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 brain SPECT.

Twenty normal volunteers (35.4 +/- 9.1 yr) each had six ECD scans at 30, 60, 120, 240, 360 and 480 min postinjection. Each scan was acquired for 24 min using a triple-head SPECT system. Average counts per pixel were measured from frontal, temporal, parietal, occipital, 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 ratio measurements.

The average ECD clearance was 4.3%/hr. There was a significant regional variation in the ECD clearance, being higher 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 significant regional variation with time. The overall G/W ratio was 2.13 at 30 min and became progressively lower after 2 hr, reaching 1.78 at 8 hr. All 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 ratios declined with time except for thalamus where it increased progressively from 0.955 to 1.120 at 8 hr.

ECD clears from normal brain slowly and shows a significant regional variation. As a result, G/W contrast begins to decrease after 2 hr and the gray-matter activity pattern becomes significantly different after 4 hr. Therefore, the optimal imaging time may be between 30-120 min. However, images obtained up to 4 hr still maintain the initial gray-matter activity pattern.