Overview of Doppler Renal Assessment, Protocols, and Interpretation
Doppler ultrasound is a widely used tool for checking your kidney system and any health issues that may be connected. Think of this tool as a unique kind of medical camera that uses sound waves to take pictures of your organs and blood flow. It’s a good choice because it’s not invasive (meaning it doesn’t require surgery), isn’t too expensive, and is generally well-received by patients.
However, one of the challenges with this tool is that it requires a skilled operator and can take quite a bit of time. Also, understanding the results from a renal Doppler ultrasound can be a bit tough, especially if you’re not familiar with the technical terms and basic ideas used.
Despite these challenges, the American College of Radiology thinks it’s a valuable tool, so much so that their guidelines often recommend using renal Doppler ultrasound first, before any other tests. It’s especially useful in patients who have issues with their kidney function, or who have had a kidney transplant. This is because in some situations, using dye for CT scans or MRI scans can cause problems.
This article discusses the blood vessel structure of the kidneys, why and how this particular type of ultrasound is performed, and a brief insight into common health issues found using this tool.
Anatomy and Physiology of Doppler Renal Assessment, Protocols, and Interpretation
The kidneys are important organs that are located deep inside your body, towards your back. They get about 20% of the blood that your heart pumps. Each kidney gets blood of its own through a special blood vessel known as a renal artery. This blood vessel branches off from the large blood vessel- the aorta, that runs down the centre of your body. Do keep in mind though, that about 30% individuals have an extra renal artery and about 10-15% have it on both sides. The renal arteries, which are usually 4-6 cm long and about 5-6 mm in diameter, take different paths to reach each kidney.
The renal artery for the right kidney crosses behind a large vein- the inferior vena cava, to reach the right kidney, whereas the one for the left kidney takes a more straight route to reach the left kidney. Along their way, these renal arteries also supply small amount of blood to the adrenal glands (small hormone producing glands that sit on top of the kidneys), the early part of the ureter (the tube that carries urine from the kidneys to the bladder), and the outer covering of the kidney, called the renal capsule. However, these small branches are usually not seen on ultrasound or other imaging because of their small size.
Just before they enter the kidneys, the renal arteries branch out into five smaller vessels. These are the apical, superior, middle, inferior and posterior segmental arteries. These go on to supply specific areas of the kidney, finally ending up in the little filtering units called glomeruli.
The renal arteries are usually behind the renal veins- the blood vessels that carry blood away from the kidneys. The renal vein for left kidney is usually longer than the one for the right and runs between the aorta and another large blood vessel, the superior mesenteric artery, before joining the IVC. Interestingly, in majority of individuals, this vein collects blood from the left adrenal and gonadal veins (the veins that carry blood away from ovaries or testes) and in many, from the veins that supply the back and the abdominal muscles. Some people may also have the congenital (present from birth) condition of having two left renal veins that encircle the aorta, which is seen in about 17% of the population. The renal vein for the right kidney is shorter and merges with the IVC more on the side. In comparison to the left, the adrenal and gonadal veins of the right side drain into the renal vein less often, in only 7% and 31% of the cases respectively.
Why do People Need Doppler Renal Assessment, Protocols, and Interpretation
Doppler ultrasound, a type of scan that examines the flow of blood through a blood vessel, is vital in the review of both original and transplanted kidneys. It plays a crucial role in evaluating kidney health and identifying potential problems.
This type of ultrasound might be needed in people with high blood pressure, especially if doctors suspect it’s caused by kidney problems. People already known to have kidney diseases, who are under medical care or have had previous treatments to widen out their blood vessels, might also need this scan. It is also used to listen to noises in the blood vessels in the abdominal or flank area, check if a blood vessel has ballooned up (aneurysm) or if there is a hookup between an artery and a vein that’s not supposed to be there (arteriovenous fistula). Furthermore, it helps to examine the root causes of sudden kidney failure and inspect blood flow in patients with previously diagnosed conditions that can deprive the kidneys of blood flow, such as a tear in the wall of the aorta or trauma. This ultrasound can also help in investigating uneven kidney sizes and in confirming if there is a blood clot in the vein that drains the kidney.
In case of people with a kidney transplant, a Doppler ultrasound is used to set baseline values for blood flow indicators. It’s also applied if complications arise like tenderness, a sudden rise in creatinine (a waste product indicating kidney function), reduced or no urine output, bloody urine, or if the tubes connecting the kidney and bladder are dilated. Also, it helps in determining if there are blood flow issues in a transplanted kidney, examining for complications after a biopsy (a procedure where a small sample of kidney tissue is removed for testing), and assessing for a disease where white blood cells grow excessively (lymphoproliferative disease).
When a Person Should Avoid Doppler Renal Assessment, Protocols, and Interpretation
There aren’t any clear situations where a renal Doppler ultrasound, a procedure to check the blood flow in your kidneys, cannot be performed. However, the test may not produce clear or useful results in some people. These people might include those who are obese, those who have too much gas in the intestine, or those with a complex kidney structure – like having a horseshoe kidney, multiple kidney blood vessels, or twisted vessels. The test might also be challenging for people with heart or aorta (the main blood vessel in the body) issues, and for very sick people who have difficulty following directions – for example, if they have trouble controlling their breathing.
The task of conducting this test is even more difficult because the location of our kidneys, which are deep within a space in the body called retroperitoneum, further complicates the process. The fact that the kidney’s blood vessels are relatively small in size also contributes to the challenge of conducting this ultrasound test smoothly.
Equipment used for Doppler Renal Assessment, Protocols, and Interpretation
When using an ultrasound machine for what is known as a Doppler Ultrasound examination, the machine will need to have a special function known as Duplex scanning. Duplex scanning is a process where the machine shows a 2D image, like a normal ultrasound picture, but also collects information about blood flow from your body at the same time.
This scanning process typically uses three kinds of Doppler technology. One is color Doppler, which creates a colorful image to discern direction and rate of blood flow. The second is power Doppler, which is helpful in spotting subtle and slow blood flow, but it doesn’t provide information about flow direction or precise measurements. Lastly, there’s spectral Doppler, which illustrates how fast blood is flowing over a span of time using a waveform or a graph-like image.
The selection of the probe, or the part of the machine that touches your body, depends on the patient’s body type and size. Generally, for adult patients, a lower frequency probe, often called a curvilinear transducer, is used because it can reach deeper areas in the body like kidneys and renal arteries. For thinner or pediatric (children) patients, a higher frequency probe called a linear transducer can be used for better detection of blood flow.
Who is needed to perform Doppler Renal Assessment, Protocols, and Interpretation?
Doctors who read and interpret kidney ultrasound results should be highly knowledgeable in this area. This means they should understand why and when the test is needed, what it can and can’t show, and the detailed structure and function of the kidneys. They should also be able to connect other medical information with the results from the ultrasound.
To ensure they stay up-to-date with the latest knowledge and skills, these doctors should regularly participate in continued medical education relevant to their practice. Similarly, ultrasound technicians, who are skilled professionals operating the ultrasound machine, should also receive proper ongoing training and keep learning about the latest updates in their field.
Both doctors and ultrasound technicians need to meet certain requirements to be certified, which means they have proven their knowledge and skills in their area of work. This way, you can be confident that you’re in good hands when you go for a kidney ultrasound test.
Preparing for Doppler Renal Assessment, Protocols, and Interpretation
Before undergoing a renal Doppler Ultrasound, which is a test that checks the blood flow in the kidneys, patients are advised not to eat or drink anything for eight hours. This includes not using chewing gums or tobacco as these can lead to the swallowing of air, which can interfere with the quality of the ultrasound images. But, for kidney transplant check-ups, patients are not required to follow this rule. If a patient needs to take necessary medications before their test, they can do so with a small amount of water. A substance known as Simethicone, which has shown to help break down gas in the stomach, can potentially improve the ultrasound images, especially for patients who are overweight. However, it is not routinely used as it’s not yet clear if the cost outweighs the benefit.
How is Doppler Renal Assessment, Protocols, and Interpretation performed
An examination of your kidneys’ blood supply will usually begin with a test that uses sound waves to create images of your kidneys. These images can show the size, position, and texture of your kidneys. They are also used to check for renal abnormalities, like potential fluid collections, especially if you’ve had a kidney transplant.
Next, doctors use Doppler images (which show the flow of blood within vessels) to evaluate the blood flow to and from your kidneys. They do this in three areas of the renal arteries (that carry blood to the kidneys); proximal (near to where the arteries begin), mid, and distal (far from where the arteries begin). They also look at where each renal artery comes from the aorta, the main blood vessel in your body. This part of the sonography can also help to identify any duplicate renal arteries.
When doctors see high velocities (fast movement) in the blood flow, they pay special attention. This is because this fast flow can show up as an aliasing error. Aliasing, in this case, is the appearance of reverse flow within central areas shown on the Doppler image. It implies turbulent flow in the artery which could be a sign of stenosis (narrowing of the blood vessel) or arteriovenous fistula (an abnormal connection between the artery and vein). Aliasing can be corrected by increasing the velocity scale on the Doppler ultrasound machine to exceed the peak velocity of the vessel being studied.
Next, spectral Doppler is used to measure the peak systolic velocity (PSV) or the maximum speed that blood attains during a heartbeat cycle. It is evaluated in the abdominal aorta near the renal arteries, as well as in different portions of the renal artery. The measurements are used to calculate the ratio of the speed of blood flow in the renal arteries to that in the abdominal aorta. The normal value for this ratio should be less than 3.5.
The Spectral Doppler then measures blood flow within the kidney using segmental and interlobar arteries (arteries within the kidneys) in three parts; the upper area, the middle section, and the lower area of the kidney. During this process, doctors observe two more criteria. One is the acceleration time, which is the time taken for blood to accelerate from the beginning to the peak of systole (or the contraction phase of the heart). The other is the acceleration index, defined as the slope of the systolic upstroke (the speed of the initial increase in blood flow at the start of systole).
Apart from the above, the Spectral Doppler also calculates the resistive index (RI), a measure used to evaluate blood flow and pressure in the vessels of the kidneys. This index helps in identifying any resistance or obstruction to this blood flow. The normal RI value ranges from 0.5 to 0.7.
Doctors may also employ a technique called Power Doppler, which is extremely sensitive to blood flow and doesn’t depend much on the angle at which the probe is held. This is particularly useful when they need to check the overall blood supply to the kidney or to identify areas of the kidney that are not receiving enough blood.
For patients who have received a kidney transplant, a few more factors come into play. Transplanted kidneys are in a slightly different location, situated in either the right or left iliac fossa (groove). This location allows for the use of high-frequency probes that provide more detailed images. When examining the vessels outside of the transplanted kidneys, it’s crucial to be familiar with the surgical anatomy, since there are multiple surgical variations that could exist.
Possible Complications of Doppler Renal Assessment, Protocols, and Interpretation
This examination typically does not have any major side effects or issues. However, it’s worth noting that several factors might limit its usefulness and the ability to properly analyze the findings.
What Else Should I Know About Doppler Renal Assessment, Protocols, and Interpretation?
Understanding how a special type of ultrasound, called a renal Doppler ultrasound, works is essential in diagnosing and studying kidney diseases. Let’s look at a few common kidney problems and how this tool helps to analyze them.
Renal artery stenosis (RAS) is the main cause of secondary hypertension, which means it’s high blood pressure caused by another condition, often atherosclerosis. It is also the leading problem that emerges in kidney transplants. The Doppler ultrasound shows direct and indirect signs of RAS. Direct signs are found at the site of the stenosis, or narrowing of the arteries. These signs might include an elevated peak systolic velocity (PSV), a greater renal/aortic PSV ratio, lack of ultrasound signal indicating obstruction, and turbulence and spectral broadening, which are types of blood flow disturbances. Indirect signs may be observed away from the stenosis site, particularly, a “parvus-tardus” waveform, which is a subtly and delayed increase in blood flow, is usually noted in the peripheral (outer) kidney blood vessels.
Renal artery thrombosis (RAT) is relatively uncommon but, if not treated promptly, it can lead to severe outcomes, especially in situations where the kidney doesn’t have multiple arteries. The Doppler imaging shows a lack of blood flow within the renal artery and an abnormal waveform pattern. It’s important to carefully adjust the Doppler settings to avoid false-positive results. But, it is worth mentioning that ultrasound has less sensitivity in detecting small infarctions, blood clot-caused tissue death, compared to CT or MRI scans. An acutely affected kidney might look enlarged and irregular on the ultrasound. There might be no color or flow in the affected kidney tissue, if it is a localized infarction, this presents as dark wedge-shaped regions.
Renal vein thrombosis (RVT) can be either bland or tumor thrombus and includes partial blockage of the vein versus complete obstruction. In transplanted kidneys, RVT is a disastrous complication, with many patients developing subsequent graft failure. Symptoms of RVT are blood in urine or signs of kidney failure such as increasing creatinine or no urine. The ultrasound might show a bigger kidney due to associated vein congestion. Color Doppler of the vein displays either filling defects indicating partial blockage or complete lack of flow indicating obstruction.
And finally, Pseudoaneurysms (PSAs) are often the result of iatrogenic trauma like biopsies or operations and they may present as simple or complex cystic growths. If a PSA is identified, it’s necessary to evaluate surrounding blood clot, external blood flow, and an arteriovenous fistula (AVF) which is an abnormal connection between an artery and a vein, that might coexist. Findings suggesting the presence of an AVF include arterialized venous flow in the draining vein.
