Diagnostic ultrasound ( PDFDrive )

368 PART II Abdominal and Pelvic Sonography
Renal Artery Stenosis
Hypertension may be primary (95%-99%) or secondary (1%-5%).
he vast majority of patients with secondary hypertension have
renovascular disease. Renovascular disease is most frequently
caused by atherosclerosis (66%), and the majority of the remaining
cases largely result from ibromuscular dysplasia. 276 Many
diferent imaging techniques have been used in an efort to detect
patients with renovascular hypertension. hese include intravenous
and intraarterial digital subtraction angiography, captopril
renal scintigraphy, duplex and color Doppler ultrasound, CT
angiography, and magnetic resonance angiography.
Numerous studies and clinical experience in ultrasound
laboratories have suggested the utility of Doppler ultrasound
as an initial screening examination for renal vascular hypertension.
he screening approach may involve (1) detection of abnormal
Doppler signals at or just distal to the stenosis or (2) detection
of abnormal Doppler signals in the intrarenal vasculature. Typical
direct criteria for hemodynamically signiicant stenosis (>60%-
70%) include a focal peak systolic velocity of higher than 200 cm/
sec, poststenotic turbulence, and a renal artery to aortic ratio of
greater than 3.5. 277
Evaluation of the main renal arteries in their entirety is usually
not possible with ultrasound. It is estimated that the main renal
arteries are not seen in up to 42% of patients. 278 In addition, 14%
to 24% of patients have accessory renal arteries that are usually
not detected sonographically. herefore, evaluation of the main
renal arteries as a screening approach for renal artery stenosis
oten fails, particularly in diicult-to-scan patients. he second
approach is to interrogate the intrarenal vasculature. Normally,
there is a steep upstroke in systole with a second small peak in
early systole. A tardus-parvus waveform downstream from a
stenosis refers to a slowed systolic acceleration with low amplitude
of the systolic peak (Fig. 9.83). To evaluate the delayed upstroke,
two measurements are taken:
• Acceleration time: time from start of systole to peak systole
• Acceleration index: slope of the systolic upstroke
An acceleration time greater than 0.07 second and a slope of
systolic upstroke less than 3 m/sec 2 are suggested as thresholds
to assess for renal artery stenosis. 278 Simple recognition of the
change in pattern may be adequate 279 (Fig. 9.84). Pharmacologic
manipulation with captopril 280 can enhance the waveform
abnormalities in patients with renal artery stenosis. he use of
intravascular contrast agents increases the technical success rate
for the evaluation of renal artery stenosis. 281 hey may also play
a role in the assessment and follow-up of patients undergoing
renal artery angioplasty and stent placement. 282
Despite abundant literature indicating the promise of either
direct or indirect Doppler approaches, both techniques can be
tedious in hard-to-scan patients. Widely varying reported sensitivities
and speciities for both approaches for the detection of
“signiicant” renal arterial stenosis have also called into question
FIG. 9.83 Schematic Diagram of Renal Artery Doppler Tracings.
Right side, Tracing from a normal renal artery. Note early systolic
peak. Middle, Tracing shows high-velocity low measured at the stenosis.
Left side, Tracing shows the dampened tardus-parvus waveform downstream
from the stenosis. (With permission from Mitty HA, Shapiro RS,
Parsons RB, Silberzweig JE. Renovascular hypertension. Radiol Clin
North Am. 1996;34:1017-1036. 278 )
A
B
FIG. 9.84 Renal Artery Stenosis. (A) Intrarenal spectral waveform shows a tardus-parvus signal with a prolonged acceleration time and low
resistive index (RI). (B) Waveform at the origin of the renal artery from the aorta shows a high peak velocity of 410 cm/sec with an RI of 0.43.