Errors due to instability of the input in the PET continuous inhalation method for CBF and CMRO/sub 2/ determination

The simple models used to compute the physiological parameters from the equilibrium inhalation method require that the arterial level of radioactivity be kept constant during the breathup and imaging periods. Drifts in the input of labeled gas or in breathing dynamics can cause drifts in these input concentrations. In order to characterize the effects of such drifts the authors have carried out a computer simulation study under the following assumptions: 1. drifts are such that the various arterial blood concentrations have the form: C(t)=C/sub o/+mt. This is the first term of a Taylor expansion in t and hence is representative of all slowly varying drifts; 2. drift begins after initial equilibrium is established and continues to the end of the study. Studies were carried out for three different cases: 1. effects of drift on flow measurements; 2. Effects on OEF and CMRO/sub 2/ of a drift in flow measurement, and 3. Effects on OEF and CMRO/sub 2/ of a drift in O/sub 2/ measurement. Systematic errors in flow tend to be minimum at zero flow and increase slowly with flow. Systematic errors in OEF and CMRO/sub 2/ increase rapidly as these parameters become small and tend to level off to constant values as they increase. For very large drift rates (10%/min), errors in flow vary from 3-30% depending on duration of drift. For more moderate drift rates (eg. 2%/min) errors range up to 7% for the longest drift times. For relatively stable operation errors in OEF and CMRO/sub 2/ are less than 15% for E> 0.2 and increase rapidly for lower values of E