Abstract
Background:
Seven-tesla magnetic resonance imaging (7T MRI) non-invasively detects microstructural changes associated with chronic kidney disease (CKD). To obtain high resolution, it is often necessary to use animal-specific coils and to minimize artifacts from breathing and heartbeat even under anesthesia. However, in feline medicine, this ultra-high field imaging has not been investigated either in vivo or even in ex vivo using explanted organs.

Aim:
This study aimed to use a high-resolution ex-vivo imaging approach to characterize microstructural changes in a kidney from a cat with CKD compared to a background-matched control, and to correlate the MRI findings with histopathology, focusing on the relationship between the Apparent Diffusion Coefficient (ADC) and interstitial fibrosis.

Methods:
We imaged the post-mortem kidneys from a cat with IRIS Stage 2 CKD and a background-matched control using a 7T MRI system. High-resolution T2-weighted imaging (T2WI) and DWI sequences were acquired to generate Apparent Diffusion Coefficient (ADC) maps. We measured mean ADC values in the cortex and medulla and correlated these values with the degree of interstitial fibrosis, assessed by Masson's trichrome staining.

Results:
On T2-weighted images, the CKD kidney showed a loss of corticomedullary differentiation, whereas the control kidney maintained clear definition. The mean ADC value in the renal cortex of the CKD cat (432 ± 8 μm²/sec) was lower than in the non-CKD cat (609 ± 15 μm²/sec). A lower ADC value was also found in the outer medulla of the CKD kidney. In contrast, the ADC value in the inner medulla was higher in the CKD kidney (1088 ± 19 μm²/sec) compared to the control (962 ± 26 μm²/sec). Histopathology confirmed extensive interstitial fibrosis in the CKD kidney, but only minimal fibrosis in the control kidney.

Conclusion:
Our results demonstrate that high-resolution ex-vivo MRI can detect microstructural changes in the feline CKD kidney. In particular, we found that a decreased cortical ADC value is strongly associated with interstitial fibrosis. These findings suggest that DWI could be a promising non-invasive tool for assessing the pathology of feline CKD and provide a basis for future studies in living animals.

Key words: Feline chronic kidney disease (CKD); Magnetic resonance imaging (MRI); Apparent diffusion coefficient (ADC); Interstitial fibrosis; Ex vivo.