Equilibration of 6-[F-18]fluoro-L-m-tyrosine between plasma and erythrocytes

Intracranial or intraventricular blood pools have been suggested as noninva sive sources of an input function for quantitative PET. These techniques me asure the concentration of the tracer in whole blood, but the concentration in plasma depends on the equilibration of the tracer between plasma and er ythrocytes. Methods: FDG, 6-[F-18]fluoro-L-m-tyrosine (FmT), or its major m etabolite, 6-[F-18]fluoro-3-hydroxyphenylacetic acid (FHPAA), was added to blood samples obtained from healthy fasting volunteers along with radioiodi nated human serum albumin (RIHSA). Samples were incubated at 37 degrees C f or times between 10 s and 2 h and then plunged into an ice bath and centrif uged. Whole blood and plasma were counted for F-18 and I-125 activities. Th e resulting time courses were fit to successively more complex models, eval uated using an F test. Results: All radioactivity associated with RIHSA rem ained in the plasma, whereas FDG equilibrated instantaneously between plasm a and erythrocytes. FmT took about 1 h to equilibrate between plasma and er ythrocytes; this time course could be described by a single exponential wit h a half-life of 10 min. FHPAA equilibrated within the first 5 min of the s tudy. Conclusion: Our results show that, unlike FDG, the partitioning of Fm T between plasma and erythrocytes is a relatively slow process. We present an analytic correction that may be applied to the measured time course of r adioactivity in whole blood to obtain the time course of the tracer in plas ma.