Imaging the Urinary Tract
Norman Ackerman, DVM, DAC
Veterinary Radiology Consulting
Huntsville, Al, USA
Urinary tract imaging depends on expense, time, equipment availability, familiarity and confidence in the image method. Radiography is widely available, quick, relatively inexpensive and can be performed by technicians. Ultrasound is expensive, time consuming, usually requires veterinarian participation and is subject to many artifacts. Nuclear imaging, computed tomography and magnetic resonance are limited.
Radiography. Technicians can obtain abdominal radiographs without patient preparation and or supervision. Radiographs provide a global image, are most valuable when clinical signs are vague and provide additional information such as discospondylitis secondary to urinary tract infection or pulmonary metastasis. The radiograph can be examined later and can be shown to the client.
Selective Abdominal Compression utilizes a wooden paddle to displace the intestines and isolate the kidneys or bladder.With the patient laterally or dorsally recumbent the organ is palpated and isolated from the overlying intestines. The paddle holds it in place preventing the intestines from moving into the area, the technician's hand is removed and the radiograph is exposed. Reduce the radiographic technique to compensate for reduced thickness beneath the paddle. Wear protective devices and collimate the x-ray beam to the area of interest. Sedation is rarely needed.
Positional Maneuvers. Comparison of right and left lateral views is useful when mineralization or calculi are suspected. Gravity plus a horizontally directed x-ray beam concentrate the calculi or move intestines off the bladder. A lateral radiograph with the rear limbs pulled cranially helps evaluate the canine male urethra.
Additional Radiographs help determine if a structure observed on the initial radiograph is artifact or pathologic. When a mineral opacity is seen which could represent urinary calculi, soft tissue mineralization or gi foreign material a radiograph obtained after a time interval, or after an enema can be useful.
Special Procedures: Excretory Urography (EU, IVP), evaluates renal and ureteral size, shape, and position. Quality is affected by glomerular filtration and urine specific gravity. Be cautious in patients with dehydration, diabetes, iodine allergy, Bence-Jones proteinuria, and renal and hepatic failure. Retching and/or vomiting during or immediately post contrast injection is common; hives is rare. Contrast induced renal failure is reported in dogs and cats. In any contrast reaction the patient should be fluid diuresed. Renal dysfunction does not predict undesirable reactions. A 12-hour fast, laxatives and/or enemas are recommended. Pre-injection radiographs ensure an empty GI tract and serve as a baseline. An intravenous bolus of water-soluble contrast media with an iodine content equivalent to 660 mg iodine/kg body weight is used. Non-ionic contrast may be substituted but is expensive and does not reduce contrast induced renal failure. Anesthesia (sedation) does not increase the risk and may facilitate patient positioning reducing radiation exposure. These animals may still retch or vomit following contrast administration. Ventrodorsal and lateral radiographs are taken immediately and at 5, 10, 20, and 30 minutes after injection. Abdominal compression may be used to interfere with ureteral peristalsis and distend the renal collecting system. If urine specific gravity is low administering a second or third dose of contrast material may be helpful. Exceeding 1980 mg iodine/kg body weight offers no additional benefit. There is no correlation between serum urea nitrogen level and urogram quality. Increased urine specific gravity, false positive urine protein determinations, cellular morphology changes, bacterial growth inhibition and unusual crystals occur after contrast injection. The EU may be combined with a pneumocystogram when ectopic ureters are suspected. Contrast material may increase renal echo intensity and contrast induced diuresis causes ureteral dilation; therefore the ultrasound exam precedes the contrast study.
Nephropyelocentesis and antegrade ureterography. Urine samples may be obtained and contrast injected into the renal pelvis following percutaneous needle puncture. This has a limited value but is useful for evaluation of ureteral strictures and ectopia.
Cystography. A positive contrast, pneumo-, or double contrast cystogram may be used. Complications include bladder or urethral trauma or infection. Inflate the bladder slowly and stop when the distended bladder is palpated or the animal exhibits pain or discomfort. Bladder ultrasound should precede cystography. Double contrast cystography is best for bladder evaluation. Empty the bladder, distend it with air and inject contrast (cat 0.5cc; large dog, 3cc) through the urinary catheter. Lateral and VD views are adequate, obliques are often recommended. Bladder distention can mask signs of mild to moderate cystitis.
Positive-contrast cystography. Hematuria or bladder rupture are contraindications for pneumo- and double contrast cystograms. Positive-contrast cystography will delineate the rupture site. Empty the bladder and distend it with dilute contrast media (50 ml contrast with 250 ml saline). Oblique views are recommended; lateral and VD views are sufficient.
Retrograde urethrography will evaluate the entire urethra. Dilute contrast (1/2 cc/kg) with a maximum per injection of 20 cc in large male dogs is used. Avoid producing air bubbles that may be mistaken for radiolucent calculi. Use a Foley catheter with the tip placed just beyond the urethral papilla in the female or within the penile urethra in the male. Inject the contrast manually exposing the radiograph as the contrast is injected. Lateral and VD views should be obtained. A contrast injection precedes each radiograph. In male dogs the VD view should be obliqued to avoid superimposition of the distal and proximal urethra. Bladder distension prior to urethrography is not recommended.Use sedation to avoid patient motion, facilitate urethral catheterization and decrease the risk of infection. Complications include urethral trauma or infection. With urethral mucosal rupture or ulcer, contrast may enter the systemic circulation. This is no problem when using positive contrast; using air, death from air embolism has occurred. Retrograde urethrography has little value in evaluation of prostatic disease.
Vaginourethrography. If ectopic ureter, urethral rectal or rectovaginal fistula is suspected, vaginourethrography may be performed by placing a balloon tipped catheter into the vagina distal to the urethral papillae. The urethra will fill as contrast is injected. A nontraumatic forceps keeps the catheter in place and prevents contrast leaking. Anesthesia is usually required.
Ultrasound. Kidneys are easily identified. When within the rib cage evaluation may be difficult. Renal size may be accurately measured; renal volume computation is not useful. The renal pelvis, proximal ureter and renal vein can be identified. The ureter can rarely be traced beyond the renal pelvis. Renal vessels can be examined with color flow or Doppler ultrasound. The animal's size, the transducer and machine used affect the kidney's appearance. Kidney orientation to the transducer markedly alters its appearance and artifacts can be created when oblique planes are used. The canine renal cortex is brighter than the medulla with an intensity slightly less than liver and markedly less than spleen. The renal medulla has a few internal echoes but is more homogenous and hypoechoic relative to the cortex. The renal medulla's echo intensity can be misinterpreted as hydronephrosis. A hyperechoic line at the corticomedullary junction may be observed in normal dogs. Renal pelvic recesses appear as bright, evenly spaced, round or linear echoes. These may shadow and mimic calcification. The renal pelvis or proximal ureter is visible as an anechoic "v" or "y" shaped structure. Fat within the renal hilus produces a hyperechoic region. When compared to adults renal cortical echogenicity is greater in puppies up to 2 weeks old; relative renal size is greater in puppies up to 12 weeks old. Cortical echogenicity is variable in cats. Renal cortical fat makes it markedly hyperechoic when compared to the medulla. A linear hyperechoic zone within the renal medulla of older cats has been associated with microscopic mineral deposits within the tubules between the corticomedullary junction and the renal crest. Feline renal length ranges from 3.0 to 4.3 cm. Mild to moderate renal pelvic dilation can be observed following intravenous fluid administration. Doppler ultrasound can be used to distinguish the ureter from vessels and to characterize renal blood flow. Spectral waveform analysis discriminates between normal and abnormal flow; determination of flow velocity is an indirect measurement of blood pressure. Resistive index (measures resistance to renal blood flow) is elevated in acute renal failure and renal dysplasia.
Urinary Bladder. Ultrasound is ideal for bladder exam. A small bladder can be identified. Anechoic urine contrasts well with the bladder wall. Transducer pressure will distort the bladder. Slice thickness, refraction, side and grating lobe artifacts create the appearance of cellular material in the bladder. Wall thickness varies from 2.3 mm in minimally distended to 1.4 mm in moderately distended bladders. It increases with increasing body weight. Focal thickening can be identified at the trigone where each ureter enters the bladder. Swirling may be observed secondary to urine propulsion from the ureter into the bladder. This is often observed when the urine specific gravity in the bladder differs from that in the ureter or when highly cellular or crystalline material is present. It can be observed using color Doppler. Ultrasound can guide a cystocentesis or needle aspiration of a bladder mass. Tumor seeding along the needle tract may occur.
Ureter. The normal ureter can be identified entering the bladder. Ectopic ureters can be detected. These are often dilated. Absence of bladder thickening at the ureteral vesical junction can be used as a sign of ectopic ureter. Ureteral calculi can be identified. The ureter is usually distended proximal to the stone and may be traced to the renal pelvis.
Urethra. Ultrasound is rarely used for urethral evaluation. The membranous urethra in the male can be evaluated from the ischial arch through the os penis. The female urethra can be evaluated beyond the bladder neck by bladder compression to distend the proximal urethra.
Nuclear Imaging provides functional and anatomic information. Renal size, shape and location, blood flow, glomerular filtration and tubular function, ureteral patency and bladder emptying can be studied. Scintigraphy can determine each kidney's contribution to total GFR. Nephrectomy was performed when the affected kidney contributed less than 33% to the total GFR. Radioisotope white blood cell labeling has been used to detect renal infection. Equipment availability is limited, technical aspects are demanding and quality control is extremely important.
Computed Tomography and Magnetic Resonance use different technology but both provide cross sectional imaging. Respiratory motion degrades the image. Improved technology decreasing image acquisition time has improved the images. The equipment expense and availability have limited these techniques to special cases.
Choosing An Imaging Technique. Technique selection depends on expense, availability, familiarity, confidence and utility. Radiography is the least expensive and most available, can be obtained quickly, technicians can perform the study and the image can be evaluated at a convenient time. Most veterinarians are familiar with and have confidence in radiography. Radiography is limited since the organ's internal architecture cannot be studied. Radiography is the best choice when the symptoms are nonspecific and the lesion's site unclear. Radiography permits evaluation of the entire abdomen including the external soft tissues and bones. Ultrasound is expensive, but is useful in discriminating between cystic and solid masses, is well suited for bladder evaluation, less useful in discriminating among renal masses and in discriminating among the diffuse renal diseases. Ultrasound is useful for detecting urinary tract mineralization or calculi, for cystocentesis, fine needle aspirate or biopsy but is inefficient in imaging ureters or urethra. Radiographic contrast procedures are ideal for imaging the urethra and ureter. EU is a good substitute if ultrasound is not available. Ultrasound is superior to cystography except in bladder rupture when a positive contrast cystogram should be performed.Scintigraphy, expensive and rarely available, produces poor anatomic detail, is most useful for physiologic studies. Magnetic resonance and computed tomography are expensive. Take an x-ray first, followed by ultrasound and then a contrast study. If the disease is limited to or the clinical signs point directly to the urinary tract ultrasound should be the procedure of choice.
Norman Ackerman, DVM, DACVR