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 on¹³N-labeled ammonia (¹³N-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 of¹³N-ammonia with⁸²Rb and⁶²Cu-labeled PTSM (⁶²Cu-PTSM) to identify tracer-specific patterns that may be important in the clinical interpretation of cardiac flow studies. Methods and Results: Twenty-eight healthy volunteers underwent PET imaging at rest with either¹³N-ammonia (n=14),⁸²Rb (n=8), or⁶²Cu-PTSM (n=6). Eight subjects given¹³N-ammonia also underwent imaging after adenosine. Activity measured in the posterolateral wall on transaxial images was significantly lower than in the septum for¹³N-ammonia, both at rest (p<0.005) and after adenosine (p<0.05). No differences were detected for⁸²Rb or⁶²Cu-PTSM. The septum/posterolateral wall activity ratios for¹³N-ammonia,⁸²Rb, and⁶²Cu-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 for¹³N-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). For⁶²Cu-PTSM, activity in the inferior wall was greater than that in other regions (p<0.005). No regional differences were detected for⁸²Rb. Conclusions: The relatively increased wall activity with¹³N-ammonia and⁶²Cu-PTSM is most likely due to cross-contamination of activity from the liver. The significant reduction in activity in the lateral wall with¹³N-ammonia, which persists after adenosine, is most likely related to regional heterogeneity in¹³N-ammonia retention and may reflect regional differences in metabolic-trapping mechanisms for¹³N-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 clinical¹³N-ammonia PET studies.