Order of CT stroke protocol (CTA before or after CTP): impact on image quality

The purpose of this study was to determine the appropriate order of CT angiography and CT perfusion in a multimodal stroke CT protocol. Forty patients with clinical suspicion of an acute cerebral infarct underwent non-enhanced CT (NECT), CT angiography (CTA), and CT perfusion (CTP). Twenty examinations were performed with CTP before CTA (group 1) and 20 in reversed order (group 2). Mean densities were determined at baseline and peak enhancement of CTP, as well as on source images of CTA in defined brain regions. Contrast of extra-/intracranial arteries and veins was rated according to a 5-point scale (1 = excellent, 5 = poor). CT-perfusion maps were assessed by determining the mean transit time (MTT), cerebral blood flow (CBF), and blood volume (CBV) in identical regions. Mean densities between both groups were not significantly different for CTA and CTP at peak enhancement. At CTP baseline, mean densities between groups 1 and 2 were different for all points except for GM and WM. There was no significant difference between both groups for the mean delta (the difference between baseline and peak enhancement), as well as for mean MTT, CBV, and CBF. Subjective evaluation of the CTA quality revealed no difference between both protocols, except for the extracranial venous contrast, which was less severe in group 2. Reversal of CT stroke protocol had no significant influence on quantitative parameters of CTP. Subjective quality of extracranial venous contrast was rated to be superior when CTA was performed before CTP.