Cortex Calling: Better Imaging, Better Kidney Clarity

Comparison of B-mode, conventional color Doppler, and superb microvascular imaging for evaluating renal cortical thickness in dogs without and with chronic kidney disease

Jiwoo Koh, Younghoon Song, Dong-Gil Lee, Aryung Nam, Kidong Eom, Jaehwan Kim

Background
Chronic kidney disease (CKD) in dogs is commonly diagnosed using clinical staging from the International Renal Interest Society (IRIS) and standard ultrasonographic imaging. While B-mode ultrasound is frequently used to assess renal abnormalities, its limitations—especially in visualizing the corticomedullary junction—prompted evaluation of enhanced imaging methods. This study aimed to compare renal cortical thickness (RCT), normalized to aortic diameter (RCT:Ao ratio), across B-mode, color Doppler imaging (CDI), and superb microvascular imaging (SMI) to determine the most reliable technique for assessing renal function in dogs with CKD.

Methods
An observational study included 85 dogs—36 controls and 49 with CKD stages 1–4 based on IRIS guidelines. Ultrasonographic exams were performed using three modalities (B-mode, CDI, SMI) across sagittal, transverse, and dorsal planes. Measurements included renal dimensions and RCT:Ao ratios. Clinical, biochemical, and imaging data were analyzed for correlation with CKD stage, and ROC analysis was conducted to determine diagnostic performance and cutoff values by stage.

Results
The RCT:Ao ratio measured by CDI and SMI showed significantly stronger correlations with IRIS CKD stages and serum creatinine levels than B-mode. RCT:Ao ratios in SMI and CDI had nearly identical performance, both outperforming B-mode across all imaging planes. Optimal diagnostic cutoffs for CDI/SMI ranged from 0.62 (stage 1) to 0.44 (stage 4). Imaging planes influenced RCT values, with sagittal plane measurements being lower than dorsal or transverse ones. A hyperechoic layer, possibly representing a cortical anisotropic artifact, was significantly more common in the control group and may indicate healthy kidney status.

Limitations
The study’s relatively small sample size, particularly in advanced CKD stages, and the use of two ultrasound devices may limit generalizability. Additionally, glomerular filtration rate (GFR), a direct renal function measure, was not included. Standardization of reference values across imaging planes was also not established.

Conclusions
Color Doppler and SMI offer superior diagnostic accuracy for assessing renal cortical thickness in dogs with CKD. The RCT:Ao ratio using these methods correlates more strongly with CKD severity and provides more consistent data across imaging planes. Adoption of these advanced imaging techniques may enhance early diagnosis and monitoring of CKD, especially when standardized per imaging plane.

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