90 using a 18F-FDG 3-compartment model and >0.99 for Patlak analysis. The 18F-FDG rate constants \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(K_{1}^{{\ast}}\) \end{document}, \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(k_{2}^{{\ast}}\) \end{document}, \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(k_{3}^{{\ast}}\) \end{document}, and \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(k_{4}^{{\ast}}\) \end{document}, obtained for the 4 mouse brains, were comparable. The cerebral glucose metabolic rates obtained from 4 normoglycemic mice were 21.5 ± 4.3 μmol/min/100 g (mean ± SD) under the influence of 1.5% isoflurane. By generating the whole-body parametric images of \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(K_{FDG}^{{\ast}}\) \end{document} (mL/min/g), the uptake constant of 18F-FDG, we obtained similar pixel values as those obtained from the conventional regional analysis using tissue time–activity curves. Conclusion: With an automated microfluidic blood-sampling device, our studies showed that quantitative small-animal PET can be performed in mice routinely, reliably, and safely in a small-animal PET facility.

Quantification of Cerebral Glucose Metabolic Rate in Mice Using 18F-FDG and Small-Animal PET

The aim of this study was to evaluate various methods for estimating the metabolic rate of glucose utilization in the mouse brain (cMRglc) using small-animal PET and reliable blood curves derived by a microfluidic blood sampler. Typical values of 18F-FDG rate constants of normal mouse cerebral cortex were estimated and used for cMRglc calculations. The feasibility of using the image-derived liver time–activity curve as a surrogate input function in various quantification methods was also evaluated. Methods: Thirteen normoglycemic C57BL/6 mice were studied. Eighteen blood samples were taken from the femoral artery by the microfluidic blood sampler. Tissue time–activity curves were derived from PET images. cMRglc values were calculated using 2 different input functions (one derived from the blood samples [IFblood] and the other from the liver time–activity curve [IFliver]) in various quantification methods, which included the 3-compartment 18F-FDG model (from which the 18F-FDG rate constants were derived), the Patlak analysis, and operational equations. The estimated cMRglc value based on IFblood and the 3-compartment model served as a standard for comparisons with the cMRglc values calculated by the other methods. Results: The values of \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{K}_{1}^{{\ast}}\) \end{document}, \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(k_{2}^{{\ast}}\) \end{document}, \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(k_{3}^{{\ast}}\) \end{document}, \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(k_{4}^{{\ast}}\) \end{document}, and \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{K}_{\mathrm{FDG}}^{{\ast}}\) \end{document} estimated by IFblood and the 3-compartment model were 0.22 ± 0.05 mL/min/g, 0.48 ± 0.09 min−1, 0.06 ± 0.02 min−1, 0.025 ± 0.010 min−1, and 0.024 ± 0.007 mL/min/g, respectively. The standard cMRglc value was, therefore, 40.6 ± 13.3 μmol/100 g/min (lumped constant = 0.6). No significant difference between the standard cMRglc and the cMRglc estimated by the operational equation that includes \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(k_{4}^{{\ast}}\) \end{document} was observed. The standard cMRglc was also found to have strong correlations (r > 0.8) with the cMRglc value estimated by the use of IFliver in the 3-compartment model and with those estimated by the Patlak analysis (using either IFblood or IFliver). Conclusion: The 18F-FDG rate constants of normal mouse cerebral cortex were determined. These values can be used in the \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(k_{4}^{{\ast}}\) \end{document}-included operational equation to calculate cMRglc. IFliver can be used to estimate cMRglc in most methods included in this study, with proper linear corrections applied. The validity of using the Patlak analysis for estimating cMRglc in mouse PET studies was also confirmed.