The presence/absence, degree and the speed of development of collateral flow in acute stroke strongly correlate with patient outcome. So far, DSA was the only reliable way to assess the colateral circulation. Of late, however, with the improved CT angiogram technology, excellent visualisation of teh medium sized vessels is possible and the cortical/leptomeningeal vessels ca be reliably seen. This has led to visual  assessment of the degree of the leptomeningeal collateral.
In this article from Alberta, an objective scoring system with clincal correlation has been published and is welcome. It should pave the path for better non-invasive assessment of the patient.
 Link to the article in journal's website