Diagnostic Ultrasound - Abdomen and Pelvis

Acute Pyelonephritis
TERMINOLOGY
Abbreviations
• Acute pyelonephritis (AP)
Definitions
• Renal parenchymal infection
• Different from pyelitis (inflammation of renal pelvis) or
pyonephrosis (infection plus collecting system obstruction)
IMAGING
General Features
• Best diagnostic clue
○ Findings of AP are almost always asymmetric
– Renal enlargement with loss of corticomedullary (CM)
differentiation
– Geographic areas of altered echogenicity, attenuation,
or signal depending on modality
– Urothelial thickening
○ Perinephric inflammatory changes may be subtle but are
almost always present
• Location
○ Usually unilateral
○ No side or polar predominance
○ Infection affects entire cortex, from sinus to capsule
– Pelvis and ureter may or may not be involved
Ultrasonographic Findings
• Grayscale ultrasound
○ In general, ultrasound is much more sensitive for causes
(obstruction) and complications (abscess) of
pyelonephritis than for pyelonephritis itself, which is a
clinical diagnosis
○ Many kidneys with pyelonephritis will be sonographically
normal
○ Asymmetric global or focal swelling
○ Altered echogenicity and diminished corticomedullary
differentiation
○ Microabscesses or areas of necrosis can emerge after 1-2
weeks of infection
○ Perinephric fat may be unusually echogenic due to
edema and inflammatory cell infiltrate
○ Thin, anechoic rim of perinephric fluid sometimes seen
○ Foci of gas in parenchyma (rare) could indicate
emphysematous pyelonephritis
– Treat as urologic emergency
○ Pyelonephritis plus collecting system dilatation indicate
pyonephrosis (pus under pressure), a surgical emergency
• Color Doppler
○ Although kidney is inflamed, parenchyma is usually
hypovascular
○ Slight elevation in arcuate artery resistive index can be
seen but is not as specific or sensitive as diagnostic
criterion
○ Renal vein thrombosis is an occasional complication
CT Findings
• NECT
○ May see perinephric stranding or renal enlargement
○ May be occult
○ Important to check for stones
– Stone may cause outflow obstruction
– Tiny fragment can be nidus of infection after
lithotripsy
• CECT
○ Global or focal renal enlargement, sometimes with sinus
obliteration
○ Altered nephrogram, classically striated, best seen in
excretory phase
– Wedge-shaped areas of low attenuation may mimic
infarcts
– Striated nephrogram arises from partial obstruction of
tubules by white blood cells and debris
○ Urothelial thickening in pelvis or ureter
○ Perinephric or periureteric fat stranding or edema
○ Look for asymmetric thickening of renal fascia
Nuclear Medicine Findings
• No utility in acute setting
○ DMSA cortical scans can be used to assess scarring;
usually seen in kids with chronic reflux
○ Technetium-99 DTPA or MAG-3 scans provide functional
information in kidney damaged by chronic infection
Imaging Recommendations
• Best imaging tool
○ CT is most sensitive for subtle parenchymal changes in
acute pyelonephritis, but with increased radiation and
expense; usually not necessary
○ Ultrasound is useful to rule out obstruction or abscess,
particularly in children or other radiation-sensitive
contexts
• Protocol advice
○ Initial investigation by ultrasound followed by CT if
needed (for delineation of complications)
○ Watch for asymmetry between kidneys over phases of
enhancement and excretion on multiphasic CT or MR
DIFFERENTIAL DIAGNOSIS
Acute Tubular Necrosis (ATN)
• Due to ischemia or nephrotoxicity
• AP usually not associated with decline in renal function, but
ATN is
Lymphoma
• Diffuse: Enlarged kidney and ↓ echogenicity
• Multifocal: Enlarged kidney and hypoechoic masses
• Look for adjacent retroperitoneal adenopathy
Acute Renal Infarction
• Global or segmental vascular defect on color Doppler
imaging in normal/enlarged kidney
PATHOLOGY
General Features
• Etiology
○ Most common organism: Escherichia coli
○ Route of spread of infection: Ascending (85%) >
hematogenous (15%)
○ Risk factors include obstruction, ureteric reflux, diabetes,
pregnancy, lower UTI
Diagnoses: Urinary Tract
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