Estimation of ¹²³I-IMP arterial blood activity using ¹²³I-IMP acquisition data from the lungs and brain without any blood sampling: validation of its usefulness for quantification of regional cerebral blood flow.
OBJECTIVE
The conventional methods for the estimation of regional cerebral blood flow (rCBF) using ¹²³I-labeled N-isopropyl-p-iodoamphetamine (¹²³I IMP) autoradiography (ARG) require continuous or 1-point arterial blood sampling. Patients who need rCBF quantification benefit from the avoidance of arterial puncture. In this study, we attempted to develop a method without any blood sampling to estimate ¹²³I IMP activity in the arterial blood sample at 10 minutes after injection of ¹²³I IMP (Ca10) for the purpose of rCBF quantification. For the evaluation of validity of this method, the mean of rCBFs in various regions of the brain (mean CBF) calculated by ¹²³I IMP ARG method using the estimated Ca10 was compared with that calculated using the Ca10 directly measured with the actual arterial blood sample. Both groups of the mean CBF values were also compared with those measured by O-15 H₂O PET ARG method.


METHODS
I-123 IMP ARG study was applied to 23 patients, and O-15 H₂O PET ARG was applied to 20 patients of them. Dynamic images of the lungs, time series of static images of the brain, and brain SPECT images were acquired after injection of ¹²³I IMP. Arterial blood sampling was done 10 minutes after injection of ¹²³I IMP. Multiple regression analysis was used to estimate Ca10 using 5 parameters from the lung washout counts, time series of brain static counts, and brain SPECT average counts as the explanatory variables and the Ca10 directly measured with the actual arterial blood sample as the objective variable, and the regression equation was calculated.


RESULTS
The regression equation was calculated by multiple regression analysis as follows: Estimated Ca10 = (2.09 × 10⁻² · LW3) - (2.29 × 10⁻⁴ · Cb5) - (9.87 × 10⁻³ · Cbpre-SPECT) + (1.06 · CbSPECTav) + (1.03 × 10⁻² · Cbpost-SPECT) + 165 (counts/s/g), where LW3: lung washout count at 3 minutes after injection, Cb5: brain count at 5 minutes, Cb pre-SPECT: brain count before SPECT, Cb SPECT av: average brain count during SPECT, and Cb post-SPECT: brain count after SPECT. The estimated Ca10 values closely correlated with the directly measured Ca10 values (r = 0.907, P < 0.01). The mean CBF values (mL/min/100 g) calculated by ¹²³I IMP ARG method using the estimated Ca10 also closely correlated with those calculated using the directly measured Ca10 (r = 0.818, P < 0.01). The mean CBF values calculated by the ¹²³I IMP ARG method using either the directly measured or the estimated Ca10 significantly correlated (r = 0.698 and 0.590, respectively; P < 0.01) with those measured by O-15 H₂O PET ARG method.


CONCLUSIONS
The ¹²³I IMP arterial blood activity can be estimated reliably without any blood sampling using the ¹²³I IMP acquisition data from the lungs and brain. This method can serve for a convenient and noninvasive rCBF quantification technique instead of the conventional methods requiring arterial blood sampling.