Development and validation of a direct-comparison method for cardiac ^sup 123^I-metaiodobenzylguanidine washout rates derived from late 3-hour and 4-hour imaging

Purpose The washout rate (WR) has been used in ^sup 123^I-metaiodobenzylguanidine (MIBG) imaging to evaluate cardiac sympathetic innervation. However, WR varies depending on the time between the early and late MIBG scans. Late scans are performed at either 3 or 4 hours after injection of MIBG. The aim of this study was to directly compare the WR at 3 hours (WR^sub 3h^) with the WR at 4 hours (WR^sub 4h^). Methods We hypothesized that the cardiac count would reduce linearly between the 3-hour and 4-hour scans. A linear regression model for cardiac counts at two time-points was generated. We enrolled a total of 96 patients who underwent planar ^sup 123^I-MIBG scintigraphy early (15 min) and during the late phase at both 3 and 4 hours. Patients were randomly divided into two groups: a model-creation group (group 1) and a clinical validation group (group 2). Cardiac counts at 15 minutes (count^sub early^), 3 hours (count^sub 3h^) and 4 hours (count^sub 4h^) were measured. Cardiac count^sub 4h^ was mathematically estimated using the linear regression model from count^sub early^ and count^sub 3h^. Results In group 1, the actual cardiac count^sub 4h^/count^sub early^ was highly significantly correlated with count^sub 3h^/count^sub early^ (r=0.979). In group 2, the average estimated count^sub 4h^ was 92.8±31.9, and there was no significant difference between this value and the actual count^sub 4h^ (91.9±31.9). Bland-Altman analysis revealed a small bias of -0.9 with 95 % limits of agreement of -6.2 and +4.3. WR^sub 4h^ calculated using the estimated cardiac count^sub 4h^ was comparable to the actual WR^sub 4h^ (24.3±9.6 % vs. 25.1±9.7 %, p=ns). Bland-Altman analysis and the intraclass correlation coefficient showed that there was excellent agreement between the estimated and actual WR^sub 4h^. Conclusion The linear regression model that we used accurately estimated cardiac count^sub 4h^ using count^sub early^ and count^sub 3h^. Moreover, WR^sub 4h^ that was mathematically calculated using the estimated count^sub 4h^ was comparable to the actual WR^sub 4h^.