Objectives:

• To identify signs that indicate pyelonephritis in the appropriate clinical setting

• To assess for common complications of pyelonephritis

• To identify enhancement patterns of the kidney

Protocolling:

• Contrast enhanced CT to assess for complicated disease/complications

Key Points:

• CT signs of pyelonephritis:

  • Enlargement of kidneys

  • Perinephric stranding/fluid

  • Altered enhancement - most commonly striated nephrogram

  • Urothelial thickening/enhancement

  • Can be normal, pyelonephritis is a clinical diagnosis!

• Pyelonephritis is a clinical diagnosis that does not routinely require imaging. CT considered if:

  • History of obstruction, stones, or urologic surgery

  • Persistent symptoms after 2-3 days on antibiotics

  • Diabetic, elderly, immunocompromised

• Complications of pyelonephritis:

  • Abscess

  • Pyonephrosis - obstructed collecting system with clinical signs of infection

    • On CT will see obstructed/dilated collecting system and possibly higher density fluid within renal pelvis or other signs of infection

  • Renal vein thrombosis

  • Emphysematous pyelonephritis - gas within the parenchyma, a surgical emergency (note that gas within the collecting system, emphysematous pyelitis, is generally treated medically)

Enhancement patterns of the kidney (suggest “Wolin EA, Hartman DS, Olson JR. Nephrographic and Pyelographic Analysis of CT Urography: Differential Diagnosis. AJR 2013;200:1197-1203 ”)

• Absent nephrogram (lack of functioning kidney):

  • No blood in = Acute arterial occlusion

  • No blood out = Complete renal vein occlusion (uncommon, most renal vein thrombi are incomplete)

  • No urine out = Complete long standing obstruction (uncommon, usually some residual function)

  • Non-functioning kidney (congenital or acquired).

• Unilateral delayed nephrogram/pyelogram (slow urine out):

  • Slow urine out = Partially obstructed collecting system (stone, tumor, clot, compression)

  • Slow blood in = renal artery stenosis, large subcapsular fluid collection

  • Slow blood out = partial renal vein occlusion

  • Unilateral poor renal function = i.e. acute pyelo

• Bilateral persistent nephrogram/delayed pyelogram

  • Systemic hypotension (look for other shock complex findings)

  • Bilateral intrarenal obstruction (crystal disease, protein, tumor lysis syndrome)

  • Much less common: bilateral renal artery stenosis, partial renal vein thrombosis, bilateral collecting system obstruction

• Rim nephrogram (only retained subcapsular enhancement)

  • Acute renal artery occlusion/global infarction - most specific sign for vascular compromise. Usually seen days to weeks after infarct

• Reverse rim nephrogram (enhancement centrally, no cortical enhancement)

  • Acute cortical necrosis (rare form of acute renal failure).

• Striated nephrogram (alternating low and normal enhancing bands, radially arranged)

  • Unilateral: acute pyelonephritis, ureteric obstruction, renal vein thrombosis

  • Bilateral: acute pyelonephritis, tubular obstruction, hypotension, polycystic kidney disease.

• Spotted nephrogram (segmental areas of non-enhancement due to tiny infarctions)

  • Intrarenal vasculitis

  • Embolic disease

  • Pyelonephritis (though, classically a striated nephrogram)