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PubMed This is a summary of 18 peer-reviewed journal articles Updated
Nephrology

Reading Your Results: Diagnostic Tests and Imaging

At a Glance

Renal artery atheroma is usually evaluated first with Doppler ultrasound. CTA or MRA can provide more detail, while angiography with a pressure wire may show whether narrowing limits blood flow; kidney size and resistive index help show how healthy the kidney tissue is.

Diagnosing renal artery atheroma is a multi-step process that moves from non-invasive “screening” tests to more detailed, and sometimes invasive, “confirmatory” tests. The goal of these tests is not just to see if there is a blockage, but to determine if that blockage is actually limiting blood flow or causing your high blood pressure [1][2].

The First Look: Duplex Doppler Ultrasound

A Duplex Doppler ultrasound is almost always the first test performed. It is non-invasive and uses sound waves to measure how fast blood is moving through your renal arteries [3].

When interpreting your ultrasound report, you will likely see two key numbers. These are imperfect estimates and laboratories use varying cutoffs:

  • Peak Systolic Velocity (PSV): This is the top speed of the blood as it passes through the narrowing. Just like water moves faster through a pinched hose, blood moves faster through a narrow artery. A PSV of 180–200 cm/s or higher often suggests at least 50% narrowing [3][4]. If the PSV is above 250 cm/s, the narrowing may be severe [1].
  • Renal-Aortic Ratio (RAR): This compares the speed of blood in your renal artery to the speed of blood in your aorta (the body’s main artery). An elevated RAR (such as above 3.5) is another clue of significant narrowing [4][5].

Advanced Imaging: CTA and MRA

If the ultrasound is unclear, your doctor may order a CT Angiogram (CTA) or an MR Angiogram (MRA). These provide highly detailed, 3D pictures of the blood vessels.

The Contrast Question

Both CTA and MRA often require “contrast dye” to make the blood vessels visible. If you already have kidney disease, this dye requires careful consideration:

  • CTA (Iodinated Contrast): This dye can cause a drop in kidney function in a risk-benefit scenario, especially if your eGFR (a measure of kidney health) is below 30 [6][7]. Doctors may use IV fluids before the test to protect your kidneys [7].
  • MRA (Gadolinium): Modern “Group II” gadolinium agents are now considered to carry a very low risk for a condition called nephrogenic systemic fibrosis (NSF) [8][9]. However, agent selection and eGFR review is still necessary. Non-contrast MRA is also an option if your doctor wants to avoid dye entirely [10].

The Invasive Option: Renal Angiography

A renal angiogram is an invasive procedure where a small tube (catheter) is threaded through your blood vessels directly to the kidney. Because of risks like bleeding or contrast injury, it is generally reserved for situations where stenting is actively being considered [2].

Because looks can be deceiving, doctors sometimes use a tool called renal Fractional Flow Reserve (rFFR) or a “pressure wire” during the angiogram. This involves placing a tiny sensor across the blockage to measure the pressure drop [11]. A significant drop (such as an rFFR below 0.80) helps specialists confirm the blockage is severe enough to restrict blood flow [12][13].

Signs of Kidney Health and “Resistive Index”

Beyond the blockage itself, your report will describe the health of the kidney tissue:

  • Kidney Size: A healthy adult kidney is usually 10–12 cm long. If a kidney is significantly smaller (e.g., under 8–9 cm), it may have suffered damage from a lack of blood flow [14][15].
  • Resistive Index (RI): This is a measurement of the resistance to blood flow within the small vessels of the kidney itself. An RI below 0.80 is generally a good sign, suggesting the kidney tissue is still healthy and might recover if blood flow is restored [16][17]. An RI higher than 0.80 can suggest there is “scarring” or damage within the kidney that a stent might not be able to fix [16][18].

Checklist: Is Your Imaging Report Complete?

A high-quality imaging report should give you and your doctor a clear picture. Ensure your report or doctor’s summary includes:

  1. [ ] The Velocity: Specific PSV numbers for both the left and right renal arteries.
  2. [ ] The Percentage: An estimate of how narrowed the artery is (e.g., 50–69% or >70%).
  3. [ ] Kidney Measurements: The exact length of both kidneys in centimeters.
  4. [ ] The Waveform: Descriptions like “tardus-parvus,” which is a specific slow-motion blood flow pattern seen downstream from a severe blockage [3].
  5. [ ] Resistive Index (RI): A measurement for both kidneys.

Common questions in this guide

What do high PSV and RAR values mean on a renal artery ultrasound?
A high peak systolic velocity (PSV) or renal-aortic ratio (RAR) can suggest significant narrowing in a renal artery. These measurements are estimates, and laboratories use different cutoffs, so your clinician should interpret them alongside the rest of the ultrasound and your health information.
What does the kidney resistive index tell me?
The resistive index (RI) estimates resistance to blood flow in the kidney’s smaller vessels. An RI below 0.80 is generally more reassuring about kidney tissue health, while a higher value can suggest scarring or damage that may not improve with a stent.
Are CTA or MRA contrast agents safe if I have kidney disease?
Safety depends on your kidney function, especially your eGFR, and on the contrast agent used. Iodinated contrast for CTA requires particular care when eGFR is below 30, while modern Group II gadolinium agents used for MRA have a very low risk of nephrogenic systemic fibrosis; non-contrast MRA may be an option.
Why would I need a renal angiogram with a pressure wire?
A renal angiogram is invasive and is usually reserved for situations in which stenting is being considered. A pressure wire, sometimes called renal fractional flow reserve or rFFR, measures the pressure change across the narrowing; a value below 0.80 can support that the blockage is restricting blood flow.
What should be included in a renal artery imaging report?
The report should include the PSV for both renal arteries, the estimated percentage of narrowing, the length of each kidney, any abnormal downstream waveform such as tardus-parvus, and the resistive index for both kidneys. These details help your clinician judge the severity of narrowing and the condition of the kidney tissue.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.What was my Peak Systolic Velocity (PSV) and my Renal-Aortic Ratio (RAR) during the ultrasound?
  2. 2.What is the size of each of my kidneys, and is there any thinning of the outer layer (cortex)?
  3. 3.What was my Resistive Index (RI), and what does that number tell you about my kidney's long-term health?
  4. 4.Do my current eGFR and protein levels make a CTA or MRA risky for my kidneys?
  5. 5.If we do an angiogram, will you use a pressure wire (rFFR) to see if the blockage is actually affecting my blood flow?

Questions For You

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References

References (18)
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    Renal Artery Stenosis As Etiology of Recurrent Flash Pulmonary Edema and Role of Imaging in Timely Diagnosis and Management.

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    Ultrasound diagnostics of renal artery stenosis: Stenosis criteria, CEUS and recurrent in-stent stenosis.

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This page is for informational purposes only and does not constitute medical advice. Your clinician should interpret your renal artery imaging, kidney measurements, and blood tests in the context of your overall health.

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