There is a large spectrum of pathologies caused by infection and inflammation in the abdominal cavity, often due to perforation, ischemia, or post-surgical complications that are responsible for abdominal sepsis, and being familiar with the major imaging findings and modalities is fundamental for an accurate diagnosis.
1. Imaging Modalities: CT and US
2. The most common pathologies:
2.1. Digestive and hepatobiliary:
2.1.1. Hepatic abscess
The hepatic abscesses are collections of necrotic inflammatory tissue caused by parasites, fungi, or bacteria (most common), normally affecting patients who have prior comorbidities such as immunodeficiency, diabetes, cirrhosis, advanced age, malignancy, etc. They can be divided into three categories based on the underlying conditions: infectious, malignant, and iatrogenic. (Fig 1)
In the case of suspicion, abdominal CT should be the initial diagnostic imaging modality, showing a variable appearance with the classical presentation as centrally hypoattenuating lesions with or without gas and peripherally enhancing that can be associated with segmental, wedge-shaped or circumferential perfusion abnormalities, with early enhancement," double target sign" (central low attenuation lesion surrounded - abscess – by a high attenuation inner rim – abscess membrane – and a low attenuation outer ring - edema). When they are multiple, they present a "cluster sign" (Fig 2).
2.1.2. Acute appendicitis
Acute appendicitis is the most frequent abdominal surgical emergency in the world, and the use of diagnostic imaging has decreased negative appendectomies and hospital costs. Despite the undoubted usefulness of US, the abdominal contrast-enhanced CT should be the gold standard imaging modality in non-pregnant adults when it is suspected because it can also identify other potential causes of abdominal pain and complications.
The main findings of CT appendicitis are: increased appendiceal diameter (≥8-9 mm), wall thickening (>3 mm), enhancement and stratification if no gangrene, cecal thickening, periappendiceal inflammation (fat stranding, thickening of the lateroconal fascia or mesoappendix, phlegmon, periappendiceal fluid and lymphadenopathy) and complications (abscess, extraluminal gas), appendicolith (high risk of perforation), some of this signs are shown in the examples (Fig 3 and Fig 4).
2.1.3. Acute cholecystitis:
Acute cholecystitis is the most common cause of acute pain in the right upper quadrant and a frequent cause of abdominal sepsis. The US should be the first imaging modality performed in the case of its suspicion; only if it is inconclusive should a CT be performed in the emergency context.
The main findings of US in acute cholecystitis are: gallbladder distention and wall thickening (>3 mm) and pericholecystic fluid. The most sensitive combination is cholelithiasis and sonographic Murphy sign, and less specific is the presence of sludge (Fig 5).
2.1.4. Acute diverticulitis
Acute colonic diverticulitis is a common cause of emergency consultation and urgent gastrointestinal surgery. CT is suggested as the initial imaging modality because it is a highly sensitive and specific modality for its diagnosis and complications (e.g., perforation, abscess, pylephlebitis, bowel obstruction, bleeding, fistula), as well as for the exclusion of alternate causes of pathology.
The main findings of acute diverticulitis are: segmental thickening of bowel wall, hyperenhancing diverticular wall and colonic wall, pericolic stranding, and complications as diverticular perforation (gas extraluminal), abscess formation, and fistula (chronic) (Fig 6).
Traditionally, the severity of diverticulitis was defined according to the Hinchey classification based on clinical and radiological criteria; however, in 2020, the World Society of Emergency Surgery (WSES) published an update based only on radiological criteria (Fig 7).
Surgery treatment is recommended in patients who fail medical management, in whom carcinoma cannot be excluded, who have recurrent events (≥2), who develop fistulas, and in those who progress to Hinchey stage III/IV and WSES 3/4.
2.1.5. Acute pancreatitis
Pancreatitis is a multifactorial inflammatory disease of the pancreas that can be a potentially life-threatening pathology. Its diagnosis is based on a combination of at least two criteria: acute epigastric pain, increased lipase/amylase >3 times the upper normal of limit and/or characteristic imaging features.
This pathology is classified by the Revised Atlanta Classification in two subtypes: interstitial edematous pancreatitis (90%) and necrotizing pancreatitis.
The main findings of acute pancreatitis are: focal or diffuse parenchymal enlargement (edema), indistinct pancreatic margins owing to inflammation, surrounding retroperitoneal fat stranding, lack of parenchymal enhancement (better 1 week after symptom onset to differentiate edema and necrosis) and pancreatitis complications (infected necrosis which is difficult to distinguish from aseptic liquefactive necrosis, abscess formation, hemorrhage, retroperitoneal fat necrosis that mimics carcinomatosis, vascular complication as aneurysms) (Fig 8).
2.1.6. Colitis and enteritis
They are intestinal inflammatory conditions that may involve one or more bowel segments secondary to multiple etiologies (inflammatory, ischemic and infectious - bacterial, virus, parasites and fungi).
As it represents a broad spectrum of pathologies and affections of the intestine, it also has varied radiological presentations, with contrast-enhanced CT being the most appropriate technique for assessing this pathology and its complications. Some of the CT findings are: dilated loops, thickening of the wall, increased density of the peri-intestinal fat, regional adenopathies, free fluid, and also increased peristalsis (US) (Fig 9).
2.1.7. Intestinal perforation due to occlusion
Bowel obstruction (large or small bowel, complete or incomplete) is a common cause of emergency surgery. The radiologist has an important role in confirming the diagnosis, localizing the site of obstruction, identifying the underlying cause (adhesions, hernias, malignancies, volvulus, etc.), and, ultimately, recognizing the complications (ischemia, perforation, abscesses) that can lead to abdominal sepsis.
The main findings of intestinal perforation due to occlusion are: dilated bowel loops proximal to a transition point (small bowel >3.0 cm, large bowel >5 cm), collapsed or normal caliber bowel distal to the transitional point, bowel wall thickening, surrounding mesenteric fat stranding, twisting of the mesentery in cases of volvulus, bowel ischemia, pneumoperitoneum in case of perforation and also abscesses (Fig 10).
2.2. Genitourinary:
2.2.1. Acute pyelonephritis
Although it is a clinical diagnosis, imaging has an important role in cases of patients not responsive to treatment, especially those with associated risk factors (diabetic, immunocompromised, pregnant, suspected septic focus, and concomitant renal colic). Most are secondary to ascending infection (85% of cases), and only a few are through hematogenous spread.
Contrast-enhanced nephrographic phase CT for acute pyelonephritis are the gold standard. The main findings are: striated nephogram (alternating bands of perfused and non-perfused renal parenchyma), patchy areas of low attenuation (edema) with wedge-shaped cortical hypoenhancement, perinephric fat stranding, renal enlargement and thickened urothelium in the collecting system, hyperenhancement of the pelvis/ureter in the cases of pyelitis, gas formation in emphysematous pyelonephritis and abscess formation (if complicated) (Fig 11).
2.2.2. Prostatic infections
Infections in the prostate gland can occur contiguously, such as from urethra or bladder infections or secondary to invasive procedures. Although it is a clinical diagnosis, imaging is useful to evaluate abscess formation.
Contrast-enhanced CT is the best imaging tool if abscess (predilection for peripheral zone) is suspected, especially in the emergency context, and we will find diffusely enlarged prostate, low-attenuation areas with rim-enhancing (abscess), and periprostatic fat stranding (Fig 12).
2.2.3. Pelvic inflammatory disease (PID)
PID is a common mimicker of appendicitis and diverticulitis and comprises a broad spectrum of pathologies, often caused by sexually transmitted infections (chlamydia, gonorrhea), intrauterine devices, etc. Due to nonspecific symptoms, contrast-enhanced CT is frequently used, especially when complications like tubo-ovarian abscess are suspected.
Contrast-enhanced CT is the chosen technique in the emergency context, and the main findings include thickening of the uterosacral ligaments, pelvic fat stranding with obscuration of fascial planes, reactive lymphadenopathy, and pelvic free fluid.
Pelvic Actinomycosis is a rare but aggressive form of PID linked to long-term IUD use, often mimicking pelvic malignancies (e.g., ovarian cancer, invasive cervical carcinoma, carcinomatosis) due to its infiltrative nature. The spread across tissue planes, particularly retroperitoneal extension into the psoas muscle, may suggest the diagnosis. The main CT findings are: infiltrative pelvic mass with enhanced soft-tissue density, pelvic abscesses with rim enhancement, fat stranding and inflammatory changes in adjacent tissues, and fistula formation (bladder or rectum) (Fig 13).