When and how do you use imaging investigations like renal ultrasound and CT KUB to assess renal function and potential obstructive uropathy?

Imaging and Investigations in Acute Kidney Injury

When to Order Renal Ultrasound

Renal ultrasound should be ordered as the first-line imaging modality for all patients with unexplained acute kidney injury to exclude obstructive uropathy, which is a reversible mechanical cause of renal failure. 1, 2

Primary Indications:

• Unexplained AKI: To rule out hydronephrosis as the indicator of urinary tract obstruction 1, 2
• Suspected obstruction: Based on clinical presentation (anuria, flank pain, bladder distension) 3
• Bilateral renal failure: More urgent evaluation warranted as bilateral obstruction poses greater risk to overall renal function 3
• Solitary kidney with dysfunction: Critical to assess for any reversible obstruction 3, 4

What You're Looking For:

• Hydronephrosis: Presence or absence is the primary finding indicating obstructive uropathy 1, 2
• Renal size: Small kidneys (<9 cm) suggest chronic kidney disease; normal or enlarged kidneys suggest acute process 2, 5
• Increased echogenicity: Indicates chronic renal parenchymal disease, though this is nonspecific 2, 5
• Bladder distension: May indicate lower urinary tract obstruction 1

Important Caveat:

While ultrasound is highly sensitive for detecting hydronephrosis, obstructive uropathy is found in only about 1% of ICU patients with acute renal failure, whereas 30-40% have sonographic evidence of chronic renal failure 1. This low yield should not deter its use, as missing obstruction has severe consequences for morbidity and mortality.

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Findings Suggesting Obstructive Uropathy

Ultrasound Findings:

• Hydronephrosis: Dilation of the renal pelvis and calyces, graded by severity (mild, moderate, severe) based on extent of dilation and parenchymal thickness 6
• Hydroureter: Dilated ureter visible on imaging suggests level of obstruction 6
• Bladder wall thickening with distension: Suggests chronic bladder outlet obstruction 1

Doppler Ultrasound Findings:

• Elevated resistive index (RI) >0.70: Indicates intrarenal vasoconstriction from obstruction (except in peracute stage) 1, 7
• RI difference ≥0.04 between kidneys: May indicate pathologic obstruction in the kidney with higher RI 2, 7
• Important limitation: Elevated RI is not specific to obstruction—it can be elevated with age, atherosclerosis, hypertension, and intrinsic renal disease 1, 7

Clinical Context:

• Post-void residual >150 mL: Suggests bladder outlet obstruction if bilateral hydronephrosis present 3
• Anuria or severe oliguria: Suggests complete bilateral obstruction or obstruction of solitary kidney 8

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When CT KUB is Indicated in AKI

Unenhanced CT abdomen and pelvis (CT KUB) is indicated when ultrasound detects hydronephrosis and you need to determine the level and cause of obstruction, or when ultrasound is nondiagnostic due to body habitus. 1

Specific Indications:

• Obstructive urolithiasis suspected: CT without contrast is particularly useful as the primary concern, especially with unilateral hydronephrosis 1
• Characterization of US-detected hydronephrosis: To determine exact level and etiology of obstruction 1
• Ultrasound nondiagnostic: Due to patient body habitus or technical limitations 1
• Suspected malignant obstruction: CT with contrast helps identify pelvic masses causing bilateral hydronephrosis 1

Preferred Advanced Imaging Options:

• CT Urography (CTU) without and with IV contrast: Provides near-comprehensive evaluation of genitourinary tract with both morphological and functional information 1, 3
• MAG3 renal scan: The de facto standard of care for diagnosing true obstruction versus non-obstructive hydronephrosis 1, 4
• MR Urography (MRU): Alternative when IV contrast contraindicated or radiation exposure a concern 1, 3

Important Distinction:

CT is generally not considered first-line imaging for AKI 1. Ultrasound should be performed first, and CT reserved for characterization of findings or when ultrasound is inadequate.

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Assessing Renal Perfusion

Doppler Ultrasound Assessment:

• Resistive Index (RI) measurement: Calculate RI = (peak systolic velocity - end diastolic velocity) / peak systolic velocity in intrarenal arteries 1, 7
• Normal RI: <0.70 1, 2, 7
• Elevated RI: >0.70 suggests decreased renal perfusion from various causes including obstruction, intrinsic renal disease, or renovascular disease 1, 7
• Serial RI measurements: Can predict progression to CKD and mortality in critically ill patients, though largely a research tool currently 1

Urine Indices for Perfusion Assessment:

While not explicitly detailed in the imaging guidelines provided, traditional urine indices help differentiate prerenal from intrinsic renal causes:

• Fractional excretion of sodium (FENa): <1% suggests prerenal azotemia (hypoperfusion)
• Urine sodium: <20 mEq/L suggests prerenal state
• Urine osmolality: >500 mOsm/kg suggests intact concentrating ability (prerenal)

Advanced Perfusion Imaging:

• Renal scintigraphy (MAG3 or DTPA): Provides functional information about renal perfusion and split kidney function 1, 4
• MAG3 preferred over DTPA: More efficiently extracted by kidney, especially in patients with reduced function 1, 4
• Color Doppler for renovascular disease: Can detect arterial stenosis or venous thrombosis when suspected 5

Clinical Algorithm:

1. Start with renal ultrasound with Doppler to assess for hydronephrosis and measure RI 1, 2
2. If hydronephrosis present: Proceed to CT KUB or CTU to determine level/cause of obstruction 1
3. If no hydronephrosis but elevated RI: Consider intrinsic renal disease or renovascular pathology 1, 7
4. For functional assessment of obstruction: MAG3 renal scan is gold standard to differentiate obstructive from non-obstructive hydronephrosis 1, 4
5. Urgent decompression indicated: If pyonephrosis (infection with obstruction) or bilateral obstruction threatening renal function 3

Common Pitfalls:

• Not all hydronephrosis is obstructive: Diuretic renography (MAG3 scan) needed to confirm true obstruction 1, 4, 7
• RI elevation is nonspecific: Cannot distinguish between obstruction, intrinsic disease, or vascular pathology based on RI alone 1, 7
• Peracute obstruction may have normal RI: Vasoconstriction takes time to develop 7
• Ultrasound less useful for identifying specific etiology: CT or MRI needed for detailed anatomic evaluation 2