REGIONAL DIFFERENCES IN TECHNETIUM-99M-ECD CLEARANCE ON BRAIN SPECT IN HEALTHY-SUBJECTS

The aim of this study was to evaluate the in vivo stability of ECD bra in SPECT. Methods: Twenty normal volunteers (35.4 +/- 9.1 yr) each had six ECD scans at 30, 60, 120, 240, 360 and 480 min postinjection, Eac h scan was acquired for 24 min using a triple-head SPECT system, Avera ge counts per pixel were measured from frontal, temporal, parietal, oc cipital, cerebellum, basal ganglia, thalamus and white matter regions, ECD clearance rates were calculated by fitting regional time activity data to a monoexponential equation. Regional gray-to-white matter (G/ W) and gray-to-cerebellum (G/C) ratios were calculated for each scan, Analysis of variance was used to compare regional ECD clearance and ra tio measurements. Results: The average ECD clearance was 4.3%/hr. Ther e was a significant regional variation in the ECD clearance, being hig her for occipital (6.34%/hr) but lower for both white matter (2.39%/hr ) and thalamus (2.45%/hr). Both G/W and G/C ratios showed a significan t regional variation with time. The overall G/W ratio was 2.13 at 30 m in and became progressively lower after 2 hr, reaching 1.78 at 8 hr. A ll regional G/W ratios declined with time except for thalamus where it remained constant at 2.15. The overall G/C ratio was 0.984 at 30 min but it declined after 4 hr, reaching 0.955 at 8 hr. All regional G/C r atios declined with time except for thalamus where it increased progre ssively from 0.955 to 1.120 at 8 hr. Conclusion: ECD clears from norma l brain slowly and shows a significant regional variation, As a result , G/W contrast begins to decrease after 2 hr and the gray-matter activ ity pattern becomes significantly different after 4 hr. Therefore, the optimal imaging time may be between 30-120 min. However, images obtai ned up to 4 hr still maintain the initial gray-matter activity pattern .