الفهرس 
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Abstract
Acute GIB is a serious and often life-threatening condition. It is typically categorized as either upper or lower depending on the anatomic location of the bleeding site.
Upper gastrointestinal bleeding (UGIB) may involve the esophagus, stomach, and duodenum. It carries a mortality rate of approximately 10%. Its common causes in order of frequency are: Erosions or ulcers, variceal bleeding, Mallory-Weiss tear, vascular lesions, and neoplasms.
Lower gastrointestinal bleeding (LGIB) may involve the small bowel, colon, and rectum. It is less common than UGIB and accounts for approximately 30% of all GIB. It carries a mortality rate of 3.6%. Its common causes in order of frequency are: Diverticular disease, angiodysplasia, neoplasms, colitis, and benign anorectal lesions.
Available diagnostic procedures are either of low diagnostic accuracy such as barium examinations and scintigraphy, invasive such as catheter angiography or insensitive in localizing small bowel lesions such as endoscopy and capsule endoscopy.
The introduction of multi-detector CT has markedly reduced acquisition times, and this minimizes image degradation from movement artifact caused by peristalsis or respiration, thus all vascular territories can be imaged simultaneously during the contrast bolus.
There are two features were considered diagnostic of acute GIB: first, presence of extravasation of CM into the bowel lumen that progressed from one phase to the other, being present in the arterial but not in the native phase, or that progressed from the arterial to the portal venous phase, and second, extravasated CM with attenuation levels greater than 90 HU.
The extravasated CM is seen within the bowel lumen in form of linear, jet like, swirled, ellipsoid, or pooled configurations or may fill the entire bowel lumen, resulting in a hyperattenuating loop through comparing sequentially acquired unenhanced CT scans and CT angiograms without rigid adherence to attenuation analysis.
Two other minor but useful CT findings suggestive of acute massive GIB are focal dilatation of fluid-filled bowel segment noted on contrast enhanced CT scan and acute hematoma on unenhanced CT scan .
Evaluation of GIB with CTA may provide concurrent localization of active hemorrhage and diagnosis of the underlying cause, and can also have important treatment and management implications, as it provides high quality thin section data with postprocessing techniques as MPR, CPR, MIP, SR, VR, and endoluminal imaging which lead to accurate localization of the anatomical site of bleeding, as well as depicting the specific bleeding vessel, leading to rapid targeted embolization without the need for preliminary angiography of all territories.
Negative CT image may be either: True or false. True negative image may occur if bleeding has ceased spontaneously. While false negative image may occur in cases of acute mild GIB, also if there is an artifact from metallic clips or implants, or existing high-density material within the bowel lumen; comparison with the unenhanced image may be helpful to identify these cases.
CT has several advantages over other more conventional radiological techniques in the initial imaging of patients presenting with acute GIB. First, CT is relatively non invasive and widely available, while CCA is both invasive, and only available in units with a vascular radiology team. The second advantage of CT is that in the absence of oral CM, so it is very sensitive for detection of extravasated intravenously administrated CM within the bowel lumen with accurate localization of the bleeding.
Conversely, False negatives may occur with CCA if the appropriate vascular territory is not selected.
Another advantage of CT over strictly endoluminal procedures such as endoscopy and capsule endoscopy is the ability to evaluate the pathology precisely with regard to extraluminal abnormalities, feeding and draining vessels, and the anatomical region with its relationship to surrounding structures.
Ultimately, CTA has the potential to profoundly impact the initial evaluation and subsequent treatment of patients who present with acute GIB and should be considered as a first-line diagnostic examination in the appropriate clinical setting.
On conclusion, MDCT provides highly sensitive and accurate diagnosis in acute GIB as it enables the identification of bleeding source and etiology in patients with acute severe upper and lower GIB. Also CT allows the depiction of indirect signs suggesting the origin and cause of hemorrhage in patients with acute mild GIB