Comparison of technetium-99m-HMPAO and xenon-133 measurements of regional cerebral blood flow by SPECT

This study compares ^99mTc-HMPAO count ratios and derived regional cerebral blood flow (rCBF) to ¹³³Xe rCBF ratios and true rCBF (ml/min/100 g), respectively. Methods: Technetium-99m-HMPAO distribution was evaluated in 14 patients and 5 normal control subjects. Immediately after ¹³³Xe SPECT, subjects received 22 ± 4mCi ^99mTc-HMPAO, and images were acquired 15 min after injection. rCBF (mVmin/100 g, ¹³³Xe) or regional count density (^99mTc-HMPAO) were extracted from 24 ROI located 6 cm above the cantho- meatal line. These data were also normalized to global cerebral blood flow (gCBF) for ¹³³Xe or to global count density (gCD) for ^99mTc-HMPAO. Technetium-99m-HMPAO ROI data also were expressed in units of ml/min/100 g by relating gCD to gCBF. Comparisons between ¹³³Xe and ^99mTc-HMPAO were evaluated using a Bonferroni-corrected paired t-test and by linear regression analysis. Results: Profile plots demonstrated agreement in the pattern of relative distribution between rCBF ratios (¹³³Xe) and count density ratios (^99mTc-HMPAO). Regression analysis indicated a significant correlation (r = 0.78), with a modest slope (0.52) and a large intercept (0.48). A closer correlation (r = 0.92) was found for the comparison between rCBF (¹³³Xe) and derived ^99mTc-HMPAO rCBF. The slope was closer to one (0.82) and the intercept closer to zero. This relationship was also examined during high rCBF after a subset of these subjects (n = 7) was injected intravenously with 1 g acetazolamide. Again, profile plots and regression analysis demonstrated agreement in the pattern of distribution (ratios) between ¹³³Xe and ^99mTc-HMPAO (r = 0.66). However, the slope was reduced and the intercept increased relative to resting data. Absolute flow correlations showed some improvement relative to the ratio data (r = 0.77). Conclusion: The distribution of ^99mTc-HMPAO is linearly related to rCBF measured by ¹³³Xe SPECT, although our data suggest that ^99mTc-HMPAO mildly underestimates rCBF above 80 ml/min/100 g. These results are similar to our previous comparison of ^99mTc-ECD and ¹³³Xe.