Mobile stroke units debuted in two U.S. cities during the past year, and already they are saving critical minutes when it comes to treating stroke patients, according to a pair of studies released Wednesday. The mobile units, each of which resemble an ordinary ambulance and cost about $1 million, are essentially specialized emergency rooms on wheels. They include unique equipment, such as a portable CT scanner to help determine what type of stroke a patient is experiencing and how best to treat it. They also include lab testing equipment and clot-busting drugs that can greatly improve the prognosis for patients who receive them soon after the onset of symptoms.