Imaging modalities of choice in an acute setting are ultrasonography (US) and Computed Tomography (CT) [3].
Magnetic Resonance Imaging (MRI) is an option for pregnant patients or patients with severe kidney failure where iodine contrast is restricted.
Imaging should be performed if [4]:
- Persistent fever after 72 hours of antibiotic treatment.
- Decreased renal function.
- Risk factors such as badly controlled diabetes,
immunodeficiency and renal transplant.
- Suspected structural or functional abnormalities.
- Severe symptoms and life-threatening situations (sepsis).
Fig. 1
No follow-up image should be done unless new clinical indications arise [3].
PYELONEPHRITIS:
When the infection comes from the urinary tract,
the bacteria enter from the collecting system into the renal tubules causing inflammation.
This inflammation produces debris that may obstruct the tubules and inflammatory changes that migrate into the renal interstitium causing oedema and vasospasm [3].These three factors make the kidney more oedematous,
with less ability to concentrate and excrete contrast.
In those cases where the bacteria come directly from the blood,
one can see small abscess formations that are peripheral,
and are thought to be the different foci of the infection.
As the infection spreads it may take on the whole kidney and be hard to differentiate one from the other.
US: First imaging procedure to be performed when indicated,
especially due to its availability.
The goal is to report any findings that may change the therapeutic path (like hydronephrosis or abscess formation).
Most of the ultrasounds performed appear as normal [5],
due to a low sensibility to parenchymal changes.
Nevertheless one may be able to find:
- Ill-defined parenchymal areas that are normally hypoechoic (oedema) or hyperechoic (bleeding).
- Loss of corticomedullary differentiation.
- Thickened parenchyma.
- Loss of renal sinus fat.
- Kidney enlargement.
Hydronephrosis,
perinephric fluid and collections may also be seen.
The use of Doppler may show hypoperfusion (due to the vasospasm).
Enhanced US is more sensible to parenchymal changes,
as it may show the hypoperfused areas that do not enhance.
Fig. 2
CT: Gold standard procedure since it provides both anatomical and physiological information [3].
In most cases a CT with contrast in nephrogenic phase is enough for diagnosis.
Depending on the clinical situation it might be helpful to add a non-contrast phase to search for lithiasis,
an excretory phase when obstruction is suspected or an arterial phase when in search for ischaemic areas.
The findings related to pyelonephritis are:
- Wedge-shaped areas or streaky zones of lower enhancement from papilla to renal cortex. Fig. 3
- Delayed enhancement of affected areas after 3-6 hours of contrast administration.
- Renal enlargement and striated perinephric fat.
- Thickening of Gerota fascia.
Complications or related pathologies are identified when there are CT findings of gas,
calculi,
abscesses,
obstruction or haemorrhage… The specific findings will be discussed in the corresponding complication.
MRI: In an acute context,
it is normally reserved for those patients who shouldn’t receive radiation (pregnant women) or are not able to receive iodine contrast [3].
When possible,
it is preferred to administer gadolinium.
Signals of pyelonephritis are similar to those of CT:
- Areas that appear with low intensity in T1-weighted images,
high intensity in T2-weighted images and that do not enhance in dynamic study.
- Enlarged kidney.
Antibiotic therapy is essential in the treatment of acute pyelonephritis and prevents progression of the infection.
If suspicion of complications arises,
the patient should be hospitalized and given broad-spectrum antibiotics[6].
In Fig. 4 we can find the possible outcomes from a pyelonephritis and other entities that are associated with it.
Fig. 4
References: Department of Radiology, University Hospital 12 de Octubre, Madrid/ Spain
HYDRONEPHROSIS:
In many patients,
pyelonephritis can be associated with obstruction of the collecting system and lead to hydronephrosis.
It can cause acute renal function or persistence of symptoms after correct antibiotic therapy,
and may need a different approach.
When suspected,
the first imaging procedure should be an ultrasound.
US: Collecting system is dilated with anechoic liquid inside and the walls may appear thickened.
Sometimes the cause is visible.
Fig. 5
CT: Dilated system with low attenuation (near water attenuation) liquid inside.
Fat stranding surrounding the collecting system may appear.
Fig. 5
MRI: Dilated collecting system hyperintense in T2 that normally does not show restriction in DWI.
Ureteral double- J stent or a nephrostomy tube may be necessary to restore urine flow.
When the pressure of the collecting system is too high,
there might be a rupture of the wall and a collection can form,
this is called a urinoma.
Perinephric liquid will be seen and in an excretory phase of contrasted CT the collection will fill with contrast,
demonstrating the connection with the collecting system.
PYONEPHROSIS:
Pyonephrosis results from an obstructed and infected collecting system.
It should be suspected when a patient with known obstructed system starts with pyelonephritis symptoms. Ultrasound is the imaging of choice.
US: Dilated collecting system with echogenic material inside,
where fluid-fluid levels might be seen.
Fig. 6
CT: Obstructed collecting system with higher attenuation values inside than usual.
Sometimes it may be hard to differentiate hydronephrosis from pyonephrosis.
MRI: May be differentiated from hydronephrosis by restriction in DWI.
Drainage is the most effective treatment.
When guided by US or CT,
the need for nephrectomy decreases [7].
ABSCESS:
Renal corticomedullary abscess is caused by ascending spread of bacteria whereas if they are located in the cortex it is caused by haematogenous spread of bacteria [8].
Abscesses are formed after inflammation areas that progress into necrosis and confine themselves.
The most common risk factors are diabetes mellitus,
nephrolithiasis,
and ureteral obstruction.
US: Defined renal mass (hyperechoic or hypoechoic) with internal echoes,
possible posterior acoustic enhancement and no vascular flow on Doppler imaging.
CT: Geographic low attenuation collections (0-20 Hounsfield units) with rim enhancement.
Fig. 7
MRI: Geographic lesion hyperintense in T2-weighted images,
hypointense in T1-weighted images and has restriction on DWI.
A corticomedullary abscess is more likely to extend to the renal capsule and perforate,
thus forming a perinephric abscess (Fig. 8).
Other signs of pyelonephritis or the clinical context may help in differentiating it from an angiomyolipoma rupture (Wünderlich syndrome)
Abscesses can also be accompanied by pylephlebitis of the renal vein (Fig. 9) or thrombus of renal artery and infarctions (Fig. 10).
Medical treatment alone should be limited to hemodynamically stable patients with abscess < 3cm.
Percutaneous or surgical drainage should be performed in patients with signs of hemodynamic instability or large renal abscesses (≥3 cm)[8].
If perinephric abscess or infected urinoma is present percutaneous perinephric drainage should be placed.
EMPHYSEMATOUS PYELONEPHRITIS:
Emphysematous pyelonephritis is a life-threatening situation caused by a necrotizing infection of the kidney with gas formation.
Most of the patients have poorly controlled diabetes or are immunocompromised.
It has a fulminating course and can lead to death if not treated promptly.
US: Gas is seen as high echogenic areas with dirty shadowing.
CT: Modality of choice:
- Bubbly or streaky lines of gas surrounding or inside the parenchyma.
- Rim-like or crescent-shaped gas collections in the perinephric area.
- Gas in the renal vein or inferior vena cava and along the psoas muscle.
Fig. 11
- Other pyelonephritis signs.
The most recent classification of radiologic features is from Huang and Tseng,
in 2000 [9]:
Class 1 - Gas confined to the collecting system (emphysematous pyelitis).
Class 2 - Gas confined to the renal parenchyma alone.
Class 3A - Perinephric extension of gas or abscess.
Class 3B - Extension of gas beyond the Gerota fascia.
Class 4 - Bilateral EPN or EPN in a solitary kidney.
Resuscitative measures and early antibiotic treatment are necessary in all patients.
Percutaneous drainage is preferred mainly in patients with compromised renal function and Class 1 or 2.
In the rest nephrectomy is the treatment of choice [9].
EMPHYSEMATOUS PYELITIS:
It is a less aggressive situation where gas appears only in the collecting system as part of the necrotizing infection.
It may be referred to as Class 1 of emphysematous pyelonephritis or as a different entity.
Other air sources such as trauma or surgery should be excluded[3].
It affects more diabetic patients,
women and obstructed kidneys.
CT again is the modality of choice due to its higher sensibility to detect gas:
- Gas within the collecting system.
- Dilated system with inflammation changes (thickened wall or stranding fat around the collecting system).
- Absence of gas in the renal parenchyma.
US may be able to depict the gas bubbles as hyperechoic foci but may not be sensitive enough.
Fig. 12
CHRONIC PYELONEPHRITIS:
Chronic pyelonephritis is characterized by renal inflammation and fibrosis induced by recurrent or persistent renal infection or causes of urinary tract obstruction.
It is more frequent in patients with major anatomic anomalies.
Radiological features are:
- Renal scarring: a well-defined area with absence or cortical thinning.
- Atrophy and renal asymmetry.
- Hypertrophy of the respected parenchyma.
- Dilated caliceal system.
There are two special types of chronic pyelonephritis,
xanthogranulomatous pyelonephritis and tuberculosis.
XANTHOGRANULOMATOUS PYELONEPHRITIS (XGP):
It is a chronic inflammatory granulomatous disorder in which a destructive mass invades the renal parenchyma.
An obstructive calculus present in nearly 80% of the cases.
The exact etiology is unknown,
but it is thought to be caused by long-term renal obstruction and an infection.
Proteus or Escherichia coli are the most frequent bacteria involved[10].
Symptoms are non-specific,
such as low-grade fever and flank pain.
Pyuria,
hematuria and positive urine cultures may be found.
The radiologic features are:
US: enlarged kidney with multiple hypoechoic masses; irregular,
thinned parenchyma; and a dilated collecting system.
CT:
- Non-functioning hypoattenuated enlarged kidney.
- Central calculus.
Fig. 13
- Calicial expansion.
- Perinephric fat stranding.
- No contrast excretion.
- Extrarrenal extension: Psoas abscess,
cutaneous or colonic fistula formation.
MRI: It shows the same radiologic features as CT.
This disorder affects the kidney diffusely but may be focal in 10% of patients.
In these focal presentations,
differential diagnoses should be done with abscess or renal neoplasm and it should be regarded as malignant until proven otherwise.
Fig. 14
The treatment is surgical,
with a nephrectomy and antibiotics[10].Medical care rarely suffices for treatment.
TUBERCULOSIS (TB):
Urinary tract is the most common site of infection of extrapulmonary TB.
Symptoms are non-specific such as fever,
weakness.
In this case haematuria and pyuria are present,
with a negative urine culture unless specific mycobacteria cultures are done.
Cultures and histologic analysis is necessary for diagnosis[11].
Radiologic findings may be absent but when present may help in the diagnosis.
They result from the combination of papillary necrosis and parenchymal destruction[3].
While intravenous urography remains the primary modality,
findings of urinary tuberculosis are also detectable using US,
CT and MRI[11].
US: Irregularly shaped kidney with dilated calices and hyperechoic parenchymal foci due to calcifications.
CT: It may also detect the involvement of other abdominal organs.
Findings are:
- Hypodense papillae
- Cortical low-attenuated masses that may have a calcified rim Fig. 15
- Dilated calices
- Wall thickening of the collecting system with strictures due to fibrosis Fig. 16
- Renal scarring and calcification (rim-like or diffuse).
MRI: May be useful if fistulae or tuberculous tracts are suspected.
Specific anti-TB antibiotics are the treatment of choice.