Inclusion Criteria:  IC1. Patients diagnosed with an AAD or acute on chronic aortic dissection, with a new diagnosis of malperfusion syndrome (Primary Cohort), by meeting both of the following criteria:  Imaging findings indicating reduced flow to the Celiac Trunk, Superior mesenteric artery, either renal artery or either iliac artery  Clinical stigmata of end organ ischemia (abdominal pain, distended abdomen, oliguria/anuria, reduced pulses, signs of limb ischemia) correlating with imaging findings -OR- Laboratory findings suggestive of end organ ischemia (lactic acidosis, elevated LFTs, Elevated Creatinine, Rhabdomyolysis, Electrolyte derangements) correlating with imaging findings  IC2. Patients diagnosed with an AAD undergoing surgical repair without evidence of malperfusion syndrome preoperatively, but who develop malperfusion syndrome due to dynamic flow changes after true lumen flow is reinstituted intraoperatively as indicated by new clinical signs or new laboratory results meeting the following criteria:  New Clinical signs include: Loss of femoral pulses, distended abdomen, reduced urine output, dusky/cold extremities New Laboratory results include: Rising lactate (>50% above baseline), Rising Creatinine, Metabolic Acidosis, Rising LFTs  Exclusion Criteria:  EC1. Subject has not been diagnosed with AAD, or acute on chronic aortic dissection EC2. Subject has never had combined imaging and clinical assessment, or laboratory results suggestive of malperfusion syndrome EC3. Subject is not hemodynamically stable to undergo IVUS evaluation EC4. Subject has anatomy or pre-existing condition precluding safe use of IVUS evaluation EC5. Subject has a pre-existing condition that may explain evidence of malperfusion (ie. Dialysis patient with severe renal stenosis).