Page 3 of 3Pearls & Pitfalls
for Ultrasound Evaluation of the Bladder and Prostate
1. Finding the Bladder: The urinary bladder is located in the pelvis, but with retention may protrude into the lower abdomen. Start with your probe just above the symphisis pubis and aim slightly inferiorly initially in case the bladder is decompressed. Urine in the bladder will appear as an area of dark black fluid (anechoic), and there will usually be some degree of hyperechoic posterior enhancement behind a fluid filled structure such as the bladder (see image). Measure the bladder size in three different planes.
2. Bladder Volume = (A)(B)(C)/2
Post-void residual bladder volume should be measured immediately after the patient completes a spontaneous void or at least an attempt to do so. To estimate the bladder volume, measure in 3 different planes (height, width, depth) and divide the product of these 3 measurements by 2.
Bladder volume = A x B x C/2, where A, B and C are the height, width and depth. A normal post-void residual urine should be less than 50-100ml.
3. Finding the Prostate Gland: The normal prostate gland is hypoechoic (gray) and is located deep and inferior to the bladder. It is usually round or apple-shaped and may have minor internal irregularities and occasionally calcifications that may shadow. Although it is not within our scope of practice to use bedside ultrasonography to assess the prostate gland, it is important to know what normal and abnormal prostates look like so that the appropriate follow up can be obtained. In situations where an obvious abnormality is visualized, it is important to arrange definitive evaluation of the prostate and surrounding structures. Many urologists will obtain a trans-rectal ultrasound for a more comprehensive evaluation of what you saw on your bedside scan. Note that bladder masses and intraluminal clots can often appear similar to an enlarged prostate abutting the bladder wall and that further evaluation or imaging is warranted when an abnormal mass is seen on bedside ultrasonography.
4. Prostate Size = (A)(B)(C)/2 or (A)(A)(C)/2 The normal prostate size in a young male is approximately 20 grams or 20cc which correlates to two finger-breadths or less. In an older male, a normal prostate size is approximately 30 grams. Urinary retention is rare if the prostate is under 40 grams and degree of urinary symptoms tends to correlate with size. However, this is not always true. If you assume the density of the prostate is similar to water, then the size of this patient’s prostate is approximately
5. 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. Find more image-based case studies at www.epmonthly.com
Brady Pregerson manages a free on-line EM Ultrasound Image Library and is the author of the Tarascon Emergency Department Quick Reference Guide. For more information visit 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.