A renal artery Doppler ultrasound uses sound waves to watch blood flowing through the arteries that feed your kidneys, with no needles, dye, or radiation. For you, it comes down to lying still, breathing on cue, and putting up with some firm pressure on your belly. It is one of the longer ultrasounds, because these arteries sit deep behind the bowel and are slow to find. Arriving prepared, usually with an empty stomach, does more than anything else to give clear pictures and avoid a repeat visit.
How to Prepare for the Scan
Many imaging centers ask you to have nothing to eat or drink for 8 hours before a kidney artery ultrasound.1InsideRadiology. Ultrasound Food, drink, and the gas they create in the bowel can hide the deep kidney arteries from the sound waves.2InsideRadiology. Ultrasound Booking a morning slot lets you fast overnight instead of through the day.
A general kidney ultrasound, which looks at the kidneys themselves, usually needs no fasting.3Johns Hopkins Medicine. Kidney Ultrasound The artery scan looks at small vessels deep in the belly, which is why fasting is often required for it. One comparison found that fasting did not clearly reduce the amount of bowel gas seen on abdominal ultrasound.4PubMed Central. Is fasting a necessary preparation for abdominal ultrasound? So follow your own clinic’s instructions even if a friend’s scan needed none.
If you take blood pressure pills, ask the clinic whether to take them with a small sip of water. People on insulin or diabetes tablets should ask how to adjust their doses while fasting, and it helps to bring a snack to eat right after the scan. Bring a list of your medicines and any earlier kidney scan reports too.
Wear a loose two-piece outfit so your belly can be uncovered easily.
If you accidentally eat or drink something within the fasting window, call the imaging center before you go. Staff can tell you whether to proceed or rebook, which beats a scan with poor pictures. Chewing gum, smoking, and fizzy drinks can add swallowed air to the stomach and bowel, so many centers ask you to avoid them during the fast; check what yours says.
What Happens During the Scan
You start lying on your back. Your belly is uncovered and a clear gel is applied so sound waves pass from the probe into your body without air in between. The gel will feel cool and slightly sticky on your skin. The gel wipes off easily afterward, and a towel or tissue is normally provided. The sonographer, the person doing the scan, slides the probe up and down the middle of your belly to find each kidney artery.5Ultrasoundpaedia. Renal Arteries normal The probe is pressed fairly firmly to get close to these deep vessels,6PubMed Central. Standards of the Polish Ultrasound Society – Update. Ultrasound examination of renal arteries and scanning may be done from the front or the sides of the belly.7UC Davis Health. Renal artery duplex scanning You may be rolled onto one side, sometimes with your arm behind your head, to get around bowel gas and see the arteries better.8PubMed Central. Standards of the Polish Ultrasound Society – Update. Ultrasound examination of renal arteries9Ultrasoundpaedia. Renal Arteries normal
Your breathing is part of the test. Most of the time you will be asked to take shallow, relaxed breaths.10PubMed Central. Standards of the Polish Ultrasound Society – Update. Ultrasound examination of renal arteries When the machine records blood flow, you will be asked to hold your breath to keep the picture still.11PubMed Central. Standards of the Polish Ultrasound Society – Update. Ultrasound examination of renal arteries Sometimes the breath is held after breathing in and sometimes after breathing out, depending on where the artery shows best.12Ultrasoundpaedia. Renal Arteries normal On your side, you may need to take a deep breath and hold it for 10 seconds or more.13PubMed Central. Standards of the Polish Ultrasound Society – Update. Ultrasound examination of renal arteries Practicing a few comfortable breath holds at home can help.
The screen shows gray images with red and blue color where blood is moving. That color is not a photo of blood but a computer display of the direction and rough speed of flow. Red and blue generally show flow toward or away from the probe, not oxygen-rich or oxygen-poor blood as many people assume. You may also hear a pulsing whooshing sound, which is the same flow information turned into audio, rising with each heartbeat. The sonographer may stay quiet while measuring and usually won’t give results on the spot.
The scan takes time because the sonographer has to follow each kidney artery from where it leaves the main abdominal artery to where it enters the kidney, then repeat this on the other side. Bowel gas and the depth of the arteries mean the probe often has to be tilted or moved to a new spot to get a usable reading. Another breath hold or a change of position is routine and does not mean something has been found.
Does It Hurt and How Long Does It Take
The scan is not painful.14Virginia Heart. What to Expect From a Renal Artery Doppler Ultrasound The probe pressing on your belly can cause slight discomfort, though, especially if you’re tender there.15Montgomery Cardiovascular Associates, P.C.. Renal Artery Ultrasound – Montgomery Cardiovascular Tell the sonographer if it hurts, because they can adjust the pressure or your position.16Virginia Heart. What to Expect From a Renal Artery Doppler Ultrasound If your belly is sore from surgery, a recent illness, or other tenderness, say so before the scan starts so the pressure can be planned from the beginning.
Some centers schedule about 45 minutes,17Montgomery Cardiovascular Associates, P.C.. Renal Artery Ultrasound – Montgomery Cardiovascular while others say a complete study can take one to two hours, with about 90 minutes being common.18Virginia Heart. What to Expect From a Renal Artery Doppler Ultrasound19UC Davis Health. Renal artery duplex scanning A scan lasting an hour or more can leave your back stiff from lying still, so ask for a pillow or a small position change if you need one.
No contrast dye is injected, so there is no risk of dye harming the kidneys, unlike with CT or angiography.20UC Davis Health. Renal artery duplex scanning You can drive and eat normally right afterward.
Why Your Doctor Ordered It
- High blood pressure, especially when it is hard to control and difficulty holding your breath or following directions can limit the test.21Sonographic Tendencies. Renal Artery Doppler
- A whooshing sound heard over the belly with a stethoscope22Sonographic Tendencies. Renal Artery Doppler
- Abnormal kidney blood tests, such as raised creatinine or urea (BUN)23Sonographic Tendencies. Renal Artery Doppler
- Follow-up of a narrowing that is already known24Sonographic Tendencies. Renal Artery Doppler
- Blood in the urine, which can point to a rare compressed vein25Sonographic Tendencies. Renal Artery Doppler
The main target is renal artery stenosis, a narrowing of a kidney artery that cuts blood flow to the kidney. Most narrowings come from fatty buildup in the artery wall, the same process behind heart attacks.26Sonography. Ultrasound of Renal Artery Fibromuscular Dysplasia—A Case Study and Review About 10-20% of cases come from fibromuscular dysplasia, an uncommon artery-wall condition seen mostly in women aged 20 to 60.27Sonography. Ultrasound of Renal Artery Fibromuscular Dysplasia—A Case Study and Review
A narrowed artery raises blood pressure because the kidney senses low blood flow and reacts as if blood pressure were too low. It then releases hormones that push blood pressure up. That is why several pills may still fail to control the pressure.
The whooshing sound over the belly is called a bruit. It is the sound of blood moving unevenly through a narrowed artery, but it is a clue rather than proof, since other vessels in the belly can also make it. A doctor who hears one will usually confirm it with an imaging test such as this scan.
Understanding Your Results
Blood speeds up as it squeezes through a narrowed spot, just as water does through a pinched hose. So your report may list peak systolic velocity (PSV), the top speed of blood with each heartbeat, in cm/s.28PubMed Central. Ultrasound diagnostics of renal artery stenosis Many labs flag readings above about 180-200 cm/s as a sign of significant narrowing.29PubMed Central. Ultrasound diagnostics of renal artery stenosis
The report may also compare kidney artery speed with speed in the main abdominal artery, and a ratio above about 3.5 suggests serious narrowing.30PubMed Central. Ultrasound diagnostics of renal artery stenosis A high ratio supports the speed reading when both point the same way, though if the abdominal artery is itself diseased, the comparison is less reliable.
A single number above a lab’s cutoff is a flag for further review, not a final diagnosis. Cutoffs differ between labs, so your report should be read against the limits your own lab uses. Top blood speed can also be affected by your heart rate and by how well the sonographer lined up the probe with the artery.
Inside the kidney, a normal artery shows a sharp rise in flow with each heartbeat.31PubMed Central. Tardus-parvus waveform the only initial clue to mid-aortic syndrome- a rare cause of youth onset hypertension: A case report and a comprehensive review Past a tight narrowing, the flow rises slowly and peaks lower.32PubMed Central. Tardus-parvus waveform the only initial clue to mid-aortic syndrome- a rare cause of youth onset hypertension: A case report and a comprehensive review A resistive index measures how hard blood flows into kidney tissue, and higher values tend to go with diabetes, high blood pressure, and kidney disease.33anzsnevents.com. What to Expect From a Renal Artery Doppler Ultrasound34PubMed Central. Ultrasound Doppler renal resistive index: a useful tool for the management of the hypertensive patient It can’t tell which kidney disease you have, though.35PubMed Central. Current Insights into the Significance of the Renal Resistive Index in Kidney and Cardiovascular Disease
After your scan, a specialist doctor reviews the images and sends a report.36Virginia Heart. What to Expect From a Renal Artery Doppler Ultrasound You or your doctor will be contacted with results and next steps.37Virginia Heart. What to Expect From a Renal Artery Doppler Ultrasound Narrowing of 60% or more may lead to referral to a specialist, though many narrowings are managed with medicines.38UC Davis Health. Renal artery duplex scanning A normal result often means your doctor will look at other causes of high blood pressure.
How Accurate the Test Is
Overall, the scan catches about 85% of significant narrowings and correctly clears about 92% of normal arteries.39PubMed. Comparative accuracy of renal duplex sonographic parameters in the diagnosis of renal artery stenosis: paired and unpaired analysis So a normal result is reassuring but not absolute, and false alarms are fairly uncommon, though they happen. Results vary by lab and by the skill of the person doing the scan, so these figures are averages rather than guarantees.
In one comparison, the scan found 63% of narrowings, while CT angiography found 96%.40PubMed. Comparison of Doppler US and CT angiography for evaluation of renal artery stenosis CT is not blocked by bowel gas and body size the way sound waves are, which gives it a clearer view of small and extra arteries. The trade-off is radiation and dye, which is why ultrasound is often tried first.
Part of the gap comes from anatomy: about 20% of kidneys have an extra small artery, which often can’t be seen, so narrowing in it can be missed.41PubMed Central. Detection of Segmental Branch Renal Artery Stenosis by Doppler US: A Case Report If the main artery looks normal but suspicion remains high, doctors may look for a second one with another test. If your blood pressure stays very hard to control after a normal scan, it’s reasonable to ask whether a CT scan is needed.
Obesity and gas in the bowel can blur the images.42MD Searchlight. Doppler Renal Assessment, Protocols, and Interpretation43United Diagnostic Services LLC. Renal Ultrasound – Renal Arterial Doppler – UDS Unusual kidney anatomy, such as extra or twisted arteries or a horseshoe kidney, makes the scan harder,44MD Searchlight. Doppler Renal Assessment, Protocols, and Interpretation45MD Searchlight. Doppler Renal Assessment, Protocols, and Interpretation
When the pictures are too poor to measure, the result is called inconclusive. That means the arteries couldn’t be seen clearly enough, not that narrowing was found. Your doctor may repeat the scan or choose CT or MR imaging instead.
Scans After a Kidney Artery Stent
Ultrasound is useful for checking an artery after it has been widened or stented.46Korean Journal of Radiology. Clinical Applications and Diagnostic Pitfalls of Renal Artery Ultrasound It matters most when blood pressure or kidney function isn’t well controlled at follow-up.47Korean Journal of Radiology. Clinical Applications and Diagnostic Pitfalls of Renal Artery Ultrasound The metal stent can block part of the signal, so the sonographer measures speed just beyond it.48Korean Journal of Radiology. Clinical Applications and Diagnostic Pitfalls of Renal Artery Ultrasound Early renarrowing may not show up inside the kidney, so the artery itself has to be scanned.49Korean Journal of Radiology. Clinical Applications and Diagnostic Pitfalls of Renal Artery Ultrasound
Speeds recorded in a stented artery can run higher than in an untreated one, even when the stent is open. Labs should adjust their speed limits for stented arteries, because normal limits can overstate narrowing.50PubMed. Ultrasound velocity criteria for renal in-stent restenosis
Children, Pregnancy, and Kidney Transplants
Guidelines recommend the scan for checking possible narrowing in normal-weight children aged 8 and over who can cooperate.51PubMed Central. Utility of Doppler sonography for renal artery stenosis screening in obese children with hypertension Younger children have to lie still and hold their breath on request, which is hard, and extra body weight can limit the view. When ultrasound cannot give a clear answer, other imaging may be chosen. To prepare a child, explain the breath holds and practice them together beforehand, and bring something to keep them busy during a long session. The scan also helps diagnose and follow kidney problems in high blood pressure during pregnancy.52PubMed Central. Maternal renal artery Doppler sonographic changes in pregnancy-induced hypertension in South West Nigeria
After a transplant, it is a key test for problems with the new kidney’s blood vessels.53PubMed Central. Renal transplant vascular complications: the role of Doppler ultrasound The new kidney sits low in the pelvis, so the probe is placed there, and these scans are often shorter because the kidney is close to the skin.