“Detecting extraluminal disease in a bleeding patient maybe helpful in influencing treatment and goals of care—for example, in a case of a bleeding small- bowel neoplasm with metastatic disease. CT may also be used to identify complications of hypoperfusion (eg, signs of bowel ischemia or infarction) or the risk of organ disease exacerbation due to hypoperfusion (eg, findings of heart failure, arterial stenosis or coronary atherosclerosis, or renal parenchymal disease).” 
CT for Evaluation of Acute Gastrointestinal Bleeding 
Michael L.Wells et al.
Radiographics. 2018 Jul-Aug;38(4):1089-1107

“Urgent CT is useful for determining the optimal timing of colonoscopy in cases of acute LGIB. CE-CT may be used to depict the presence and location of active hemorrhage and provides useful information for subsequent colonoscopy, especially in patients with diverticular bleeding.”
Urgent computed tomography for determining the optimal timing of colonoscopy in patients with acute lower gastrointestinal bleeding.
Nakatsu S et al.
Intern Med. 2015;54(6):553-8

“The rate of detection of the bleeding source on colonoscopy was significantly higher in the patients with extravasation on CE-CT than in those without extravasation on CE-CT (68% vs. 20%, respectively; p<0.001).”
Urgent computed tomography for determining the optimal timing of colonoscopy in patients with acute lower gastrointestinal bleeding.
Nakatsu S et al.
Intern Med. 2015;54(6):553-8

“Both CTA and RBC scintigraphy can be used to identify active bleeding in 38% of cases. However, the site of bleeding is localized with CTA in a significantly higher proportion of studies.” 
Localizing Acute Lower Gastrointestinal Hemorrhage: CT Angiography Versus Tagged RBC Scintigraphy 
Feuerstein JD et al.
AJR 2016; 207:578–584

“The average time to complete an RBC scin- tigraphic examination was 3 hours 9 minutes after the order was placed. CTA examinations were completed an average of 1 hour 41 minutes after the initial order (p < 0.001). In the CTA group, 32 of 45 (71%) examinations were completed within 2 hours of initial order, com- pared with 31 of 90 (34%) RBC scintigraphic examinations (p < 0.001).” 
Localizing Acute Lower Gastrointestinal Hemorrhage: CT Angiography Versus Tagged RBC Scintigraphy 
Feuerstein JD et al.
AJR 2016; 207:578–584

”The results show that in the clinical setting of a patient presenting with GI bleeding requiring catheter-directed embolization, CTA reduces the time to angiography as compared to tagged RBC scanning. Furthermore, the overall time from the decision to order a radiologic study to catheter angiography is faster with CTA. Given the time from diagnostic study completion to angiography was not statistically significant between the two groups, it can be inferred that the limiting step was with diagnosis.”
Time to conventional angiography in gastrointestinal bleeding: CT angiography compared to tagged RBC scan 
Hsu MJ et al.
Abdominal Radiol (in press ) 2019

“Although nuclear scintigraphy and CTA had similar sensitivity and specificity, localization of hemorrhage site by CTA was more precise and consistent with angiography findings. As a pre-angiography test, compared with nuclear scintigraphy, CTA reduced overall the number of imaging studies required (mean [SD] number per patient admission, 2.1 [0.3] vs 2.5 [0.8]; P = .005).”
Arteriography for Lower Gastrointestinal Hemorrhage: Role of Preceding Abdominal Computed Tomographic Angiogram in Diagnosis and Localization.
Jacovides CL et al.
JAMA Surg 2015 Jul;150(7):650-6.

“A positive result at CT angiography is predictive of a positive result at subsequently performed fluoroscopic angiography. Sun et al found that among 26 patients with positive CT angiography results who subsequently underwent fluoroscopic angiography, the fluoroscopic findings were positive in 85% of cases. Compared with RBC scintigraphy, CT angiography has similar capability in the prediction of positive fluoroscopic angiography results but is better for localizing the site of bleeding.”
CT for Evaluation of Acute Gastrointestinal Bleeding 
Michael L.Wells et al.
Radiographics. 2018 Jul-Aug;38(4):1089-1107

Scan Protocol Selection

• Arterial phase imaging
• Venous phase imaging
• Role of non-contrast CT
• Role of delayed phase imaging

“In addition, comparison with unenhanced images allows differentiation of active hemorrhage from other high-attenuation material that may be present within the gastrointestinal tract at the time of CT angiographic evaluation, thereby preventing false-positive interpretation. Therefore, high-attenuation material detected within the bowel lumen at CT angiography that was not present at unenhanced CT performed immediately prior to the CT angiography is, in our experience, diagnostic for acute gastrointestinal hemorrhage.”
Acute Gastrointestinal Bleeding: Emerging Role of Multidetector CT Angiography and Review of Current Imaging Techniques
Laing CJ et al.
Radiographics. 2007 Jul-Aug;27(4):1055-70

“Most investigators agree that it is useful to obtain a precontrast study prior to the injection of IV contrast. This will help the differentiation of potential extravasated IV contrast from other high-density materials in the bowel, such as pills, sutures, or old oral contrast, etc.”
Acute gastrointestinal bleeding: the potential role of 64 MDCT and 3D imaging in the diagnosis 
Horton KM, Jeffrey RB Jr.,Federle MP, Fishman EK
Emerg Radiol (2009) 16:349–356

“Acute gastrointestinal tract bleeding (GIB) remains an important cause of morbidity and mortality. In the United States, >750,000 patients visit the emergency department each year with GIB, and in nearly half of those visits, the source of GIB is in the lower gastrointestinal tract. Despite advances in management, the mortality rate for patients with GIB remains at approximately 10% but increases to 40% in cases of massive bleeding associated with hemodynamic instability or the requirement for transfusion of >4 units of blood. Acute lower GIB is defined as bleeding into the large bowel or bleeding into the small-bowel distal to the ligament of Treitz.”
ACR Appropriateness Criteria Bleeding: 2021 Update
Karuppasamy K et al.
J Am Coll Radiol 2021;18:S139-S152

”Approximately 75% of episodes of acute lower GIB due to diverticulosis stop spontaneously, especially in patients requiring transfusions of <4 units of blood over a 24-hour period. Hence, in a substantial number of patients with acute lower GIB, conservative management is likely to be sufficient. For these patients, no immediate interventions are required, and diagnostic tests to identify the source of lower GIB can be arranged electively.”
ACR Appropriateness Criteria Radiologic Management of Lower Gastrointestinal Tract Bleeding: 2021 Update
Karuppasamy K et al.
J Am Coll Radiol 2021;18:S139-S152

ACR Appropriateness Criteria Radiologic Management of Lower Gastrointestinal Tract Bleeding: 2021 Update
Karuppasamy K et al.
J Am Coll Radiol 2021;18:S139-S152

Variant 2: CTA of the abdomen and pelvis without and with IV contrast or transcatheter arteriography/ embolization is usually appropriate as the next step for a hemodynamically unstable patient with active lower GIB or a patient who has required >5 units of blood within 24 hours. These procedures are equivalent alternatives (ie, only one procedure will be ordered to provide the clinical information to effectively manage the patient’s care). Diagnostic/ therapeutic colonoscopy may be appropriate for this clinical scenario, but the experts could not agree on the exact appropriateness category. 
ACR Appropriateness Criteria Radiologic Management of Lower Gastrointestinal Tract Bleeding: 2021 Update
Karuppasamy K et al.
J Am Coll Radiol 2021;18:S139-S152

ACR Appropriateness Criteria Radiologic Management of Lower Gastrointestinal Tract Bleeding: 2021 Update
Karuppasamy K et al.
J Am Coll Radiol 2021;18:S139-S152

”The most common CT pitfall that mimics active contrast extravasation is the presence of hyperattenuating material within bowel loops, most frequently hyperattenuating colonic fecal material, retained or inadvertently administered positive oral contrast material, or prior pill ingestion.”
Gastrointestinal Bleeding at CT Angiography and CT Enterography: Imaging Atlas and Glossary of Terms 
Flavius F. Guglielmo et al.
RadioGraphics 2021; 41:1632–1656

OBJECTIVES: To compare the respective capabilities of the arterial, the portal, and the combined set in the detection and localization of acute gastrointestinal (GI) bleeding with 64-section computed tomography (CT).
CONCLUSIONS: Using 64-section CT, the diagnostic performance was not different among the arterial, the portal, and the combined set for the detection and localization of acute GI bleeding.
Diagnosis of acute gastrointestinal bleeding: comparison of the arterial, the portal, and the combined set using 64-section computed tomography.
Kim JW et al
J Comput Assist Tomogr. 2011 Mar-Apr;35(2):206-11

“Both observers correctly detected the bleeding site in 81.3% and 84.4% on the arterial set, in 81.3% and 84.4% on the portal set, and in 84.4% and 84.4% on the combined set, respectively.”
Diagnosis of acute gastrointestinal bleeding: comparison of the arterial, the portal, and the combined set using 64-section computed tomography.
Kim JW et al
J Comput Assist Tomogr. 2011 Mar-Apr;35(2):206-11