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 C60 curvilinear probe with a supine patient at 30 degrees elevation. The elevation is important for the pelvis exam later.
Transverse View: Find the pubic symphysis with a finger, pre-apply gel in the area above the symphysis, and place the probe in this area with the indicator examiner left. Start with a high depth setting.
A full bladder is easy to see but caudad fanning is needed for smaller bladders. The shape of the transverse bladder is not always rectangular as in the following very full bladder.
The above image was from a young woman so immediately posterior to the bladder is her uterus, the hyperechoic tissue beneath the uterus is fascia, and further posterior is the rectum.
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 are usually the most sensitive abdominal locations to collect free flowing intraperitoneal fluid. This will be covered more below.
Depth and Gain: Details of the bladder wall are important so adjust the depth to focus just on the bladder. Make sure the gain is correct for the bladder, which means getting the urine anechoic. Thicker abdominal walls make imaging more difficult. There may be some inconsequential hyperechoic artifacts near the anterior wall of a full bladder. Posterior acoustic enhancement will be covered below in the pelvis discussion.
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 in the following image of a very full bladder. Save the image for later calculation.
Uterine distortion: A larger uterus may indent part 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 lateral to the indentation. The following image shows a uterine indentation in which the depth was 4.9 in the center. If the depth was measured laterally away from the uterus it was 6.1 cm which was probably more accurate.
After measuring the transverse bladder, carefully slide/fan cephalad to caudad through the bladder looking for abnormalities.
Bladder jets: Bladder jets may be seen easily in the transverse plane or only with searching and waiting, depending on how much urine a patient is producing at the time. The ureters enter the bladder at its cephalad and posterior aspect. Here is a patient with jets easily seen in 2D. Only the left side jet is seen in this view, but both jets were seen in real time.
Color Doppler (regular or power) can enhance viewing the jets, as in the same patient.
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.
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 back 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 a normal, modestly full bladder that was triangular in shape in the longitudinal plane. The measurement was taken about in the middle of the bladder
Next is a very 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.
After the longitudinal measurement, carefully slide/fan from right to left through the bladder, looking for abnormalities.
Bladder volume calculation: Getting the bladder width, depth, and length is not difficult and there is strong agreement between different examiners 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 rarely a cube, so, a correction factor is 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, but for bladders that are very large the 0.5 correction factor will be used because these bladders become more ellipsoid.
Bladder volume abnormalities: There are two frequent questions in clinic. Is there urinary retention? 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 bladder walls with a thin layer of fluid in between. 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.
If a bladder is small at the start, the patient at least doesn’t have much urinary retention, but the bladder walls and posterior pelvic structures cannot be adequately seen. Patients can be asked to drink 250-500 mL of water and wait 30-60 minutes and the bladder will often significantly distend.
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 POCUS 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.
Infrequently, a stone at the ureteral/bladder junction may be visible with 2D and be accentuated with color Doppler and a “twinkling” appearance as was described in the previous chapter for kidney stones. The twinkling seems to be associated with incomplete obstruction and a better prognosis than a stone without twinkling with CD.
In men, the prostate and seminal vesicles lie extraperitoneal, 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.
Imaging the prostate: The probe must be angled caudad of the bladder in both the transverse and sagittal planes to bring the prostate into view. The transverse view is the more important. The seminal vesicles may or may not be seen a little cephalad and lateral to the prostate. With a reasonably full bladder, pull away from the symphysis and use the urine as the acoustic window to visualize the prostate. Posterior acoustic enhancement from a full bladder can partially obscure the area so adjust the gain or just far field gain. It is important to get the prostate into optimal focus in the center of the screen and then use magnification to see it well.
The following clip is from a patient who was thought to have prostatitis, which may explain why the prostate is so distinct. The right and left lobes are easily seen, as well as the urethra running through the middle of the prostate. The rectum is the round structure immediately below the prostate. You can see the hypoechoic outer wall of the rectum.
In most patients the distinction between the prostate and the rectum is not this clear, and that creates the possibility of including the rectum in the 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. This requires optimizing the gain, especially turning it down to emphasize the hypoechoic gland area. The depth of the prostate is usually more distinct than the width, but unfortunately, the width is the more important measurement. For most patients, over estimating the prostate size may be a worse error than under estimation. Make several measurements of prostates that are enlarged. Here is a mildly enlarged prostate in a magnified, transverse view.
Prostate volume calculation: Transabdominal ultrasound sizing of the prostate has agreed well with transrectal ultrasound sizing for clinical purposes. A normal prostate has traditionally been called the size of a walnut while very enlarged glands can be the size of lemons, baseballs, tennis balls, and even softballs. This may be nice to describe to patients, when cm and mL don’t communicate well.
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 0.5
Obviously, the width is the most important measurement so make this as accurate as possible. The prostate volume in the patient shown above was 30 mL. The upper normal cutoff for prostate volume is about 25 mL.
Next 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? Men with symptoms consistent with benign prostatic hyperplasia (BPH) should have an increase in prostate size. The relationship is strong enough that finasteride is not generally recommended for symptomatic men until prostates are over 40 mL. 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.
Another interesting and potential use of prostate volume is the calculation of PSA density to interpret mildly elevated PSA values and not push the panic button. Divide the patient’s PSA value by his estimated prostate volume in mL. This process reduces the number of men who need further prostate diagnostics. The following cutoffs gave 95% sensitivity for prostate cancer for the noted PSA value range:
PSA of 2-4, Density cutoff = 0.05
PSA of 4-10, Density cutoff = 0.1
PSA of 10-20 Density cutoff = 0.19
Overestimating the prostate volume will falsely lower the density estimation, which could cause false negatives for prostate cancer. So, only do the density calculation when the prostate measurements are good ones.
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. The POCUS exam will never be good enough for exact diagnosis with these abnormalities and patients would always need urologic referral if greater clarity is needed for focal prostate abnormalities.
Longitudinal prostate 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. To be optimally sensitive for free fluid, this exam could even be done on a standing patient. 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. Here is the relevant anatomy.
Caution: POCUS equipment, 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 and distinguishing benign from malignant is difficult. Transvaginal ultrasound and gynecology referral would 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.
Transverse view: 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 two ovaries can be seen further lateral to the bladder.
Multigravid patients will have larger uteri. A normal uterus can be more lateral than central. In the following image, the uterus was more on the left side.
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. The right iliac vessels can be seen to the right side of the ovary.
Next is a transverse view of a pre-menopausal, multigravid uterus with a left ovary lateral to the bladder containing 2-3 modest sided cysts.
Longitudinal view: The longitudinal view in a normal woman looks like the following.
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). Again, with a specific question about free fluid in the pelvis, this exam could be done with the patient standing.
By sliding transversely across the area the ovaries can usually be seen and the width of the uterus determined. Here is the previous multigravida uterus with the probe moved lateral to show the left ovary with the few cysts.