[A limitation of the split-dose method for evaluating rCBF changes using 99mTc-ECD and SPECT].

Abstract

The purpose of the study is to validate the split-dose method corrected with dose ratio of 99mTc-ECD for brain perfusion scan. A dose of 600 MBq of 99mTc-ECD was divided into two with various dose ratios from 1:1 to 1:4, and injected to eleven patients with various cerebral diseases. A lesser dose of 99mTc-ECD was injected under a control state for the first SPECT scan, and 15 min SPECT scan was performed 10 min after injection with a triple-head high resolution gamma camera. After the scan, the other dose of 99mTc-ECD was injected under the same control state and the second SPECT scan was performed as same as above. A ratio of the activity of the first scan to the net activity of the second scan corrected by dose ratio, defined as K, was measured in brain regions of each subject. Expected value of K was 1, but the value was distributed with large variations in each subject. The mean % error of the K value was 10.4 +/- 4.9%. Hence it is considered that activity changes by more than 20% from the control values should be required to detect a significant rCBF change in an activation SPECT study. Then, we proposed a new method in which the activity of both two SPECT scans was normalized by cerebellar or occipital activity and compared. The ratio obtained by the proposed method came closer to 1 with less variations and with less mean % error in comparison with those of K value obtained by the dose-correction method. Although the proposed method has a limitation in the use of an activation study loaded with Diamox, it may be useful to evaluate an alteration of rCBF in the study such as postural testing or finger-moving test.