Parametric polar maps of regional myocardial β-adrenoceptor density

Quantification of myocardial β-adrenoceptor density (B(max)) is of interest in cardiac diseases in which altered function of the sympathetic nervous system is thought to play a pathophysiological role. PET provides an unrivaled means of taking regional measurements of cardiac microcirculatory function, tissue metabolism and autonomic nervous system activity. Measurements in small regional areas may be biased because of increased noise levels. This study examined the parametric polar map approach for the regional quantification of B(max). Methods: Dynamic PET with parametric polar map imaging was performed in 10 healthy volunteers and 4 patients with hypertrophic cardiomyopathy using (S)-[¹¹C]-(4-(3-tertiarybutylamino-2- hydroxypropoxy)-benzidimazole-2)-on hydrochloride (CGP)-12177 and a double- injection protocol. Time-activity curves were corrected for partial volume, spill-over and wall motion effects. The mean B(max) of the left ventricle was calculated in two ways. First, the average time-activity curve of all segments, having the highest achievable signal-to-noise ratio, was used to calculate B(max(mTAC)) (the myocardial beta-adrenoceptor density of the left ventricle calculated using the average time-activity curve). The bias in B(max(mTAC)) introduced by noise is minimal. Second, an estimate of whole- heart receptor density was calculated using the polar map method by averaging the values of B(max) obtained for 576 individual segments. In these calculations, three different filters (3 x 5, 3 x 9 and 3 x 13 segments) were used to smooth the time-activity curves before calculating B(max). Mean values of whole-left-ventricular receptor density obtained by averaging regional values using the different filters (B(max(PMF1/2/3))) were compared with B(max(mTAC)) to assess bias introduced by the polar map approach. Segments with a calculated B(max) outside the range 0.1-50 pmol/g were considered unreliable and were excluded from the analysis. Results: The differences between the two methods of calculating B(max) were small (7.8%, 4.8% and 3.2%, with the three filters, respectively). Reliable results were obtained in >95% of the segments and in 9 volunteers and all 4 patients. Conclusion: When using PET for the quantification of β-adrenoceptor density, the regional variation in B(max) can be reliably assessed using the parametric polar map approach.