Overview of Ultrasound of the Urinary Tract
Point-of-care ultrasound (PoC US), or a type of ultrasound used right at the patient’s bedside, is often used in the emergency department for people experiencing severe side pain. This type of pain led to approximately 2 million emergency department visits each year in the United States. Kidney stones are found in about 20% of these patients. CT scans were typically the primary way to diagnose kidney stones, however, PoC US is becoming increasingly popular.
PoC US can quite accurately detect kidney stones, especially when performed by experienced doctors or in cases of moderate or severe hydronephrosis (a condition where the kidneys are swollen due to a build-up of urine). Compared to CT scans, PoC US has the added advantage of reducing the patient’s exposure to radiation without a significant increase in missed diagnoses, complications, pain scores, serious adverse events, hospitalizations, or return emergency department visits.
Kidney stones are quite common, with men having a chance of more than 12% and women a 6% chance of getting them in their lifetime. Moreover, once an individual gets kidney stones, there is a 50 – 60% chance that they will get them again within 10 years. With the increasing rate of kidney stones in recent years, it becomes important to consider PoC US as a preferential diagnostic tool given its lower radiation exposure risk. This article focuses on the use of PoC US in the emergency department for diagnosing kidney stones. Other uses such as measuring bladder volume or confirming the placement of a urinary catheter are also briefly covered.
Anatomy and Physiology of Ultrasound of the Urinary Tract
The kidneys are placed towards the back of the body, specifically between the lower chest and the middle area of your backbones. The kidney on the right is a bit lower than the left one due to the placement of the liver. When looking at the kidneys, we can make out three main sections – the cortex, medulla, and hilum. If the kidneys are viewed using an ultrasound, you’d see that the cortex appears uniform and is similar in image brightness to the liver tissue. The medullary pyramids, which are triangular structures in the medulla, appear darker on ultrasound with surrounding tissue that is continuous with the cortex. The kidneys have a central system that is made up of minor and major calyces – small cup-like structures that help transport urine. These are so well blended with the fatty region of the renal pelvis that when kidneys function normally, they’re hard to distinguish on ultrasound.
The ureters, which are tubes that carry urine from the kidneys to the bladder, follow a somewhat inward and downward path in the rear region of the abdominal space. The ureters connect to the bladder from its back-lower part at the ureterovesical junctions (UVJs). Sometimes, these UVJs can be visualized in a view that’s been taken from the side of the bladder as little protrusions or “humps” that push into the bladder cavity from the wall at the back. When looked at this way, the bladder seems rectangular and is filled with urine which can’t reflect sound waves and hence appears black on an ultrasound image. It is positioned just above the public bone. With a front view of the bladder, it looks more triangular.
Why do People Need Ultrasound of the Urinary Tract
If you’re experiencing sudden, severe pain on one side of your lower back or belly, along with feelings of sickness such as nausea, vomiting, and sometimes the presence of blood in your urine, there’s a chance you may have a urinary stone blocking your ureter (the tube that connects your kidneys to your bladder). This condition is known as ureteral obstruction and it is one of the main reasons doctors use ultrasound imaging tests on the urinary system.
The ultrasound can detect a condition known as hydronephrosis, which is essentially a buildup of urine in the kidneys because something is preventing it from draining into the bladder. This can cause the kidneys to swell, which will show as a large dark area within the usually light grey-colored kidney on the ultrasound image. Mild hydronephrosis is when the parts of the kidneys where urine gets collected are enlarged, moderate hydronephrosis is when these become even bigger and start to merge into each other, and severe hydronephrosis is when these become one single large dark area and the kidney tissue is getting squeezed. Severe hydronephrosis can have serious consequences if the cause is not discovered and treated.
While hydronephrosis in the setting of a severe backache supports the diagnosis of a urinary stone, this condition can also be seen in other scenarios like pregnancy or urinary retention especially in elderly people. Thus, other features in the ultrasound have to be studied very carefully to avoid confusion with normal structures or other conditions that can resemble hydronephrosis like medullary pyramids, blood vessels near the kidney, cysts near the kidney pelvis, and a developmental variant whereby the kidney pelvis is outside the kidney sinus.
Seeing the stone itself in the ureter can be tricky because gas in the bowel can create shadows on the ultrasound image. However, sometimes stones can be detected inside the kidney, where the ureter meets the kidney, or at the lower end of the ureter where it meets the bladder. Stones appear really bright with a shadow behind them. Checking for ureteral jets, streams of urine flowing from the ureter into the bladder, can also be helpful as their presence rules out total blockage of the ureter. However, partial blockage is still possible.
Ultrasound can also confirm if you’re unable to pass urine (urinary retention) by measuring the size of your bladder and calculating its volume. This follows a certain formula where the width, depth, and height of the bladder are multiplied by each other and then multiplied by 0.75.
Additionally, if a tube (called a Foley catheter) has been inserted into your bladder to help drain the urine, an ultrasound can verify that it has been placed correctly by identifying the catheter balloon inside the bladder. Ultrasound imaging can also support the placement of such a tube, especially in challenging situations.
When a Person Should Avoid Ultrasound of the Urinary Tract
There are no specific reasons why someone should not have an ultrasound of the abdomen and pelvis. An ultrasound is a safe and non-invasive procedure that uses sound waves to create images of organs and structures inside the body.
Equipment used for Ultrasound of the Urinary Tract
A scan of the stomach and pelvis should be carried out using a tool called a curvilinear transducer, set at a low frequency (1 to 5 MHz). This helps the doctor to clearly see the structures deep within your body.
Who is needed to perform Ultrasound of the Urinary Tract?
A medical professional who has received the necessary training can conduct an ultrasound scan of your urinary system. This includes organs like your kidneys and bladder. It is recommended by the American College of Emergency Physicians that these professionals should correctly carry out and understand at least 25 ultrasound scans of the kidneys and bladder. They should do this before they are considered fully qualified or competent in this type of examination. In simple terms, this ensures that the person doing your ultrasound scan knows what they’re doing and can accurately read the scan results.
Preparing for Ultrasound of the Urinary Tract
The patient should be lying flat on their back on a stretchable bed with their stomach fully visible. Towels should be placed around the waist area and over the clothing to keep them clean from gel. For those performing the procedure who are right-handed, the ultrasound machine should be positioned on the patient’s right side, hooked up and ready to go. If possible, the room lights should be dimmed to improve visibility on the ultrasound machine’s screen.
How is Ultrasound of the Urinary Tract performed
The most suitable type of ultrasound to examine deep structures in the abdomen and pelvic area is with a low-frequency curvilinear transducer. If that’s not available, a phased array transducer may also work. The setting on the ultrasound machine should be set to “Abdominal.”
When it comes to scanning kidneys, it’s helpful to first check the side that’s not having any symptoms. This gives the doctors an idea of what the patient’s kidneys look like normally. For the right kidney, the ultrasound probe is placed on the side of the body (the mid-axillary line), with the probe pointed upwards towards the head at the lower edge of the ribs. On the screen, you’ll see the edge of the liver at the top, its bottom edge towards the left and the right kidney can be seen towards the bottom right, beneath the liver. It’s trickier to fully see the left kidney because it’s not next to the liver which provides a good view and there may be air in the stomach or bowels which can block the view. To view the left kidney, the probe is placed on the left side of the back, above the level of the right kidney. The left kidney will be seen next to the spleen.
To better see behind the ribs, the ultrasound probe may be tilted slightly in the direction following the shape of the spaces between the ribs. Also, having the patient take a deep breath and hold it can widen the spaces between their ribs and give a clearer view of the kidneys. If needed, having the patient lie on their right side could help to better see the left kidney. On both sides, the probe should switch between the front and back of the entire kidney to see all parts.
For the bladder, the ultrasound probe should be placed on the body just above the pubic bone in a vertical line with the top of the probe pointing up the body. If the bladder is full, it’ll be seen as a hollow, dark shape (filled with urine) taking up most the front part of the image. To get a cross-sectional view, the probe is turned 90 degrees to the left, so the top of the probe is to the right side of the patient. While moving the probe in this angle, doctors should check for UVJs (the places where the bladder connects to the kidneys) to look for any kidney stones.
Possible Complications of Ultrasound of the Urinary Tract
Using ultrasound technology to check out your belly and pelvis area is generally safe, with almost no side-effects or risks. During the examination, you might feel a bit uncomfortable because of the pressure applied by the ultrasound wand or ‘transducer’. The gel used during the procedure could also feel a bit chilly, but don’t worry, it is wiped off with a clean, dry towel once the examination is over.
What Else Should I Know About Ultrasound of the Urinary Tract?
An ultrasound of the urinary tract is a quick, affordable, and safe imaging technique that is extremely effective in detecting moderate and severe hydronephrosis, a condition indicating the presence of kidney stones. This method works very well when done by a skilled professional. Consequently, it can speed up the diagnosis and treatment of obstructive uropathy, a condition where urine cannot easily drain from your kidneys to your bladder. The procedure also limits harm by reducing exposure to potentially harmful radiation from unnecessary repeated CT scans.