Effect of Dose Reduction in CT Perfusion Scans on Cerebral Blood Flow and Volume Computed with Three Perfusion Software Packages: Analysis with a Digital Phantom

Purpose: The calculation of perfusion maps from dynamic CT sequences may depend on the software package used and the dose that was used during acquisition. The purpose of this work was to study this using a hybrid phantom consisting of intensity curves from patient data combined with realistic noise. Methods and Materials: We included CTP data from 10 patients with acute ischemic stroke (320 multi-detector row, 80 kV, 24 volumes, total scanning time of 202 s, cumulative dose of 2250 mAs). In each scan regions of interest were indicated in white matter (WM) and gray matter (GM). Tissue curves were averaged over these regions and used in a digital phantom. Noise patterns from an anthropomorphic head phantom scanned in the same scanner were added to the tissue curves to simulate an acquisition at 80, 70, 60 and 50% of the original dose. Also noise was added to the tissue curves to simulate 100% dose, these were used as a reference standard. The constructed phantom data set was analyzed with three perfusion software packages developed by Vital Images (VI), TeraRecon (TR) and Fraunhofer MEVIS (FM). With these packages the cerebral blood flow (CBF) and cerebral blood volume (CBV) were calculated. For each software package and each tissue type, ratios were calculated between the perfusion values at the simulated dose level and the reference standard. Wilcoxon ranked sum tests were used to determine if the ratios deviate from the reference. Results: CBF and CBV values of GM and WM increased significantly for dose reduction for al perfusion software packages (p<0.05).The CBF values in GM and WM increased significantly with dose reduction in al perfusion software packages (p<0.05). The mean patient CBF ratios calculated with the software packages ranged from 1.14 at 80% dose to 1.47 at 50% dose in GM. In WM, mean CBF values ranged from 1.14 up to 1.51 at 80 and 50% dose, respectively. The CBV values showed similar results. Here, mean ratios ranged from 1.18 to 1.49 in GM and 1.15 to 1.50 in WM, again at 80 and 50% dose. Conclusion: Cerebral blood flow and cerebral blood volume are strongly dependent on radiation dose in CT perfusion imaging. Dose reduction may lead to errors in perfusion values calculated with most software packages. Clinical relevance/application: Radiologists have to be aware of this effect and have to interpret absolute CT perfusion values with care if dose reduction is performed