An alternative approach to estimation of the brain perfusion index for measurement of cerebral blood flow using technetium-99m compounds

Cerebral blood flow (CBF) has been quantified non-invasively using the brain perfusion index (BPI) determined from radionuclide angiographic data generated by technetium-99m hexamethylpropylene amine oxime (Tc-99m-HMPAO) or technetium-99m ethyl cysteinate dimer (Tc-99m-ECD). The BPI is generally calculated using graphical analysis (GA). In the present study, BPI was measured using spectral analysis (SA), and its usefulness evaluated in comparison with GA. The BPI was calculated from the sum of spectral data obtained by SA. We applied this method to radionuclide angiographic data collected from the bilateral brain hemispheres of 20 patients with various brain diseases using Tc-99m-HMPAO and from those of 20 patients using Tc-99m-ECD. We also measured BPI using GA. The BPI values obtained by SA (BPIS) (x) and by GA (BPIG) (y) correlated closely (y=0.708x+0.038, r=0.945 for Tc-99m-HMPAO and y=0.559x+0.093, r=0.931 for Tc-99m-ECD). However, the BPIG values were underestimated by 22.9%+/-6.6% (mean+/-SD) for Tc-99m-HMPAO and by 27.9%+/-7.5% for Tc-99m-ECD as compared with the BPIS values. The extent of underestimation tended to increase with increasing BPIS values. These findings were considered to be a result of the BPIG values being affected by the first-pass extraction fraction of the tracer. We also compared the BPIS and BPIG values with those of CBF measured using N-isopropyl-p-[I-123]iodoamphetamine (CBFIMP) in 16 patients (six for Tc-99m-HMPAO and ten for Tc-99m-ECD). Although both BPIS and BPIG values correlated significantly with the CBFIMP values, the correlation coefficient in BPIS was always better than that in BPIG (r=0.869 for Tc-99m-HMPAO and r=0.929 for Tc-99m-ECD in BPIS, r=0.629 for Tc-99m-HMPAO and r=0.856 for Tc-99m-ECD in BPIG). These results suggest that SA can provide a more reliable BPI for quantifying CBF using Tc-99m-HMPAO or Tc-99m-ECD than the conventional method using GA. Our method will be useful especially when using a tracer with a low first-pass extraction fraction and/or when performing activation studies using pharmacological intervention.