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Pearls & Pitfalls for Evaluating the Kidneys
1. Getting Started: Use the low frequency (5 to 1 MHz) phased array or curvilinear transducer. Remember that lower frequency transducers enable better visualization of deeper structures. Start with the right kidney, which is easier to find, by placing your probe in the coronal plane at the right flank. Use the liver as a window and obtain a view of the kidney in the long axis. You may need to tilt your probe in an oblique angle to get in between the ribs.
2. Be Thorough: Image the entire organ using a fanning motion. Don’t just go for one pretty static shot through the middle. If you have a difficult time viewing either kidney, have the patient take a deep breath and then halt their inspiration. This brings both kidneys inferiorly. Obtain both long-axis and short-axis views of both kidneys. Any abnormal findings should be confirmed with comprehensive imaging either during that same ED visit or as an outpatient if the patient is stable for discharge.
3. Look for the Hydro: Hydronephrosis is seen as a black or anechoic area in the center of the kidney and usually represents downstream obstruction.
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.
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 a normal anatomy and even more essential to pick up an aneurysm before it ruptures.
6. Don’t Worry about Details: During an emergency bedside ultrasound you don’t need to visualize the actual kidney stone or determine if there are ureteral jets into the bladder. The main focus of a bedside renal ultrasound is to determine if hydronephrosis is present. However, if there is a major abnormal finding, like it this case, take your time and get a good look at the sonographic abnormalities so that you are more likely to recognize abnormal scans when you see them.
7. Avoid Pitfalls: The best way to minimize errors is through experience, so scan lots of normal kidneys, even when you are using the machine for other indications.
Brady Pregerson manages a free online EM Ultrasound Image Library and is the author of the Tarascon Emergency Department Quick Reference Guide. For more info visit EMResource.org.
Teresa S. Wu is the Assoc. Residency Director and Director of Ultrasound and Simulation Based Training for the Maricopa EM Program in Phoenix.