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Acute gastrointestinal (GI) bleeding is a common cause of both emergency department visits and hospitalizations in the USA and can have a high morbidity and mortality if not treated rapidly. Imaging is playing an increasing role in both the diagnosis and management of GI bleeding. In particular, CT angiography (CTA) is a promising initial test for acute GI bleeding as it is universally available, can be performed rapidly and may provide diagnostic information to guide management. The purpose of this review was to provide an overview of the uses of imaging in the diagnosis and management of acute GI bleeding, with a focus on CTA. INTRODUCTION Acute gastrointestinal (GI) bleeding is a common cause of both emergency department visits and hospital admissions, leading to 375 hospitalizations per 100,000 individuals per year in the USA. If acute GI bleeding is not diagnosed promptly and adequately treated, morbidity and mortality are high, reaching 40% in patients who are haemodynamically unstable. Diagnosis and treatment requires a multidisciplinary approach that may involve diagnostic and interventional radiology, emergency medicine, internal medicine, gastroenterology and general surgery. Imaging is playing a growing role in the management of acute GI bleeding by localizing the source of bleeding, differentiating the underlying disease processes and aiding decisions to proceed to endovascular therapies to treat many causes of GI bleeding. The purpose of this review article is to highlight the increasing role of radiology in the diagnosis and management of acute GI bleeding, with a focus on the emerging role of multidetector CT (MDCT) angiography. We will provide an overview of the clinical diagnosis and common causes of acute GI bleeding and will review the modalities (endoscopy, radionuclide imaging, catheter angiography) traditionally used for diagnosis. We will then provide an in-depth review of the role of CT angiography (CTA), including examples of the CT appearance of many causes of acute GI bleeding, common diagnostic pitfalls and new directions in CT including dose reduction and dual-energy CT (DECT). Finally, we will propose simplified algorithms for the appropriate use of imaging in acute upper and lower GI bleeding. CLINICAL DIAGNOSIS AND COMMON CAUSES OF ACUTE GASTROINTESTINAL BLEEDING Clinical assessment and appropriate triage of patients with acute GI bleeding can be challenging. Following haemodynamic stabilization of the patient, the most important diagnostic consideration is whether the source of bleeding is in the upper GI tract (above the ligament of Treitz) or lower GI tract (below the ligament of Treitz), as patients with upper GI tract bleeding will be triaged to endoscopy and those with lower tract bleeding will generally be evaluated with imaging or colonoscopy depending on the clinical scenario. Common causes of upper GI bleeding include peptic ulcer disease (62%), arteriovenous malformations (10%), gastritis or duodenitis (8%) and oesophageal varices (6%) (Figure 1). Common causes of lower GI bleeding include diverticular disease (40%), colitis and inflammatory bowel disease (21%), neoplasia (14%), coagulopathic haemorrhage (12%) and angiodysplasia, among others. Distinguishing between upper and lower GI bleeding based solely upon clinical history and physical examination is difficult, as there is significant overlap in the clinical presentation. Most (75%) cases of GI bleeding have an upper GI tract source. Patients with upper GI bleeding classically present with haematemesis (vomiting bright red blood), “coffee ground” emesis (vomiting darker blood that has been partially digested) or melena (passage of dark faeces containing digested blood), although patients can present with haematochezia (passage of bright red blood per rectum) in cases of brisk upper GI bleeding with rapid transit time. Conversely, patients with lower GI bleeding classically present with haematochezia, but may also present with melena. Although nasogastric tube aspiration can be assessed for the presence of blood, it is neither sensitive nor specific for diagnosis of an upper GI source; so, diagnostic imaging and endoscopy are often relied upon to determine the source of bleeding. In patients presenting acutely to the emergency department with GI bleeding, several clinical factors are typically assessed to determine the appropriate diagnostic and treatment strategy, including the likely location of bleeding, rate of haemorrhage and overall hemodynamic status. DIAGNOSTIC MODALITIES Upper endoscopy and colonoscopy Endoscopy is often the first-line diagnostic and therapeutic modality in patients with acute GI bleeding, particularly if there is a suspected upper GI source. Endoscopy is highly sensitive and specific for acute upper GI bleeding with sensitivity of up to 98% and specificity of up to 100%. Endoscopy and colonoscopy provide direct visualization of the mucosa to identify the source of bleeding, enable application of haemostatic therapy and can be used for tissue sampling in cases of suspected malignancy. However, endoscopy and colonoscopy have a number of limitations. They may not be readily available in the emergency room setting. For patients with high-volume bleeding, it may be impossible to adequately visualize the source of haemorrhage with endoscopy. In addition, for those with lower GI tract bleeding, endoscopy is unable to assess the majority of the small bowel distal to the ligament of Treitz and provides limited visualization of the distal duodenum. Bowel preparation for colonoscopy can take 3–5 h, which may not be possible in patients with acute colonic haemorrhage who are clinically unstable. Even with adequate bowel preparation, sensitivity and specificity of colonoscopy in acute lower GI bleeding has been shown to be lower than that of MDCT. Figure 1. CT angiography in an 82-year-old male with bright red blood per rectum: on axial non-contrast CT (a), there is no hyperattenuating material in the bowel lumen. The axial arterial phase image (b) demonstrates active extravasation of contrast (arrows) into the second portion of the duodenum, which was found on endoscopy to arise from a duodenal ulcer. Ulcer disease is the most common cause of upper gastrointestinal bleeding. Figure 2. Tagged red blood cell images in a 78-year-old female with bright red blood per rectum demonstrating accumulation of radiotracer within the transverse colon (a, arrows). Selective catheter angiography of a branch of the right colic artery (b) shows brisk contrast extravasation (white arrow) into the lumen of the caecum, which was subsequently treated with coil embolization. BJR Wortman et al 2 of 9 birpublications.org/bjr Br J Radiol;90:20170076 Video capsule endoscopy is a technique that has been shown to be beneficial in evaluation of obscure GI bleeding (persistent bleeding with negative upper and lower endoscopy). Recently, some researchers have evaluated capsule endoscopy in assessment of acute upper GI bleeding in patients in the emergency department as a method to triage patients. Although this approach shows promise, capsule endoscopy is not currently considered a suitable substitute for endoscopy. Radionuclide imaging Radionuclide imaging for GI bleeding is generally performed with technetium-99m tagged red blood cells, with initial injection of radiotracer and subsequent gamma camera imaging. GI bleeding can be diagnosed when radiotracer activity is visualized outside of normal areas of blood pool, which either focally intensifies or moves over time in an antegrade or retrograde fashion (Figure 2). Radionuclide studies have the advantage of being highly sensitive, detecting rates of bleeding as low as 0.05–0.1 mlmin. In addition, scintigraphy can assess for bleeding over a prolonged period of time and can detect both arterial and venous haemorrhage. However, owing to their prolonged imaging times, these studies are not ideal for patients who are clinically unstable. In addition, radionuclide imaging may have limited availability in the acute care setting, particularly overnight. Radionuclide scans also often cannot provide precise anatomic localization of the site of active bleeding; this, in combination with their high sensitivity, could potentially lead to some positive radionuclide scans that are followed by negative endoscopy or catheter angiography. Catheter angiography Catheter angiography is considered the first-line imaging and treatment modality for patients who are unstable with lower GI bleeding, patients following a failed upper or lower endoscopy and patients with lower GI bleeding with a source of bleeding visualized on an additional imaging modality. For a known source of GI bleeding, selective catheter angiography of the bleeding artery and embolization can be performed (Figure 2). For an unknown bleeding source, angiography is generally performed of the celiac axis, superior mesenteric artery, and inferior mesenteric artery. Bleeding can be diagnosed confidently when active extravasation of contrast material into the bowel lumen is visualized (Figure 2), and embolization can be performed if active extravasation is visualized. In addition, for patients with upper GI bleeding, prophylactic embolization can be performed even if a site of bleeding is not identified, most commonly of the gastroduodenal artery. This has been shown to be effective, particularly in patients with large volume upper GI bleeding refractory to endoscopic treatment. Sensitivity and specificity of catheter angiography is highly variable in the literature, with sensitivity averaging 60%, and technical success rates ranging from 73–100% for lower GI bleeding and 60–100% for upper GI bleeding. Angiography has a high spatial resolution, can detect rates of bleeding as low as 0.5 ml/ min and has the added major advantage of allowing for treatment of GI bleeding. The primary disadvantage of catheter angiography is that it is an