Gabriela S. Seiler, DECVDI, DACVR
Department of Molecular Biomedical Sciences, College of Veterinary Medicine, North Carolina State University, Raleigh, NC, USA
The urinary bladder is often thought to be one of the easier organs to evaluate with ultrasound. However, there are many pitfalls for bladder ultrasound interpretation because of artifacts which are more common and more apparent in the urinary bladder, and because of mobility of intraluminal structures as well as the potentially intrapelvic location of the caudal aspect of the urinary bladder and urethra. Knowledge of ultrasound artifacts, how they can be avoided or interpreted, and good scanning technique are very important when interpreting ultrasound examinations of the urinary bladder.
As with any other ultrasound examination, it is important to prepare the skin properly by clipping the hair coat and applying alcohol and ultrasound gel. The urinary bladder, depending on body composition and degree of filling, typically has a very superficial location along the abdominal wall. High-frequency linear transducers are ideal to evaluate the wall thickness and wall layering in most animals. Curvilinear, microcurved or sector probes may be necessary to evaluate the urethra and prostate due to deeper location and better access to the pelvic inlet with a smaller footprint probe. It is important to adjust the focal zone during evaluation of the urinary bladder. For the superficially located ventral or ventrolateral (depending on patient position) bladder wall, the focal zone should be moved into a superficial position, whereas it should be moved farther down when evaluating the far bladder wall. Since the urine in the bladder lumen is poorly sound-attenuating, there is a fair amount of distal acoustic enhancement which can be corrected by adjusting the time gain compensation to a curvilinear shape, decreasing the amount of intensification of the deeper echoes.
The bladder can be examined with the patient in lateral or dorsal position, keeping in mind that bladder position relative to midline can vary with degree of filling and presence of gastrointestinal contents. The bladder should be examined in long axis and in transverse image planes, and care should be taken to follow the urethra as far as possible into the pelvic canal to determine if there are mural or intraluminal lesions.
For a complete evaluation of the urinary bladder, the regional lymph nodes, ureters and kidneys should be examined as well. The uretero-vesicular junctions are located in the caudodorsal bladder wall at the level of the trigone and are variable in their visibility and appearance. If visible, they present as focal tissue thickening or protrusion into the urinary bladder lumen. The ureters are typically poorly visible unless dilated or thickened. Examination of the kidneys is important to determine presence of obstructive lesions, infection, nodules or masses and other lesions that could result from or manifest in urinary bladder disease. The sublumbar lymph center including the medial and internal iliac lymph nodes is the closest draining lymph center and is closely associated with the bifurcation of the aorta and caudal vena cava.
As mentioned above, distal acoustic enhancement is a common artifact associated with the urinary bladder and can easily be corrected by adjusting time gain compensation. Also common is the occurrence of “pseudosludge”. This is characterized by increased echogenicity in the far field over the urinary bladder with a curvilinear surface and could be confused with real urinary sediment. The underlying cause is a combination of slice thickness artifact where a portion of the curved echogenic urinary bladder wall is averaged with the hypoechoic bladder lumen, resulting in an intermediate echogenicity along the bladder wall. The second component is called side lobe artifact. This artifact is caused by sound beams originating from the transducer that travel in a slightly different direction from the main beam. If reflected back to the transducer by a highly reflective surface, such as the descending colon, some of these sound waves may return to the transducer and result in creation of an image along the main axis of the beam, again mimicking urinary bladder sediment. This artifact is not unique to the urinary bladder but can happen anywhere in the abdomen; the difference is that these stray or side lobe echoes are visible in the otherwise anechoic urinary bladder, whereas they blend in with the abdominal contents elsewhere.
Near-field artifacts are unavoidable and are encountered throughout the abdomen as well. They are particularly disrupting in the urinary bladder though, since the thin bladder wall is in a very superficial position and is difficult to evaluate when obscured by these artifacts. Linear transducers help with improving the near field, and, as mentioned above, adjustment of the focal zone, use of harmonic imaging or image compounding can help as well.
A very useful artifact in the urinary bladder is distal acoustic shadowing caused by mineralized structures such as urinary calculi. This artifact is caused by complete absorption of sound waves by the mineralized structure with creation of a signal void or shadow distal to the surface of the calculus. If shadowing is present, then the conclusion can be drawn that there is mineralized material in the urinary bladder. Another artifact that can be used to determine presence of mineralized calculi is the twinkle artifact. When color Doppler is placed on a mineralized structure with a highly irregular surface, it will create a Doppler signal without any actual flow present.
A Few Helpful Tips and Tricks
Urinary bladder ultrasound is a very dynamic examination due to the mobile nature of the bladder contents. Repositioning the patient is therefore very helpful and commonly used. Turning the patient from one side to the other or from lateral to dorsal recumbency or vice versa has several effects. It moves different portions of the bladder wall to the surface, and areas that were previously obscured by near-field artifacts or artifacts originating from the colon can now be assessed. It can also be used to determine if any observed lesions are intraluminal and mobile or if they are sessile and associated with the bladder wall. Sediment can be moved and stirred up that way to differentiate from artifacts as described above. Can’t see the trigone because the bladder is in a too-caudal location? Try having someone do a rectal examination and have them try to push intrapelvic structures, such as the prostate, cranially. Not sure if a shadowing structure is a calculus or an aggregation of mineralized sludge? This question is sometimes difficult to answer. Repositioning the patient or rolling them from side to side a few times often stirs up sediment and results in altered shape or dissolution of an aggregate of sludge, whereas a calculus may change its location but not its size and shape. Palpation of the urinary bladder to move intraluminal contents is another good option.
Doppler ultrasound is useful not only to elicit the above-mentioned twinkle artifact but is essential to determine if a mural mass is perfused or not (tumor vs. hematoma). It is also often used to improve visibility of ureteral jets. Presence of a visible urine jet through both ureterovesicular junctions into the urinary bladder is proof of patency (e.g., in presence of trigonal urinary bladder masses) and location of the ureteral junction in order to rule out ectopic ureters in dogs with urinary incontinence. To increase the frequency and visibility of ureteral jets, a diuretic can be administered intravenously or subcutaneously, which will result in increased urine production within a few minutes.
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