Diagnostic Ultrasound - Abdomen and Pelvis

Renal Artery Stenosis
TERMINOLOGY
Abbreviations
• Renal artery stenosis (RAS)
Definitions
• Hemodynamically significant narrowing of renal arterial
lumen
IMAGING
General Features
• Best diagnostic clue
○ Poststenotic "jet" and turbulent flow on color Doppler
US
– Abnormally high peak systolic velocity with anglecorrected
spectral Doppler in main renal artery
immediately distal to stenosis
○ Diminished downstream systolic peaks
– Abnormally low peak systolic velocity in distal main
and arcuate arteries with diminished resistive indices
(RI)
• Location
○ Ostium/intramural: Primary aortic disease
○ Atherosclerosis: Within 2 cm of ostium
○ Fibromuscular dysplasia (FMD): Distal renal artery (RA),
hilar branches
Ultrasonographic Findings
• Grayscale ultrasound
○ Asymmetric kidneys (unilateral length < 8 cm or
difference > 2 cm)
○ Diffusely increased parenchymal echogenicity (limited to
chronic, severe ischemia)
○ Occasional string of beads appearance of arterial wall in
FMD (requires excellent visualization)
• Pulsed Doppler
○ Normal renal artery peak systolic velocity 75-125 cm/sec
○ Doppler criteria ≥ 50-60% diameter stenosis
– Peak systolic velocity in and immediately distal to
stenosis ≥ 180-200 cm/sec
– Renal/aortic ratio > 3.5 (peak systole in RAS/peak
systole in aorta at level of renal arteries)
– Poststenotic Doppler spectral broadening
○ Intrarenal Doppler waveform signs of significant RAS
– These downstream effects are very different from the
poststenotic high velocity jet
– Tardus et parvus waveform shape, late and small
systolic peaks
□ Prolonged acceleration time (time to peak systole)
> 0.07 seconds
□ Low resistive index < 0.5 (compare with other
kidney) due to dampened systolic peaks and normal
diastolic flow
□ Acceleration index (AI) < 3 m/s²
– Cannot accurately diagnose RAS solely by intrarenal
arterial waveform analysis
○ Doppler angle ≤ 60° essential to measure velocity
• Color Doppler
○ Color shift/color aliasing in renal artery at site of stenosis:
High velocity flow
○ Poststenotic turbulence, possibly with soft tissue
reverberation
Angiographic Findings
• Contrast enhanced MRA, CTA, or DSA
○ Atherosclerotic lesions: Focal eccentric/concentric
stenosis
○ Fibromuscular dysplasia: Most commonly serial ridges or
string of beads pattern, may have associated aneurysms
○ Focal dephasing on phase contrast MR analogous to
aliasing in poststenotic jet
Imaging Recommendations
• Best imaging tool
○ Imaging goal: Accurately diagnose ≥ 50-60% diameter
renal artery stenosis
○ Ultrasound may be used for screening, followed by
contrast-enhanced MRA or CTA
○ DSA may be needed for accurate FMD diagnosis in distal
RA, hilar branches due to higher spatial resolution
○ Duplex ultrasound problems
– Technically difficult/high exam failure rate
– Failure to recognize duplicate RAs and collaterals,
which will alter arcuate artery waveforms
– Inadequate visualization for distal/hilar RAS
– Wide reported accuracy range; best results for
proximal (atherosclerosis-related) RAS
• Protocol advice
○ Imaging for RAS (regardless of modality) is indicated
only after appropriate clinical screening
DIFFERENTIAL DIAGNOSIS
Primary Hypertension
• Renal arteries normal
Chronic Parenchymal Renal Disease Unrelated to
Renal Artery Stenosis
• Parenchymal hyperechogenicity and atrophy from
interstitial fibrosis
• Elevated resistive index (> 0.7) interlobar/arcuate arteries
• Usually bilateral unless there is a local insult such as
infection, trauma, radiation, etc.
Aortic Dissection
• Possible ostial/intramural obstruction, may be dynamic
• Dissection flap may extend into RA
• US findings: Dissection plane/2 lumens seen on color
Doppler US
Renal Artery Occlusion
• Etiology
○ Subsequent to RAS
○ Embolic
○ Primary aortic disease
• US findings
○ Absent RA on color Doppler US
○ Absent or very diminished arterial signals in kidney if
acute
○ Intrarenal Doppler signals may be normal if chronic
(collateralized)
○ Renal atrophy if chronic
Diagnoses: Urinary Tract
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