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The animal can be examined in dorsal or lateral recumbency. The transducer is placed near the pubis and moved cranially until the bladder is identified. The bladder should be examined carefully in several planes of section. The canine prostate is usually examined by applying a sector transducer cranial to the pubis and directing the beam caudally. In large dogs, a linear array probe designed for large animal use may be introduced in to the rectum.
Normal appearance: The normal bladder is smooth and well defined with anechoic contents. Most species produce urine, which appears anechoic or black on ultrasound examination. Equine urine however, contains a large concentration of calcium carbonate. These crystals reflect the ultrasound beam and therefore appear as echogenic particles suspended within the urine giving the bladder contents a cloudy appearance. Care must therefore be taken when scanning mares not to mistake the bladder with its echogenic contents for endometritis or a pyometra. It is important to identify the position of the bladder in mares at the start of the scan so it can be disregarded from the rest of the examination. The urinary bladder is situated ventral to the uterus, between it and the pelvis. It is pear shaped, tapering caudally towards the bladder neck and has a thin, smooth wall when distended. A full colon or rectum can indent the normal smooth outline dorsally.
BOVINE BLADDER EQUINE BLADDER
Using the bladder as an anatomical landmark, the prostate may be identified just caudal to the bladder neck, encircling the urethra. The flimsy prostatic capsule of dog is not well seen, making it difficult to define the exact borders of the gland on ultrasonography unless it has a distinctly different echotexture to the surrounding pelvic tissues. The normal prostate is symmetrical structure of uniform echotexture except in cases where a periurethral or hilar echogenic zone is apparent.
Abnormal findings: Cystic calculi are easily seen as echogenic masses that cast an acoustic shadow and lie on dependent part of bladder. The major advantage of USG has been that it allows an easy detection of both radiolucent and radiopaque cystoliths, because cystoliths create an ultrasonographic artifact referred to as acoustic shadowing.
Urinary calculi of all types are very reflective to the ultrasound beam. The entire beam is returned from their surface to the transducer producing a strong returning echo and this appears on the screen as a bright white or hyperechoic image representing the surface of the calculus closest to the transducer. None of the beam can penetrate the surface of the calculus and therefore a black band or acoustic shadow appears on the image beyond the surface. It is therefore not possible to obtain information about the internal structure of the calculus or any structures located beyond it. In addition to producing acoustic shadowing, urinary calculi are also mobile within the bladder. This means that their location will change if the position of the animal is altered. This can easily be confusing as the top of the screen always represents the transducer no matter what the position of the animal or orientation of the transducer. This means that the calculi will fall up the screen due to gravity when the animal is placed in sternal recumbency or examined in the standing position. If air has been introduced inadvertently during catheterisation then the bubbles may be seen to float down the screen. If sufficient pressure is applied to the bladder with the transducer during an examination, it is possible to push the descending colon into the bladder wall. The colon also appears as a hyperechoic curved structure with a distal acoustic shadow due to it content of feces and gas and care should be taken not to confuse this with a urinary calculus. Careful examination will reveal that the bladder wall can be seen passing between the transducer and the hyperechoic line confirming that the structure is out with the bladder. It is not possible to image the far wall of the bladder beyond a calculus because of the acoustic shadowing. Also, if the animal is repositioned in sternal recumbency or standing, the colon will stay in the far field dorsal to the bladder while a calculus will fall towards the near field.
Other intraluminal masses are hypoechoic, usually irregular in shape, and are highlighted by the surrounding anechoic urine. Neoplasms and polyps are adherent to the wall and so remained fixed in position even if the position of the animal is changed.
Focal fluid filled lesions in the prostate are commonly seen. Diffuse heterogeneity of the prostatic parenchyma can be associated with acute prostatitis, usually with an overall decrease in echogenicity, or with neoplasia or chronic prostatitis, usually with an overall increase in echogenicity. Prostatic hyperplasia causes little detectable change in the ultrasonographic appearance of the prostate other than for an increase in size. Para-prostatic cysts are fluid filled masses with a varying soft tissue component, which may be adjacent to the bladder and prostate, but are distinct from these organs. Benign prostatic hypertrophy, which occurs in 50% of dogs over 4 years and approximately 90% over 5 years, may produce a diffuse, mild increase in echogenicity. In castrated male, prostate is small and hypoechoic.
PHYSICS OF ULTRASOUND| MERITS| LIMITATIONS| EYE| HEART| LIVER| SPLEEN| KIDNEY| BLADDER AND PROSTRATE| PANCREAS| GI TRACT| TESTIS| REPRODUCTION AND OBSTETRICS