Evaluation of the Valiant Mona LSA Thoracic Stent Graft System in Descending TAA and Chronic Dissections

Inclusion Criteria for Thoracic Aortic Aneurysms and Penetrating Ulcers: * Subject is at least 18 years of age. * Subject understands and has signed an Informed Consent approved by the Sponsor and by the IRB for this study. * Subject must be considered a candidate for revascularization of the LSA. Subject must be able to tolerate a surgical revascularization of the LSA. * Subject has a TAA/PAU which will require coverage of the LSA and is: * a fusiform aneurysm with a diameter of ≥ 5.5 cm OR is \> 2 times the diameter of the non-aneurysmal thoracic aorta; AND/OR * a saccular aneurysm or PAU (ulcer defined as ≥ 10 mm in depth and 20 mm in diameter, or symptomatic) * Subject has a healthy, non-diseased aortic proximal seal zone of at least 20 mm from the distal end of the LCC ostium to the beginning of the disease, including at least 10 mm between the LSA and the LCC. * Subject has a non -diseased aortic proximal neck length of \>0mm distal to the LSA * Subject has a non-diseased aortic diameter between 25 mm and 42 mm * Subject has a non-diseased LSA with a diameter between 8 mm and 13 mm. * Subject has sufficient landing zone within the LSA to accommodate the BSG without occlusion of any significant vessels * Brachial, iliac or femoral artery access vessel morphology (diameter, calcification, tortuosity) that is compatible with vascular access techniques, the device, or accessories. * Introducer sheath is required for all procedures. * An iliac conduit is required for access if the above requirements are not met. Inclusion Criteria for Chronic Type B Dissections: * Subject is at least 18 years of age. * Subject understands and has signed an Informed Consent approved by the Sponsor and by the IRB for this study. * Subject must be considered a candidate for revascularization of the Left Subclavian Artery (LSA). Subject must be able to tolerate a surgical revascularization of the LSA. * Subject has a chronic type B dissection which will require coverage of the LSA. A chronic type B dissection is defined as \> 30 days from symptom onset and is complicated with an aortic diameter ≥ 5.5 cm or has progressive aortic enlargement (\> 5 mm/year). * Subject has a healthy, non-diseased aortic proximal seal zone of at least 20 mm from the distal end of the LCC ostium to the beginning of the disease, including at least 10 mm between the LSA and the LCC * Subject has a non-diseased aortic diameter between 28 mm to 44 mm. * Subject has a non-diseased LSA with a diameter between 8 mm and 13 mm. * Subject has sufficient landing zone within the LSA to accommodate the BSG without occlusion of any significant vessels * Brachial, iliac or femoral artery access vessel morphology (diameter, calcification, tortuosity) that is compatible with vascular access techniques, the device, or accessories. * Introducer sheath is required for all procedures. * An iliac conduit is required for access if these requirements are not met. Exclusion Criteria * Subjects will be excluded if they have conditions requiring prospective revascularization of the LSA including: * Dominant left vertebral artery requiring revascularization * Prior coronary artery bypass graft utilizing the left mammary artery requiring revascularization * Incomplete circle of Willis or other neurological vasculature requiring revascularization * Subject has an aneurysmal, tortuous, or atherosclerotic LSA. * Subject has an acute dissection of the descending thoracic aorta. * Subject has an intramural hematoma of the descending thoracic aorta. * Subject has prohibitive calcification, occlusive disease, or tortuosity of intended fixation sites. * Subject has circumferential calcification in the external iliac artery or in the common iliac artery with an intraluminal diameter (ID) less than 10mm at any point proximal to or at the access vessel site unless a surgical adjunctive procedure is planned. * Subject requiring an aortic conduit or direct aortic access * Subject has an aortic atheroma classified as grade IV or grade V. * Subject has had previous endovascular repair of the ascending and/or descending thoracic aorta \<30 days of implantation of investigational device or previous repair was a non-Medtronic device * Treatment with the Valiant Mona LSA Thoracic Stent Graft system would require intentional coverage of the left common carotid artery with the stent graft fabric. * Subject has significant and/or circumferential aortic mural thrombus at either the proximal or distal attachment sites that would compromise fixation and seal of the device. * Subject is a pregnant female. * Subject has a known allergy or intolerance to the device components. * Subject is in acute renal failure or has renal insufficiency with a serum creatinine ≥ 2.0 mg/dL or is on dialysis. * Subject has a body habitus which prevents adequate visualization of the aorta. * Subject has coronary artery disease with unstable angina and who has not received treatment. * Subject has a connective tissue disease (e.g. Marfan's syndrome, medial degeneration). * Subject has active systemic infection and/or a mycotic aneurysm. * Subject is currently participating in an investigational drug or device clinical trial that would interfere with the observations of this study. * Subject has other medical, social, or psychological problems that, in the opinion of the investigator, will interfere with treatment and follow-up procedures. * Subject has a life expectancy of less than 1 year. * Subject requires treatment of an infrarenal aneurysm at the time of the implantation. * Subject has had previous surgical or endovascular treatment of an infra-renal aortic aneurysm \<30 days of implantation of investigational device. * Subject has a history of bleeding diathesis, coagulopathy, or refuses blood transfusion. * Subject has had a cerebral vascular accident (CVA) within 3 months. * Subject has had a myocardial infarction (MI) within 3 months. * Subject has a known hypersensitivity or contraindication to anticoagulants or contrast media, which is not amenable to pre-treatment.