Drawing on current literature relating to ultrasonographic examination of feline kidneys, the aim is to summarise ultrasonographic technique, anatomy and changes associated with renal and perirenal diseases.
Healthy cats of three cat breeds -- Sphynx (n = 11), British Shorthair (n = 15) and Ragdoll (n = 15) -- were included in this study. All cats underwent an ultrasonographic examination to assess renal length, cortical thickness, medullary thickness and corticomedullary ratio. Of all ultrasonographic measurements, renal length showed the highest variation. For all ultrasonographic dimensions, individual and kidney side (left vs right) variation were much more pronounced than interbreed variation. Sphynx cats tended to have larger kidneys (4.09 ± 0.33 cm) than British Shorthair (3.77 ± 0.43 cm) and Ragdoll cats (3.87 ± 0.41 cm). British Shorthair cats, however, tended to have a thinner cortex (0.67 ± 0.13 cm) and medulla (0.76 ± 0.18 cm) than Sphynx (0.76 ± 0.14 cm and 0.90 ± 0.25 cm, respectively) and Ragdoll cats (0.75 ± 0.13 cm and 0.91 ± 0.22 cm, respectively). However, statistical tests did not reveal significant differences between these cat breeds. The corticomedullary ratio was similar for the three cat breeds (Sphynx: 0.93 ± 0.43; British Shorthair: 0.91 ± 0.26; Ragdoll: 0.88 ± 0.31). The left kidney (3.83 ± 0.42 cm) was significantly smaller than the right kidney (3.99 ± 0.40 cm) and showed a thicker medulla (left: 0.93 ± 0.21 cm, right: 0.79 ± 0.22 cm), and thus a lower corticomedullary ratio (left: 0.80 ± 0.23, right: 1.01 ± 0.32). For the cortical thickness, no significant difference was observed between the left (0.71 ± 0.14 cm) and right kidney (0.74 ± 0.14 cm).
Fifty-six healthy Ragdoll cats underwent an ultrasonographical examination of the urinary tract to evaluate if gender, age, bodyweight and presence of a medullary rim sign had a significant influence on renal length and corticomedullary ratio (CM). Individual variation percentage was much more pronounced for renal length in comparison with CM ratio. Mean renal length measured 3.83 ± 0.45 cm (range 2.98-5.09 cm), mean cortical thickness 0.73 ± 0.15 cm (range 0.36-1.18 cm), mean medullary thickness 0.87 ± 0.19 cm (range 0.46-1.39 cm) and mean CM ratio 0.88 ± 0.29 (range 0.29-1.78). Renal length showed a significant positive correlation with bodyweight (P <0.0001), age (P = 0.0073) and male gender (P <0.0001). Therefore, these parameters have to be kept in mind when evaluating renal length on ultrasound. The CM ratio was solely influenced by the presence of a medullary rim sign (P <0.0001). Further research, however, is needed to investigate the usefulness of the CM ratio for the detection of kidney disease by ultrasonography.
Evaluation of glomerular function is a useful part of the diagnostic approach in animals suspected of having renal disease. Time-interval and background region of interest (bg ROI) selection are determining factors when calculating the glomerular filtration ratio (GFR) based on percentage uptake of (99m)technetium-labelled diethylene triamine penta-acetic acid ((99m)Tc-DTPA). Therefore, three different time intervals (60-120 s, 120-180 s, 60-180 s) and three different bg ROIs (C-shape, caudolateral, cranial + caudal) were investigated. In addition, global GFRs based on percentage dose uptake of (99m)Tc-DTPA for the different time-intervals and bg ROIs were compared with the global GFR based on (51)chromium-ethylene diaminic tetra-acetic acid ((51)Cr-EDTA) plasma clearance in nine healthy European domestic shorthair cats. Paired Student's t-tests and linear regression analysis were used to analyse the data. Different time intervals seemed to cause significant variation (P <0.01) in absolute GFR values, regardless of the choice of bg ROI. Significant differences (P <0.01) between bg ROIs were only observed in the 120-180s time interval between the C-shape and cranial + caudal bg ROI, and between the caudolateral and cranial + caudal bg ROI. The caudolateral bg ROI in the 60-180 s time interval showed the highest correlation coefficient (r = 0.882) between (99m)Tc-DTPA and (51)Cr-EDTA, although a significant difference (P <0.05) was present between both techniques.
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