Improvement of signal-to-noise ratio using iterative reconstruction in a 99mTc-ECD split-dose injection protocol
Split-dose injection using technetium-99m ethyl cysteinate dimer (99mTc-ECD) and consecutive SPET measurements performed before and after acetazolamide (ACZ) loading was used to estimate the cerebral perfusion reserve. The disadvantage of the split-dose method is that the signal-to-noise ratio (S/N) of ACZ-loaded images is decreased by subtraction of the 1st SPET data (rest) from the 2nd SPET data (ACZ loaded). To improve the S/N of reconstructed images, we implemented an iterative reconstruction algorithm including the term of remaining radioactivity in the brain from the 1st injection. It was expected that this method (the "addition method") would improve the S/N of rest and ACZ images compared with the conventional subtraction method owing to exclusion of the subtraction process. To evaluate the effect of statistical noise, we estimated the percentage coefficient of variation (%COV) as a function of total photon counts (from 1.35 to 86.5 Mcounts/slice) by Monte Carlo simulation with equal-volume split-dose injection. The %COV of the 2nd SPET study was higher than that of the 1st (e.g. 50.3% for the 1st and 80.5% for the 2nd at a total count of 2.70 Mcounts/slice) when using the conventional subtraction method. By contrast, the %COV of the 1st and 2nd SPET studies was almost equivalent (e.g. 43.1% for the 1st and 41.4% for the 2nd at a total count of 2.70 Mcounts/slice) when using the addition method. We also determined the optimal injection dose ratio of the 2nd to the 1st SPET study, which provides the equivalent %COV value between the 1st and 2nd images. With the subtraction method, the optimal injection dose ratio of the 2nd to the 1st SPET study was approximately 2.0, while with the addition method it was approximately 1.0. The absolute value of %COV at the optimal injection dose was about 54% and 43% with the subtraction method and the addition method, respectively. The addition method gave a lower %COV value than the subtraction method even at the optimal injection dose ratio. In a clinical study, the addition method provided better quality images than the subtraction method. The ROI values of rest images estimated by the subtraction method were close to the results obtained with the addition method (ROIsub=1.01 ROIadd−0.312, r=0.999). The ROI values of the ACZ images estimated by the subtraction method also agreed with the results obtained using the addition method, but the correlation was slightly worse (ROIsub=1.03 ROIadd−2.23, r=0.995). Quantitative ROI values were quite similar between the methods. Our results demonstrated that the quality of reconstructed rest and ACZ-loaded images were significantly better with the addition method than with the conventional subtraction method. We conclude that the proposed method will be useful as a practical reconstruction algorithm to improve the S/N in an equal-volume split-dose injection protocol using 99mTc-ECD.