Influence of intravascular activity on the determination of blood flow and oxygen extraction in normal and ulcerated legs using the 15O steady-state technique and positron emission tomography.

An analysis using the 15O steady-state inhalation technique has been carried out into the effects of correction for intravascular activity on the determination of blood flow and oxygen extraction ratio (OER) in the legs of patients affected by ulceration. Transaxial images of the distribution of activity in a plane passing through the ulcer were obtained during steady-state inhalation of C15O2 and 15O2, respectively, using a single slice positron emission tomographic scanner. From these, the basic tracer model equations were solved to obtain blood flow and oxygen extraction with and without additional correction for the presence of intravascular activity. The latter was obtained from a scan following the inhalation of a tracer quantity of 11CO, which labels the red blood cell pool. Mean blood flow and OER in ulcerated regions were 0.087 ml/ml/min and 0.33, respectively. With correction for blood volume, assumed to be all venous, the mean OER decreased to 0.12. In the "mirror" region of normal legs mean blood flow was 0.011 ml/ml/min with uncorrected and corrected OER values of 0.87 and 0.75, respectively. The effect of assuming a nonzero arterial volume increases as flow and OER decrease but is uncertain due to the unknown arterial fraction. Correction for intravascular 15O is essential and generally the assumption that all blood volume is venous is adequate.