To emphasize the point-you can’t assume that because you don’t see a thrombus that a thrombus is not present unless you have optimal vessel visualization. In most case you probably will be correct but when you are wrong…

Pitfall: In cases of suspected ischemia make sure you examine the entire vessel not just its proximal portion. If you can’t define it make that clear in your report.

“Superior Mesenteric Artery Aneurysms (SMAAs) SMAAs account for 5.5% of all visceral artery aneurysms. They most commonly present with abdominal pain and GI bleeding. Aneurysms commonly affect the proximal 5 cm of the artery. SMAAs occur more commonly in men and have a rupture rate of 38% in that cohort. Pseudoaneurysms of the SMA are most often caused by arterial dissection while true aneurysms are most often caused by septic emboli.
Visceral Artery Aneurysms: Diagnosis, Surveillance, and Treatment 
Fady Ibrahim et al.
Curr Treat Options Cardio Med (2018) 20: 97

29-year-old male with a history of Loeys-Dietz syndrome who presents with abdominal pain, nausea, and vomiting. Axial (A), sagittal (B), and 3D rendering (C) CT images demonstrate fusiform aneursymal dilation of the proximal SMA measuring up to 3.5 cm (red arrows) with dilation of the more distal SMA to 1.2 cm (yellow arrows). Minimal wall thickening and fat stranding surrounding the proximal SMA. Tortuosity and dilation of the abdominal aorta and common iliac arteries.

82-year-old male with proximal celiac artery occlusion who presents with peri-umbilical abdominal pain. Axial (A) image demonstrates a calcified enhancing lesion arising from an unclear vessel (red arrow). Multiple collateral vessels arising from the pancreaticoduodenal arcade. The sagittal MIP (B) image more clearly demonstrates a saccular aneurysm of a proximal SMA branch measuring up to 1.7 cm (red arrows).

69-year-old male with abdominal pain and an incidental aneurysm found at outside hospital. Sagittal image (A) demonstrates a partially thrombosed aneurysm of the SMA (red arrow). The sagittal MIP image (B) easily depicts the aneurysm is 4.2 cm from the SMA origin. Volume rendering 3D image (C) shows the location of the aneurysm in relation to the branches of the SMA and the pancreas.

Left atrial appendage clot

“Intimal flap, thrombosed false lumen and aneurysmal dilatation are the most common CT findings of spontaneous splanchnic artery dissection. In splanchnic artery dissection, conservative management without anticoagulation had good outcome except in patients with bowel ischemia, aneurysmal dilatation three times larger than a normal segment or progression of dissection. ”
Spontaneous Dissection of the Splanchnic Arteries: CT Findings, Treatment and Occlusion
Jung SC et al.
AJR 2013;200:219-225

“The superior mesenteric artery (SMA) provides vital blood supply to the midgut, and an acute abnormality can rapidly precipitate bowel ischemia and infarction and lead to morbidity and mortality. Vascular diseases that acutely compromise the SMA threaten its tributaries and include occlusion, dissection, aneurysm rupture, pseudoaneurysm, vasculitis, and SMA branch hemorrhage into the bowel. Emergency medicine physicians rely on the radiologist to identify SMA abnormalities, to characterize them, and to detail findings that guide the appropriate triage of these patients. As such, radiologists must be cognizant of the features that influence patient treatment to assist vascular surgeons and interventional radiologists in decision making.”
High-Value Multidetector CT Angiography of the Superior Mesenteric Artery: What Emergency Medicine Physicians and Interventional Radiologists Need to Know.
Ghodasara N1, Liddell R1, Fishman EK1, Johnson PT1.
Radiographics. 2019 Mar-Apr;39(2):559-577. 

Several different classification schemes exist for isolated SMA dissections. Sakamoto et al. describe four types:

• Type 1 has a patent false lumen with entry and re-entry.
• Type 2 is a “cul-de-sac” shaped false lumen without re-entry.
• Type 3 has a thrombosed false lumen with ulcer.
• Type 4 has a completely thrombosed false lumen without ulcer.

MDCT findings direct management:

• Concomitant aortic dissection, which is managed surgically
• Length and diameter of dissection
• Presence or absence of reentry
• Involvement of distal branches
• Patency of true and false lumens
  • Compression of true lumen >80% may be an indication for stenting.
• Collateral branches of SMA
• Presence of hematoma or bowel ischemia

Treatment options include medical, surgical, and endovascular approaches.

• Conservative management with long-term anticoagulation and blood pressure control is common.
• Surgery or endovascular treatment often reserved for failed medical therapy or recurrent symptoms.
• Stent placement with goal of obliterating false lumen, covering entry point, and balloon angioplasty of distal involvement.
• Surgery indicated with bowel necrosis, impending arterial rupture, and increased size of aneurysmal dilation
  • Aorto-SMA bypass
  • Intimectomy
  • Endoaneurysmorrhaphy
  • Arteriotomy with thrombectomy