The purpose of this study is to determine if the Valiant stent graft is safe and effective in treating patients who have a thoracic dissection. A thoracic dissection is a tear in the wall of the aorta, that causes blood to flow between the layers of the aorta and force the layers apart (dissect). This condition is a medical emergency and can quickly lead to death. Since stent grafting has been an effective way to treat other aortic conditions such as aneurysms (bulge in aorta wall), it is believed that the device would be effective in treating dissections. Information will be collected on the performance of the device for 5 years.