The most common types of uroliths larger than 1 mm are readily visible and accurately measured in an in vitro setting mimicking the canine abdomen using digital radiography

Patricia DeBow, Mylène Auger, Constance Fazio, Kelsey Cline, Xiaojuan Zhu, Jody Lulich, Marie de Swarte, Donald Lamb, Adrien-Maxence Hespel

Background: Historically, cysteine and urate calculi were not considered visible radiographically. This study evaluates the visibility and measurement accuracy of pure composition uroliths using digital radiography and urinary bladder phantoms.

Study: The study was a prospective, in vitro, diagnostic accuracy design that involved 40 canine uroliths of known sizes and compositions (calcium oxalate, struvite, cystine, and urate). The uroliths were placed in phantoms of three different sizes representing small, medium, and large canine abdomens and radiographed with a direct digital radiology system.

Methods: Three board-certified veterinary radiologists, blinded to urolith presence/absence, size, number, and composition, independently evaluated each image using a DICOM viewer. They assessed the presence of a urolith and the largest diameter of the urolith using digital calipers. The measurements were repeated twice by each reader with at least a 1-month interval between measurements and re-randomization of the second set of radiographs. The reference standard for urolith composition and size was crystallography and/or infrared spectroscopy performed by the Minnesota Urolith Center.

Results: For all four types of uroliths and all three readers, the specificity and positive predictive value were 100% with an associated very high sensitivity (94.4%–98.9%) and negative predictive value (94.8%–98.9%). The only uroliths that were not detected were 1 mm in size. Calcium oxalate uroliths were the most accurately measured and struvite were the least accurately measured when compared with the reference standard. Smaller uroliths were more accurately measured than larger uroliths. Uroliths placed in smaller bladder phantoms were more accurately measured than in larger bladder phantoms.

Limitations: The main limitation of the study was its in vitro nature with the use of urinary bladder phantoms, which may not reflect the real clinical conditions and challenges of detecting and measuring uroliths in live patients. Another potential limitation was the lack of a calibration marker included in each of the radiographs, which could have affected the accuracy of the measurements.

Conclusions: The study demonstrated that urate and cystine uroliths, when placed within a phantom bladder model, were identifiable on digital radiographs. All types of uroliths greater than 1 mm in size could be visualized and accurately measured in an in vitro setting. Based on these findings, urate and cystine uroliths should now also be considered as differentials when mineral opaque uroliths are identified on radiographs.

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