Inflammatory and infectious pathology
—Peptic ulcer disease
It is mainly associated with Helicobacter pylori infection and the use of nonsteroid anti-inflammatory drugs, resulting from erosion of the gastroduodenal mucosal barrier by peptic acid. This erosion can penetrate the deeper parietal layers, ultimately leading to perforation. Duodenal ulcers are most common in the duodenal bulb, the first contact site between gastric acid and the duodenum. Postbulbar ulcers are rare and may be associated with pathologies such as Crohn's disease and Zollinger-Ellison syndrome.
Direct CT findings of uncomplicated peptic ulcer disease are luminal outpouching and focal mucosal discontinuity. Indirect signs include short segmental parietal thickening, submucosal oedema, adjacent fat stranding, and locoregional lymphadenopathy.
Haemorrhage, perforation, strictures and gastric outlet obstruction can complicate peptic ulcer disease.
Haemorrhage: Three-phase CT angiography is the most often used imaging method for diagnosing active haemorrhage. Non-enhanced CT images can show spontaneously hyperdense (30-45 HU) intraluminal material and a sentinel clot. On arterial phase images, active bleeding can be detected by an intraluminal contrast blush. On the portal venous phase images, there can be an increase in the volume or change in the size of the contrast blush.
Perforation: Imaging findings of duodenal peptic ulcer perforation on CT include pneumoperitoneum or pneumoretroperitoneum (depending on the duodenal segment affected), focal parietal thickening, periduodenal fluid, adjacent fat stranding, defects of the duodenal wall, and extraluminal extravasation of oral contrast material. 
— Duodenitis
It may have an inflammatory aetiology, such as Crohn's disease, or an infectious aetiology, most commonly Helicobacter pylori. The duodenal Crohn's disease usually affects the first and second portions of the duodenum.
Duodenitis manifests itself on CT through nonspecific findings such as mucosal hyperattenuation, submucosal oedema, wall thickening, and adjacent fat stranding. 
In the case of Crohn's disease, ulceration and stricture formation may occur early, and fistulization from adjacent affected intestinal loops may occur at more advanced stages.
—Pancreatitis
It is the inflammatory pathology that most frequently affects the duodenum. Duodenal injury can result from extrinsic compression by the enlarged pancreatic head, a fluid collection or the direct action of pancreatic enzymes. Pancreatic enzymes may induce mild to severe duodenal mural oedema, leading to gastric outlet obstruction, or disrupt the duodenum's intramural vasculature with consequent intramural hematoma in cases of severe pancreatitis. CT findings comprise duodenal wall thickening and surrounding fat standing associated with pancreatitis findings.
Paraduodenal pancreatitis is a segmental chronic pancreatitis that affects the pancreaticoduodenal space between the pancreatic head, the duodenum and the distal common bile duct. Inflammation of the paraduodenal space can range from mild stranding to a marked tumefactive infiltrate. CT may demonstrate ill-defined soft tissue attenuation with late enhancement in the pancreaticoduodenal groove, duodenal wall thickening that may present small cystic lesions ("cystic dystrophy"), fat stranding, and massive enlargement of the pancreatic head. 
—Acute cholecystitis
Due to the proximity of the gallbladder to the first and second duodenal segments, acute cholecystitis can cause secondary inflammation of the adjacent duodenum.
In cases of severe, recurrent or chronic cholecystitis, gallstone ileus may occur, resulting from erosion of the gallbladder wall by a stone with fistulization, generally at the level of the duodenum (cholecystoduodenal fistula), allowing the passage of the stone into the intestinal lumen. 
Neoplastic pathology
Neoplastic pathology may arise primarily in the duodenum or develop in other organs and secondarily affect the duodenum.
Duodenal adenocarcinoma, the most common duodenal malignancy, occurs often in the periampullary area and rarely at the level of the duodenal bulb. CT often reveals a focal concentric or asymmetrical parietal thickening and enhancement, inducing luminal narrowing, which may develop early gastric outlet obstruction or even common bile duct obstruction if the neoplasm affects the second portion of the duodenum. It can present itself as a polypoid, infiltrative, or ulcerative mass.
Lipoma is a mesenchymal mass-forming tumour comprised primarily of adipose tissue. It is a benign, usually solitary, well-circumscribed, and slow-growing tumour that may present on CT as a submucosal mass with regular, well-defined contours and homogeneous fat density. 
Periampullary tumours arise within 2 cm of the ampulla of Vater. They may originate in the ampulla of Vater, in the intrapancreatic bile duct, in the head and uncinate process of the pancreas and the duodenal mucosa.
Ampullary carcinomas generally have an intraluminal growth with ductal obstruction leading to an early clinical presentation (intermittent jaundice is often the initial presentation) and, therefore, usually have a better prognosis than adenocarcinomas of the bile ducts or pancreas. CT findings of ampullary carcinomas include wall thickening or intraluminal soft-tissue polypoid mass originating in the medial wall of the second duodenal segment, usually associated with biliary duct dilatation, with or without pancreatic duct dilatation. 
Duodenal metastases most commonly affect the periampullary region and the duodenal bulb. Duodenum metastization may result from local invasion (from tumours of the pancreatic head, stomach, gallbladder, colon and liver), hematogenous spread (from lung cancer and melanoma) or through the mesentery (spread through the mesentery). 
Vascular pathology
—Aortoenteric fistula (AEF)
It is an abnormal communication between the aorta and the intestinal tract, most often involving the duodenum (aortoduodenal fistula).
Clinical presentation is nonspecific and may include gastrointestinal bleeding, sepsis and abdominal pain.
Primary AEF is rare. It corresponds to direct communication between the native aorta and the adjacent intestine, typically associated with a mycotic aortic aneurysm and without previous aortic surgery or trauma.
Secondary AEF is more common than primary but still relatively rare. It results from aortic reconstructive surgery with or without aortic stent graft.
Three-phase CT angiography may detect specific findings such as ectopic gas adjacent to the aorta, breach of the aortic wall, active extravasation of contrast material into the duodenal lumen, and the fistulous tract between the duodenum and the aorta. Other findings include thickening of the duodenal wall adjacent to the aorta, periaortic fat stranding, soft-tissue attenuation around the aorta with effacement of the fat planes between the aorta and the duodenum, retroperitoneal hematoma, and aortic pseudoaneurysm. 
—Superior mesenteric artery (SMA) syndrome
Corresponds to compression of the third duodenal segment between the aorta and the SMA due to the narrowing of the aortomesenteric distance (less than 8-10 mm) and angle (less than 22º), which is evaluated on sagittal CT or MRI reformations. 
Congenital pathology
—Annular pancreas
It is an uncommon congenital migration anomaly of the pancreas in which the pancreatic parenchyma surrounds (completely or incompletely) the duodenum, most often the second duodenal segment. It can be an incidental finding in imaging studies or manifest with duodenal obstruction (usually in childhood) or pancreatitis (commonly in adulthood).
In the incomplete annular pancreas, pancreatic tissue is observed anterior and posterior to the duodenum, with a "crocodile jaw" appearance. This finding may be associated with duodenal narrowing and dilatation of the proximal duodenum. 
—Malrotation
Defines the abnormal positioning of the intestine in the peritoneal cavity resulting from abnormal intestinal rotation during embryogenesis. As a result, the duodenum does not cross the midline and remains to the right. The SMA is on the right of the vein, the small intestine is on the right side of the abdomen, the colon is on the left, and peritoneal bands (Ladd bands) may extend from the cecum up to the liver and peritoneum on the right side of the midline, crossing the duodenum and compressing it, with obstruction. 
Traumatic/iatrogenic complications
Iatrogenic injury to the duodenum can occur after invasive diagnostic and therapeutic procedures such as endoscopic retrograde cholangiopancreatography, esophagogastroduodenoscopy, sphincterotomy, stent placement, polypectomy and mucosectomy.
These include haemorrhage, perforation, obstruction, and infection. 