Myocardial tissue fraction--correction for partial volume effects and measure of tissue viability.

We have compared two independent methods of correcting the systematic underestimation in measurements of myocardial radiotracer concentration due to wall motion and small transmural wall thickness in cardiac PET studies. The first technique was based on measurement of the tissue fraction by fitting 15O-labeled water dynamic PET data. The other technique involved the subtraction of the C15O-blood volume scan from the transmission data, producing an image of extravascular density. In normal myocardial regions, both values were observed to be about 60% of myocardial tissue density. The tissue fraction was approximately 10% larger than the extravascular density in normal tissue regions. The ratio of alpha/Dev indicates the proportion of the total extravascular tissue for a given ROI that is perfusable by water--independent of the partial volume effect. This ratio was confirmed to be the expected value in normal tissue regions but was reduced in regions of infarction. The use of 15O-water, C15O and transmission data may aid in the differentiation between perfusable and nonperfusable tissue in the infarcted myocardium.