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The Requirement for Constant Arterial Radioactivity in the C¹⁵O₂ Steady-State Blood-Flow Model

Neuroisotope Laboratory, Montreal Neurological Institute, Montreal, Quebec, Canada

Correspondence: For reprints contact: E. Meyer, PhD, Montreal Neurological Institute, 3801 University St., Rm. 632, Montreal, Quebec, Canada H3A 2B4.

ABSTRACT

This study evaluates the discrepancy between the true CBF value and the CBF value calculated according to the C¹⁵O₂ steady-state model, for situations where the arterial input function, Ca(t), deviates considerably from its steady-state value, Ca. The fact that arterial input function and tissue 0–15 concentration are not independent variables is taken into account. Inconstant or variable arterial input functions are simulated and the corresponding tissue 0–15 concentrations calculated. The steady-state CBF values are evaluated for several temporal variations of Ca over the period of imaging, all derived from Ca(t) by simulation of various blood-sampling schemes, and are compare with the true CBF values. The study indicates that reliable CBF values are obtained by the C¹⁵O₂ steady-state method even under severely impaired "unsteady-state" conditions, provided that either the true average arterial concentration over the entire scan, or the average concentration from multiple arterial samples, is used.