Venous phase

“ CT angiography performed in the emergency setting in patients with acute lower intestinal bleeding is feasible and correctly depicts the presence and location of active or recent hemorrhage, as well as the potential cause, in the majority of patients.”
Acute Lower Intestinal Bleeding: Feasibility and Diagnostic Performance of CT Angiography
Marti M et al.
Radiology 2012; 262:109-116

“ CT angiography depicted or helped exclude active or recent bleeding with an accuracy of 98% (46 of 47 patients).”
Acute Lower Intestinal Bleeding: Feasibility and Diagnostic Performance of CT Angiography
Marti M et al.
Radiology 2012; 262:109-116

“ Rather than restricting it to cases where colonoscopy fails initially, in the majority of cases, we propose CT angiography as the first step in diagnostic evaluation of patients with substantial bleeding for confirmation of active or recent hemorrhage and correct identification of the site and cause of bleeding.”
Acute Lower Intestinal Bleeding: Feasibility and Diagnostic Performance of CT Angiography
Marti M et al.
Radiology 2012; 262:109-116

“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

“ Not only can CT be valuable in those patients for whom colonoscopy is not feasible or impractical, but it can also identify a variety of extraluminal findings which may not be visible to the endoscopist. Moreover, CT can provide a wealth of valuable information beyond the presence or absence of active contrast extravasation/bleeding, such as bowel wall inflammation, perirectal inflammation, or the presence of an underlying vascular anomaly.” 
MDCT/CTA Evaluation of Rectal Bleeding: The Role of Volume Visualization
Raman SP, Horton KM, Fishman EK
AJR 2013;201: 589-597

Rectal varices are most commonly seen in the setting of cirrhosis and portal hypertension, with a prevalence ranging between 40 – 77% of all patients with portal hypertension. Nevertheless, the clinical incidence of bleeding in patients with rectal varices is still relatively low, perhaps less than 5%. From an imaging standpoint, rectal varices appear no different from hemorrhoids, although rectal varices typically occur more proximally in the rectum (proximal to the dentate line), rather than at the level of the anus. As with hemorrhoids, large serpiginous veins can be seen both surrounding the rectum (pararectal varices) and within the rectal wall itself (rectal varices) on portal venous phase images.

As with hemorrhoids, large serpiginous veins can be seen both surrounding the rectum (pararectal varices) and within the rectal wall itself (rectal varices) on portal venous phase images. Notably, unlike a rectal AVM, these serpiginous vessels do not enhance on the arterial phase images, and no early draining vein should be present. Given that bleeding from rectal varices is venous in nature, active extravasation is almost never visualized. Although they can appear quite similar on imaging, rectal varices should not be confused with hemorrhoids: Hemorrhoids occur in the anus, rather than the rectum, have no communication with the portal circulation, are not associated with portal hypertension, and merely represent vascular cushions .

”In patients with portal hypertension, rectal varices are common, occurring in 40%–77% of patients, although significant bleeding has been reported to occur in less than 5%.They appear as serpiginous vessels within the rectal wall that are best defined in the portal venous phase. Although the imaging appearance is identical to hemorrhoids, they can be differentiated by location above the dentate line and the associated findings of portal hypertension in the abdomen and pelvis.”
Gastrointestinal Bleeding at CT Angiography and CT Enterography: Imaging Atlas and Glossary of Terms 
Flavius F. Guglielmo et al.
RadioGraphics 2021; 41:1632–1656