The main finding of this study is that CTA is a reliable, fast, and noninvasive tool to assess proximal cerebral artery diameters during the management of cerebral vasospasm after aSAH. CTA has emerged as an alternative noninvasive modality for imaging of the cerebral arteries. Its usefulness was demonstrated for imaging and therapeutic decision making for cerebral aneurysms [3–5]. In this study, we assessed the reliability of CTA as a noninvasive, alternative imaging tool for the diagnosis of cerebral arterial vasospasm. Angiographic vasospasm is seen up to 40–70 % of patients and leads to ischemic deficits in 15–36 % of patients [6–9]. It usually occurs in a delayed fashion from 48 h to 14 days after hemorrhage [10, 11]. CTA has been compared with DSA for assessment of vasospasm in several prospective [12, 13] and retrospective [1, 14–16] studies. These studies showed high accuracy of CTA for the diagnosis of severe and proximal vasospasm. However, there is little mention of how vessel diameters were measured, and inaccuracies induced by using maximum intensity projections on CTA or by the lack of a millimeter scale in DSA have not been addressed to date. By normalizing all arterial diameters to the diameter of the extradural segment of the internal carotid artery, we tried to develop an objective and easy tool for clinicians to evaluate vasospasm in patients after aneurysmal SAH. This method succeeded in all proximal artery segments independently of the time point of measurements. Moreover, our study yielded lower correlation between CTA and DSA for the diagnosis of peripheral vasospasm as compared with proximal vasospasm. This finding confirms the notion that in vasospasm, the evaluation of post-bifurcation segments A1, M2, and P2 is beyond the resolution of CTA. Another limitation is poor image quality of CTA caused by movement artifact or the presence of streak artifacts from metal devices that obscure adjacent arteries.