Emphysematous infections produce gas at abnormal locations, either in the wall of hollow organs or in the parenchyma of solid organs. This group of infections include emphysematous cholecystitis, emphysematous pyelonephritis, emphysematous pancreatitis or emphysematous cystitis.However, there are numerous causes of gas within walls and parenchyma of abdominal andpelvic organs. Aside from emphysematous infections, iatrogenic or fistula might produce abnormal gas location. Conventional radiography and ultrasound can detect the presence of abnormal gas but havelimited sensitivity and specificity. Computed tomography is the imaging technique of choice for these entities, since it allows gas detection and extension as well as ruling out other differential diagnoses.

• Emphysematous cholecystitis

Emphysematous cholecystitis is a rare form of cholecystitis where gas is located wirhin the wall or lumen of the gallbladder. It represents a surgical emergency. It is more frequent in men and in diabetic individuals. There might be no associated cholelithiasis.

If the gas is intraluminal, the ultrasound will show a hyperechogenic image within the lumen, mobile, with a posterior comet artifact. An air-fluid level can be seen on CT. If the gas is in the wall, the ultrasound will show an arch-shaped hyperechoic image, with a posterior shadow, which can be confused with a porcelain gallbladder or full of calculi.

Sonographic suspicion needs to be confirmed with CT, which is more sensitive for demonstrating air in the wall, in the form of bubbles, or linearly.

Emphysematous cholecystitis versus aerobilia could make diagnosis difficult, although in aerobilia gas would not be found in the gallbladder wall.

• Emphysematous pyelonephritis

Emphysematous pyelonephritis is a necrotizing and suppurative infection of the urinary tract caused by gas-producing microorganisms that affects the renal parenchyma and the perirenal space. It must be differentiated from emphysematous pyelitis, in which the gas is limited to the interior of the urinary tract. This affectation is less serious than emphysematous pyelonephritis, although it can also be complicated if it is accompanied by an obstructive condition.

Risk factors are chronic urinary tract infection, immunosuppression (90% of patients have diabetes) and obstructive uropathy.

In ultrasound, the fundamental finding is the presence of gas, which can condition the assessment of the kidney itself if the amount of perirenal gas is significant. In this way, hyperechogenic foci with reverberation are visualized both in the sinus and in the renal parenchyma.

In CT, gas was observed both in the urinary tract and intraparenchymal, or formation of collections with air-fluid levels. The gas may spread as far as the contralateral retroperitoneum.

• Emphysematous cystitis

It is a rare but potentially lethal urinary tract infection, as it can rapidly progress to urinary sepsis. It often occurs in diabetics and women. It is characterized by the presence of air in the bladder wall, which can be identified with different radiological techniques.

• Infected pancreatic necrosis

In necrotizing pancreatitis, infection of the necrosis occurs in 20% of patients, generally between 2 and 4 weeks after the onset of the symptoms. It has a high morbidity and mortality and often requires surgical cleaning, unlike sterile necrosis.

Infection is suspected in a necrotic collection based on the patient's clinical picture or on CT if bubbles appear within the collection. This sign is not pathognomonic: if the collection has spontaneously drained into the gastrointestinal tract or has been marsupialized during surgery, it may contain gas without indicating infection.

• Emphysematous prostatitis

Acute prostatitis usually is due to surgical manipulation of the gland and rarely diagnosed by imaging test. Emphysematous prostatitis is a rare condition characterized by gas collection and purulent exudates within the prostate. Pelvis CT scan confirms this gas presence.

• Fournier Gangrene

Fournier gangrene is a rapidly progressing necrotizing fasciitis involving the perineal, perianal, or genital region. Although the diagnosis of Fournier gangrene is often made clinically, emergency computed tomography can provide early diagnosis and assessment of the extension. Findings at CT include asymmetric fascial thickening, subcutaneous emphysema, fluid collections, and abscess formation.