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Pearls & Pitfalls for Renal Ultrasonography
1. What probes can you use? Typically, the kidneys are best visualized using a 5-2 MHz curvilinear probe or a 5-1 MHz phased array transducer. The smaller footprint of the phased array transducer enables the operator to maneuver the face of the probe in between the patient’s ribs.
2. Be Thorough: Image the entire organ using a fanning motion. Don’t just go for one isolated static shot through the middle of the kidney. Obtain complete views of the kidneys in both the longitudinal and transverse fashion. If you have a difficult time imaging the left kidney, have the patient hold their breath to bring the kidney more inferiorly. Use the spleen as your acoustic window on the left and the liver as your acoustic window on the right. Slide your transducer in an oblique angle in between the ribs to minimize the amount of rib shadowing obstructing your view of the kidney. Any abnormal findings should be confirmed with comprehensive imaging via radiology, but this can often be done as an outpatient.
3. Look for Hydro: Hydronephrosis is seen as a black or anechoic area in the center of the kidney and represents downstream obstruction. Shape is often irregular and varies depending on severity. Hydronephrosis can be categorized as mild, moderate, or severe. Cysts tend to be circular and more peripherally located than hydronephrosis.
4. Compare Sides: Because most people have two kidneys, any questionable findings on one side may benefit from a comparison view of the other kidney. If bilateral hydronephrosis is detected, image the urinary bladder to ensure there is no bladder retention or downstream obstruction.
5. Don’t Forget the Aorta: An aortic aneurysm often mimics renal colic. So always take a look at the aorta, even if your suspicion is low. It’s essential to practice looking at normal anatomy and even more essential to pick up an aneurysm before it ruptures.
6. Don’t Worry about Details: During an ED ultrasound it’s not necessarily your job to find the stone itself or to look for ureteral jets into the bladder. Concentrate on looking for hydronephrosis. Note that often times, stones may not be seen on bedside ultrasound, but their acoustic shadows may be picked up during your scan through the kidney and proximal ureter.
7. Image in Urosepsis: Don’t miss renal obstruction in the setting of urosepsis. Remember that relieving the obstruction is a critical part of treatment. These patients may have little to no pain if they are elderly and/or the obstruction has become chronic. Be especially suspicious for obstruction if the patient does not have any other risk factors for a UTI.
8. Practice: With bedside ultrasound, there is no substitute for experience. The more scans you do, the better you will be able to differentiate abnormal from normal, even when you may not be sure exactly what the abnormality is. An image library of normal and abnormal ultrasounds helps immensely, and we can help. Just go to the Soundings department on www.epmonthly.comt
Brady Pregerson manages a free on-line EM Ultrasound Image Library and is the editor of the Emergency Medicine Pocketbook series. www.EMresource.ORG
Teresa S. Wu is the Associate Residency Director, and Director of Ultrasound and Simulation Based Training for the Maricopa Emergency Medicine Program in Phoenix, Arizona.