• canine;
  • hyperechoic outer medulla;
  • medullary rim sign;
  • renal;
  • ultrasound

Ultrasound findings of the canine kidney include a hyperechoic cortex and a hypo to anechoic medulla. In this study, the sonographic appearance of the outer renal medulla in dogs without evidence of renal disease is described. Dogs that underwent abdominal ultrasound over a 6-month period were subjected to review and then divided into six groups based on body weight (kg): < 4.9, 5.0–9.9, 10–19.9, 20–29.9, 30–39.9, and ≥ 40. Chi-square analysis was used to determine if the frequency of a hyperechoic outer medulla was significantly different between weight groups, sex, and age (P-value < 0.05). Of the 145 dogs that met the inclusion criteria, 45 had a hyperechoic outer medulla relative to the cortex and inner medulla. In the remaining dogs, the outer medulla was isoechoic to the cortex. Dogs less than 5 kg had the highest frequency of a hyperechoic outer medulla (P < 0.0001) and dogs greater than 40 kg did not have a hyperechoic outer medulla (P < 0.0001). Sex had no influence on the presence or absence of the hyperechoic outer medulla; however, younger dogs were overrepresented (6.4 ± 0.6 years compared with 7.8 ± 0.4 years; P = 0.04). Ultrasound descriptions of the canine kidney have not taken into account the contributions of the renal cortex and outer medulla. Based on this study of dogs with no clinically significant renal disease, the outer medulla can be isoechoic or hyperechoic to the cortex and a hyperechoic outer medulla is more commonly seen in small breed dogs.