Background: Recent reports on 13N-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 of 13N-ammonia with 82Rb and 62Cu-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 either 13N-ammonia (n = 14), 82Rb (n = 8), or 62Cu-PTSM (n = 6). Eight subjects given 13N-ammonia also underwent imaging after adenosine. Activity measured in the posterolateral wall on transaxial images was significantly lower than in the septum for 13N-ammonia, both at rest (p < 0.005) and after adenosine (p < 0.05). No differences were detected for 82Rb or 62Cu-PTSM. The septum/posterolateral wall activity ratios for 13N-ammonia, 82Rb, and 62Cu-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 13N-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 62Cu-PTSM, activity in the inferior wall was greater than that in other regions (p < 0.005). No regional differences were detected for 82Rb.
Conclusions: The relatively increased wall activity with 13N-ammonia and 62Cu-PTSM is most likely due to cross-contamination of activity from the liver. The significant reduction in activity in the lateral wall with 13N-ammonia, which persists after adenosine, is most likely related to regional heterogeneity in 13N-ammonia retention and may reflect regional differences in metabolic-trapping mechanisms for 13N-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 13N-ammonia PET studies.