Duplex Scanning of Renal Arteries: RI, RAR, PSV, and Ultrasound Criteria for Stenosis in Renovascular Hypertension
Calculation and Normal Range of the Resistance Index (RI)
RI is calculated using the formula: (peak systolic velocity − end diastolic velocity) / peak systolic velocity. The index reflects renal vascular resistance (RVR). According to most studies, normal RI values range from 0.58 to 0.64 (EFSUMB, 2024); authors observed normal RI values of 0.60±0.03 in adults. According to Hagen-Ansert (2023), a normal or low RI is <0.7 (for adults 0.5–0.7).
In children during the first year of life, RI values vary significantly: in preterm infants — up to 0.9; in newborns — 0.6–0.8; by the end of the first year, values approach those of adults (0.5–0.7). It should be noted that cardiac pathology, aortic coarctation, and patent ductus arteriosus can affect RI.
Ultrasound Criteria for Renal Artery Stenosis (RAS)
Direct criteria for significant stenosis (EFSUMB, 2024):
| Parameter | Threshold | Source |
|---|---|---|
| PSV in the stenosis zone | >1.8–2 m/s | EFSUMB, 2024 |
| RAR (PSV of stenosis / PSV of aorta at the level of renal arteries) | >3.5 | EFSUMB, 2024 |
| Post-stenotic turbulence | Present in significant stenosis | EFSUMB, 2024 |
According to Hagen-Ansert (2023), normal renal artery PSV should be less than 180 cm/s, and RAR should be less than 3.5:1. An example is given of a spectral pattern in ostial severe stenosis of the right renal artery with a PSV of 4.1 m/s and an end diastolic velocity of 1 m/s (EFSUMB, 2024).
Technical Aspects of Spectral Analysis
For correct spectral analysis, the wall filter should be set to the lowest possible values to avoid eliminating low-velocity diastolic flow. The pulse repetition frequency (PRF) should be set to the lowest possible to avoid aliasing. Blood flow velocities decrease distally.
Indirect (Intrarenal) Assessment Methods
Indirect methods include the study of segmental or arcuate arteries with measurement of RI and acceleration time. Normal acceleration time is less than 70 ms (Hagen-Ansert, 2023), with criteria being the same for children and adults. A normal renal artery spectral waveform is characterized by a sharp systolic rise and continuous diastolic flow without elevated velocities.
Frequently asked questions
What PSV is considered diagnostic for renal artery stenosis?
The threshold PSV in the stenosis zone is more than 1.8–2 m/s (EFSUMB, 2024). Normally, renal artery PSV is <180 cm/s (Hagen-Ansert, 2023).
What RAR value indicates significant stenosis?
RAR >3.5 is considered diagnostic for RAS (EFSUMB, 2024). RAR is the ratio of the highest PSV in the stenosis zone to the PSV of the aorta at the level of the renal arteries.
What are the normal RI values in adults?
In most studies, normal RI is 0.58–0.64; authors observed 0.60±0.03 (EFSUMB, 2024). Normal or low RI is <0.7 (Hagen-Ansert, 2023).
What is the normal acceleration time (AT)?
Normal acceleration time is less than 70 ms, with criteria being the same for children and adults (Hagen-Ansert, 2023).
How to set the device for correct measurement of diastolic flow?
The wall filter should be set to the lowest possible values to avoid eliminating low diastolic flow, and PRF should be set to the lowest possible to avoid aliasing (EFSUMB, 2024).