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Background
The most common causes of acute non-variceal upper and lower gastrointestinal (GI) bleeds are peptic ulcer disease and diverticulosis, respectively. Less common etiologies of GI bleeding include vascular abnormalities, angiodysplasia, iatrogenic injury, neoplasms, inflammatory bowel disease, and polyps, among others. Although there is significant literature detailing the efficacy of transarterial embolization of the gastroduodenal artery, there is considerably less published on embolization of other vessels supplying the GI tract, including the ileocolic, right gastric, and right colic, and pancreaticoduodenal arteries, which will be discussed in this educational exhibit.

Educational Goals / Teaching Points
Vascular anatomy of the upper and lower gastrointestinal tracts is complex. Three-phase GI bleed CT protocol (noncontrast, arterial and venous) is essential for the localization of the source of bleeding and the potential etiology. Angiographic interrogation of the mesenteric vascular beds must be systematic. Upper GI bleeding must include interrogation of the celiac trunk and main branches including the gastroduodenal artery (GDA) which is one of the most common sources of bleeding. Lower GI bleeding interrogation must include the superior and inferior mesenteric arteries (SMA and IMA). The majority of cases of non-variceal GI bleeding are attributed to peptic ulcer disease. Radiologists must be aware of uncommon causes of GI bleeding like visceral artery aneurysms, iatrogenic, right diverticular disease, and neoplasms. Microcatheters should be always used for the interrogation of the distal mesenteric vascular beds. Active extravasation may not be angiographically seen in cases with a positive CT examination. Empiric embolization of the anatomic region is a reasonable alternative for unstable patients. Treatment options of mesenteric vascular bed bleeding include embolic agents like coils, gelatin sponges, and polyvinyl alcohol. The operator must be able to make a therapeutic selection based on the etiology of the bleeding. Complications of embolization of the mesenteric vascular beds include non-target embolization and rebleeding.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
For suspected upper GI bleeding (UGIB), the celiac artery is commonly imaged first as a majority of UGIB is caused by gastroduodenal ulcers which are supplied by branches of the celiac artery. If angiographically negative, selective left gastric artery (LGA) and gastroduodenal artery (GDA) evaluation is typically done. If the source of bleeding is thought to be in the small bowel or if no evidence of bleeding is seen upon interrogation of the celiac artery or its branches, the superior mesenteric artery (SMA) and inferior mesenteric artery (IMA) are evaluated next.

Conclusion
Knowledge of the vascular anatomy of the upper and lower GI tracts is essential for identifying uncommon sources of GI bleeding as well as potential complications. Familiarity with the diagnostic approaches and endovascular treatment options of less common GI bleeds is critical in effectively managing these bleeds when they are encountered.