Imaging is essential to managing emphysematous pyelonephritis —a life-threatening, fulminant, necrotizing upper urinary tract infection associated with gas within the kidney—if an early diagnosis is to be made and a potentially devastating outcome is to be avoided. [1, 2, 3, 4, 5]
Some confusion exists regarding terminology for conditions involving renal gas. The term emphysematous pyelonephritis should be reserved for renal tract infections with intraparenchymal renal gas. Gas confined to the renal pelvis should be called emphysematous pyelitis, and gas confined to the perinephric space should be called perinephric emphysema. [6, 7, 2, 8, 9, 10, 11]
Emphysematous pyelonephritis is rare, but the frequency is higher in patients who are immunocompromised, especially patients with diabetes, who account for 87-97% of patients (see the images below). 
Lu and associates conducted a study to determine the clinical characteristics and prognostic factors of patients with emphysematous pyelonephritis. The study concluded that (1) low albumin levels, (2) an initial presentation of shock, (3) bacteremia, (4) indications for hemodialysis, or (5) polymicrobial infection represent prognostic factors for mortality in patients with emphysematous pyelonephritis. Any 2 or more of these of these prognostic factors carried a high risk of mortality, and these patients should be considered for more aggressive management. 
Plain abdominal radiography is the initial examination of choice in emphysematous pyelonephritis because this modality better depicts air in the renal collecting system and it is much more specific than ultrasonography. In practice, however, ultrasonography may be the initial examination performed. (See the image below.) [13, 14, 15, 16, 17, 18]
CT scan findings are diagnostic of the presence of air within the renal tract, and CT images also elegantly depict the renal and perirenal anatomy and the spread of infection to the perinephric tissues. [15, 4]
Renal function is depressed or even absent on the affected side in emphysematous pyelonephritis, and radionuclide study is an excellent modality for assessing differential function when nephrectomy is contemplated. Scintigraphy has been used to evaluate responses to antimicrobial therapy. Intravenous urography may be necessary if renal intervention is contemplated.
Limitations of techniques
Plain radiographs are good for depicting air within the renal collecting system, but nonspecificity is a problem because of the superimposition of gas from the bowel. Moreover, gas in the retroperitoneum and gas within a renal or perinephric abscess may mimic emphysematous pyelonephritis.
Similarly, ultrasonography is limited because gas within the kidney and/or renal pelvis mimics renal calculi and produces artifact due to reverberation echoes and shadowing.
CT scans do not always depict other causes of intrarenal air, such as reflux of air from the bladder and bronchorenal, enterorenal, or cutaneorenal fistulae. These may occur with xanthogranulomatous pyelonephritis and focal renal abscesses. 
Radionuclide studies are nonspecific; therefore, they have a limited role in the evaluation of emphysematous pyelonephritis. In addition, radionuclide imaging suffers from a lack of availability. However, it does not result in false-positive or false-negative diagnoses.
Magnetic resonance imaging (MRI) is not the modality of choice in the diagnosis of emphysematous pyelonephritis. MRI findings are a signal void on T1- and T2-weighted images. However, signal voids on MRI scans may occur with renal calculi or rapidly flowing blood, creating false-positive results. Perinephric and intraparenchymal fluid collections are demonstrated well on MRI. [15, 16]
Emphysema is part of the differential diagnosis of emphysematous pyelonephritis. Other conditions to be considered include the following:
Retroperitoneal perforation of an abdominal viscus
Psoas abscess secondary to gas-forming organisms
Reflux of air from the bladder
Bronchorenal, enterorenal, or cutaneorenal fistulae - As may occur with xanthogranulomatous pyelonephritis
Air in a focal renal abscess - Not life threatening
Gas in the renal parenchyma may be seen in conditions other than emphysematous pyelonephritis on CT scanning. Intraparenchymal renal gas may be seen following urologic intervention such as that for a nephrostomy insertion or may result from a fistulous communication between the gastrointestinal tract and the renal collecting system. These situations do not represent clinical emergencies and are not life-threatening, as in emphysematous pyelonephritis. With the increasing use of abdominal CT scanning, radiologists, especially in the emergency setting, should be aware of this comparatively rare finding and should be familiar with its differential diagnosis.
Emphysematous pyelonephritis has been described as a presenting feature of a urinary bladder adenocarcinoma in a middle-aged, nondiabetic patient. 
Plain radiographs in patients with emphysematous pyelonephritis may show bubbles of gas within the region of the renal bed and in the upper renal collecting system. These may be diagnostic in the appropriate clinical setting.  Gas within the collecting system without evidence of renal parenchymal gas may be seen in patients with diabetes and does not have the same ominous prognosis (see the images below). Acute renal edema with obliteration of the renal pelvis can be seen.
Intravenous urography shows significant renal enlargement associated with delayed or absent excretion, and retrograde pyelography can be used to establish the presence of ureteral obstruction.
Plain radiographic findings may be diagnostic in ill patients with diabetes who have signs of acute pyelonephritis.
Renal fossa gas may be confused with gastrointestinal gas. A false-positive diagnosis of emphysematous pyelonephritis may occur with retroperitoneal gas and a psoas abscess secondary to gas-forming organisms. Reflux of air from the bladder and bronchorenal, enterorenal, or cutaneorenal fistulae (as may occur with xanthogranulomatous pyelonephritis  ) may also lead to false-positive findings. Air can be seen in focal renal abscesses.
CT scanning is the most reliable and sensitive modality in diagnosing emphysematous pyelonephritis. [15, 4, 21] Intraparenchymal, intracalyceal, and intrapelvic gas and extension into the perinephric space are readily identified on nonenhanced CT scans;  mottled areas of low attenuation extend radially along the pyramids, and pus may occasionally be seen extending into the renal veins. [22, 23, 24]
Two subtypes of emphysematous pyelonephritis based on CT scan appearances have been described.  Type I (33% of patients) is characterized by parenchymal destruction with either absence of fluid collection or presence of streaky or mottled gas radiating from the medulla to the cortex. A crescent of subcapsular or perinephric gas may be present. The absence of fluid collection implies a poor immune response. The mortality rate is high, at 69%. [12, 13]
Type II (66% of patients) typically has a confined, bubbly, intrarenal gas pattern—probably within abscesses associated with renal and perinephric fluid collection—and gas within the renal pelvis. The mortality rate in type II is 18%. [12, 13]
Conversion from type I to type II emphysematous pyelonephritis has been described.  Wan et al correlated imaging findings of types I and II disease with clinical course and prognosis and showed that the radiologic differentiation between the 2 types is important due to the prognostic difference.  That is, the mortality rate for type I disease was higher than that for type II (69% vs 18%, respectively, as stated above), with type I emphysematous pyelonephritis tending to have a more fulminant course and a significantly shorter interval from clinical onset to death. (See the images below.)
Emphysematous pyelonephritis should be differentiated from reflux of air from the bladder and bronchorenal, enterorenal, or cutaneorenal fistulae (as may occur with xanthogranulomatous pyelonephritis  ). Air also can be seen in focal renal abscesses, but it is not life threatening.
Ultrasonography is usually the first imaging modality for assessing renal pathology. The ultrasonographic findings often guide clinicians in choosing the next modality, such as CT scanning, to achieve a more specific diagnosis. [15, 18]
Intrarenal gas causes high-amplitude echoes within the renal sinus/renal parenchyma associated with dirty acoustic shadowing, and ring-down artifacts may result from air bubbles trapped in fluid. In addition, shadowing from gas bubbles in the perinephric space may be seen, making visualization of the kidney difficult (see the following image). Perinephric fluid, if any, tends to be obscured by gas.
Gas within the kidney and/or renal pelvis mimics renal calculi. In select patients, particularly those with diabetes in whom ultrasonograms suggest renal calculi, obtaining a coned radiograph of the renal area is worthwhile to preclude missing the diagnosis of emphysematous pyelonephritis.