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.

Anatomy

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

Considerations in Renal US

Patients who are severely dehydrated may have a falsely negative ultrasound for hydronephrosis, so intravenous hydration may be required for accurate evaluation of the collecting system.²

One must also consider that the actual renal calculus is often not seen on ultrasonography. For this reason, spiral CT (computed tomography) continues to be the modality of choice for visualizing renal calculi. In fact, Sheafor et al. found that spiral CT was more sensitive (96%) compared with ultrasound (sensitivity, 61%) in the detection of renal calculi.³

However, renal ultrasonography allows for early diagnosis and therefore treatment of renal calculi, prior to CT. It may also serve to reduce radiation from repeated CTs in the setting of recurrent episodes of renal colic. Patients with known renal calculi, without urinary tract infection, and with evidence of hydronephrosis on ultrasound may not require spiral CT. These patients may be discharged from the emergency department with a urine strainer, pain medications, and close urology follow-up.

Clinical Indications for Performing Bladder Imaging

Some of the main indications for bedside bladder ultrasound examination are to obtain images that are important to assess for specific conditions:

• Infection – bladder wall thickening.
• Obstruction – bladder calculi, visualization of the vesico-ureteral junctions.
• Retention – pre- and postvoid residual volumes.

Performing a Bladder Ultrasound

Patient Positioning

The patient should be placed in a supine position and the bladder should be moderately to completely full at the time of assessment, which provides an acoustic window. This also aids in the visualization of the prostate or uterus and ovaries.

Bladder Scanning

A low-frequency 3.5- to 5-mHZ transducer should be placed suprapubically, and the bladder should be scanned in the longitudinal and transverse plane.

Anatomy

Within the bladder, the ureteral and urethral orifices demarcate the trigone and the uretheral orifice denotes the bladder neck. The neck and trigone remain constant in shape and position, but the remainder of the bladder wall changes shape and position depending on the volume of urine within it. The bladder wall is smooth with uniform thickness. While in the transverse plane (indicator to the patient’s right), sweep through from the superior dome to the base of the bladder (Fig. 10, 11).

Infection

Cystitis is commonly found in women when the urethra is colonized by rectal flora such as E. coli. It also occurs in men due to obstructive pathology or prostatitis. Patients are found to have irritable bladder and hematuria. Some noted findings in the bladder are mucosal edema and decreased bladder capacity.

On sonography, the most typical finding is diffuse bladder wall thickening (Fig. 12, 13). Occasional pseudopolyps may form if cystitis is focal.⁴

Other types of cystitis that have sonographic findings include emphysematous (air within the bladder wall) and chronic cystitis, which leads to Brunn’s nests and cyst formation and possibly eventual adenocarcinoma, all of which cause thickening of the bladder wall.

Other causes of thickened bladder wall on ultrasound are many, and it is difficult if not impossible to differentiate among them based solely on imaging.

Obstruction

Obstruction of the bladder most commonly occurs from bladder calculi,

either from migration of stones from the ureters or from urinary stasis (Fig. 14A, 14B). Urinary stasis is often linked to outlet obstruction, cystocele, neurogenic bladder, prostate enlargement, foreign body, or medications. Ureteral obstruction from a stone or other pathology can be noted by utilizing gray scale or color Doppler evaluation to identify ureteral jets (Fig. 15). On sonography, a mobile, echogenic focus with distal acoustic shadowing will be seen.⁴

Color Doppler Evaluation

Transabdominal evaluation of the ureteral orifices for jets is helpful to assess for obstruction. By using the color Doppler feature over the bladder, low-level echoes can be seen of urine entering the bladder from the ureteral orifice. The jet frequency may range from one per minute to continuous flow; the appearance of jet flow should be symmetrical in a healthy patient. This confirms renal function and ureteral patency. Patients with high-grade obstruction will have asymmetric jets. An accurate evaluation of jets requires a minimum of 10 minutes.⁵ In an emergency setting, the cost of this time must be taken into account.

Retention

Urinary retention has many causes. The use of bladder ultrasound in an emergency setting can assist in diagnosing urinary retention and aid in estimation of amount of urine in the bladder.

What Is the Bladder Volume?

In a study by Chan, bladder volume estimations were calculated using ultrasound with catheterized bladder volumes when urinary retention was suspected clinically; the correlation of the two measurements was highly significant, which supports using ultrasound in calculating bladder volume.⁶ Estimation of bladder volume can be obtained by using the automated volume calculations on your machine or by hand. A common formula to use is (0.75 × width × length × height) (Fig. 16A, 16B).

Conclusion

Bedside ultrasound of the bladder is useful in detecting possible infection, obstruction, and urinary retention. It can also be used in detecting ureteral jets for ureteral patency and estimating bladder volume.

References

1. Gorelik U, Ulish Y, Yagil Y. The use of standard imaging techniques and their diagnostic value in the workup of renal colic in the setting of intractable flank pain. Urology 1996;47:637-42.
2. Noble VE, Nelson BP. Manual of Emergency and Critical Care Ultrasound, 2nd ed. New York: Cambridge University Press, 2011.
3. Sheafor DH, Hertzberg BS, Freed KS, et al. Nonenhanced helical CT and US in the emergency evaluation of patients with renal colic: Prospective comparison. Radiology 2000;217:792-7.
4. Rumack CM, Wilson SR, Charboneau JW, Levine D, eds. Diagnostic Ultrasound. Philadelphia: Elsevier, 2005.
5. Delair SM, Kurzrock EA. Clinical utility of ureteral jets: Disparate opinions. J. Endourol. 2006;20(2):111-4.
6. Chan H. Noninvasive bladder volume measurement. J. Neurosci. Nurs. 1993;25:309-12.