Simultaneous in vitro and in vivo validation of nitrogen-13-ammonia for the assessment of regional myocardial blood flow

Measurement of myocardial blood flow by ¹³NH₃ relies heavily on the assessment of both the input function and the variable tissue extraction fraction. In six open-chest dogs, myocardial and arterial ¹³NH₃ activity was measured both by in vitro sampling and by in vivo positron emission tomography (PET). Regional myocardial blood flow was forced to vary in the range 0.2-5 ml/min/g and actual values were assessed by in vitro counting of ¹⁵³Gd microspheres. The ammonia input function was processed by: (a) total curve integration; (b) curve integration for 2 min; (c) integral of a fitted curve (gamma variate in vivo and exponential of the downslope in vitro). Method C brought to regional flow values which best approximated microspheres data. The in vitro correlation allows for correcting in vivo values for the flow-dependent extraction fraction. The method can be easily applied for regional myocardial blood flow measurements with PET in human studies.