HETEROGENEITY OF REGIONAL NITROGEN 13-LABELED AMMONIA TRACER DISTRIBUTION IN THE NORMAL HUMAN HEART - COMPARISON WITH RUBIDIUM-82 AND COPPER-62-LABELED PTSM

Background. Recent reports on N-13-labeled ammonia (N-13-ammonia) positron emission tomographic (PET) imaging have suggested a relative reduction of measured tracer activity in the posterolateral wall. Such inhomogeneity of tracer distribution could potentially affect accuracy for detection of disease. The aim of this study was to compare the regional distribution of N-13-ammonia with Rb-82 and Cu-62-labeled PTSM (Cu-62-PTSM) to identify tracer-specific patterns that may be important in.the clinical interpretation of cardiac how studies. Methods and Results. Twenty-eight healthy volunteers underwent PET imaging at rest with either N-13-ammonia (n = 14), Rb-82 (n = s), or Cu-62-PTSM (n = 6). Eight subjects given N-13-ammonia also underwent imaging after adenosine. Activity measured in the posterolateral wall on transaxial images was significantly lower than in the septum for N-13-ammonia, both at rest (p < 0.005) and after adenosine (p < 0.05). No differences were detected for Rb-82 or Cu-62-PTSM. The septum/posterolateral wall activity ratios for N-13-ammonia, Rb-82, and Cu-62-PTSM were 1.15 +/- 0.07, 1.00 +/- 0.06, and 0.97 +/- 0.08, respectively (p < 0.001). Regional analysis of image data showed the percent of maximal activity data for N-13-ammonia in the lateral wall to be less than that of other regions (p < 0.001) and in the inferior wall to be greater than in the anterior and lateral walls (p < 0.001). For Cu-62-PTSM, activity in the inferior wall was greater than that in other regions (p < 0.005). No regional differences were detected for Rb-82. Conclusions. The relatively increased wall activity with N-13-ammonia and Cu-62-PTSM is most likely due to cross-contamination of activity from the liver. The significant reduction in activity in the lateral wall with N-13-ammonia, which persists after adenosine, is most likely related to regional heterogeneity in N-13-ammonia retention and may reflect regional differences in metabolic-trapping mechanisms for N-13-ammonia. Further investigation is required to elucidate the underlying mechanism of this phenomenon. Reduced tracer retention in the lateral wall segment as a normal variant must be considered when evaluating clinical N-13-ammonia PET studies.