Bladder and Pelvis
The utility of being able to gain information from a subset of pelvic structures is important in both the inpatient and outpatient setting.
The urinary bladder lies extraperitoneal and immediately cephalad to the pubic bone. The bladder has its own importance but is also an acoustic window to see reproductive organs and the posterior pelvic space. Details of these extra-bladder structures are discussed later in this chapter.
It is probably better to systematically examine all aspects of the bladder in the transverse and longitudinal planes and then go back and examine the structures posterior to the bladder as a separate task. The following anatomy is pertinent.
IMAGING THE BLADDER
Use the curvilinear probe. Many patients have excellent views of the bladder and pelvis in the standing position and this position can also be quicker and easier for patients, but the supine position is traditional. The standing position will also be optimal to detect pelvic fluid. Be careful with pressure on the bladder at the start because a full bladder may be uncomfortable. Drape the patient’s undergarments with a washcloth.
Optimization: Details of the bladder wall are important so adjust the depth and sector width to focus just on the bladder. The focus position can be the middle of the bladder. Make sure the gain is correct for the bladder, which means getting the urine mostly anechoic. There may be some hyperechoic artifacts near the anterior wall of a full bladder that can be eliminated with the TGC control to better see the wall.
Transverse View: Place the probe superior to the pubis with the indicator physician left.
A full bladder is easy to see but caudad fanning is needed for smaller bladders. Here is a distended bladder in transverse view. Notice the modest artifact below the anterior wall and the intense posterior acoustic enhancement because of the large volume of urine.
Trap: Next is a patient with a large amount of ascites who had a Foley catheter. An initial thought could be a full bladder with a plugged Foley.
The distinctive fluid filled Foley catheter bulb is visible posterior to the anechoic fluid. A variety of abdominal views in this patient showed a large amount of ascites. The ascites had pushed the collapsed bladder against the symphysis pubis and down into the pelvis on top of the Foley. Notice the thin layer of fluid under the Foley. This is ascites in the rectovesicular space (this could be a man or a woman with a hysterectomy) as shown in the following diagram.
The rectovesicular and rectouterine spaces can be very sensitive locations to image free intraperitoneal fluid, particularly with a standing patient. This will be covered more below.
Transverse bladder dimensions: Find the image in which the transverse bladder appears the largest and freeze. Measure the lumen WIDTH (left to right on the screen) and the DEPTH (top to bottom), as was done in the transverse distended bladder image above.
Bladder distortion: A larger uterus or prostate may indent part of the posterior wall of the cephalad bladder. The depth of the whole bladder is probably underestimated if the depth measurement is taken in the narrowed central portion. Instead, measure to the depth of the lateral part of the wall.
After measuring the transverse bladder and storing the image, carefully slide/fan cephalad to caudad through the bladder looking for wall abnormalities.
Bladder jets: Bladder jets may be seen easily in the transverse view or only with searching and waiting (one minute is probably the reasonable maximum), depending on how much urine a patient is producing at the time. The ureters drain into the bladder near the beginning of the urethra in the bladder trigone. This is located on the posterior and more caudad wall of the bladder. However, as the bladder distends, the ureters spread apart from each other and from the urethra. They remain posterior but may move more cephalad. The best probe position is at the anterior, cephalad bladder with the beam angled diagonally through the bladder to the posterior caudad wall. This plane should pass through any jets entering the bladder. Here is a patient with only a modestly full bladder and a jet easily seen in B-mode with a lucky parasagittal position.
Color power Doppler can enhance viewing the jets, as in the following woman who had mild right hydronephrosis from a stone. Only a left sided bladder jet was seen in her bladder.
Bladder jets are useful in patients with possible unilateral hydronephrosis. A visualized bladder jet on a suspect side indicates that the ureter is not completely blocked, making the situation non-urgent. An absent jet, with a jet present on the other side, indicates a likely obstructed ureter and the need for more urgent intervention.
When a kidney stone becomes stuck at the ureteral-vesicular junction, it can be visualized at the posterior wall. In addition, Color or power Doppler are described as showing a characteristic twinkling artifact. The following was taken from a 5 Min Sono podcast and shows a patient with right-sided hydronephrosis and a stone visible at the right posterior bladder wall. Color was applied and the twinkling was seen. As briefly discussed in the Kidney chapter, twinkling seems related to an uneven, calcified surface.
Longitudinal bladder shape and length: Rotate the probe to the sagittal position, make the bladder as large as possible, and tilt the probe caudad to bring in the caudad wall. The shape of the bladder in the longitudinal orientation is variable. Normal, moderately full bladders often appear triangular in shape while very full bladders may appear ellipsoid or cuboid. A standard measurement technique is needed for the cephalad-caudad LENGTH of the bladder but there isn’t clarity about this in the literature. We will be consistent and measure the horizontal cephalad-caudad dimension through the middle of the bladder, regardless of its shape.
Here is the longitudinal view of the distended bladder that was shown above along with a correctly performed length measurement. This bladder appears roughly cuboid in this plane.
Next is a distended bladder with an ellipsoid shape in the sagittal plane. The length should have been measured about a cm lower to be in the middle of the bladder, but the distance was about the same over the whole middle range of this bladder.
Store the longitudinal measurement so Venue can calculate the volume and then carefully slide from right to left through the bladder, looking for wall abnormalities.
Bladder volume calculation:
Getting the bladder width, depth, and length is not difficult and there is strong agreement between different physicians on the same patient. Almost every bladder volume equation begins with multiplying the Width x Depth x Length to get the volume of a cube. Literature agreement stops at this point.
The bladder is infrequently a cube, so, a correction factor is usually needed, and this gets messy. Every correction factor is based on a geometric shape that might approximate the bladder and at least 8 equations have been used. Each equation makes errors in some patients. Trying to choose the shape of a patient’s bladder and use the equation for that shape hasn’t been shown to be superior, although it makes sense. In every study (all modestly sized), each equation has had an overall error of at least ± 25%, but this is probably acceptable for how these volume estimates should be used.
There are two major contenders for the better correction factor. Some ED ultrasound leaders chose a correction factor of 0.75 based on a 1993 study. Another group of non-ED ultrasound physicians chose 0.5 from the equation for a prolate ellipsoid, which many very enlarged bladders resemble.
Given that no equation has been overall better, the 0.75 correction factor is probably reasonable for the great majority of bladders seen in clinic that are roughly triangular in the sagittal view, but for bladders that are very large the 0.5 correction factor will be used because these bladders usually become more ellipsoid. An unusual cuboid bladder requires no correction factor. Venue calculates each of these volumes and the physician decides which to use. The first distended bladder shown above was mostly cuboid, but that calculation was not displayed because it was not part of the Venue setup when the exam was done.
Bladder volume abnormalities: There are two frequent questions in clinic. “Is there urinary retention” and “What is the residual urine volume after urination?” The bladder size varies with patient height, but for most normal adults, the beginning of the urge to urinate occurs at about 200 mL of urine volume. The discomfort with any given amount of urine varies from patient to patient. Acute urinary retention is more uncomfortable than slowly progressive chronic retention. A bladder with > 400 mL of urine indicates urinary retention, voluntary or otherwise. Here is a gigantic bladder that was chronic.
After urination, normal younger adult bladders will have less than 50 mL, which shows as a difficult to find bladder with only a thin layer of fluid between the walls. In older adults, there is some weakening of the detrusor muscle so typical post-void residuals may be up to 100 mL without concern. Any patient with a bladder question should first be examined with a full bladder (to measure the volume and be able to see the bladder walls and the structures under the bladder) and then the post-void bladder can be re-examined. Important information may be missed when patients are brought only for post-void residual measurement.
Bladder wall abnormalities: The bladder wall can be thickened (> 4 mm) from tumor or inflammation. Below is a longitudinal bladder showing a thickened wall from infiltrating tumor.
Bladder diverticuli can be single or multiple, small or large, but they are additional anechoic structures out beyond the typical wall of the bladder. There is usually a visible narrow channel between the main bladder and the diverticulum. There is no characteristic size, number, or location so we decided there wasn’t a representative image to show.
Stones in the bladder are dependent, hyperechoic with posterior shadowing, and obvious. Blood clots and tumors are the main important things that IMBUS will image along the bladder wall. Blood clots are usually not adherent to the wall and move with position change. Tumors of the wall will not move. Here is a clinic patient with hematuria. The first clip is the transverse view of the bladder, showing structures on the side wall defying gravity.
Here is the same bladder in the sagittal plane.
These masses did not move with position change and were found to be carcinoma.
However, here is a patient who had only a partly full bladder and in the sagittal plane, something looked suspicious in the anterior, cephalad wall. Transverse views suggested this might be on the left side. Instead of immediately generating a urology consult for cystoscopy, this patient was asked to come back with a full bladder for a better exam and the abnormality was no longer seen.
Finally, here is a large mass in the bladder that seemed to be coming from the prostate. The patient was very ill, was hospitalized, and also found to have a brain mass. He died within a week without definition of the tumor type.
In men, the prostate and seminal vesicles lie posterior and caudal to the bladder and directly anterior to the rectum, hence the ability of a rectal exam to feel the posterior surface of the prostate. The bladder is the sonic window to the prostate and a standing position can work well for prostate imaging, particularly when bladder volume is only modest. The standing position will also optimize fluid detection in the pelvis.
Transverse view of the Prostate
The probe must be angled caudad of the bladder in the transverse plane to bring the prostate into view. The seminal vesicles may or may not be seen cephalad and lateral to the prostate. We think it is important to stay near the pubic symphysis and try for as perpendicular a plane into the prostate as possible. A more oblique angle may falsely increase the measurement of the prostate depth. Posterior acoustic enhancement from a full bladder can partially obscure the area so adjust the gain using TGC for the region of the prostate. It is important to optimize the prostate image with depth, width, and focus position. Accuracy with prostate size estimation is completely dependent on the quality of the view, which requires high-end equipment and examiner experience.
Here is an optimized transverse prostate image. Notice the rectum below the prostate. The width and the depth of the prostate were measured. The width was a few mm too generous on the left side of the image but this doesn’t matter clinically.
Be careful not to include the rectum in the depth measurement of the prostate. Fan slowly through the prostate and find a thin, slightly hyperechoic line that is the fascia separating the prostate from the rectum.
The prostate has a thin “true capsule” and a thicker “false capsule”. In addition, around the top part of the prostate is the bladder wall (up to 4 mm or more). All of this creates a hyperechoic “rind”, particularly around the anterior gland. Only the WIDTH and DEPTH of the hypoechoic gland should be measured. The depth of the prostate is usually more distinct than the width, but unfortunately, the width is the more important measurement. For most patients, overestimating the prostate size may be a worse error than under estimation. Make several measurements of prostates that are enlarged. Here is a moderately enlarged prostate with measurements.
Prostate volume calculation
Transabdominal ultrasound sizing of the prostate has agreed well with transrectal ultrasound sizing for clinical purposes, but this depends on the quality of the machine and the physician technique.
The formula for a prolate spheroid, which the enlarged prostate resembles, only requires the WIDTH and DEPTH in the transverse plane. This is fortunate because the caudad border of the prostate in the longitudinal plane is often indistinct. The formula is:
Volume = Width2 x Depth x 0.52
Obviously, the width is the most important measurement so make this as accurate as possible. The prostate volume in the first patient shown above was about 60 mL while the second patient’s volume was 45 mL. The upper normal cutoff for prostate volume is 25 mL. Venue will calculate this volume from the measurements. Next is a patient with only mild prostate enlargement (34 mL).
Finally, here is the current ANGMA clinic record for a large prostate with a volume of 220 mL.
Prostate compression: Bladder volumes of greater than 400 mL are able to compress the prostate. In one study, 50 mL prostates were reduced to 41 mL with bladder volumes over 400 mL.
What to do with prostate volume? Improvement in ultrasound technology is increasing our accuracy with prostate imaging. Nevertheless, we still think that categorizing prostates as normal, mildly, moderately, and severely enlarged is better than worrying about differences of single digits in a measurement. Men with symptoms consistent with benign prostatic hyperplasia (BPH) should have some increase in prostate size. The relationship is strong enough that finasteride is not generally recommended for symptomatic men until prostates are over 40 mL. For us, that would be a prostate that is moderately increased. Tamsulosin could be used for symptoms at smaller gland size but be suspicious of a BPH diagnosis with a gland that isn’t over 30 mL.
Focal hypoechoic or hyperechoic areas can be seen in a prostate and these can be infection related or tumor. Calcifications can accompany hyperplasia or tumor. Ultrasound can’t determine the exact diagnosis with these abnormalities and patients would need urologic referral if greater clarity is needed about focal prostate abnormalities.
Longitudinal male pelvis view
Rotate the probe to the sagittal position and take the depth out beyond the prostate. Don’t measure the longitudinal prostate but look at it and in particular observe the area cephalad of the prostate and posterior to the bladder, which is where free fluid in the pelvis should be best seen. The sagittal view is better than the transverse view for seeing this fluid. As noted above, the standing patient position is best to detect fluid. Here is an example where the rectovesicular space contained fluid. The seminal vesicles were also seen cephalad of the prostate, but these are still in the extraperitoneal space.
Here is a clip from a younger man with an almost invisible prostate who had modest free fluid in the retrovesicle space.
Imaging the ovaries, uterus, or vagina well requires a urine filled bladder. As with the prostate, the standing position may be better when bladder volume is only modest. Here is the relevant anatomy.
Caution: IMBUS skill and experience can never be good enough to evaluate a young woman in early pregnancy. Such a patient with pelvic complaints needs an immediate OB or ED visit. Enlarged and heterogenous uteri from fibroids are common in older women and distinguishing benign from malignant is difficult. Formal transvaginal and transabdominal ultrasound and gynecology referral might be needed. Finally, enlarged and cystic ovarian structures in post-menopausal women would definitely be important findings needing formal imaging and gynecology followup. Follicular cysts in menstruating women may be up to 2.5 cm in width and these could be followed in clinic for resolution.
As with the male pelvis, the standing patient position can work well for the female pelvis, but the supine position is more standard. The standing position optimizes the detection of pelvic fluid. Here is a transverse view of a nulligravida patient with a very full bladder. The probe was angled cephalad through the bladder to view the uterus, which contained an IUD in the center. The patient’s right ovary is in view.
Multigravid patients will have larger uteri and a normal uterus can be more lateral than central. The ovaries are usually located lateral near the iliac vessels but are not always easy to see. However, if the following image were obtained in a post-menopausal woman, there would be a cancer concern. The uterus is heterogenous and enlarged and there is a 2.5 cm cyst in the right ovary.
Here is the longitudinal view in a normal gravid, premenopausal, non-pregnant woman. The vagina is immediately posterior to bladder on the right side of the screen. The rectum is below the vagina. The uterus is just below the bladder on the left side of the screen. The uterine stripe can be seen intermittently in the center of the uterus. The cervix is at the end of the uterine stripe.
The very important rectouterine space that might contain small amounts of intra-peritoneal fluid is shown in the following image (arrow pointing to the fluid).By sliding transversely across the area, the ovaries can usually be seen, and the width of the uterus determined. Here is the previous premenopausal patient with the probe moved lateral to show the normal left ovary with several small cysts.
Next is the right ovary in the same patient showing a dominant cyst about 2 cm in length with several smaller cysts. The iliac artery and vein are anterior to the ovary. These cysts can be a normal part of the menstrual cycle and usually resolve spontaneously without symptoms. No follow up would be needed in a premenopausal woman who was not pregnant unless there were symptoms. This same appearance in a postmenopausal woman could still be normal but would warrant short interval followup to see regression and not growth.