For most stroke patients, receiving a clot-dissolving drug shortly after arriving at a hospital can reduce the effects of stroke and limit permanent disabilities. But for some patients with a certain type of stroke, such a drug can actually increase bleeding in the brain. Stroke experts say the best way to tell which patients should get the drug is by having a CT scan of their heads read within 45 minutes of their landing in the emergency room. But a rule that would call for a CT scan within 45 minutes was rejected last fall by a quasi-governmental group that sets medical guidelines used by Medicare to evaluate and reimburse U.S. hospitals. The group, known as the National Quality Forum, said the vague wording of the rule raised too many questions.