URINARY TRACT INFECTIONS IMAGING

RENAL TRACT INFLAMMATION AND INFECTION
The most common urinary tract infections are bacterial in origin, with viral and fungal infections being comparatively rare. The diagnosis is made by urinalysis after the patient presents with symptoms of dysuria, haematuria and/or suprapubic or renal angle pain. The origin of the infection may be via the blood stream (haematogenous) or the urethra (ascending). Ascending infections are more common in women due to their short urethra. Ultrasound is often requested, particularly in children, to identify any unsuspected renal pathology which may be associated with the infection, for example a duplex collecting system, pelvic kidney.

Common conditions which may be identified on ultrasound include renal cystic diseases, calculi, obstructive uropathy, reflux and anatomical variants. The infection may be either acute or chronic. Ultrasound signs of renal infection may be absent altogether, and this is the commonest scenario as the infective episode has often been successfully treated with antibiotics by the time the ultrasound scan is performed. The infection may be confined to the bladder, that is cystitis, in which case low-level echoes and/or hyperechoic debris may be identified, or may have progressed to the kidneys. Scarring and/or cortical thinning may be present in cases of repeated infections.

Pyelonephritis - Acute pyelonephritis
Acute inflammation of the kidney rarely results in any ultrasound abnormality. Occasionally the kidney may be enlarged and hypoechoic, the contrast between the kidney and the hepatic or splenic parenchyma increasing due to oedema, but the ultrasound changes are generally subtle. The normally clear differentiation between the cortex and the medullary pyramids may become indistinct, but again may go unrecognized. CT is useful for detecting subtle inflammatory changes within the kidney.

Chronic pyelonephritis
This chronic inflammatory state is usually the result of frequent previous inflammatory/infective episodes. The kidney may be small and often has focal scarring present. Scar tissue has the appearance of a hyperechoic, linear lesion which affects the smooth renal outline and crosses the renal cortex. (Do not confuse focal scarring with fetal lobulation: the latter is smooth, thin, continuous with the capsule and forms an indentation between the pyramids.) The renal cortex is frequently thin in chronic pyelonephritis and may appear abnormally hyperechoic. Bladder diverticula Repeated infections can cause the bladder wall to thicken and become trabeculated. In such cases, a bladder diverticulum may form, making treatment of subsequent infections particularly difficult. The diverticulum may harbour debris or stones and may fail to empty properly, often enlarging as the urine refluxes into it when the patient micturates.

Focal pyelonephritis
The presence of acute infection within the kidney may progress in focal regions of the renal parenchyma. This phenomenon is particularly associated with diabetics. The ultrasonic changes are subtle, as in diffuse pyelonephritis, but it is possible to detect a slight change in echogenicity when it is surrounded by normal-looking parenchyma. Focal pyelonephritis (sometimes called focal nephronia) may be either hypo- or hyperechoic compared with normal renal tissue. Depending on the size of the lesion, it may cause a mass effect, mimicking a renal tumour. The outline of the kidney is preserved, however. The patient presents with fever and tenderness on the affected side and frequently has a history of urinary tract infection. A focal renal mass under these circumstances is highly suggestive of focal pyelonephritis and is also well demonstrated on CT. It usually responds to antibiotic therapy and resolution of the lesion can be monitored with ultrasound scans. Focal pyelonephritis can progress to form an abscess in the kidney, which can normally be treated by percutaneous drainage and antibiotics.

Renal abscess
A renal abscess is generally a progression of focal inflammation within the kidney. The area liquefies and may enlarge to form a complex mass with distal acoustic enhancement. Low-level echoes from pus may fill the abscess cavity, giving it the appearance of increased echogenicity, but it may also be hypoechoic. The margins of the abscess may be ill-defined at first but may develop a more obvious capsule as the lesion becomes established, this capsule often has an easily identifiable thick rim. Flow may be seen in the inflammatory capsule with colour Doppler, but not in the liquefied centre. A renal abscess may mimic a lymphoma as both may be hypoechoic on ultrasound, and both may have either single or multiple foci. The abscess may be intrarenal, subcapsular or perirenal. Frequently, drainage under ultrasound guidance is the preferred treatment; gradual resolution of the abscess can also be monitored with ultrasound.

Tuberculosis (TB)
Renal TB is an uncommon finding and a difficult diagnosis to make on ultrasound. The subtle inflammatory changes which affect the calyces in the early stages are best demonstrated with CT. In the later stages ultrasound may show calcific foci and obstructed calyces as a result of thickened inflammatory calyceal walls, calcification and debris. TB frequently spreads to other adjacent sites in the abdomen, including the psoas muscle and gastrointestinal tract. The differential diagnosis is xanthogranulomatous pyelonephritis, which is often indistinguishable from TB on ultrasound, or a necrotic renal neoplasm.

Xanthogranulomatous pyelonephritis (XGP)
This condition (which gets its name from the yellow colour of the kidney) is the result of renal obstruction by calculi in the pelvicalyceal system. Frequently, a staghorn calculus is responsible. The kidney becomes chronically infected and the calyces enlarge and become filled with infected debris. The cortex may be eroded and thin. On ultrasound, these appearances are similar to TB or to a pyonephrosis. The latter is usually accompanied by a more severe, acute pain and fever whereas XGP or TB has a lower-grade, chronic pain. CT may differentiate TB from XGP and is also more sensitive to extrarenal spread of disease.

Hydatid cysts
The Echinococcus parasite spends part of its life cycle in dogs. The larvae may be transmitted to humans through contact with dog faeces, finding their way to the lungs, liver and, less frequently, the kidneys. The parasite forms a cyst which has a thickened wall, often with smaller, peripheral daughter cysts. Frequently the main cyst contains echoes. The condition is rare in the UK, but may be diagnosed when small, grape-like cysts are passed in the urine.