Renal ultrasonography has replaced more invasive radiographic assessments such as IVP (intravenous pyelogram) in the diagnosis of the more common kidney complaints. In the emergency department, bedside renal ultrasound has allowed the physician to quickly and accurately assess the kidneys and the bladder for obstruction. Gorelik et al. found that in the diagnosis of renal calculus, the sensitivity of renal ultrasound alone was 93% and specificity 83%.1 When combined with KUB, the specificity increased to 100%.
Chief complaints that can lead to the diagnosis of renal pathology include flank pain, abdominal pain, back pain, urinary retention, dysuria, and/or hematuria. The emergency physician can easily bring the ultrasound machine to the bedside for quick assessment of the kidneys and the bladder to evaluate for renal pathology.
Probe Selection and Technique
Use a low-frequency 3- to 5-mHZ curvilinear probe with color Doppler. It is necessary to view both kidneys in the transverse and longitudinal planes, fanning through the superior and inferior poles to allow for full visualization of the entire kidney. The transducer mark should face toward the patient’s head for a longitudinal view, then be turned 90 degrees to view the transverse plane of the kidney (Fig. 1A, 1B). The physician must keep in mind that the liver on the right side will cause the right kidney to be positioned more caudal than the left. Therefore, when scanning the left kidney, the transducer should be placed cephalad and posterior, as compared to the right side.
The renal medulla points inward toward the central pelvis, which collects into the ureter of each kidney (Fig. 2). The renal pelvis is hypoechoic, or white, compared with the cortex. In normal kidneys, the renal pelvis has an organized cotton ball appearance (Fig. 3-5).
There are two main questions that can easily be answered with bedside renal ultrasonography: Is there hydronephrosis? What is the bladder volume?2
Is There Hydronephrosis?
As hydronephrosis develops, one can imagine the cotton center of the pelvis is stretched (Fig. 6, 7). This creates a “thinned-out” appearance to the inner white layer. In severe hydronephrosis, the cotton center is stretched to its capacity, leaving only a thin white line as a remnant of the once-organized center. In its place, the renal pelvis takes on a “bear claw” appearance2 (Fig. 8, 9).