CT of the feline ureters for suspected ureteral obstruction

Why ureteral obstruction in cats needs precise imaging

Ureteral calculi in cats cause vague signs. In 163 affected cats (Kyles et al., Journal of the American Veterinary Medical Association, 2005), clinical signs tended to be non-specific and included inappetence, vomiting, lethargy and weight loss. Azotaemia was present in 134 of 162 cats (83%), and 91 of 93 analysed calculi (98%) contained calcium oxalate. The authors recommend abdominal imaging in every cat with chronic non-specific signs or with acute or chronic renal failure.

Calculi are not the only cause. In 72 cats with 103 obstructed kidneys (Merindol et al., Journal of Veterinary Internal Medicine, 2023), obstruction was due to uroliths in 73%, strictures in 13.5% and pyonephrosis in 13.5% of kidneys. Dried solidified blood calculi can also obstruct the ureter. In a series of cats with these calculi (Westropp et al., Journal of Veterinary Internal Medicine, 2006), no radio-opaque calculi were visible on radiographs, including contrast studies, and ultrasound reports did not record discrete calculi.

Ultrasound first, CT for the full map

Ultrasound is the usual first test. In 122 azotaemic cats presented as emergencies (Beeston et al., Journal of Small Animal Practice, 2023), point-of-care ultrasound abnormalities were found in 60 of 74 obstructed cats (81.1%) and in 18 of 48 non-obstructed cats (37.5%). Cats with renal pelvis dilation were 39 times more likely to have ureteral obstruction than non-obstructive acute kidney injury. Radiography and ultrasound together revealed calculi in 66 of 73 cats (90%) whose diagnosis was confirmed at surgery or necropsy.

A prospective study (Testault et al., Journal of Veterinary Internal Medicine, 2021) compared ultrasound with non-enhanced CT, performed within six hours of each other, in 51 cats (101 ureters) with at least one ureteral calculus:

Finding	Ultrasound	Non-enhanced CT
Calculi detected	90	126
Ureters with calculi	57	70
Calculi in the distal third of the ureter	17	42
Ureters in which only this modality found calculi	0	13

• CT also found bilateral calculi in significantly more cats (19 cats on CT).
• The difference was significant in the proximal and distal ureter but not in the middle third.
• The authors explain that a ureter that is not dilated is hard to follow on ultrasound, and that the narrow caudal abdomen, the colon and the bladder can hide the distal ureter.
• An ultrasound renal pelvis above 5 mm and a ureter above 3 mm were always associated with calculi, but values overlapped between groups.

The authors suggest CT when urolithiasis is suspected but ultrasound shows no pelvic or ureteral dilation.

Why the number and position of calculi matter

Location affects the treatment decision. With medical management, success was reported in 23% (17 of 75) of kidneys obstructed by uroliths. Distal position and smaller size (median length 1.85 mm) were significantly associated with success. A count of every calculus, its side and its level, in both ureters, is therefore useful before choosing between medical management and intervention.

Circumcaval ureter: a feline variant that CT shows well

In a circumcaval (retrocaval) ureter, the ureter passes dorsal to the caudal vena cava. It was present in 106 of 301 cat cadavers, or 35.2% (Bélanger et al., American Journal of Veterinary Research, 2014), most often on the right. On contrast-enhanced CT of 272 cats (Pey et al., Veterinary Radiology & Ultrasound, 2015), 68 had a presumed preureteral vena cava, always with a “reverse-J” course of the ureter at L4–5. Affected cats had a higher risk of concurrent urinary signs (odds ratio 3.00).

Among 128 cats with ureteral obstruction (Steinhaus et al., Journal of Veterinary Internal Medicine), 22 (17%) had a circumcaval ureter. Strictures caused the obstruction more often in these cats than in cats with normal anatomy (40% versus 17%). Strictures in another series of 10 cats (Zaid et al., Journal of Veterinary Internal Medicine, 2011) were linked to previous ureteral surgery, inflammation, a circumcaval ureter, impacted calculi or unknown causes.

Ureter CT protocol at a glance

The comparison with ultrasound used non-enhanced CT, performed under sedation, or under general anaesthesia when a procedure was already planned, as described in the prospective CT study. Its authors note that intravenous urography and contrast-enhanced CT are rarely used in ureteral obstruction, because of the potential nephrotoxicity of iodinated contrast and reduced filtration.

What the radiologist looks at

• Every calculus, in both ureters. The number, side and level of each calculus, since position and size relate to the success of medical management.
• A circumcaval ureter on each side, because strictures caused obstruction more often in these cats.
• Calculi in a row. Adjacent calculi can be counted as one. Ultrasound found 6 calculi that CT did not count, all in ureters where CT saw at least one calculus.
• Retroperitoneal fluid. Effusion or inflammation can make the whole ureter hard to follow on non-enhanced CT.

What your report will include

Every Veheri case is read by a radiologist or neurologist, and when you order you can choose Diplomate-only reporting or the full specialist panel. You can chat directly with the specialist assigned to your case, and a simplified pet owner report helps you explain the findings to the owner.

When you take the report forward:

• Detection is not obstruction. The CT study recorded calculi, not whether the ureter was obstructed, and had no surgical or necropsy confirmation.
• Non-mineral calculi can be missed: the CT attenuation of dried solidified blood calculi is unknown, and they may be missed.
• Medical management was more often successful with distal and smaller calculi, so the position and size in the report feed directly into that decision.
• A circumcaval ureter with obstruction raises the possibility of a stricture rather than a calculus.

What to include when you request a ureteral CT report

• signalment: age, breed and sex
• creatinine, potassium and calcium, and whether the cat is azotaemic
• ultrasound findings: renal pelvis size, ureteral diameter and where dilation ends
• previous calculi, stone analysis, ureteral surgery or stents
• whether medical management is being considered or has been tried
• the question: number, side and level of all calculi in both ureters, and any circumcaval ureter

How Veheri can help

Veheri’s radiologists report veterinary CT studies remotely for clinics, and we offer expert support for CT protocols, positioning and acquisition. For dogs, see CT of the ureters in dogs, and for abdominal scanning, see optimising routine abdominal CT protocols.