A basilar arterial scan was a type of medical test used in the 24th century. It was used to determine the existence and extent of damage to the basilar artery, which can be a fatal condition if left untreated.
In 2371, a version of Julian Bashir from a few hours in the future told Miles O'Brien that he had died due to damage of his basilar artery, which had not been detected because a scan had not been performed. He instructed O'Brien to ask his past self to run such a scan, which proved to save O'Brien's life. (DS9: "Visionary")