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Equilibration of 6-[¹⁸F]Fluoro-L-m-Tyrosine Between Plasma and Erythrocytes

Theoretical Biology, Institute for Advanced Study, Princeton, New Jersey; Department of Physics and Astronomy, McMaster University, Hamilton, Ontario; and Department of Nuclear Medicine, McMaster University Medical Centre, Hamilton, Ontario, Canada

Intracranial or intraventricular blood pools have been suggested as noninvasive sources of an input function for quantitative PET. These techniques measure the concentration of the tracer in whole blood, but the concentration in plasma depends on the equilibration of the tracer between plasma and erythrocytes. Methods: FDG, 6-[¹⁸F]fluoro-L-m-tyrosine (FmT), or its major metabolite, 6-[¹⁸F]fluoro-3-hydroxyphenylacetic acid (FHPAA), was added to blood samples obtained from healthy fasting volunteers along with radioiodinated human serum albumin (RIHSA). Samples were incubated at 37°C for times between 10 s and 2 h and then plunged into an ice bath and centrifuged. Whole blood and plasma were counted for ¹⁸F and ¹²⁵I activities. The resulting time courses were fit to successively more complex models, evaluated using an F test. Results: All radioactivity associated with RIHSA remained in the plasma, whereas FDG equilibrated instantaneously between plasma and erythrocytes. FmT took about 1 h to equilibrate between plasma and erythrocytes; this time course could be described by a single exponential with a half-life of 10 min. FHPAA equilibrated within the first 5 min of the study. Conclusion: Our results show that, unlike FDG, the partitioning of FmT between plasma and erythrocytes is a relatively slow process. We present an analytic correction that may be applied to the measured time course of radioactivity in whole blood to obtain the time course of the tracer in plasma.

Key Words: PET • input function • plasma • FmT • hematocrit

Received Jul. 26, 1999; revision accepted Feb. 16, 2000.

For correspondence or reprints contact: Claude Nahmias, PhD, Department of Nuclear Medicine, HSC-1P, McMaster University Medical Centre, 1200 Main St. W., Hamilton, Ontario, L8N 3Z5, Canada.

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